]> An in-depth exploration of the art of shell scripting Mendel Cooper

thegrendel@theriver.com

2.1.3 30 September 2003 0.1 14 June 2000 mc Initial release. 0.2 30 October 2000 mc Bugs fixed, plus much additional material and more example scripts. 0.3 12 February 2001 mc Another major update. 0.4 08 July 2001 mc More bugfixes, much more material, more scripts - a complete revision and expansion of the book. 0.5 03 September 2001 mc Major update. Bugfixes, material added, chapters and sections reorganized. 1.0 14 October 2001 mc Bugfixes, reorganization, material added. Stable release. 1.1 06 January 2002 mc Bugfixes, material and scripts added. 1.2 31 March 2002 mc Bugfixes, material and scripts added. 1.3 02 June 2002 mc 'TANGERINE' release: A few bugfixes, much more material and scripts added. 1.4 16 June 2002 mc 'MANGO' release: Quite a number of typos fixed, more material and scripts added. 1.5 13 July 2002 mc 'PAPAYA' release: A few bugfixes, much more material and scripts added. 1.6 29 September 2002 mc 'POMEGRANATE' release: some bugfixes, more material, one more script added. 1.7 05 January 2003 mc 'COCONUT' release: a couple of bugfixes, more material, one more script. 1.8 10 May 2003 mc 'BREADFRUIT' release: a number of bugfixes, more scripts and material. 1.9 21 June 2003 mc 'PERSIMMON' release: bugfixes and more material. 2.0 24 August 2003 mc 'GOOSEBERRY' release: Major update. 2.1 14 September 2003 mc 'HUCKLEBERRY' release: bugfixes and more material. 2.1.3 30 September 2003 mc Intermediate update, leading up to 'CRANBERRY' release. This tutorial assumes no previous knowledge of scripting or programming, but progresses rapidly toward an intermediate/advanced level of instruction . . . all the while sneaking in little snippets of UNIX wisdom and lore. It serves as a textbook, a manual for self-study, and a reference and source of knowledge on shell scripting techniques. The exercises and heavily-commented examples invite active reader participation, under the premise that the only way to really learn scripting is to write scripts. This book is suitable for classroom use as a general introduction to programming concepts. The latest update of this document, as an archived, bzip2-ed tarball including both the SGML source and rendered HTML, may be downloaded from the author's home site. See the change log for a revision history. For Anita, the source of all the magic The shell is a command interpreter. More than just the insulating layer between the operating system kernel and the user, it's also a fairly powerful programming language. A shell program, called a script, is an easy-to-use tool for building applications by gluing together system calls, tools, utilities, and compiled binaries. Virtually the entire repertoire of UNIX commands, utilities, and tools is available for invocation by a shell script. If that were not enough, internal shell commands, such as testing and loop constructs, give additional power and flexibility to scripts. Shell scripts lend themselves exceptionally well to administrative system tasks and other routine repetitive jobs not requiring the bells and whistles of a full-blown tightly structured programming language. A working knowledge of shell scripting is essential to anyone wishing to become reasonably proficient at system administration, even if they do not anticipate ever having to actually write a script. Consider that as a Linux machine boots up, it executes the shell scripts in /etc/rc.d to restore the system configuration and set up services. A detailed understanding of these startup scripts is important for analyzing the behavior of a system, and possibly modifying it. Writing shell scripts is not hard to learn, since the scripts can be built in bite-sized sections and there is only a fairly small set of shell-specific operators and options These are referred to as builtins, features internal to the shell. to learn. The syntax is simple and straightforward, similar to that of invoking and chaining together utilities at the command line, and there are only a few rules to learn. Most short scripts work right the first time, and debugging even the longer ones is straightforward. A shell script is a quick and dirty method of prototyping a complex application. Getting even a limited subset of the functionality to work in a shell script, even if slowly, is often a useful first stage in project development. This way, the structure of the application can be tested and played with, and the major pitfalls found before proceeding to the final coding in C, C++, Java, or Perl. Shell scripting hearkens back to the classical UNIX philosophy of breaking complex projects into simpler subtasks, of chaining together components and utilities. Many consider this a better, or at least more esthetically pleasing approach to problem solving than using one of the new generation of high powered all-in-one languages, such as Perl, which attempt to be all things to all people, but at the cost of forcing you to alter your thinking processes to fit the tool. When not to use shell scripts resource-intensive tasks, especially where speed is a factor (sorting, hashing, etc.) procedures involving heavy-duty math operations, especially floating point arithmetic, arbitrary precision calculations, or complex numbers (use C++ or FORTRAN instead) cross-platform portability required (use C instead) complex applications, where structured programming is a necessity (need type-checking of variables, function prototypes, etc.) mission-critical applications upon which you are betting the ranch, or the future of the company situations where security is important, where you need to guarantee the integrity of your system and protect against intrusion, cracking, and vandalism project consists of subcomponents with interlocking dependencies extensive file operations required (Bash is limited to serial file access, and that only in a particularly clumsy and inefficient line-by-line fashion) need multi-dimensional arrays need data structures, such as linked lists or trees need to generate or manipulate graphics or GUIs need direct access to system hardware need port or socket I/O need to use libraries or interface with legacy code proprietary, closed-source applications (shell scripts put the source code right out in the open for all the world to see) If any of the above applies, consider a more powerful scripting language, perhaps Perl, Tcl, Python, Ruby, or possibly a high-level compiled language such as C, C++, or Java. Even then, prototyping the application as a shell script might still be a useful development step. We will be using Bash, an acronym for Bourne-Again Shell and a pun on Stephen Bourne's now classic Bourne Shell. Bash has become a de facto standard for shell scripting on all flavors of UNIX. Most of the principles dealt with in this book apply equally well to scripting with other shells, such as the Korn Shell, from which Bash derives some of its features, Many of the features of ksh88, and even a few from the updated ksh93 have been merged into Bash. and the C Shell and its variants. (Note that C Shell programming is not recommended due to certain inherent problems, as pointed out in an October, 1993 Usenet post by Tom Christiansen.) What follows is a tutorial on shell scripting. It relies heavily on examples to illustrate various features of the shell. The example scripts work -- they've been tested -- and some of them are even useful in real life. The reader can play with the actual working code of the examples in the source archive (scriptname.sh), By convention, user-written shell scripts that are Bourne shell compliant generally take a name with a .sh extension. System scripts, such as those found in /etc/rc.d, do not follow this guideline. give them execute permission (chmod u+rx scriptname), then run them to see what happens. Should the source archive not be available, then cut-and-paste from the HTML, pdf, or text rendered versions. Be aware that some of the scripts below introduce features before they are explained, and this may require the reader to temporarily skip ahead for enlightenment. Unless otherwise noted, the author of this book wrote the example scripts that follow. In the simplest case, a script is nothing more than a list of system commands stored in a file. At the very least, this saves the effort of retyping that particular sequence of commands each time it is invoked. &ex1; There is nothing unusual here, just a set of commands that could just as easily be invoked one by one from the command line on the console or in an xterm. The advantages of placing the commands in a script go beyond not having to retype them time and again. The script can easily be modified, customized, or generalized for a particular application. &ex2; Since you may not wish to wipe out the entire system log, this variant of the first script keeps the last section of the message log intact. You will constantly discover ways of refining previously written scripts for increased effectiveness. The sha-bang sha-bang ( #! #!) at the head of a script tells your system that this file is a set of commands to be fed to the command interpreter indicated. The #! is actually a two-byte Some flavors of UNIX (those based on 4.2BSD) take a four-byte magic number, requiring a blank after the ! -- #! /bin/sh. magic number magic number, a special marker that designates a file type, or in this case an executable shell script (see man magic for more details on this fascinating topic). Immediately following the sha-bang is a path name. This is the path to the program that interprets the commands in the script, whether it be a shell, a programming language, or a utility. This command interpreter then executes the commands in the script, starting at the top (line 1 of the script), ignoring comments. The #! line in a shell script will be the first thing the command interpreter (sh or bash) sees. Since this line begins with a #, it will be correctly interpreted as a comment when the command interpreter finally executes the script. The line has already served its purpose - calling the command interpreter. If, in fact, the script includes an extra #! line, then bash will interpret it as a comment. #!/bin/bash echo "Part 1 of script." a=1 #!/bin/bash # This does *not* launch a new script. echo "Part 2 of script." echo $a # Value of $a stays at 1. #!/bin/sh #!/bin/bash #!/usr/bin/perl #!/usr/bin/tcl #!/bin/sed -f #!/usr/awk -f Each of the above script header lines calls a different command interpreter, be it /bin/sh, the default shell (bash in a Linux system) or otherwise. This allows some cute tricks. #!/bin/rm # Self-deleting script. # Nothing much seems to happen when you run this... except that the file disappears. WHATEVER=65 echo "This line will never print (betcha!)." exit $WHATEVER # Doesn't matter. The script will not exit here. Also, try starting a README file with a #!/bin/more, and making it executable. The result is a self-listing documentation file. Using #!/bin/sh, the default Bourne Shell in most commercial variants of UNIX, makes the script portable to non-Linux machines, though you may have to sacrifice a few Bash-specific features. The script will, however, conform to the POSIX Portable Operating System Interface, an attempt to standardize UNIX-like OSes. sh standard. Note that the path given at the sha-bang must be correct, otherwise an error message -- usually Command not found -- will be the only result of running the script. #! can be omitted if the script consists only of a set of generic system commands, using no internal shell directives. The second example, above, requires the initial #!, since the variable assignment line, lines=50, uses a shell-specific construct. Note again that #!/bin/sh invokes the default shell interpreter, which defaults to /bin/bash on a Linux machine. This tutorial encourages a modular approach to constructing a script. Make note of and collect boilerplate code snippets that might be useful in future scripts. Eventually you can build a quite extensive library of nifty routines. As an example, the following script prolog tests whether the script has been invoked with the correct number of parameters. if [ $# -ne Number_of_expected args ] then echo "Usage: `basename $0` whatever" exit $WRONG_ARGS fi Having written the script, you can invoke it by sh scriptname, Caution: invoking a Bash script by sh scriptname turns off Bash-specific extensions, and the script may therefore fail to execute. or alternatively bash scriptname. (Not recommended is using sh <scriptname, since this effectively disables reading from stdin within the script.) Much more convenient is to make the script itself directly executable with a chmod. Either: chmod 555 scriptname (gives everyone read/execute permission) A script needs read, as well as execute permission for it to run, since the shell needs to be able to read it. or chmod +rx scriptname (gives everyone read/execute permission) chmod u+rx scriptname (gives only the script owner read/execute permission) Having made the script executable, you may now test it by ./scriptname. Why not simply invoke the script with scriptname? If the directory you are in ($PWD) is where scriptname is located, why doesn't this work? This fails because, for security reasons, the current directory, . is not included in a user's $PATH. It is therefore necessary to explicitly invoke the script in the current directory with a ./scriptname. If it begins with a sha-bang line, invoking the script calls the correct command interpreter to run it. As a final step, after testing and debugging, you would likely want to move it to /usr/local/bin (as root, of course), to make the script available to yourself and all other users as a system-wide executable. The script could then be invoked by simply typing scriptname [ENTER] from the command line. System administrators often write scripts to automate common tasks. Give several instances where such scripts would be useful. Write a script that upon invocation shows the time and date, lists all logged-in users, and gives the system uptime. The script then saves this information to a logfile. # # special character # comment Lines beginning with a # (with the exception of #!) are comments. # This line is a comment. Comments may also occur at the end of a command. echo "A comment will follow." # Comment here. Comments may also follow whitespace at the beginning of a line. # A tab precedes this comment. A command may not follow a comment on the same line. There is no method of terminating the comment, in order for live code to begin on the same line. Use a new line for the next command. Of course, an escaped # in an echo statement does not begin a comment. Likewise, a # appears in certain parameter substitution constructs and in numerical constant expressions. echo "The # here does not begin a comment." echo 'The # here does not begin a comment.' echo The \# here does not begin a comment. echo The # here begins a comment. echo ${PATH#*:} # Parameter substitution, not a comment. echo $(( 2#101011 )) # Base conversion, not a comment. # Thanks, S.C. The standard quoting and escape characters (" ' \) escape the #. Certain pattern matching operations also use the #. ; ; special character ; separator [Semicolon] Permits putting two or more commands on the same line. echo hello; echo there if [ -x "$filename" ]; then # Note that "if" and "then" need separation. # Why? echo "File $filename exists."; cp $filename $filename.bak else echo "File $filename not found."; touch $filename fi; echo "File test complete." Note that the ; sometimes needs to be escaped. ;; ;; special character case ;; [Double semicolon] case "$variable" in abc) echo "$variable = abc" ;; xyz) echo "$variable = xyz" ;; esac . . special character . dot command source [period] Equivalent to source (see ). This is a bash builtin. . . special character . filename part of a filename When working with filenames, a dot is the prefix of a hidden file, a file that an ls will not normally show. bash$ touch .hidden-file bash$ ls -l total 10 -rw-r--r-- 1 bozo 4034 Jul 18 22:04 data1.addressbook -rw-r--r-- 1 bozo 4602 May 25 13:58 data1.addressbook.bak -rw-r--r-- 1 bozo 877 Dec 17 2000 employment.addressbook bash$ ls -al total 14 drwxrwxr-x 2 bozo bozo 1024 Aug 29 20:54 ./ drwx------ 52 bozo bozo 3072 Aug 29 20:51 ../ -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.addressbook -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.addressbook.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.addressbook -rw-rw-r-- 1 bozo bozo 0 Aug 29 20:54 .hidden-file When considering directory names, a single dot represents the current working directory, and two dots denote the parent directory. bash$ pwd /home/bozo/projects bash$ cd . bash$ pwd /home/bozo/projects bash$ cd .. bash$ pwd /home/bozo/ The dot often appears as the destination (directory) of a file movement command. bash$ cp /home/bozo/current_work/junk/* . . . special character . character match match single character When matching characters, as part of a regular expression, a dot matches a single character. " [double quote] "STRING" preserves (from interpretation) most of the special characters within STRING. See also . ' [single quote] 'STRING' preserves all special characters within STRING. This is a stronger form of quoting than using ". See also . , The comma operator links together a series of arithmetic operations. All are evaluated, but only the last one is returned. let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". \ [backslash] \X escapes the character X. This has the effect of quoting X, equivalent to 'X'. The \ may be used to quote " and ', so they are expressed literally. See for an in-depth explanation of escaped characters. / [forward slash] Separates the components of a filename (as in /home/bozo/projects/Makefile). This is also the division arithmetic operator. ` [backticks] `command` makes available the output of command for setting a variable. This is also known as backticks or backquotes. : : special character : null command true endless loop [colon] This is the shell equivalent of a NOP (no op, a do-nothing operation). It may be considered a synonym for the shell builtin true. The : command is a itself a Bash builtin, and its exit status is true (0). : echo $? # 0 Endless loop: while : do operation-1 operation-2 ... operation-n done # Same as: # while true # do # ... # done Placeholder in if/then test: if condition then : # Do nothing and branch ahead else take-some-action fi Provide a placeholder where a binary operation is expected, see and default parameters. : ${username=`whoami`} # ${username=`whoami`} without the leading : gives an error # unless "username" is a command or builtin... Provide a placeholder where a command is expected in a here document. See . Evaluate string of variables using parameter substitution (as in ). : ${HOSTNAME?} ${USER?} ${MAIL?} #Prints error message if one or more of essential environmental variables not set. Variable expansion / substring replacement. In combination with the > redirection operator, truncates a file to zero length, without changing its permissions. If the file did not previously exist, creates it. : > data.xxx # File "data.xxx" now empty. # Same effect as cat /dev/null >data.xxx # However, this does not fork a new process, since ":" is a builtin. See also . In combination with the >> redirection operator, updates a file access/modification time (: >> new_file). If the file did not previously exist, creates it. This is equivalent to touch. This applies to regular files, not pipes, symlinks, and certain special files. May be used to begin a comment line, although this is not recommended. Using # for a comment turns off error checking for the remainder of that line, so almost anything may be appear in a comment. However, this is not the case with :. : This is a comment that generates an error, ( if [ $x -eq 3] ). The : also serves as a field separator, in /etc/passwd, and in the $PATH variable. bash$ echo $PATH /usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games ! ! special character ! not logical not The ! operator inverts the exit status of the command to which it is applied (see ). It also inverts the meaning of a test operator. This can, for example, change the sense of equal ( = ) to not-equal ( != ). The ! operator is a Bash keyword. In a different context, the ! also appears in indirect variable references. In yet another context, from the command line, the ! invokes the Bash history mechanism (see ). Note that within a script, the history mechanism is disabled. * * special character * wild card globbing regular expression [asterisk] The * character serves as a wild card for filename expansion in globbing. By itself, it matches every filename in a given directory. bash$ echo * abs-book.sgml add-drive.sh agram.sh alias.sh The * also represents any number (or zero) characters in a regular expression. * * special character * multiplication exponentiation arithmetic operator In the context of arithmetic operations, the * denotes multiplication. A double asterisk, **, is the exponentiation operator. ? ? special character ? test operator test token Within certain expressions, the ? indicates a test for a condition. In a double parentheses construct, the ? serves as a C-style trinary operator. See . In a parameter substitution expression, the ? tests whether a variable has been set. ? ? special character ? wild card globbing regular expression The ? character serves as a single-character wild card for filename expansion in globbing, as well as representing one character in an extended regular expression. $ $ special character $ variable substitution var1=5 var2=23skidoo echo $var1 # 5 echo $var2 # 23skidoo A $ prefixing a variable name indicates the value the variable holds. $ $ special character $ regular expression end of line In a regular expression, a $ addresses the end of a line of text. ${} ${} special character ${} parameter substitution $* $@ $* special character $* $@ positional parameters $@ $? $? special character ? exit status variable exit status The $? variable holds the exit status of a command, a function, or of the script itself. $$ $$ special character $$ process id variable process id The $$ variable holds the process id of the script in which it appears. () (a=hello; echo $a) A listing of commands within parentheses starts a subshell. Variables inside parentheses, within the subshell, are not visible to the rest of the script. The parent process, the script, cannot read variables created in the child process, the subshell. a=123 ( a=321; ) echo "a = $a" # a = 123 # "a" within parentheses acts like a local variable. Array=(element1 element2 element3) {xxx,yyy,zzz..} special character {} brace expansion {xxx,yyy,zzz,...} grep Linux file*.{txt,htm*} # Finds all instances of the word "Linux" # in the files "fileA.txt", "file2.txt", "fileR.html", "file-87.htm", etc. A command may act upon a comma-separated list of file specs within braces. The shell does the brace expansion. The command itself acts upon the result of the expansion. Filename expansion (globbing) applies to the file specs between the braces. No spaces allowed within the braces unless the spaces are quoted or escaped. echo {file1,file2}\ :{\ A," B",' C'} file1 : A file1 : B file1 : C file2 : A file2 : B file2 : C {} {} special character {} block of code [curly brackets] Also referred to as an inline group, this construct, in effect, creates an anonymous function. However, unlike a function, the variables in a code block remain visible to the remainder of the script. bash$ { local a; a=123; } bash: local: can only be used in a function a=123 { a=321; } echo "a = $a" # a = 321 (value inside code block) # Thanks, S.C. The code block enclosed in braces may have I/O redirected to and from it. &ex8; &rpmcheck; Unlike a command group within (parentheses), as above, a code block enclosed by {braces} will not normally launch a subshell. Exception: a code block in braces as part of a pipe may be run as a subshell. ls | { read firstline; read secondline; } # Error. The code block in braces runs as a subshell, # so the output of "ls" cannot be passed to variables within the block. echo "First line is $firstline; second line is $secondline" # Will not work. # Thanks, S.C. {} \; Mostly used in find constructs. This is not a shell builtin. The ; ends the -exec option of a find command sequence. It needs to be escaped to protect it from interpretation by the shell. [ ] [] special character [ ] test Test expression between [ ]. Note that [ is part of the shell builtin test (and a synonym for it), not a link to the external command /usr/bin/test. [[ ]] [[]] special character [[ ]] test Test expression between [[ ]] (shell keyword). See the discussion on the [[ ... ]] construct. [ ] [ ] special character array_element[ ] array element In the context of an array, brackets set off the numbering of each element of that array. Array[1]=slot_1 echo ${Array[1]} [ ] [ ] special character character range regular expression As part of a regular expression, brackets delineate a range of characters to match. (( )) (( )) special character (( )) integer comparison Expand and evaluate integer expression between (( )). See the discussion on the (( ... )) construct. > &> >& >> < > >& >> < special character > special character >& special character >> special character < redirection scriptname >filename redirects the output of scriptname to file filename. Overwrite filename if it already exists. command &>filename redirects both the stdout and the stderr of command to filename. command >&2 redirects stdout of command to stderr. scriptname >>filename appends the output of scriptname to file filename. If filename does not already exist, it will be created. (command)> <(command) In a different context, the < and > characters act as string comparison operators. In yet another context, the < and > characters act as integer comparison operators. See also . << < > < special character < > ASCII comparison > veg1=carrots veg2=tomatoes if [[ "$veg1" < "$veg2" ]] then echo "Although $veg1 precede $veg2 in the dictionary," echo "this implies nothing about my culinary preferences." else echo "What kind of dictionary are you using, anyhow?" fi \< \> \< regular expression \< > word boundary > bash$ grep '\<the\>' textfile | | special character | pipe Passes the output of previous command to the input of the next one, or to the shell. This is a method of chaining commands together. echo ls -l | sh # Passes the output of "echo ls -l" to the shell, #+ with the same result as a simple "ls -l". cat *.lst | sort | uniq # Merges and sorts all ".lst" files, then deletes duplicate lines. A pipe, as a classic method of interprocess communication, sends the stdout of one process to the stdin of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a filter (a command that transforms its input) for processing. cat $filename | grep $search_word The output of a command or commands may be piped to a script. #!/bin/bash # uppercase.sh : Changes input to uppercase. tr 'a-z' 'A-Z' # Letter ranges must be quoted #+ to prevent filename generation from single-letter filenames. exit 0 Now, let us pipe the output of ls -l to this script. bash$ ls -l | ./uppercase.sh -RW-RW-R-- 1 BOZO BOZO 109 APR 7 19:49 1.TXT -RW-RW-R-- 1 BOZO BOZO 109 APR 14 16:48 2.TXT -RW-R--R-- 1 BOZO BOZO 725 APR 20 20:56 DATA-FILE The stdout of each process in a pipe must be read as the stdin of the next. If this is not the case, the data stream will block, and the pipe will not behave as expected. cat file1 file2 | ls -l | sort # The output from "cat file1 file2" disappears. A pipe runs as a child process, and therefore cannot alter script variables. variable="initial_value" echo "new_value" | read variable echo "variable = $variable" # variable = initial_value If one of the commands in the pipe aborts, this prematurely terminates execution of the pipe. Called a broken pipe, this condition sends a SIGPIPE signal. >| >| special character >| redirection force noclobber This will forcibly overwrite an existing file. || || special character || or logical operator In a test construct, the || operator causes a return of 0 (success) if either of the linked test conditions is true. & A command followed by an & will run in the background. bash$ sleep 10 & [1] 850 [1]+ Done sleep 10 Within a script, commands and even loops may run in the background. &bgloop; A command run in the background within a script may cause the script to hang, waiting for a keystroke. Fortunately, there is a remedy for this. && && special character && and logical operator In a test construct, the && operator causes a return of 0 (success) only if both the linked test conditions are true. - Option flag for a command or filter. Prefix for an operator. COMMAND -[Option1][Option2][...] ls -al sort -dfu $filename set -- $variable if [ $file1 -ot $file2 ] then echo "File $file1 is older than $file2." fi if [ "$a" -eq "$b" ] then echo "$a is equal to $b." fi if [ "$c" -eq 24 -a "$d" -eq 47 ] then echo "$c equals 24 and $d equals 47." fi - - special character - redirection from/to stdin/stdout [dash] (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) # Move entire file tree from one directory to another # [courtesy Alan Cox <a.cox@swansea.ac.uk>, with a minor change] # 1) cd /source/directory Source directory, where the files to be moved are. # 2) && "And-list": if the 'cd' operation successful, then execute the next command. # 3) tar cf - . The 'c' option 'tar' archiving command creates a new archive, # the 'f' (file) option, followed by '-' designates the target file as stdout, # and do it in current directory tree ('.'). # 4) | Piped to... # 5) ( ... ) a subshell # 6) cd /dest/directory Change to the destination directory. # 7) && "And-list", as above # 8) tar xpvf - Unarchive ('x'), preserve ownership and file permissions ('p'), # and send verbose messages to stdout ('v'), # reading data from stdin ('f' followed by '-'). # # Note that 'x' is a command, and 'p', 'v', 'f' are options. # Whew! # More elegant than, but equivalent to: # cd source-directory # tar cf - . | (cd ../target-directory; tar xzf -) # # cp -a /source/directory /dest also has same effect. bunzip2 linux-2.4.3.tar.bz2 | tar xvf - # --uncompress tar file-- | --then pass it to "tar"-- # If "tar" has not been patched to handle "bunzip2", # this needs to be done in two discrete steps, using a pipe. # The purpose of the exercise is to unarchive "bzipped" kernel source. Note that in this context the - is not itself a Bash operator, but rather an option recognized by certain UNIX utilities that write to stdout, such as tar, cat, etc. bash$ echo "whatever" | cat - whatever Where a filename is expected, - redirects output to stdout (sometimes seen with tar cf), or accepts input from stdin, rather than from a file. This is a method of using a file-oriented utility as a filter in a pipe. bash$ file Usage: file [-bciknvzL] [-f namefile] [-m magicfiles] file... By itself on the command line, file fails with an error message. Add a - for a more useful result. This causes the shell to await user input. bash$ file - abc standard input: ASCII text bash$ file - #!/bin/bash standard input: Bourne-Again shell script text executable Now the command accepts input from stdin and analyzes it. The - can be used to pipe stdout to other commands. This permits such stunts as prepending lines to a file. Using diff to compare a file with a section of another: grep Linux file1 | diff file2 - Finally, a real-world example using - with tar. &ex58; Filenames beginning with - may cause problems when coupled with the - redirection operator. A script should check for this and add an appropriate prefix to such filenames, for example ./-FILENAME, $PWD/-FILENAME, or $PATHNAME/-FILENAME. If the value of a variable begins with a -, this may likewise create problems. var="-n" echo $var # Has the effect of "echo -n", and outputs nothing. - [dash] cd - changes to the previous working directory. This uses the $OLDPWD environmental variable. Do not confuse the - used in this sense with the - redirection operator just discussed. The interpretation of the - depends on the context in which it appears. - Minus sign in an arithmetic operation. = Assignment operator a=28 echo $a # 28 In a different context, the = is a string comparison operator. + Addition arithmetic operator. In a different context, the + is a Regular Expression operator. + Option flag for a command or filter. Certain commands and builtins use the + to enable certain options and the - to disable them. % Modulo (remainder of a division) arithmetic operation. In a different context, the % is a pattern matching operator. ~ [tilde] This corresponds to the $HOME internal variable. ~bozo is bozo's home directory, and ls ~bozo lists the contents of it. ~/ is the current user's home directory, and ls ~/ lists the contents of it. bash$ echo ~bozo /home/bozo bash$ echo ~ /home/bozo bash$ echo ~/ /home/bozo/ bash$ echo ~: /home/bozo: bash$ echo ~nonexistent-user ~nonexistent-user ~+ This corresponds to the $PWD internal variable. ~- This corresponds to the $OLDPWD internal variable. ^ ^ special character ^ regular expression beginning of line In a regular expression, a ^ addresses the beginning of a line of text. Control Characters A control character is a CONTROL + key combination. Ctl-C Terminate a foreground job. Ctl-D Log out from a shell (similar to exit). EOF (end of file). This also terminates input from stdin. Ctl-G BEL (beep). Ctl-H Backspace. #!/bin/bash # Embedding Ctl-H in a string. a="^H^H" # Two Ctl-H's (backspaces). echo "abcdef" # abcdef echo -n "abcdef$a " # abcd f # Space at end ^ ^ Backspaces twice. echo -n "abcdef$a" # abcdef # No space at end Doesn't backspace (why?). # Results may not be quite as expected. echo; echo Ctl-J Carriage return. Ctl-L Formfeed (clear the terminal screen). This has the same effect as the clear command. Ctl-M Newline. Ctl-U Erase a line of input. Ctl-Z Pause a foreground job. Whitespace Whitespace consists of either spaces, tabs, blank lines, or any combination thereof. In some contexts, such as variable assignment, whitespace is not permitted, and results in a syntax error. Blank lines have no effect on the action of a script, and are therefore useful for visually separating functional sections. $IFS, the special variable separating fields of input to certain commands, defaults to whitespace. Variables are at the heart of every programming and scripting language. They appear in arithmetic operations and manipulation of quantities, string parsing, and are indispensable for working in the abstract with symbols - tokens that represent something else. A variable is nothing more than a location or set of locations in computer memory holding an item of data. The name of a variable is a placeholder for its value, the data it holds. Referencing its value is called variable substitution. $ $ variable $ variable substitution Let us carefully distinguish between the name of a variable and its value. If variable1 is the name of a variable, then $variable1 is a reference to its value, the data item it contains. The only time a variable appears naked, without the $ prefix, is when declared or assigned, when unset, when exported, or in the special case of a variable representing a signal (see ). Assignment may be with an = (as in var1=27), in a read statement, and at the head of a loop (for var2 in 1 2 3). Enclosing a referenced value in double quotes (" ") does not interfere with variable substitution. This is called partial quoting, sometimes referred to as weak quoting. Using single quotes (' ') causes the variable name to be used literally, and no substitution will take place. This is full quoting, sometimes referred to as strong quoting. See for a detailed discussion. Note that $variable is actually a simplified alternate form of ${variable}. In contexts where the $variable syntax causes an error, the longer form may work (see , below). &ex9; An uninitialized variable has a null value - no assigned value at all (not zero!). Using a variable before assigning a value to it will usually cause problems. It is nevertheless possible to perform arithmetic operations on an uninitialized variable. echo "$uninitialized" # (blank line) let "uninitialized += 5" # Add 5 to it. echo "$uninitialized" # 5 # Conclusion: # An uninitialized variable has no value, however #+ it acts as if it were 0 in an arithmetic operation. # This is undocumented (and probably non-portable) behavior. See also . = = variable assignment the assignment operator (no space before & after) Do not confuse this with = and -eq, which test, rather than assign! Note that = can be either an assignment or a test operator, depending on context. &ex15; &ex16; Variable assignment using the $(...) mechanism (a newer method than backquotes) # From /etc/rc.d/rc.local R=$(cat /etc/redhat-release) arch=$(uname -m) Unlike many other programming languages, Bash does not segregate its variables by type. Essentially, Bash variables are character strings, but, depending on context, Bash permits integer operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits. &intorstring; Untyped variables are both a blessing and a curse. They permit more flexibility in scripting (enough rope to hang yourself!) and make it easier to grind out lines of code. However, they permit errors to creep in and encourage sloppy programming habits. The burden is on the programmer to keep track of what type the script variables are. Bash will not do it for you. local variables variable local variables visible only within a code block or function (see also local variables in functions) environmental variables variable environmental variables that affect the behavior of the shell and user interface In a more general context, each process has an environment, that is, a group of variables that hold information that the process may reference. In this sense, the shell behaves like any other process. Every time a shell starts, it creates shell variables that correspond to its own environmental variables. Updating or adding new environmental variables causes the shell to update its environment, and all the shell's child processes (the commands it executes) inherit this environment. The space allotted to the environment is limited. Creating too many environmental variables or ones that use up excessive space may cause problems. bash$ eval "`seq 10000 | sed -e 's/.*/export var&=ZZZZZZZZZZZZZZ/'`" bash$ du bash: /usr/bin/du: Argument list too long (Thank you, S. C. for the clarification, and for providing the above example.) If a script sets environmental variables, they need to be exported, that is, reported to the environment local to the script. This is the function of the export command. A script can export variables only to child processes, that is, only to commands or processes which that particular script initiates. A script invoked from the command line cannot export variables back to the command line environment. Child processes cannot export variables back to the parent processes that spawned them. --- positional parameters parameter positional arguments passed to the script from the command line - $0, $1, $2, $3... $0 is the name of the script itself, $1 is the first argument, $2 the second, $3 the third, and so forth. The process calling the script sets the $0 parameter. By convention, this parameter is the name of the script. See the manpage for execv. After $9, the arguments must be enclosed in brackets, for example, ${10}, ${11}, ${12}. The special variables $* and $@ denote all the positional parameters. &ex17; The bracket notation for positional parameters leads to a fairly simple way of referencing the last argument passed to a script on the command line. This also requires indirect referencing. args=$# # Number of args passed. lastarg=${!args} # Note that lastarg=${!$#} doesn't work. Some scripts can perform different operations, depending on which name they are invoked with. For this to work, the script needs to check $0, the name it was invoked by. There must also exist symbolic links to all the alternate names of the script. If a script expects a command line parameter but is invoked without one, this may cause a null variable assignment, generally an undesirable result. One way to prevent this is to append an extra character to both sides of the assignment statement using the expected positional parameter. variable1_=$1_ # This will prevent an error, even if positional parameter is absent. critical_argument01=$variable1_ # The extra character can be stripped off later, if desired, like so. variable1=${variable1_/_/} # Side effects only if $variable1_ begins with "_". # This uses one of the parameter substitution templates discussed in Chapter 9. # Leaving out the replacement pattern results in a deletion. # A more straightforward way of dealing with this is #+ to simply test whether expected positional parameters have been passed. if [ -z $1 ] then exit $POS_PARAMS_MISSING fi --- &ex18; --- shift command shift The shift command reassigns the positional parameters, in effect shifting them to the left one notch. $1 <--- $2, $2 <--- $3, $3 <--- $4, etc. The old $1 disappears, but $0 (the script name) does not change. If you use a large number of positional parameters to a script, shift lets you access those past 10, although {bracket} notation also permits this. &ex19; The shift command also works on parameters passed to a function. See . " special character " ' special character ' quote \ special character \ escape Quoting means just that, bracketing a string in quotes. This has the effect of protecting special characters in the string from reinterpretation or expansion by the shell or shell script. (A character is special if it has an interpretation other than its literal meaning, such as the wild card character, *.) bash$ ls -l [Vv]* -rw-rw-r-- 1 bozo bozo 324 Apr 2 15:05 VIEWDATA.BAT -rw-rw-r-- 1 bozo bozo 507 May 4 14:25 vartrace.sh -rw-rw-r-- 1 bozo bozo 539 Apr 14 17:11 viewdata.sh bash$ ls -l '[Vv]*' ls: [Vv]*: No such file or directory Certain programs and utilities can still reinterpret or expand special characters in a quoted string. This is an important use of quoting, protecting a command-line parameter from the shell, but still letting the calling program expand it. bash$ grep '[Ff]irst' *.txt file1.txt:This is the first line of file1.txt. file2.txt:This is the First line of file2.txt. Note that the unquoted grep [Ff]irst *.txt works under the Bash shell, but not under tcsh. When referencing a variable, it is generally advisable to enclose it in double quotes (" "). This preserves all special characters within the variable name, except $, ` (backquote), and \ (escape). Encapsulating ! within double quotes gives an error when used from the command line. Apparently this is interpreted as a history command. Within a script, though, this problem does not occur. Of more concern is the inconsistent behavior of \ within double quotes. bash$ echo hello\! hello! bash$ echo "hello\!" hello\! bash$ echo -e x\ty xty bash$ echo -e "x\ty" x y (Thank you, Wayne Pollock, for pointing this out.) Keeping $ as a special character within double quotes permits referencing a quoted variable ("$variable"), that is, replacing the variable with its value (see , above). Use double quotes to prevent word splitting. Word splitting, in this context, means dividing a character string into a number of separate and discrete arguments. An argument enclosed in double quotes presents itself as a single word, even if it contains whitespace separators. variable1="a variable containing five words" COMMAND This is $variable1 # Executes COMMAND with 7 arguments: # "This" "is" "a" "variable" "containing" "five" "words" COMMAND "This is $variable1" # Executes COMMAND with 1 argument: # "This is a variable containing five words" variable2="" # Empty. COMMAND $variable2 $variable2 $variable2 # Executes COMMAND with no arguments. COMMAND "$variable2" "$variable2" "$variable2" # Executes COMMAND with 3 empty arguments. COMMAND "$variable2 $variable2 $variable2" # Executes COMMAND with 1 argument (2 spaces). # Thanks, S.C. Enclosing the arguments to an echo statement in double quotes is necessary only when word splitting is an issue. &weirdvars; Single quotes (' ') operate similarly to double quotes, but do not permit referencing variables, since the special meaning of $ is turned off. Within single quotes, every special character except ' gets interpreted literally. Consider single quotes (full quoting) to be a stricter method of quoting than double quotes (partial quoting). Since even the escape character (\) gets a literal interpretation within single quotes, trying to enclose a single quote within single quotes will not yield the expected result. echo "Why can't I write 's between single quotes" echo # The roundabout method. echo 'Why can'\''t I write '"'"'s between single quotes' # |-------| |----------| |-----------------------| # Three single-quoted strings, with escaped and quoted single quotes between. # This example courtesy of Stephane Chazelas. Escaping is a method of quoting single characters. The escape (\) preceding a character tells the shell to interpret that character literally. With certain commands and utilities, such as echo and sed, escaping a character may have the opposite effect - it can toggle on a special meaning for that character. used with echo and sed \n \n escaped character \n newline means newline \r \r escaped character \r carriage return means return \t \t escaped character \t tabulation means tab \v \v escaped character \v vertical tabulation means vertical tab \b \b escaped character \b backspace means backspace \a \a escaped character \a alert beep flash means alert (beep or flash) \0xx \0xx escaped character \0xx octal ASCII translates to the octal ASCII equivalent of 0xx &escaped; See for another example of the $' ' string expansion construct. \" \" escaped character \" quote gives the quote its literal meaning echo "Hello" # Hello echo "\"Hello\", he said." # "Hello", he said. \$ \$ escaped character \$ dollar gives the dollar sign its literal meaning (variable name following \$ will not be referenced) echo "\$variable01" # results in $variable01 \\ \\ escaped character \\ double backslash gives the backslash its literal meaning echo "\\" # Results in \ # Whereas . . . echo "\" # Invokes secondary prompt from the command line. # In a script, gives an error message. The behavior of \ depends on whether it is itself escaped, quoted, or appearing within command substitution or a here document. # Simple escaping and quoting echo \z # z echo \\z # \z echo '\z' # \z echo '\\z' # \\z echo "\z" # \z echo "\\z" # \z # Command substitution echo `echo \z` # z echo `echo \\z` # z echo `echo \\\z` # \z echo `echo \\\\z` # \z echo `echo \\\\\\z` # \z echo `echo \\\\\\\z` # \\z echo `echo "\z"` # \z echo `echo "\\z"` # \z # Here document cat <<EOF \z EOF # \z cat <<EOF \\z EOF # \z # These examples supplied by Stephane Chazelas. Elements of a string assigned to a variable may be escaped, but the escape character alone may not be assigned to a variable. variable=\ echo "$variable" # Will not work - gives an error message: # test.sh: : command not found # A "naked" escape cannot safely be assigned to a variable. # # What actually happens here is that the "\" escapes the newline and #+ the effect is variable=echo "$variable" #+ invalid variable assignment variable=\ 23skidoo echo "$variable" # 23skidoo # This works, since the second line #+ is a valid variable assignment. variable=\ # \^ escape followed by space echo "$variable" # space variable=\\ echo "$variable" # \ variable=\\\ echo "$variable" # Will not work - gives an error message: # test.sh: \: command not found # # First escape escapes second one, but the third one is left "naked", #+ with same result as first instance, above. variable=\\\\ echo "$variable" # \\ # Second and fourth escapes escaped. # This is o.k. Escaping a space can prevent word splitting in a command's argument list. file_list="/bin/cat /bin/gzip /bin/more /usr/bin/less /usr/bin/emacs-20.7" # List of files as argument(s) to a command. # Add two files to the list, and list all. ls -l /usr/X11R6/bin/xsetroot /sbin/dump $file_list echo "-------------------------------------------------------------------------" # What happens if we escape a couple of spaces? ls -l /usr/X11R6/bin/xsetroot\ /sbin/dump\ $file_list # Error: the first three files concatenated into a single argument to 'ls -l' # because the two escaped spaces prevent argument (word) splitting. The escape also provides a means of writing a multi-line command. Normally, each separate line constitutes a different command, but an escape at the end of a line escapes the newline character, and the command sequence continues on to the next line. (cd /source/directory && tar cf - . ) | \ (cd /dest/directory && tar xpvf -) # Repeating Alan Cox's directory tree copy command, # but split into two lines for increased legibility. # As an alternative: tar cf - -C /source/directory . | tar xpvf - -C /dest/directory # See note below. # (Thanks, Stephane Chazelas.) If a script line ends with a |, a pipe character, then a \, an escape, is not strictly necessary. It is, however, good programming practice to always escape the end of a line of code that continues to the following line. echo "foo bar" #foo #bar echo echo 'foo bar' # No difference yet. #foo #bar echo echo foo\ bar # Newline escaped. #foobar echo echo "foo\ bar" # Same here, as \ still interpreted as escape within weak quotes. #foobar echo echo 'foo\ bar' # Escape character \ taken literally because of strong quoting. #foo\ #bar # Examples suggested by Stephane Chazelas. Chet Ramey ...there are dark corners in the Bourne shell, and people use all of them. The exit command exit exit command may be used to terminate a script, just as in a C program. It can also return a value, which is available to the script's parent process. Every command returns an exit status exit status (sometimes referred to as a return status return status ). A successful command returns a 0, while an unsuccessful one returns a non-zero value that usually may be interpreted as an error code. Well-behaved UNIX commands, programs, and utilities return a 0 exit code upon successful completion, though there are some exceptions. Likewise, functions within a script and the script itself return an exit status. The last command executed in the function or script determines the exit status. Within a script, an exit nnn command may be used to deliver an nnn exit status to the shell (nnn must be a decimal number in the 0 - 255 range). When a script ends with an exit that has no parameter, the exit status of the script is the exit status of the last command executed in the script (not counting the exit). $? variable $? $? reads the exit status of the last command executed. After a function returns, $? gives the exit status of the last command executed in the function. This is Bash's way of giving functions a return value. After a script terminates, a $? from the command line gives the exit status of the script, that is, the last command executed in the script, which is, by convention, 0 on success or an integer in the range 1 - 255 on error. &ex5; $? is especially useful for testing the result of a command in a script (see and ). The !, the logical not qualifier, reverses the outcome of a test or command, and this affects its exit status. true # the "true" builtin. echo "exit status of \"true\" = $?" # 0 ! true echo "exit status of \"! true\" = $?" # 1 # Note that the "!" needs a space. # !true leads to a "command not found" error # Thanks, S.C. Certain exit status codes have reserved meanings and should not be user-specified in a script. if test if then else if elif Every reasonably complete programming language can test for a condition, then act according to the result of the test. Bash has the test command, various bracket and parenthesis operators, and the if/then construct. An if/then construct tests whether the exit status of a list of commands is 0 (since 0 means success by UNIX convention), and if so, executes one or more commands. There exists a dedicated command called [ (left bracket special character). It is a synonym for test, and a builtin for efficiency reasons. This command considers its arguments as comparison expressions or file tests and returns an exit status corresponding to the result of the comparison (0 for true, 1 for false). With version 2.02, Bash introduced the [[ ... ]] extended test command, which performs comparisons in a manner more familiar to programmers from other languages. Note that [[ is a keyword, not a command. Bash sees [[ $a -lt $b ]] as a single element, which returns an exit status. The (( ... )) and let ... constructs also return an exit status of 0 if the arithmetic expressions they evaluate expand to a non-zero value. These arithmetic expansion constructs may therefore be used to perform arithmetic comparisons. let "1<2" returns 0 (as "1<2" expands to "1") (( 0 && 1 )) returns 1 (as "0 && 1" expands to "0") An if can test any command, not just conditions enclosed within brackets. if cmp a b &> /dev/null # Suppress output. then echo "Files a and b are identical." else echo "Files a and b differ." fi if grep -q Bash file then echo "File contains at least one occurrence of Bash." fi if COMMAND_WHOSE_EXIT_STATUS_IS_0_UNLESS_ERROR_OCCURRED then echo "Command succeeded." else echo "Command failed." fi An if/then construct can contain nested comparisons and tests. if echo "Next *if* is part of the comparison for the first *if*." if [[ $comparison = "integer" ]] then (( a < b )) else [[ $a < $b ]] fi then echo '$a is less than $b' fi This detailed if-test explanation courtesy of Stephane Chazelas. &ex10; Explain the behavior of , above. if [ condition-true ] then command 1 command 2 ... else # Optional (may be left out if not needed). # Adds default code block executing if original condition tests false. command 3 command 4 ... fi When if and then are on same line in a condition test, a semicolon must terminate the if statement. Both if and then are keywords. Keywords (or commands) begin statements, and before a new statement on the same line begins, the old one must terminate. if [ -x "$filename" ]; then elif elif is a contraction for else if. The effect is to nest an inner if/then construct within an outer one. if [ condition1 ] then command1 command2 command3 elif [ condition2 ] # Same as else if then command4 command5 else default-command fi test test test [ special character [ ] special character ] The if test condition-true construct is the exact equivalent of if [ condition-true ]. As it happens, the left bracket, [ , is a token which invokes the test command. The closing right bracket, ] , in an if/test should not therefore be strictly necessary, however newer versions of Bash require it. The test command is a Bash builtin which tests file types and compares strings. Therefore, in a Bash script, test does not call the external /usr/bin/test binary, which is part of the sh-utils package. Likewise, [ does not call /usr/bin/[, which is linked to /usr/bin/test. bash$ type test test is a shell builtin bash$ type '[' [ is a shell builtin bash$ type '[[' [[ is a shell keyword bash$ type ']]' ]] is a shell keyword bash$ type ']' bash: type: ]: not found &ex11; test test test [[ special character [[ ]] special character ]] The [[ ]] construct is the more versatile Bash version of [ ]. This is the extended test command, adopted from ksh88. No filename expansion or word splitting takes place between [[ and ]], but there is parameter expansion and command substitution. file=/etc/passwd if [[ -e $file ]] then echo "Password file exists." fi Using the [[ ... ]] test construct, rather than [ ... ] can prevent many logic errors in scripts. For example, the &&, ||, <, and > operators work within a [[ ]] test, despite giving an error within a [ ] construct. Following an if, neither the test command nor the test brackets ( [ ] or [[ ]] ) are strictly necessary. dir=/home/bozo if cd "$dir" 2>/dev/null; then # "2>/dev/null" hides error message. echo "Now in $dir." else echo "Can't change to $dir." fi The "if COMMAND" construct returns the exit status of COMMAND. Similarly, a condition within test brackets may stand alone without an if, when used in combination with a list construct. var1=20 var2=22 [ "$var1" -ne "$var2" ] && echo "$var1 is not equal to $var2" home=/home/bozo [ -d "$home" ] || echo "$home directory does not exist." test test test (( special character )) (( special character )) The (( )) construct expands and evaluates an arithmetic expression. If the expression evaluates as zero, it returns an exit status of 1, or false. A non-zero expression returns an exit status of 0, or true. This is in marked contrast to using the test and [ ] constructs previously discussed. &arithtests; -e file exists -f file is a regular file (not a directory or device file) -s file is not zero size -d file is a directory -b file is a block device (floppy, cdrom, etc.) -c file is a character device (keyboard, modem, sound card, etc.) -p file is a pipe -h file is a symbolic link -L file is a symbolic link -S file is a socket -t file (descriptor) is associated with a terminal device This test option may be used to check whether the stdin ([ -t 0 ]) or stdout ([ -t 1 ]) in a given script is a terminal. -r file has read permission (for the user running the test) -w file has write permission (for the user running the test) -x file has execute permission (for the user running the test) -g set-group-id (sgid) flag set on file or directory If a directory has the sgid flag set, then a file created within that directory belongs to the group that owns the directory, not necessarily to the group of the user who created the file. This may be useful for a directory shared by a workgroup. -u set-user-id (suid) flag set on file A binary owned by root with set-user-id flag set runs with root privileges, even when an ordinary user invokes it. Be aware that suid binaries may open security holes and that the suid flag has no effect on shell scripts. This is useful for executables (such as pppd and cdrecord) that need to access system hardware. Lacking the suid flag, these binaries could not be invoked by a non-root user. -rwsr-xr-t 1 root 178236 Oct 2 2000 /usr/sbin/pppd A file with the suid flag set shows an s in its permissions. -k sticky bit set Commonly known as the sticky bit, the save-text-mode flag is a special type of file permission. If a file has this flag set, that file will be kept in cache memory, for quicker access. On modern UNIX systems, the sticky bit is no longer used for files, only on directories. If set on a directory, it restricts write permission. Setting the sticky bit adds a t to the permissions on the file or directory listing. drwxrwxrwt 7 root 1024 May 19 21:26 tmp/ If a user does not own a directory that has the sticky bit set, but has write permission in that directory, he can only delete files in it that he owns. This keeps users from inadvertently overwriting or deleting each other's files in a publicly accessible directory, such as /tmp. -O you are owner of file -G group-id of file same as yours -N file modified since it was last read f1 -nt f2 file f1 is newer than f2 f1 -ot f2 file f1 is older than f2 f1 -ef f2 files f1 and f2 are hard links to the same file ! not -- reverses the sense of the tests above (returns true if condition absent). &brokenlink; , , , , and also illustrate uses of the file test operators. -eq is equal to if [ "$a" -eq "$b" ] -ne is not equal to if [ "$a" -ne "$b" ] -gt is greater than if [ "$a" -gt "$b" ] -ge is greater than or equal to if [ "$a" -ge "$b" ] -lt is less than if [ "$a" -lt "$b" ] -le is less than or equal to if [ "$a" -le "$b" ] < is less than (within double parentheses) (("$a" < "$b")) <= is less than or equal to (within double parentheses) (("$a" <= "$b")) > is greater than (within double parentheses) (("$a" > "$b")) >= is greater than or equal to (within double parentheses) (("$a" >= "$b")) = is equal to if [ "$a" = "$b" ] == is equal to if [ "$a" == "$b" ] This is a synonym for =. [[ $a == z* ]] # true if $a starts with an "z" (pattern matching) [[ $a == "z*" ]] # true if $a is equal to z* [ $a == z* ] # file globbing and word splitting take place [ "$a" == "z*" ] # true if $a is equal to z* # Thanks, S.C. != is not equal to if [ "$a" != "$b" ] This operator uses pattern matching within a [[ ... ]] construct. < is less than, in ASCII alphabetical order if [[ "$a" < "$b" ]] if [ "$a" \< "$b" ] Note that the < needs to be escaped within a [ ] construct. > is greater than, in ASCII alphabetical order if [[ "$a" > "$b" ]] if [ "$a" \> "$b" ] Note that the > needs to be escaped within a [ ] construct. See for an application of this comparison operator. -z string is null, that is, has zero length -n string is not null. The -n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see ) normally works, however, this is an unsafe practice. Always quote a tested string. As S.C. points out, in a compound test, even quoting the string variable might not suffice. [ -n "$string" -o "$a" = "$b" ] may cause an error with some versions of Bash if $string is empty. The safe way is to append an extra character to possibly empty variables, [ "x$string" != x -o "x$a" = "x$b" ] (the x's cancel out). &ex13; &strtest; &ex14; -a logical and exp1 -a exp2 returns true if both exp1 and exp2 are true. -o logical or exp1 -o exp2 returns true if either exp1 or exp2 are true. These are similar to the Bash comparison operators && and ||, used within double brackets. [[ condition1 && condition2 ]] The -o and -a operators work with the test command or occur within single test brackets. if [ "$exp1" -a "$exp2" ] Refer to and to see compound comparison operators in action. Condition tests using the if/then construct may be nested. The net result is identical to using the && compound comparison operator above. if [ condition1 ] then if [ condition2 ] then do-something # But only if both "condition1" and "condition2" valid. fi fi See for an example of nested if/then condition tests. The systemwide xinitrc file can be used to launch the X server. This file contains quite a number of if/then tests, as the following excerpt shows. if [ -f $HOME/.Xclients ]; then exec $HOME/.Xclients elif [ -f /etc/X11/xinit/Xclients ]; then exec /etc/X11/xinit/Xclients else # failsafe settings. Although we should never get here # (we provide fallbacks in Xclients as well) it can't hurt. xclock -geometry 100x100-5+5 & xterm -geometry 80x50-50+150 & if [ -f /usr/bin/netscape -a -f /usr/share/doc/HTML/index.html ]; then netscape /usr/share/doc/HTML/index.html & fi fi Explain the test constructs in the above excerpt, then examine the entire file, /etc/X11/xinit/xinitrc, and analyze the if/then test constructs there. You may need to refer ahead to the discussions of grep, sed, and regular expressions. variable assignment Initializing or changing the value of a variable = operation = = All-purpose assignment operator, which works for both arithmetic and string assignments. var=27 category=minerals # No spaces allowed after the "=". Do not confuse the = assignment operator with the = test operator. # = as a test operator if [ "$string1" = "$string2" ] # if [ "X$string1" = "X$string2" ] is safer, # to prevent an error message should one of the variables be empty. # (The prepended "X" characters cancel out.) then command fi expr command expr let command let + + operation + addition plus plus - - operation - subtraction minus minus * * operation * multiplication multiplication / / operation / division division ** ** operation ** exponentiation exponentiation # Bash, version 2.02, introduced the "**" exponentiation operator. let "z=5**3" echo "z = $z" # z = 125 % % operation % modulo modulo, or mod (returns the remainder of an integer division operation) bash$ echo `expr 5 % 3` 2 This operator finds use in, among other things, generating numbers within a specific range (see and ) and formatting program output (see and ). It can even be used to generate prime numbers, (see ). Modulo turns up surprisingly often in various numerical recipes. &gcd; += += operation += plus-equal plus-equal (increment variable by a constant) let "var += 5" results in var being incremented by 5. -= -= operation -= minus-equal minus-equal (decrement variable by a constant) *= *= operation *= times-equal times-equal (multiply variable by a constant) let "var *= 4" results in var being multiplied by 4. /= /= operation /= slash-equal slash-equal (divide variable by a constant) %= %= operation %= mod-equal mod-equal (remainder of dividing variable by a constant) Arithmetic operators often occur in an expr or let expression. &arithops; Integer variables in Bash are actually signed long (32-bit) integers, in the range of -2147483648 to 2147483647. An operation that takes a variable outside these limits will give an erroneous result. a=2147483646 echo "a = $a" # a = 2147483646 let "a+=1" # Increment "a". echo "a = $a" # a = 2147483647 let "a+=1" # increment "a" again, past the limit. echo "a = $a" # a = -2147483648 # ERROR (out of range) Bash does not understand floating point arithmetic. It treats numbers containing a decimal point as strings. a=1.5 let "b = $a + 1.3" # Error. # t2.sh: let: b = 1.5 + 1.3: syntax error in expression (error token is ".5 + 1.3") echo "b = $b" # b=1 Use bc in scripts that that need floating point calculations or math library functions. The bitwise operators seldom make an appearance in shell scripts. Their chief use seems to be manipulating and testing values read from ports or sockets. Bit flipping is more relevant to compiled languages, such as C and C++, which run fast enough to permit its use on the fly. << << operation << left shift bitwise left shift (multiplies by 2 for each shift position) <<= <<= operation <<= left-shift-equal left-shift-equal let "var <<= 2" results in var left-shifted 2 bits (multiplied by 4) >> >> operation >> right shift bitwise right shift (divides by 2 for each shift position) >>= >>= operation >>= right-shift-equal right-shift-equal (inverse of <<=) & & operation & AND bitwise bitwise and &= &= operation &= and-equal bitwise and-equal | | operation | OR bitwise bitwise OR |= |= operation |= OR-equal bitwise OR-equal ~ ~ operation ~ negate bitwise negate ! ! operation ! NOT bitwise NOT ^ ^ operation ^ XOR bitwise XOR ^= ^= operation ^= XOR-equal bitwise XOR-equal && && operator && AND logical and (logical) if [ $condition1 ] && [ $condition2 ] # Same as: if [ $condition1 -a $condition2 ] # Returns true if both condition1 and condition2 hold true... if [[ $condition1 && $condition2 ]] # Also works. # Note that && operator not permitted within [ ... ] construct. && may also, depending on context, be used in an and list to concatenate commands. || || operator || OR logical or (logical) if [ $condition1 ] || [ $condition2 ] # Same as: if [ $condition1 -o $condition2 ] # Returns true if either condition1 or condition2 holds true... if [[ $condition1 || $condition2 ]] # Also works. # Note that || operator not permitted within [ ... ] construct. Bash tests the exit status of each statement linked with a logical operator. &andor; The && and || operators also find use in an arithmetic context. bash$ echo $(( 1 && 2 )) $((3 && 0)) $((4 || 0)) $((0 || 0)) 1 0 1 0 , , operation , linking comma operator The comma operator chains together two or more arithmetic operations. All the operations are evaluated (with possible side effects), but only the last operation is returned. let "t1 = ((5 + 3, 7 - 1, 15 - 4))" echo "t1 = $t1" # t1 = 11 let "t2 = ((a = 9, 15 / 3))" # Set "a" and calculate "t2". echo "t2 = $t2 a = $a" # t2 = 5 a = 9 The comma operator finds use mainly in for loops. See . A shell script interprets a number as decimal (base 10), unless that number has a special prefix or notation. A number preceded by a 0 is octal (base 8). A number preceded by 0x is hexadecimal (base 16). A number with an embedded # evaluates as BASE#NUMBER (with range and notational restrictions). &numbers; Used properly, variables can add power and flexibility to scripts. This requires learning their subtleties and nuances. Builtin variables variables affecting bash script behavior $BASH $BASH variable $BASH path to bash the path to the Bash binary itself bash$ echo $BASH /bin/bash $BASH_ENV $BASH_ENV variable $BASH_ENV an environmental variable pointing to a Bash startup file to be read when a script is invoked $BASH_VERSINFO[n] $BASH_VERSINFO variable version information a 6-element array containing version information about the installed release of Bash. This is similar to $BASH_VERSION, below, but a bit more detailed. # Bash version info: for n in 0 1 2 3 4 5 do echo "BASH_VERSINFO[$n] = ${BASH_VERSINFO[$n]}" done # BASH_VERSINFO[0] = 2 # Major version no. # BASH_VERSINFO[1] = 05 # Minor version no. # BASH_VERSINFO[2] = 8 # Patch level. # BASH_VERSINFO[3] = 1 # Build version. # BASH_VERSINFO[4] = release # Release status. # BASH_VERSINFO[5] = i386-redhat-linux-gnu # Architecture # (same as $MACHTYPE). $BASH_VERSION $BASH_VERSION variable $BASH_VERSION the version of Bash installed on the system bash$ echo $BASH_VERSION 2.04.12(1)-release tcsh% echo $BASH_VERSION BASH_VERSION: Undefined variable. Checking $BASH_VERSION is a good method of determining which shell is running. $SHELL does not necessarily give the correct answer. $DIRSTACK $DIRSTACK variable $DIRSTACK directory stack directory stack the top value in the directory stack (affected by pushd and popd) This builtin variable corresponds to the dirs command, however dirs shows the entire contents of the directory stack. $EDITOR $EDITOR variable $EDITOR editor the default editor invoked by a script, usually vi or emacs. $EUID $EUID variable $EUID effective user ID effective user ID number Identification number of whatever identity the current user has assumed, perhaps by means of su. The $EUID is not necessarily the same as the $UID. $FUNCNAME $FUNCNAME variable function name name of the current function xyz23 () { echo "$FUNCNAME now executing." # xyz23 now executing. } xyz23 echo "FUNCNAME = $FUNCNAME" # FUNCNAME = # Null value outside a function. $GLOBIGNORE $GLOBIGNORE variable globbing ignore A list of filename patterns to be excluded from matching in globbing. $GROUPS $GROUPS variable $GROUPS groups groups current user belongs to This is a listing (array) of the group id numbers for current user, as recorded in /etc/passwd. root# echo $GROUPS 0 root# echo ${GROUPS[1]} 1 root# echo ${GROUPS[5]} 6 $HOME $HOME variable $HOME home directory directory home home directory of the user, usually /home/username (see ) $HOSTNAME $HOSTNAME variable $HOSTNAME system name variable name The hostname command assigns the system name at bootup in an init script. However, the gethostname() function sets the Bash internal variable $HOSTNAME. See also . $HOSTTYPE $HOSTTYPE variable $HOSTTYPE host type host type Like $MACHTYPE, identifies the system hardware. bash$ echo $HOSTTYPE i686 $IFS $IFS variable $IFS input field separator input field separator This defaults to whitespace (space, tab, and newline), but may be changed, for example, to parse a comma-separated data file. Note that $* uses the first character held in $IFS. See . bash$ echo $IFS | cat -vte $ bash$ bash -c 'set w x y z; IFS=":-;"; echo "$*"' w:x:y:z $IFS does not handle whitespace the same as it does other characters. &ifsh; (Thanks, S. C., for clarification and examples.) $IGNOREEOF $IGNOREEOF variable $IGNOREEOF Ignore EOF ignore EOF: how many end-of-files (control-D) the shell will ignore before logging out. $LC_COLLATE $LC_COLLATE variable $LC_COLLATE lowercase collate Often set in the .bashrc or /etc/profile files, this variable controls collation order in filename expansion and pattern matching. If mishandled, LC_COLLATE can cause unexpected results in filename globbing. As of version 2.05 of Bash, filename globbing no longer distinguishes between lowercase and uppercase letters in a character range between brackets. For example, ls [A-M]* would match both File1.txt and file1.txt. To revert to the customary behavior of bracket matching, set LC_COLLATE to C by an export LC_COLLATE=C in /etc/profile and/or ~/.bashrc. $LC_CTYPE $LC_CTYPE variable $LC_CTYPE lowercase character type This internal variable controls character interpretation in globbing and pattern matching. $LINENO $LINENO variable $LINENO line number This variable is the line number of the shell script in which this variable appears. It has significance only within the script in which it appears, and is chiefly useful for debugging purposes. # *** BEGIN DEBUG BLOCK *** last_cmd_arg=$_ # Save it. echo "At line number $LINENO, variable \"v1\" = $v1" echo "Last command argument processed = $last_cmd_arg" # *** END DEBUG BLOCK *** $MACHTYPE $MACHTYPE variable $MACHTYPE machine type machine type Identifies the system hardware. bash$ echo $MACHTYPE i686 $OLDPWD $OLDPWD variable $OLDPWD previous working directory directory working old working directory (OLD-print-working-directory, previous directory you were in) $OSTYPE $OSTYPE variable $OSTYPE os type operating system type bash$ echo $OSTYPE linux $PATH $PATH variable $PATH path to binaries path to binaries, usually /usr/bin/, /usr/X11R6/bin/, /usr/local/bin, etc. When given a command, the shell automatically does a hash table search on the directories listed in the path for the executable. The path is stored in the environmental variable, $PATH, a list of directories, separated by colons. Normally, the system stores the $PATH definition in /etc/profile and/or ~/.bashrc (see ). bash$ echo $PATH /bin:/usr/bin:/usr/local/bin:/usr/X11R6/bin:/sbin:/usr/sbin PATH=${PATH}:/opt/bin appends the /opt/bin directory to the current path. In a script, it may be expedient to temporarily add a directory to the path in this way. When the script exits, this restores the original $PATH (a child process, such as a script, may not change the environment of the parent process, the shell). The current working directory, ./, is usually omitted from the $PATH as a security measure. $PIPESTATUS $PIPESTATUS variable pipe Exit status of last executed foreground pipe. Interestingly enough, this does not give the same result as the exit status of the last executed command. bash$ echo $PIPESTATUS 0 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $PIPESTATUS 141 bash$ ls -al | bogus_command bash: bogus_command: command not found bash$ echo $? 127 The $PIPESTATUS variable may contain an erroneous 0 value in a login shell. tcsh% bash bash$ who | grep nobody | sort bash$ echo ${PIPESTATUS[*]} 0 The above lines contained in a script would produce the expected 0 1 0 output. Thank you, Wayne Pollock for pointing this out and supplying the above example. $PPID $PPID variable $PPID process id The $PPID of a process is the process ID (pid) of its parent process. The PID of the currently running script is $$, of course. Compare this with the pidof command. $PROMPT_COMMAND $PROMPT_COMMAND variable prompt A variable holding a command to be executed just before the primary prompt, $PS1 is to be displayed. $PS1 $PS1 variable $PS1 prompt This is the main prompt, seen at the command line. $PS2 $PS2 variable $PS2 prompt secondary The secondary prompt, seen when additional input is expected. It displays as >. $PS3 $PS3 variable $PS3 prompt tertiary The tertiary prompt, displayed in a select loop (see ). $PS4 $PS4 variable $PS4 prompt quartenary The quartenary prompt, shown at the beginning of each line of output when invoking a script with the -x option. It displays as +. $PWD $PWD variable $PWD working directory directory working working directory (directory you are in at the time) This is the analog to the pwd builtin command. &wipedir; $REPLY $REPLY variable $REPLY default value of read reply read The default value when a variable is not supplied to read. Also applicable to select menus, but only supplies the item number of the variable chosen, not the value of the variable itself. &reply; $SECONDS $SECONDS variable $SECONDS seconds execution time runtime seconds The number of seconds the script has been running. &seconds; $SHELLOPTS $SHELLOPTS variable $SHELLOPTS shell options the list of enabled shell options, a readonly variable bash$ echo $SHELLOPTS braceexpand:hashall:histexpand:monitor:history:interactive-comments:emacs $SHLVL $SHLVL variable $SHLVL shell level Shell level, how deeply Bash is nested. If, at the command line, $SHLVL is 1, then in a script it will increment to 2. $TMOUT $TMOUT variable $TMOUT timeout interval If the $TMOUT environmental variable is set to a non-zero value time, then the shell prompt will time out after time seconds. This will cause a logout. Unfortunately, this works only while waiting for input at the shell prompt console or in an xterm. While it would be nice to speculate on the uses of this internal variable for timed input, for example in combination with read, $TMOUT does not work in that context and is virtually useless for shell scripting. (Reportedly the ksh version of a timed read does work.) Implementing timed input in a script is certainly possible, but may require complex machinations. One method is to set up a timing loop to signal the script when it times out. This also requires a signal handling routine to trap (see ) the interrupt generated by the timing loop (whew!). &tmdin; An alternative is using stty. &timeout; Perhaps the simplest method is using the -t option to read. &tout; $UID $UID variable $UID user ID user ID number current user's user identification number, as recorded in /etc/passwd This is the current user's real id, even if she has temporarily assumed another identity through su. $UID is a readonly variable, not subject to change from the command line or within a script, and is the counterpart to the id builtin. &amiroot; See also . The variables $ENV, $LOGNAME, $MAIL, $TERM, $USER, and $USERNAME are not Bash builtins. These are, however, often set as environmental variables in one of the Bash startup files. $SHELL, the name of the user's login shell, may be set from /etc/passwd or in an init script, and it is likewise not a Bash builtin. tcsh% echo $LOGNAME bozo tcsh% echo $SHELL /bin/tcsh tcsh% echo $TERM rxvt bash$ echo $LOGNAME bozo bash$ echo $SHELL /bin/tcsh bash$ echo $TERM rxvt $0, $1, $2, etc. $0 variable $0 positional parameter parameter positional positional parameters, passed from command line to script, passed to a function, or set to a variable (see and ) $# $# variable $# positional parameter number of parameter positional number of number of command line arguments The words argument and parameter are often used interchangeably. In the context of this document, they have the same precise meaning, that of a variable passed to a script or function. or positional parameters (see ) $* $* variable $* positional parameter all parameter positional all All of the positional parameters, seen as a single word $* must be quoted. $@ $@ variable $* positional parameter all Same as $*, but each parameter is a quoted string, that is, the parameters are passed on intact, without interpretation or expansion. This means, among other things, that each parameter in the argument list is seen as a separate word. Of course, $@ should be quoted. &arglist; Following a shift, the $@ holds the remaining command-line parameters, lacking the previous $1, which was lost. #!/bin/bash # Invoke with ./scriptname 1 2 3 4 5 echo "$@" # 1 2 3 4 5 shift echo "$@" # 2 3 4 5 shift echo "$@" # 3 4 5 # Each "shift" loses parameter $1. # "$@" then contains the remaining parameters. The $@ special parameter finds use as a tool for filtering input into shell scripts. The cat "$@" construction accepts input to a script either from stdin or from files given as parameters to the script. See and . The $* and $@ parameters sometimes display inconsistent and puzzling behavior, depending on the setting of $IFS. &incompat; The $@ and $* parameters differ only when between double quotes. &ifsempty; $- $- variable $- flags Flags passed to script (using set). See . This was originally a ksh construct adopted into Bash, and unfortunately it does not seem to work reliably in Bash scripts. One possible use for it is to have a script self-test whether it is interactive. $! $! variable $! PID last job background PID (process ID) of last job run in background LOG=$0.log COMMAND1="sleep 100" echo "Logging PIDs background commands for script: $0" >> "$LOG" # So they can be monitored, and killed as necessary. echo >> "$LOG" # Logging commands. echo -n "PID of \"$COMMAND1\": " >> "$LOG" ${COMMAND1} & echo $! >> "$LOG" # PID of "sleep 100": 1506 # Thank you, Jacques Lederer, for suggesting this. $_ $_ variable $_ underscore last argument Special variable set to last argument of previous command executed. #!/bin/bash echo $_ # /bin/bash # Just called /bin/bash to run the script. du >/dev/null # So no output from command. echo $_ # du ls -al >/dev/null # So no output from command. echo $_ # -al (last argument) : echo $_ # : $? $? variable $? exit status Exit status of a command, function, or the script itself (see ) $$ $$ variable $$ PID of script Process ID of the script itself. The $$ variable often finds use in scripts to construct unique temp file names (see , , , and ). This is usually simpler than invoking mktemp. Bash supports a surprising number of string manipulation operations. Unfortunately, these tools lack a unified focus. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. This results in inconsistent command syntax and overlap of functionality, not to mention confusion. ${#string} string length parameter substitution expr length $string string length expr expr "$string" : '.*' string length expr stringZ=abcABC123ABCabc echo ${#stringZ} # 15 echo `expr length $stringZ` # 15 echo `expr "$stringZ" : '.*'` # 15 ¶graphspace; expr match "$string" '$substring' substring length expr $substring is a regular expression. expr "$string" : '$substring' substring length expr $substring is a regular expression. stringZ=abcABC123ABCabc # |------| echo `expr match "$stringZ" 'abc[A-Z]*.2'` # 8 echo `expr "$stringZ" : 'abc[A-Z]*.2'` # 8 expr index $string $substring substring index expr Numerical position in $string of first character in $substring that matches. stringZ=abcABC123ABCabc echo `expr index "$stringZ" C12` # 6 # C position. echo `expr index "$stringZ" 1c` # 3 # 'c' (in #3 position) matches before '1'. This is the near equivalent of strchr() in C. ${string:position} substring extraction Extracts substring from $string at $position. If the $string parameter is * or @, then this extracts the positional parameters, This applies to either command line arguments or parameters passed to a function. starting at $position. ${string:position:length} substring extraction Extracts $length characters of substring from $string at $position. stringZ=abcABC123ABCabc # 0123456789..... # 0-based indexing. echo ${stringZ:0} # abcABC123ABCabc echo ${stringZ:1} # bcABC123ABCabc echo ${stringZ:7} # 23ABCabc echo ${stringZ:7:3} # 23A # Three characters of substring. # Is it possible to index from the right end of the string? echo ${stringZ:-4} # abcABC123ABCabc # Defaults to full string, as in ${parameter:-default}. # However . . . echo ${stringZ:(-4)} # Cabc echo ${stringZ: -4} # Cabc # Now, it works. # Parentheses or added space "escape" the position parameter. # Thank you, Dan Jacobson, for pointing this out. If the $string parameter is * or @, then this extracts a maximum of $length positional parameters, starting at $position. echo ${*:2} # Echoes second and following positional parameters. echo ${@:2} # Same as above. echo ${*:2:3} # Echoes three positional parameters, starting at second. expr substr $string $position $length substring extraction expr Extracts $length characters from $string starting at $position. stringZ=abcABC123ABCabc # 123456789...... # 1-based indexing. echo `expr substr $stringZ 1 2` # ab echo `expr substr $stringZ 4 3` # ABC expr match "$string" '\($substring\)' substring extraction expr Extracts $substring at beginning of $string, where $substring is a regular expression. expr "$string" : '\($substring\)' substring extraction expr Extracts $substring at beginning of $string, where $substring is a regular expression. stringZ=abcABC123ABCabc # ======= echo `expr match "$stringZ" '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 echo `expr "$stringZ" : '\(.[b-c]*[A-Z]..[0-9]\)'` # abcABC1 echo `expr "$stringZ" : '\(.......\)'` # abcABC1 # All of the above forms give an identical result. expr match "$string" '.*\($substring\)' substring extraction expr Extracts $substring at end of $string, where $substring is a regular expression. expr "$string" : '.*\($substring\)' substring extraction expr Extracts $substring at end of $string, where $substring is a regular expression. stringZ=abcABC123ABCabc # ====== echo `expr match "$stringZ" '.*\([A-C][A-C][A-C][a-c]*\)'` # ABCabc echo `expr "$stringZ" : '.*\(......\)'` # ABCabc ${string#substring} substring removal Strips shortest match of $substring from front of $string. ${string##substring} substring removal Strips longest match of $substring from front of $string. stringZ=abcABC123ABCabc # |----| # |----------| echo ${stringZ#a*C} # 123ABCabc # Strip out shortest match between 'a' and 'C'. echo ${stringZ##a*C} # abc # Strip out longest match between 'a' and 'C'. ${string%substring} substring removal Strips shortest match of $substring from back of $string. ${string%%substring} substring removal Strips longest match of $substring from back of $string. stringZ=abcABC123ABCabc # || # |------------| echo ${stringZ%b*c} # abcABC123ABCa # Strip out shortest match between 'b' and 'c', from back of $stringZ. echo ${stringZ%%b*c} # a # Strip out longest match between 'b' and 'c', from back of $stringZ. &cvt; ${string/substring/replacement} substring replacement Replace first match of $substring with $replacement. ${string//substring/replacement} substring replacement Replace all matches of $substring with $replacement. stringZ=abcABC123ABCabc echo ${stringZ/abc/xyz} # xyzABC123ABCabc # Replaces first match of 'abc' with 'xyz'. echo ${stringZ//abc/xyz} # xyzABC123ABCxyz # Replaces all matches of 'abc' with # 'xyz'. ${string/#substring/replacement} substring replacement If $substring matches front end of $string, substitute $replacement for $substring. ${string/%substring/replacement} substring replacement If $substring matches back end of $string, substitute $replacement for $substring. stringZ=abcABC123ABCabc echo ${stringZ/#abc/XYZ} # XYZABC123ABCabc # Replaces front-end match of 'abc' with 'XYZ'. echo ${stringZ/%abc/XYZ} # abcABC123ABCXYZ # Replaces back-end match of 'abc' with 'XYZ'. A Bash script may invoke the string manipulation facilities of awk as an alternative to using its built-in operations. &substringex; For more on string manipulation in scripts, refer to and the relevant section of the expr command listing. For script examples, see: ${parameter} Same as $parameter, i.e., value of the variable parameter. In certain contexts, only the less ambiguous ${parameter} form works. May be used for concatenating variables with strings. your_id=${USER}-on-${HOSTNAME} echo "$your_id" # echo "Old \$PATH = $PATH" PATH=${PATH}:/opt/bin #Add /opt/bin to $PATH for duration of script. echo "New \$PATH = $PATH" ${parameter-default} ${parameter:-default} If parameter not set, use default. echo ${username-`whoami`} # Echoes the result of `whoami`, if variable $username is still unset. ${parameter-default} and ${parameter:-default} are almost equivalent. The extra : makes a difference only when parameter has been declared, but is null. ¶msub; The default parameter construct finds use in providing missing command-line arguments in scripts. DEFAULT_FILENAME=generic.data filename=${1:-$DEFAULT_FILENAME} # If not otherwise specified, the following command block operates #+ on the file "generic.data". # # Commands follow. See also , , and . Compare this method with using an and list to supply a default command-line argument. ${parameter=default} ${parameter:=default} If parameter not set, set it to default. Both forms nearly equivalent. The : makes a difference only when $parameter has been declared and is null, If $parameter is null in a non-interactive script, it will terminate with a 127 exit status (the Bash error code code for command not found). as above. echo ${username=`whoami`} # Variable "username" is now set to `whoami`. ${parameter+alt_value} ${parameter:+alt_value} If parameter set, use alt_value, else use null string. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, see below. echo "###### \${parameter+alt_value} ########" echo a=${param1+xyz} echo "a = $a" # a = param2= a=${param2+xyz} echo "a = $a" # a = xyz param3=123 a=${param3+xyz} echo "a = $a" # a = xyz echo echo "###### \${parameter:+alt_value} ########" echo a=${param4:+xyz} echo "a = $a" # a = param5= a=${param5:+xyz} echo "a = $a" # a = # Different result from a=${param5+xyz} param6=123 a=${param6+xyz} echo "a = $a" # a = xyz ${parameter?err_msg} ${parameter:?err_msg} If parameter set, use it, else print err_msg. Both forms nearly equivalent. The : makes a difference only when parameter has been declared and is null, as above. &ex6; &usagemessage; The following expressions are the complement to the match in expr string operations (see ). These particular ones are used mostly in parsing file path names. ${#var} String length (number of characters in $var). For an array, ${#array} is the length of the first element in the array. Exceptions: ${#*} and ${#@} give the number of positional parameters. For an array, ${#array[*]} and ${#array[@]} give the number of elements in the array. &length; ${var#Pattern} ${var##Pattern} Remove from $var the shortest/longest part of $Pattern that matches the front end of $var. A usage illustration from : # Function from "days-between.sh" example. # Strips leading zero(s) from argument passed. strip_leading_zero () # Better to strip possible leading zero(s) { # from day and/or month val=${1#0} # since otherwise Bash will interpret them return $val # as octal values (POSIX.2, sect 2.9.2.1). } Another usage illustration: echo `basename $PWD` # Basename of current working directory. echo "${PWD##*/}" # Basename of current working directory. echo echo `basename $0` # Name of script. echo $0 # Name of script. echo "${0##*/}" # Name of script. echo filename=test.data echo "${filename##*.}" # data # Extension of filename. ${var%Pattern} ${var%%Pattern} Remove from $var the shortest/longest part of $Pattern that matches the back end of $var. Version 2 of Bash adds additional options. &pattmatching; &rfe; These constructs have been adopted from ksh. ${var:pos} Variable var expanded, starting from offset pos. ${var:pos:len} Expansion to a max of len characters of variable var, from offset pos. See for an example of the creative use of this operator. ${var/Pattern/Replacement} First match of Pattern, within var replaced with Replacement. If Replacement is omitted, then the first match of Pattern is replaced by nothing, that is, deleted. ${var//Pattern/Replacement} All matches of Pattern, within var replaced with Replacement. As above, if Replacement is omitted, then all occurrences of Pattern are replaced by nothing, that is, deleted. &ex7; ${var/#Pattern/Replacement} If prefix of var matches Pattern, then substitute Replacement for Pattern. ${var/%Pattern/Replacement} If suffix of var matches Pattern, then substitute Replacement for Pattern. &varmatch; ${!varprefix*} ${!varprefix@} Matches all previously declared variables beginning with varprefix. xyz23=whatever xyz24= a=${!xyz*} # Expands to names of declared variables beginning with "xyz". echo "a = $a" # a = xyz23 xyz24 a=${!xyz@} # Same as above. echo "a = $a" # a = xyz23 xyz24 # Bash, version 2.04, adds this feature. declare typeset command declare command typeset The declare or typeset builtins (they are exact synonyms) permit restricting the properties of variables. This is a very weak form of the typing available in certain programming languages. The declare command is specific to version 2 or later of Bash. The typeset command also works in ksh scripts. -r readonly declare -r var1 (declare -r var1 works the same as readonly var1) This is the rough equivalent of the C const type qualifier. An attempt to change the value of a readonly variable fails with an error message. -i integer declare -i number # The script will treat subsequent occurrences of "number" as an integer. number=3 echo "Number = $number" # Number = 3 number=three echo "Number = $number" # Number = 0 # Tries to evaluate the string "three" as an integer. Certain arithmetic operations are permitted for declared integer variables without the need for expr or let. n=6/3 echo "n = $n" # n = 6/3 declare -i n n=6/3 echo "n = $n" # n = 2 -a array declare -a indices The variable indices will be treated as an array. -f functions declare -f A declare -f line with no arguments in a script causes a listing of all the functions previously defined in that script. declare -f function_name A declare -f function_name in a script lists just the function named. -x export declare -x var3 This declares a variable as available for exporting outside the environment of the script itself. var=$value declare -x var3=373 The declare command permits assigning a value to a variable in the same statement as setting its properties. &ex20; Assume that the value of a variable is the name of a second variable. Is it somehow possible to retrieve the value of this second variable from the first one? For example, if a=letter_of_alphabet and letter_of_alphabet=z, can a reference to a return z? This can indeed be done, and it is called an indirect reference. It uses the unusual eval var1=\$$var2 notation. &indref; &coltotaler2; This method of indirect referencing is a bit tricky. If the second order variable changes its value, then the first order variable must be properly dereferenced (as in the above example). Fortunately, the ${!variable} notation introduced with version 2 of Bash (see ) makes indirect referencing more intuitive. $RANDOM variable $RANDOM $RANDOM is an internal Bash function (not a constant) that returns a pseudorandom integer in the range 0 - 32767. $RANDOM should not be used to generate an encryption key. &ex21; &pickcard; Jipe points out another set of techniques for generating random numbers within a range. # Generate random number between 6 and 30. rnumber=$((RANDOM%25+6)) # Generate random number in the same 6 - 30 range, #+ but the number must be evenly divisible by 3. rnumber=$(((RANDOM%30/3+1)*3)) # Exercise: Try to figure out the pattern here. Just how random is $RANDOM? The best way to test this is to write a script that tracks the distribution of random numbers generated by $RANDOM. Let's roll a $RANDOM die a few times... &randomtest; As we have seen in the last example, it is best to reseed the RANDOM generator each time it is invoked. Using the same seed for RANDOM repeats the same series of numbers. (This mirrors the behavior of the random() function in C.) &seedingrandom; The /dev/urandom device-file provides a means of generating much more random pseudorandom numbers than the $RANDOM variable. dd if=/dev/urandom of=targetfile bs=1 count=XX creates a file of well-scattered pseudorandom numbers. However, assigning these numbers to a variable in a script requires a workaround, such as filtering through od (as in above example) or using dd (see ). There are also other means of generating pseudorandom numbers in a script. Awk provides a convenient means of doing this. &random2; Similar to the let command, the ((...)) construct permits arithmetic expansion and evaluation. In its simplest form, a=$(( 5 + 3 )) would set a to 5 + 3, or 8. However, this double parentheses construct is also a mechanism for allowing C-type manipulation of variables in Bash. &cvars; See also . Operations on code blocks are the key to structured, organized shell scripts. Looping and branching constructs provide the tools for accomplishing this. A loop is a block of code that iterates (repeats) a list of commands as long as the loop control condition is true. for (in) for in do done loop for This is the basic looping construct. It differs significantly from its C counterpart. for arg in list do  command(s) done During each pass through the loop, arg takes on the value of each successive variable in the list. for arg in "$var1" "$var2" "$var3" ... "$varN" # In pass 1 of the loop, $arg = $var1 # In pass 2 of the loop, $arg = $var2 # In pass 3 of the loop, $arg = $var3 # ... # In pass N of the loop, $arg = $varN # Arguments in [list] quoted to prevent possible word splitting. The argument list may contain wild cards. If do is on same line as for, there needs to be a semicolon after list. for arg in list ; do &ex22; Each [list] element may contain multiple parameters. This is useful when processing parameters in groups. In such cases, use the set command (see ) to force parsing of each [list] element and assignment of each component to the positional parameters. &ex22a; A variable may supply the [list] in a for loop. &fileinfo; The [list] in a for loop may contain filename globbing, that is, using wildcards for filename expansion. &listglob; Omitting the in [list] part of a for loop causes the loop to operate on $@, the list of arguments given on the command line to the script. A particularly clever illustration of this is . &ex23; It is possible to use command substitution to generate the [list] in a for loop. See also , and . &forloopcmd; This is a somewhat more complex example of using command substitution to create the [list]. &bingrep; More of the same. &userlist; A final example of the [list] resulting from command substitution. &findstring; The output of a for loop may be piped to a command or commands. &symlinks; The stdout of a loop may be redirected to a file, as this slight modification to the previous example shows. &symlinks2; There is an alternative syntax to a for loop that will look very familiar to C programmers. This requires double parentheses. &forloopc; See also , , and . --- Now, a for-loop used in a real-life context. &ex24; while while do done loop while This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is true (returns a 0 exit status). In contrast to a for loop, a while loop finds use in situations where the number of loop repetitions is not known beforehand. while condition do  command done As is the case with for/in loops, placing the do on the same line as the condition test requires a semicolon. while condition ; do Note that certain specialized while loops, as, for example, a getopts construct, deviate somewhat from the standard template given here. &ex25; &ex26; A while loop may have multiple conditions. Only the final condition determines when the loop terminates. This necessitates a slightly different loop syntax, however. &ex26a; As with a for loop, a while loop may employ C-like syntax by using the double parentheses construct (see also ). &whloopc; A while loop may have its stdin redirected to a file by a < at its end. until until do done loop until This construct tests for a condition at the top of a loop, and keeps looping as long as that condition is false (opposite of while loop). until condition-is-true do  command done Note that an until loop tests for the terminating condition at the top of the loop, differing from a similar construct in some programming languages. As is the case with for/in loops, placing the do on the same line as the condition test requires a semicolon. until condition-is-true ; do &ex27; A nested loop is a loop within a loop, an inner loop within the body of an outer one. What happens is that the first pass of the outer loop triggers the inner loop, which executes to completion. Then the second pass of the outer loop triggers the inner loop again. This repeats until the outer loop finishes. Of course, a break within either the inner or outer loop may interrupt this process. &nestedloop; See for an illustration of nested while loops, and to see a while loop nested inside an until loop. break continue loop break loop continue break continue The break and continue loop control commands These are shell builtins, whereas other loop commands, such as while and case, are keywords. correspond exactly to their counterparts in other programming languages. The break command terminates the loop (breaks out of it), while continue causes a jump to the next iteration of the loop, skipping all the remaining commands in that particular loop cycle. &ex28; The break command may optionally take a parameter. A plain break terminates only the innermost loop in which it is embedded, but a break N breaks out of N levels of loop. &breaklevels; The continue command, similar to break, optionally takes a parameter. A plain continue cuts short the current iteration within its loop and begins the next. A continue N terminates all remaining iterations at its loop level and continues with the next iteration at the loop N levels above. &continuelevels; &continuenex; The continue N construct is difficult to understand and tricky to use in any meaningful context. It is probably best avoided. The case and select constructs are technically not loops, since they do not iterate the execution of a code block. Like loops, however, they direct program flow according to conditions at the top or bottom of the block. case (in) / esac case in esac switch ;; menus The case construct is the shell equivalent of switch in C/C++. It permits branching to one of a number of code blocks, depending on condition tests. It serves as a kind of shorthand for multiple if/then/else statements and is an appropriate tool for creating menus. case "$variable" in  "$condition1" )  command  ;;  "$condition2" )  command  ;; esac Quoting the variables is not mandatory, since word splitting does not take place. Each test line ends with a right paren ). Each condition block ends with a double semicolon ;;. The entire case block terminates with an esac (case spelled backwards). &ex29; &ex30; An exceptionally clever use of case involves testing for command-line parameters. #! /bin/bash case "$1" in "") echo "Usage: ${0##*/} <filename>"; exit 65;; # No command-line parameters, # or first parameter empty. # Note that ${0##*/} is ${var##pattern} param substitution. Net result is $0. -*) FILENAME=./$1;; # If filename passed as argument ($1) starts with a dash, # replace it with ./$1 # so further commands don't interpret it as an option. * ) FILENAME=$1;; # Otherwise, $1. esac &casecmd; A case construct can filter strings for globbing patterns. &matchstring; &isalpha; select select menus The select construct, adopted from the Korn Shell, is yet another tool for building menus. select variable in list do  command  break done This prompts the user to enter one of the choices presented in the variable list. Note that select uses the PS3 prompt (#? ) by default, but that this may be changed. &ex31; If in list is omitted, then select uses the list of command line arguments ($@) passed to the script or to the function in which the select construct is embedded. Compare this to the behavior of a for variable in list construct with the in list omitted. &ex32; See also . builtin A builtin is a command contained within the Bash tool set, literally built in. This is either for performance reasons -- builtins execute faster than external commands, which usually require forking off a separate process -- or because a particular builtin needs direct access to the shell internals. When a command or the shell itself initiates (or spawns) a new subprocess to carry out a task, this is called forking. This new process is the child, and the process that forked it off is the parent. While the child process is doing its work, the parent process is still executing. Generally, a Bash builtin does not fork a subprocess when it executes within a script. An external system command or filter in a script usually will fork a subprocess. A builtin may be a synonym to a system command of the same name, but Bash reimplements it internally. For example, the Bash echo command is not the same as /bin/echo, although their behavior is almost identical. #!/bin/bash echo "This line uses the \"echo\" builtin." /bin/echo "This line uses the /bin/echo system command." A keyword is a reserved word, token or operator. Keywords have a special meaning to the shell, and indeed are the building blocks of the shell's syntax. As examples, for, while, do, and ! are keywords. Similar to a builtin, a keyword is hard-coded into Bash, but unlike a builtin, a keyword is not by itself a command, but part of a larger command structure. An exception to this is the time command, listed in the official Bash documentation as a keyword. echo echo command echo prints (to stdout) an expression or variable (see ). echo Hello echo $a An echo requires the -e option to print escaped characters. See . Normally, each echo command prints a terminal newline, but the -n option suppresses this. An echo can be used to feed a sequence of commands down a pipe. if echo "$VAR" | grep -q txt # if [[ $VAR = *txt* ]] then echo "$VAR contains the substring sequence \"txt\"" fi An echo, in combination with command substitution can set a variable. a=`echo "HELLO" | tr A-Z a-z` See also , , , and . Be aware that echo `command` deletes any linefeeds that the output of command generates. The $IFS (internal field separator) variable normally contains \n (linefeed) as one of its set of whitespace characters. Bash therefore splits the output of command at linefeeds into arguments to echo. Then echo outputs these arguments, separated by spaces. bash$ ls -l /usr/share/apps/kjezz/sounds -rw-r--r-- 1 root root 1407 Nov 7 2000 reflect.au -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au bash$ echo `ls -l /usr/share/apps/kjezz/sounds` total 40 -rw-r--r-- 1 root root 716 Nov 7 2000 reflect.au -rw-r--r-- 1 root root 362 Nov 7 2000 seconds.au This command is a shell builtin, and not the same as /bin/echo, although its behavior is similar. bash$ type -a echo echo is a shell builtin echo is /bin/echo printf printf command printf The printf, formatted print, command is an enhanced echo. It is a limited variant of the C language printf() library function, and its syntax is somewhat different. printf format-string parameter This is the Bash builtin version of the /bin/printf or /usr/bin/printf command. See the printf manpage (of the system command) for in-depth coverage. Older versions of Bash may not support printf. &ex47; Formatting error messages is a useful application of printf E_BADDIR=65 var=nonexistent_directory error() { printf "$@" >&2 # Formats positional params passed, and sents them to stderr. echo exit $E_BADDIR } cd $var || error $"Can't cd to %s." "$var" # Thanks, S.C. read read command read Reads the value of a variable from stdin, that is, interactively fetches input from the keyboard. The -a option lets read get array variables (see ). &ex36; A read without an associated variable assigns its input to the dedicated variable $REPLY. &readnovar; Normally, inputting a \ suppresses a newline during input to a read. The -r option causes an inputted \ to be interpreted literally. &readr; The read command has some interesting options that permit echoing a prompt and even reading keystrokes without hitting ENTER. # Read a keypress without hitting ENTER. read -s -n1 -p "Hit a key " keypress echo; echo "Keypress was "\"$keypress\""." # -s option means do not echo input. # -n N option means accept only N characters of input. # -p option means echo the following prompt before reading input. # Using these options is tricky, since they need to be in the correct order. The -n option to read also allows detection of the arrow keys and certain of the other unusual keys. &arrowdetect; The -t option to read permits timed input (see ). The read command may also read its variable value from a file redirected to stdin. If the file contains more than one line, only the first line is assigned to the variable. If read has more than one parameter, then each of these variables gets assigned a successive whitespace-delineated string. Caution! &readredir; Piping output to a read, using echo to set variables will fail. However, piping the output of cat does seem to work. cat file1 file2 | while read line do echo $line done cd cd command cd The familiar cd change directory command finds use in scripts where execution of a command requires being in a specified directory. (cd /source/directory && tar cf - . ) | (cd /dest/directory && tar xpvf -) [from the previously cited example by Alan Cox] The -P (physical) option to cd causes it to ignore symbolic links. cd - changes to $OLDPWD, the previous working directory. The cd command does not function as expected when presented with two forward slashes. bash$ cd // bash$ pwd // The output should, of course, be /. This is a problem both from the command line and in a script. pwd pwd command pwd $PWD variable $PWD directory working Print Working Directory. This gives the user's (or script's) current directory (see ). The effect is identical to reading the value of the builtin variable $PWD. pushd popd dirs pushd command pushd popd command popd dirs command dirs directory working bookmark This command set is a mechanism for bookmarking working directories, a means of moving back and forth through directories in an orderly manner. A pushdown stack is used to keep track of directory names. Options allow various manipulations of the directory stack. pushd dir-name pushes the path dir-name onto the directory stack and simultaneously changes the current working directory to dir-name popd removes (pops) the top directory path name off the directory stack and simultaneously changes the current working directory to that directory popped from the stack. dirs lists the contents of the directory stack (compare this with the $DIRSTACK variable). A successful pushd or popd will automatically invoke dirs. Scripts that require various changes to the current working directory without hard-coding the directory name changes can make good use of these commands. Note that the implicit $DIRSTACK array variable, accessible from within a script, holds the contents of the directory stack. &ex37; let let command let The let command carries out arithmetic operations on variables. In many cases, it functions as a less complex version of expr. &ex46; eval eval command eval eval arg1 [arg2] ... [argN] Translates into commands the arguments in a list (useful for code generation within a script). &ex43; &ex44; &rot14; Rory Winston contributed the following instance of how useful eval can be. &evalex; The eval command can be risky, and normally should be avoided when there exists a reasonable alternative. An eval $COMMANDS executes the contents of COMMANDS, which may contain such unpleasant surprises as rm -rf *. Running an eval on unfamiliar code written by persons unknown is living dangerously. set set command set The set command changes the value of internal script variables. One use for this is to toggle option flags which help determine the behavior of the script. Another application for it is to reset the positional parameters that a script sees as the result of a command (set `command`). The script can then parse the fields of the command output. &ex34; Invoking set without any options or arguments simply lists all the environmental and other variables that have been initialized. bash$ set AUTHORCOPY=/home/bozo/posts BASH=/bin/bash BASH_VERSION=$'2.05.8(1)-release' ... XAUTHORITY=/home/bozo/.Xauthority _=/etc/bashrc variable22=abc variable23=xzy Using set with the -- option explicitly assigns the contents of a variable to the positional parameters. When no variable follows the --, it unsets the positional parameters. &setpos; See also and . unset unset command unset The unset command deletes a shell variable, effectively setting it to null. Note that this command does not affect positional parameters. bash$ unset PATH bash$ echo $PATH bash$ &uns; export export command export The export command makes available variables to all child processes of the running script or shell. Unfortunately, there is no way to export variables back to the parent process, to the process that called or invoked the script or shell. One important use of export command is in startup files, to initialize and make accessible environmental variables to subsequent user processes. &coltotaler3; It is possible to initialize and export variables in the same operation, as in export var1=xxx. However, as Greg Keraunen points out, in certain situations this may have a different effect than setting a variable, then exporting it. bash$ export var=(a b); echo ${var[0]} (a b) bash$ var=(a b); export var; echo ${var[0]} a declare typeset declare command declare typeset command typeset The declare and typeset commands specify and/or restrict properties of variables. readonly readonly command readonly Same as declare -r, sets a variable as read-only, or, in effect, as a constant. Attempts to change the variable fail with an error message. This is the shell analog of the C language const type qualifier. getopts getopts command getopts $OPTIND variable $OPTIND $OPTARG variable $OPTARG This powerful tool parses command-line arguments passed to the script. This is the Bash analog of the getopt external command and the getopt library function familiar to C programmers. It permits passing and concatenating multiple options A option is an argument that acts as a flag, switching script behaviors on or off. The argument associated with a particular option indicates the behavior that the option (flag) switches on or off. and associated arguments to a script (for example scriptname -abc -e /usr/local). The getopts construct uses two implicit variables. $OPTIND is the argument pointer (OPTion INDex) and $OPTARG (OPTion ARGument) the (optional) argument attached to an option. A colon following the option name in the declaration tags that option as having an associated argument. A getopts construct usually comes packaged in a while loop, which processes the options and arguments one at a time, then decrements the implicit $OPTIND variable to step to the next. The arguments passed from the command line to the script must be preceded by a minus (-) or a plus (+). It is the prefixed - or + that lets getopts recognize command-line arguments as options. In fact, getopts will not process arguments without the prefixed - or +, and will terminate option processing at the first argument encountered lacking them. The getopts template differs slightly from the standard while loop, in that it lacks condition brackets. The getopts construct replaces the obsolete and less powerful getopt external command. while getopts ":abcde:fg" Option # Initial declaration. # a, b, c, d, e, f, and g are the options (flags) expected. # The : after option 'e' shows it will have an argument passed with it. do case $Option in a ) # Do something with variable 'a'. b ) # Do something with variable 'b'. ... e) # Do something with 'e', and also with $OPTARG, # which is the associated argument passed with option 'e'. ... g ) # Do something with variable 'g'. esac done shift $(($OPTIND - 1)) # Move argument pointer to next. # All this is not nearly as complicated as it looks <grin>. &ex33; source . (dot command) source command source . command . This command, when invoked from the command line, executes a script. Within a script, a source file-name loads the file file-name. This is the shell scripting equivalent of a C/C++ #include directive. It is useful in situations when multiple scripts use a common data file or function library. &ex38; File data-file for , above. Must be present in same directory. &ex38bis; It is even possible for a script to source itself, though this does not seem to have any practical applications. &selfsource; exit exit command exit Unconditionally terminates a script. The exit command may optionally take an integer argument, which is returned to the shell as the exit status of the script. It is good practice to end all but the simplest scripts with an exit 0, indicating a successful run. If a script terminates with an exit lacking an argument, the exit status of the script is the exit status of the last command executed in the script, not counting the exit. exec exec command exec This shell builtin replaces the current process with a specified command. Normally, when the shell encounters a command, it forks off a child process to actually execute the command. Using the exec builtin, the shell does not fork, and the command exec'ed replaces the shell. When used in a script, therefore, it forces an exit from the script when the exec'ed command terminates. For this reason, if an exec appears in a script, it would probably be the final command. &ex54; &selfexec; An exec also serves to reassign file descriptors. exec <zzz-file replaces stdin with the file zzz-file (see ). The -exec option to find is not the same as the exec shell builtin. shopt shopt command shopt This command permits changing shell options on the fly (see and ). It often appears in the Bash startup files, but also has its uses in scripts. Needs version 2 or later of Bash. shopt -s cdspell # Allows minor misspelling of directory names with 'cd' cd /hpme # Oops! Mistyped '/home'. pwd # /home # The shell corrected the misspelling. true true command true A command that returns a successful (zero) exit status, but does nothing else. # Endless loop while true # alias for ":" do operation-1 operation-2 ... operation-n # Need a way to break out of loop or script will hang. done false false command false A command that returns an unsuccessful exit status, but does nothing else. # Null loop while false do # The following code will not execute. operation-1 operation-2 ... operation-n # Nothing happens! done type [cmd] type command type variable which Similar to the which external command, type cmd gives the full pathname to cmd. Unlike which, type is a Bash builtin. The useful -a option to type identifies keywords and builtins, and also locates system commands with identical names. bash$ type '[' [ is a shell builtin bash$ type -a '[' [ is a shell builtin [ is /usr/bin/[ hash [cmds] hash command hash $PATH variable $PATH Record the path name of specified commands (in the shell hash table), so the shell or script will not need to search the $PATH on subsequent calls to those commands. When hash is called with no arguments, it simply lists the commands that have been hashed. The -r option resets the hash table. help help command help COMMAND looks up a short usage summary of the shell builtin COMMAND. This is the counterpart to whatis, but for builtins. bash$ help exit exit: exit [n] Exit the shell with a status of N. If N is omitted, the exit status is that of the last command executed. Certain of the following job control commands take a job identifier as an argument. See the table at end of the chapter. jobs jobs command jobs ps command ps Lists the jobs running in the background, giving the job number. Not as useful as ps. It is all too easy to confuse jobs and processes. Certain builtins, such as kill, disown, and wait accept either a job number or a process number as an argument. The fg, bg and jobs commands accept only a job number. bash$ sleep 100 & [1] 1384 bash $ jobs [1]+ Running sleep 100 & 1 is the job number (jobs are maintained by the current shell), and 1384 is the process number (processes are maintained by the system). To kill this job/process, either a kill %1 or a kill 1384 works. Thanks, S.C. disown disown command disown Remove job(s) from the shell's table of active jobs. fg bg fg command foreground background command bg The fg command switches a job running in the background into the foreground. The bg command restarts a suspended job, and runs it in the background. If no job number is specified, then the fg or bg command acts upon the currently running job. wait wait command wait Stop script execution until all jobs running in background have terminated, or until the job number or process id specified as an option terminates. Returns the exit status of waited-for command. You may use the wait command to prevent a script from exiting before a background job finishes executing (this would create a dreaded orphan process). &ex39; Optionally, wait can take a job identifier as an argument, for example, wait%1 or wait $PPID. See the job id table. Within a script, running a command in the background with an ampersand (&) may cause the script to hang until ENTER is hit. This seems to occur with commands that write to stdout. It can be a major annoyance. #!/bin/bash # test.sh ls -l & echo "Done." bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh _ Placing a wait after the background command seems to remedy this. #!/bin/bash # test.sh ls -l & echo "Done." wait bash$ ./test.sh Done. [bozo@localhost test-scripts]$ total 1 -rwxr-xr-x 1 bozo bozo 34 Oct 11 15:09 test.sh Redirecting the output of the command to a file or even to /dev/null also takes care of this problem. suspend suspend command suspend This has a similar effect to ControlZ, but it suspends the shell (the shell's parent process should resume it at an appropriate time). logout logout command log out Exit a login shell, optionally specifying an exit status. times times command times Gives statistics on the system time used in executing commands, in the following form: 0m0.020s 0m0.020s This capability is of very limited value, since it is uncommon to profile and benchmark shell scripts. kill kill command kill Forcibly terminate a process by sending it an appropriate terminate signal (see ). &selfdestruct; kill -l lists all the signals. A kill -9 is a sure kill, which will usually terminate a process that stubbornly refuses to die with a plain kill. Sometimes, a kill -15 works. A zombie process, that is, a process whose parent has terminated, cannot be killed (you can't kill something that is already dead), but init will generally clean it up sooner or later. command command command command The command COMMAND directive disables aliases and functions for the command COMMAND. This is one of three shell directives that effect script command processing. The others are builtin and enable. builtin builtin command builtin Invoking builtin BUILTIN_COMMAND runs the command BUILTIN_COMMAND as a shell builtin, temporarily disabling both functions and external system commands with the same name. enable enable command enable This either enables or disables a shell builtin command. As an example, enable -n kill disables the shell builtin kill, so that when Bash subsequently encounters kill, it invokes /bin/kill. The -a option to enable lists all the shell builtins, indicating whether or not they are enabled. The -f filename option lets enable load a builtin as a shared library (DLL) module from a properly compiled object file. The C source for a number of loadable builtins is typically found in the /usr/share/doc/bash-?.??/functions directory. Note that the -f option to enable is not portable to all systems. . autoload autoload command autoloader This is a port to Bash of the ksh autoloader. With autoload in place, a function with an autoload declaration will load from an external file at its first invocation. The same effect as autoload can be achieved with typeset -fu. This saves system resources. Note that autoload is not a part of the core Bash installation. It needs to be loaded in with enable -f (see above). Notation Meaning %N Job number [N] %S Invocation (command line) of job begins with string S %?S Invocation (command line) of job contains within it string S %% current job (last job stopped in foreground or started in background) %+ current job (last job stopped in foreground or started in background) %- Last job $! Last background process Standard UNIX commands make shell scripts more versatile. The power of scripts comes from coupling system commands and shell directives with simple programming constructs. ls ls command ls The basic file list command. It is all too easy to underestimate the power of this humble command. For example, using the -R, recursive option, ls provides a tree-like listing of a directory structure. Other interesting options are -S, sort listing by file size, -t, sort by file modification time, and -i, show file inodes (see ). &ex40; cat tac cat command cat tac command tac cat, an acronym for concatenate, lists a file to stdout. When combined with redirection (> or >>), it is commonly used to concatenate files. cat filename cat file.1 file.2 file.3 > file.123 The -n option to cat inserts consecutive numbers before all lines of the target file(s). The -b option numbers only the non-blank lines. The -v option echoes nonprintable characters, using ^ notation. The -s option squeezes multiple consecutive blank lines into a single blank line. See also and . tac, is the inverse of cat, listing a file backwards from its end. rev rev command rev reverses each line of a file, and outputs to stdout. This is not the same effect as tac, as it preserves the order of the lines, but flips each one around. bash$ cat file1.txt This is line 1. This is line 2. bash$ tac file1.txt This is line 2. This is line 1. bash$ rev file1.txt .1 enil si sihT .2 enil si sihT cp cp command cp This is the file copy command. cp file1 file2 copies file1 to file2, overwriting file2 if it already exists (see ). Particularly useful are the -a archive flag (for copying an entire directory tree) and the -r and -R recursive flags. mv This is the file move command. It is equivalent to a combination of cp and rm. It may be used to move multiple files to a directory, or even to rename a directory. For some examples of using mv in a script, see and . When used in a non-interactive script, mv takes the -f (force) option to bypass user input. When a directory is moved to a preexisting directory, it becomes a subdirectory of the destination directory. bash$ mv source_directory target_directory bash$ ls -lF target_directory total 1 drwxrwxr-x 2 bozo bozo 1024 May 28 19:20 source_directory/ rm rm command rm Delete (remove) a file or files. The -f option forces removal of even readonly files, and is useful for bypassing user input in a script. When used with the recursive flag -r, this command removes files all the way down the directory tree. rmdir rmdir command rmdir Remove directory. The directory must be empty of all files, including invisible dotfiles, These are files whose names begin with a dot, such as ~/.Xdefaults. Such filenames do not show up in a normal ls listing, and they cannot be deleted by an accidental rm -rf *. Dotfiles are generally used as setup and configuration files in a user's home directory. for this command to succeed. mkdir mkdir command mkdir Make directory, creates a new directory. mkdir -p project/programs/December creates the named directory. The -p option automatically creates any necessary parent directories. chmod chmod command chmod Changes the attributes of an existing file (see ). chmod +x filename # Makes "filename" executable for all users. chmod u+s filename # Sets "suid" bit on "filename" permissions. # An ordinary user may execute "filename" with same privileges as the file's owner. # (This does not apply to shell scripts.) chmod 644 filename # Makes "filename" readable/writable to owner, readable to # others # (octal mode). chmod 1777 directory-name # Gives everyone read, write, and execute permission in directory, # however also sets the "sticky bit". # This means that only the owner of the directory, # owner of the file, and, of course, root # can delete any particular file in that directory. chattr chattr command chattr Change file attributes. This has the same effect as chmod above, but with a different invocation syntax, and it works only on an ext2 filesystem. ln Creates links to pre-existings files. Most often used with the -s, symbolic or soft link flag. This permits referencing the linked file by more than one name and is a superior alternative to aliasing (see ). ln -s oldfile newfile links the previously existing oldfile to the newly created link, newfile. man info man command man info command info These commands access the manual and information pages on system commands and installed utilities. When available, the info pages usually contain a more detailed description than do the man pages. find find command find {} special character {} \; escaped character \; -exec COMMAND \; Carries out COMMAND on each file that find matches. The command sequence terminates with \; (the ; is escaped to make certain the shell passes it to find literally). If COMMAND contains {}, then find substitutes the full path name of the selected file for {}. bash$ find ~/ -name '*.txt' /home/bozo/.kde/share/apps/karm/karmdata.txt /home/bozo/misc/irmeyc.txt /home/bozo/test-scripts/1.txt find /home/bozo/projects -mtime 1 # Lists all files in /home/bozo/projects directory tree #+ that were modified within the last day. # # mtime = last modification time of the target file # ctime = last status change time (via 'chmod' or otherwise) # atime = last access time DIR=/home/bozo/junk_files find "$DIR" -type f -atime +5 -exec rm {} \; # Deletes all files in "/home/bozo/junk_files" #+ that have not been accessed in at least 5 days. # # "-type filetype", where # f = regular file # d = directory, etc. # (The 'find' manpage has a complete listing.) find /etc -exec grep '[0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*[.][0-9][0-9]*' {} \; # Finds all IP addresses (xxx.xxx.xxx.xxx) in /etc directory files. # There a few extraneous hits - how can they be filtered out? # Perhaps by: find /etc -type f -exec cat '{}' \; | tr -c '.[:digit:]' '\n' \ | grep '^[^.][^.]*\.[^.][^.]*\.[^.][^.]*\.[^.][^.]*$' # [:digit:] is one of the character classes # introduced with the POSIX 1003.2 standard. # Thanks, S.C. The -exec option to find should not be confused with the exec shell builtin. &ex57; &idelete; See , , and for scripts using find. Its manpage provides more detail on this complex and powerful command. xargs xargs command xargs A filter for feeding arguments to a command, and also a tool for assembling the commands themselves. It breaks a data stream into small enough chunks for filters and commands to process. Consider it as a powerful replacement for backquotes. In situations where backquotes fail with a too many arguments error, substituting xargs often works. Normally, xargs reads from stdin or from a pipe, but it can also be given the output of a file. The default command for xargs is echo. This means that input piped to xargs may have linefeeds and other whitespace characters stripped out. bash$ ls -l total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 bash$ ls -l | xargs total 0 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file1 -rw-rw-r-- 1 bozo bozo 0 Jan 29 23:58 file2 ls | xargs -p -l gzip gzips every file in current directory, one at a time, prompting before each operation. An interesting xargs option is -n NN, which limits to NN the number of arguments passed. ls | xargs -n 8 echo lists the files in the current directory in 8 columns. Another useful option is -0, in combination with find -print0 or grep -lZ. This allows handling arguments containing whitespace or quotes. find / -type f -print0 | xargs -0 grep -liwZ GUI | xargs -0 rm -f grep -rliwZ GUI / | xargs -0 rm -f Either of the above will remove any file containing GUI. (Thanks, S.C.) &ex41; &ex42; expr expr command expr All-purpose expression evaluator: Concatenates and evaluates the arguments according to the operation given (arguments must be separated by spaces). Operations may be arithmetic, comparison, string, or logical. expr 3 + 5 returns 8 expr 5 % 3 returns 2 expr 5 \* 3 returns 15 The multiplication operator must be escaped when used in an arithmetic expression with expr. y=`expr $y + 1` Increment a variable, with the same effect as let y=y+1 and y=$(($y+1)). This is an example of arithmetic expansion. z=`expr substr $string $position $length` Extract substring of $length characters, starting at $position. &ex45; The : operator can substitute for match. For example, b=`expr $a : [0-9]*` is the exact equivalent of b=`expr match $a [0-9]*` in the above listing. &ex45a; This example illustrates how expr uses the escaped parentheses -- \( ... \) -- grouping operator in tandem with regular expression parsing to match a substring. Perl, sed, and awk have far superior string parsing facilities. A short sed or awk subroutine within a script (see ) is an attractive alternative to using expr. See for more on string operations. date date command date Simply invoked, date prints the date and time to stdout. Where this command gets interesting is in its formatting and parsing options. &ex51; The -u option gives the UTC (Universal Coordinated Time). bash$ date Fri Mar 29 21:07:39 MST 2002 bash$ date -u Sat Mar 30 04:07:42 UTC 2002 zdump zdump command time zone dump Echoes the time in a specified time zone. bash$ zdump EST EST Tue Sep 18 22:09:22 2001 EST time time command time Outputs very verbose timing statistics for executing a command. time ls -l / gives something like this: 0.00user 0.01system 0:00.05elapsed 16%CPU (0avgtext+0avgdata 0maxresident)k 0inputs+0outputs (149major+27minor)pagefaults 0swaps See also the very similar times command in the previous section. As of version 2.0 of Bash, time became a shell reserved word, with slightly altered behavior in a pipeline. touch touch command touch Utility for updating access/modification times of a file to current system time or other specified time, but also useful for creating a new file. The command touch zzz will create a new file of zero length, named zzz, assuming that zzz did not previously exist. Time-stamping empty files in this way is useful for storing date information, for example in keeping track of modification times on a project. The touch command is equivalent to : >> newfile or >> newfile (for ordinary files). at at command at crond command crond The at job control command executes a given set of commands at a specified time. Superficially, it resembles crond, however, at is chiefly useful for one-time execution of a command set. at 2pm January 15 prompts for a set of commands to execute at that time. These commands should be shell-script compatible, since, for all practical purposes, the user is typing in an executable shell script a line at a time. Input terminates with a Ctl-D. Using either the -f option or input redirection (<), at reads a command list from a file. This file is an executable shell script, though it should, of course, be noninteractive. Particularly clever is including the run-parts command in the file to execute a different set of scripts. bash$ at 2:30 am Friday < at-jobs.list job 2 at 2000-10-27 02:30 batch batch command batch at command at The batch job control command is similar to at, but it runs a command list when the system load drops below .8. Like at, it can read commands from a file with the -f option. cal cal command cal Prints a neatly formatted monthly calendar to stdout. Will do current year or a large range of past and future years. sleep sleep command sleep This is the shell equivalent of a wait loop. It pauses for a specified number of seconds, doing nothing. It can be useful for timing or in processes running in the background, checking for a specific event every so often (polling), as in . sleep 3 # Pauses 3 seconds. The sleep command defaults to seconds, but minute, hours, or days may also be specified. sleep 3 h # Pauses 3 hours! The watch command may be a better choice than sleep for running commands at timed intervals. usleep usleep command usleep Microsleep (the u may be read as the Greek mu, or micro- prefix). This is the same as sleep, above, but sleeps in microsecond intervals. It can be used for fine-grain timing, or for polling an ongoing process at very frequent intervals. usleep 30 # Pauses 30 microseconds. This command is part of the Red Hat initscripts / rc-scripts package. The usleep command does not provide particularly accurate timing, and is therefore unsuitable for critical timing loops. hwclock clock hwclock command hwclock clock command clock The hwclock command accesses or adjusts the machine's hardware clock. Some options require root privileges. The /etc/rc.d/rc.sysinit startup file uses hwclock to set the system time from the hardware clock at bootup. The clock command is a synonym for hwclock. sort sort command sort File sorter, often used as a filter in a pipe. This command sorts a text stream or file forwards or backwards, or according to various keys or character positions. Using the -m option, it merges presorted input files. The info page lists its many capabilities and options. See , , and . tsort tsort command topological sort Topological sort, reading in pairs of whitespace-separated strings and sorting according to input patterns. uniq uniq command uniq This filter removes duplicate lines from a sorted file. It is often seen in a pipe coupled with sort. cat list-1 list-2 list-3 | sort | uniq > final.list # Concatenates the list files, # sorts them, # removes duplicate lines, # and finally writes the result to an output file. The useful -c option prefixes each line of the input file with its number of occurrences. bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times. bash$ uniq -c testfile 1 This line occurs only once. 2 This line occurs twice. 3 This line occurs three times. bash$ sort testfile | uniq -c | sort -nr 3 This line occurs three times. 2 This line occurs twice. 1 This line occurs only once. The sort INPUTFILE | uniq -c | sort -nr command string produces a frequency of occurrence listing on the INPUTFILE file (the -nr options to sort cause a reverse numerical sort). This template finds use in analysis of log files and dictionary lists, and wherever the lexical structure of a document needs to be examined. &wf; bash$ cat testfile This line occurs only once. This line occurs twice. This line occurs twice. This line occurs three times. This line occurs three times. This line occurs three times. bash$ ./wf.sh testfile 6 this 6 occurs 6 line 3 times 3 three 2 twice 1 only 1 once expand unexpand expand command expand unexpand command unexpand The expand filter converts tabs to spaces. It is often used in a pipe. The unexpand filter converts spaces to tabs. This reverses the effect of expand. cut cut command cut awk command awk A tool for extracting fields from files. It is similar to the print $N command set in awk, but more limited. It may be simpler to use cut in a script than awk. Particularly important are the -d (delimiter) and -f (field specifier) options. Using cut to obtain a listing of the mounted filesystems: cat /etc/mtab | cut -d ' ' -f1,2 Using cut to list the OS and kernel version: uname -a | cut -d" " -f1,3,11,12 Using cut to extract message headers from an e-mail folder: bash$ grep '^Subject:' read-messages | cut -c10-80 Re: Linux suitable for mission-critical apps? MAKE MILLIONS WORKING AT HOME!!! Spam complaint Re: Spam complaint Using cut to parse a file: # List all the users in /etc/passwd. FILENAME=/etc/passwd for user in $(cut -d: -f1 $FILENAME) do echo $user done # Thanks, Oleg Philon for suggesting this. cut -d ' ' -f2,3 filename is equivalent to awk -F'[ ]' '{ print $2, $3 }' filename See also . paste paste command paste cut command cut Tool for merging together different files into a single, multi-column file. In combination with cut, useful for creating system log files. join join command join Consider this a special-purpose cousin of paste. This powerful utility allows merging two files in a meaningful fashion, which essentially creates a simple version of a relational database. The join command operates on exactly two files, but pastes together only those lines with a common tagged field (usually a numerical label), and writes the result to stdout. The files to be joined should be sorted according to the tagged field for the matchups to work properly. File: 1.data 100 Shoes 200 Laces 300 Socks File: 2.data 100 $40.00 200 $1.00 300 $2.00 bash$ join 1.data 2.data File: 1.data 2.data 100 Shoes $40.00 200 Laces $1.00 300 Socks $2.00 The tagged field appears only once in the output. head head command head lists the beginning of a file to stdout (the default is 10 lines, but this can be changed). It has a number of interesting options. &scriptdetector; &rnd; See also . tail tail command tail lists the end of a file to stdout (the default is 10 lines). Commonly used to keep track of changes to a system logfile, using the -f option, which outputs lines appended to the file. &ex12; See also , and . grep grep command grep A multi-purpose file search tool that uses regular expressions. It was originally a command/filter in the venerable ed line editor, g/re/p, that is, global - regular expression - print. grep pattern file Search the target file(s) for occurrences of pattern, where pattern may be literal text or a regular expression. bash$ grep '[rst]ystem.$' osinfo.txt The GPL governs the distribution of the Linux operating system. If no target file(s) specified, grep works as a filter on stdout, as in a pipe. bash$ ps ax | grep clock 765 tty1 S 0:00 xclock 901 pts/1 S 0:00 grep clock The -i option causes a case-insensitive search. The -w option matches only whole words. The -l option lists only the files in which matches were found, but not the matching lines. The -r (recursive) option searches files in the current working directory and all subdirectories below it. The -n option lists the matching lines, together with line numbers. bash$ grep -n Linux osinfo.txt 2:This is a file containing information about Linux. 6:The GPL governs the distribution of the Linux operating system. The -v (or --invert-match) option filters out matches. grep pattern1 *.txt | grep -v pattern2 # Matches all lines in "*.txt" files containing "pattern1", # but ***not*** "pattern2". The -c (--count) option gives a numerical count of matches, rather than actually listing the matches. grep -c txt *.sgml # (number of occurrences of "txt" in "*.sgml" files) # grep -cz . # ^ dot # means count (-c) zero-separated (-z) items matching "." # that is, non-empty ones (containing at least 1 character). # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz . # 4 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '$' # 5 printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -cz '^' # 5 # printf 'a b\nc d\n\n\n\n\n\000\n\000e\000\000\nf' | grep -c '$' # 9 # By default, newline chars (\n) separate items to match. # Note that the -z option is GNU "grep" specific. # Thanks, S.C. When invoked with more than one target file given, grep specifies which file contains matches. bash$ grep Linux osinfo.txt misc.txt osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system. misc.txt:The Linux operating system is steadily gaining in popularity. To force grep to show the filename when searching only one target file, simply give /dev/null as the second file. bash$ grep Linux osinfo.txt /dev/null osinfo.txt:This is a file containing information about Linux. osinfo.txt:The GPL governs the distribution of the Linux operating system. If there is a successful match, grep returns an exit status of 0, which makes it useful in a condition test in a script, especially in combination with the -q option to suppress output. SUCCESS=0 # if grep lookup succeeds word=Linux filename=data.file grep -q "$word" "$filename" # The "-q" option causes nothing to echo to stdout. if [ $? -eq $SUCCESS ] then echo "$word found in $filename" else echo "$word not found in $filename" fi demonstrates how to use grep to search for a word pattern in a system logfile. &grp; egrep is the same as grep -E. This uses a somewhat different, extended set of regular expressions, which can make the search somewhat more flexible. fgrep is the same as grep -F. It does a literal string search (no regular expressions), which allegedly speeds things up a bit. agrep extends the capabilities of grep to approximate matching. The search string may differ by a specified number of characters from the resulting matches. This utility is not part of the core Linux distribution. To search compressed files, use zgrep, zegrep, or zfgrep. These also work on non-compressed files, though slower than plain grep, egrep, fgrep. They are handy for searching through a mixed set of files, some compressed, some not. To search bzipped files, use bzgrep. look look command look The command look works like grep, but does a lookup on a dictionary, a sorted word list. By default, look searches for a match in /usr/dict/words, but a different dictionary file may be specified. &lookup; sed awk sed command sed awk command awk Scripting languages especially suited for parsing text files and command output. May be embedded singly or in combination in pipes and shell scripts. sed Non-interactive stream editor, permits using many ex commands in batch mode. It finds many uses in shell scripts. awk Programmable file extractor and formatter, good for manipulating and/or extracting fields (columns) in structured text files. Its syntax is similar to C. wc wc command wc wc gives a word count on a file or I/O stream: bash $ wc /usr/doc/sed-3.02/README 20 127 838 /usr/doc/sed-3.02/README [20 lines 127 words 838 characters] wc -w gives only the word count. wc -l gives only the line count. wc -c gives only the character count. wc -L gives only the length of the longest line. Using wc to count how many .txt files are in current working directory: $ ls *.txt | wc -l # Will work as long as none of the "*.txt" files have a linefeed in their name. # Alternative ways of doing this are: # find . -maxdepth 1 -name \*.txt -print0 | grep -cz . # (shopt -s nullglob; set -- *.txt; echo $#) # Thanks, S.C. Using wc to total up the size of all the files whose names begin with letters in the range d - h bash$ wc [d-h]* | grep total | awk '{print $3}' 71832 Using wc to count the instances of the word Linux in the main source file for this book. bash$ grep Linux abs-book.sgml | wc -l 50 See also and . Certain commands include some of the functionality of wc as options. ... | grep foo | wc -l # This frequently used construct can be more concisely rendered. ... | grep -c foo # Just use the "-c" (or "--count") option of grep. # Thanks, S.C. tr tr command tr character translation filter. Must use quoting and/or brackets, as appropriate. Quotes prevent the shell from reinterpreting the special characters in tr command sequences. Brackets should be quoted to prevent expansion by the shell. Either tr "A-Z" "*" <filename or tr A-Z \* <filename changes all the uppercase letters in filename to asterisks (writes to stdout). On some systems this may not work, but tr A-Z '[**]' will. The -d option deletes a range of characters. echo "abcdef" # abcdef echo "abcdef" | tr -d b-d # aef tr -d 0-9 <filename # Deletes all digits from the file "filename". The --squeeze-repeats (or -s) option deletes all but the first instance of a string of consecutive characters. This option is useful for removing excess whitespace. bash$ echo "XXXXX" | tr --squeeze-repeats 'X' X The -c complement option inverts the character set to match. With this option, tr acts only upon those characters not matching the specified set. bash$ echo "acfdeb123" | tr -c b-d + +c+d+b++++ Note that tr recognizes POSIX character classes. This is only true of the GNU version of tr, not the generic version often found on commercial UNIX systems. bash$ echo "abcd2ef1" | tr '[:alpha:]' - ----2--1 &ex49; &lowercase; &du; &rot13; &cryptoquote; The tr utility has two historic variants. The BSD version does not use brackets (tr a-z A-Z), but the SysV one does (tr '[a-z]' '[A-Z]'). The GNU version of tr resembles the BSD one, so quoting letter ranges within brackets is mandatory. fold fold command fold A filter that wraps lines of input to a specified width. This is especially useful with the -s option, which breaks lines at word spaces (see and ). fmt fmt command fmt Simple-minded file formatter, used as a filter in a pipe to wrap long lines of text output. &ex50; See also . A powerful alternative to fmt is Kamil Toman's par utility, available from http://www.cs.berkeley.edu/~amc/Par/. col col command reverse line feed This deceptively named filter removes reverse line feeds from an input stream. It also attempts to replace whitespace with equivalent tabs. The chief use of col is in filtering the output from certain text processing utilities, such as groff and tbl. column column command column Column formatter. This filter transforms list-type text output into a pretty-printed table by inserting tabs at appropriate places. &colm; colrm colrm command colrm Column removal filter. This removes columns (characters) from a file and writes the file, lacking the range of specified columns, back to stdout. colrm 2 4 <filename removes the second through fourth characters from each line of the text file filename. If the file contains tabs or nonprintable characters, this may cause unpredictable behavior. In such cases, consider using expand and unexpand in a pipe preceding colrm. nl nl command fmt Line numbering filter. nl filename lists filename to stdout, but inserts consecutive numbers at the beginning of each non-blank line. If filename omitted, operates on stdin. The output of nl is very similar to cat -n, however, by default nl does not list blank lines. &lnum; pr pr command pr Print formatting filter. This will paginate files (or stdout) into sections suitable for hard copy printing or viewing on screen. Various options permit row and column manipulation, joining lines, setting margins, numbering lines, adding page headers, and merging files, among other things. The pr command combines much of the functionality of nl, paste, fold, column, and expand. pr -o 5 --width=65 fileZZZ | more gives a nice paginated listing to screen of fileZZZ with margins set at 5 and 65. A particularly useful option is -d, forcing double-spacing (same effect as sed -G). gettext gettext command localization The GNU gettext package is a set of utilities for localizing and translating the text output of programs into foreign languages. While originally intended for C programs, it now supports quite a number of programming and scripting languages. The gettext program works on shell scripts. See the info page. msgfmt msgfmt command localization A program for generating binary message catalogs. It is used for localization. iconv iconv command encoding A utility for converting file(s) to a different encoding (character set). Its chief use is for localization. recode recode command encoding Consider this a fancier version of iconv, above. This very versatile utility for converting a file to a different encoding is not part of the standard Linux installation. TeX gs TeX command TeX gs command Postscript TeX and Postscript are text markup languages used for preparing copy for printing or formatted video display. TeX is Donald Knuth's elaborate typsetting system. It is often convenient to write a shell script encapsulating all the options and arguments passed to one of these markup languages. Ghostscript (gs) is a GPL-ed Postscript interpreter. groff tbl eqn groff command groff tbl command table eqn command equation Yet another text markup and display formatting language is groff. This is the enhanced GNU version of the venerable UNIX roff/troff display and typesetting package. Manpages use groff (see ). The tbl table processing utility is considered part of groff, as its function is to convert table markup into groff commands. The eqn equation processing utility is likewise part of groff, and its function is to convert equation markup into groff commands. lex yacc lex command flex yacc command bison The lex lexical analyzer produces programs for pattern matching. This has been replaced by the nonproprietary flex on Linux systems. The yacc utility creates a parser based on a set of specifications. This has been replaced by the nonproprietary bison on Linux systems. tar tar command tar The standard UNIX archiving utility. Originally a Tape ARchiving program, it has developed into a general purpose package that can handle all manner of archiving with all types of destination devices, ranging from tape drives to regular files to even stdout (see ). GNU tar has been patched to accept various compression filters, such as tar czvf archive_name.tar.gz *, which recursively archives and gzips all files in a directory tree except dotfiles in the current working directory ($PWD). A tar czvf archive_name.tar.gz * will include dotfiles in directories below the current working directory. This is an undocumented GNU tar feature. Some useful tar options: -c create (a new archive) -x extract (files from existing archive) --delete delete (files from existing archive) This option will not work on magnetic tape devices. -r append (files to existing archive) -A append (tar files to existing archive) -t list (contents of existing archive) -u update archive -d compare archive with specified filesystem -z gzip the archive (compress or uncompress, depending on whether combined with the -c or -x) option -j bzip2 the archive It may be difficult to recover data from a corrupted gzipped tar archive. When archiving important files, make multiple backups. shar shar command archive Shell archiving utility. The files in a shell archive are concatenated without compression, and the resultant archive is essentially a shell script, complete with #!/bin/sh header, and containing all the necessary unarchiving commands. Shar archives still show up in Internet newsgroups, but otherwise shar has been pretty well replaced by tar/gzip. The unshar command unpacks shar archives. ar ar command archive Creation and manipulation utility for archives, mainly used for binary object file libraries. rpm rpm command package manager The Red Hat Package Manager, or rpm utility provides a wrapper for source or binary archives. It includes commands for installing and checking the integrity of packages, among other things. A simple rpm -i package_name.rpm usually suffices to install a package, though there are many more options available. An rpm -qa gives a complete list of all installed rpm packages on a given system. An rpm -qa package_name lists only the package(s) corresponding to package_name. bash$ rpm -qa redhat-logos-1.1.3-1 glibc-2.2.4-13 cracklib-2.7-12 dosfstools-2.7-1 gdbm-1.8.0-10 ksymoops-2.4.1-1 mktemp-1.5-11 perl-5.6.0-17 reiserfs-utils-3.x.0j-2 ... bash$ rpm -qa docbook-utils docbook-utils-0.6.9-2 bash$ rpm -qa docbook | grep docbook docbook-dtd31-sgml-1.0-10 docbook-style-dsssl-1.64-3 docbook-dtd30-sgml-1.0-10 docbook-dtd40-sgml-1.0-11 docbook-utils-pdf-0.6.9-2 docbook-dtd41-sgml-1.0-10 docbook-utils-0.6.9-2 cpio cpio command cpio This specialized archiving copy command (copy input and output) is rarely seen any more, having been supplanted by tar/gzip. It still has its uses, such as moving a directory tree. &ex48; rpm2cpio rpm command cpio This command extracts a cpio archive from an rpm one. &derpm; gzip gzip command gzip The standard GNU/UNIX compression utility, replacing the inferior and proprietary compress. The corresponding decompression command is gunzip, which is the equivalent of gzip -d. The zcat filter decompresses a gzipped file to stdout, as possible input to a pipe or redirection. This is, in effect, a cat command that works on compressed files (including files processed with the older compress utility). The zcat command is equivalent to gzip -dc. On some commercial UNIX systems, zcat is a synonym for uncompress -c, and will not work on gzipped files. See also . bzip2 bzip2 command bzip2 An alternate compression utility, usually more efficient (but slower) than gzip, especially on large files. The corresponding decompression command is bunzip2. Newer versions of tar have been patched with bzip2 support. compress uncompress compress command compress uncompress command uncompress This is an older, proprietary compression utility found in commercial UNIX distributions. The more efficient gzip has largely replaced it. Linux distributions generally include a compress workalike for compatibility, although gunzip can unarchive files treated with compress. The znew command transforms compressed files into gzipped ones. sq sq command sq Yet another compression utility, a filter that works only on sorted ASCII word lists. It uses the standard invocation syntax for a filter, sq < input-file > output-file. Fast, but not nearly as efficient as gzip. The corresponding uncompression filter is unsq, invoked like sq. The output of sq may be piped to gzip for further compression. zip unzip zip command pkzip.exe unzip command unzip Cross-platform file archiving and compression utility compatible with DOS pkzip.exe. Zipped archives seem to be a more acceptable medium of exchange on the Internet than tarballs. unarc unarj unrar unarc command arc.exe unarj command arj.exe unrar command rar.exe These Linux utilities permit unpacking archives compressed with the DOS arc.exe, arj.exe, and rar.exe programs. file file command file A utility for identifying file types. The command file file-name will return a file specification for file-name, such as ascii text or data. It references the magic numbers found in /usr/share/magic, /etc/magic, or /usr/lib/magic, depending on the Linux/UNIX distribution. The -f option causes file to run in batch mode, to read from a designated file a list of filenames to analyze. The -z option, when used on a compressed target file, forces an attempt to analyze the uncompressed file type. bash$ file test.tar.gz test.tar.gz: gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix bash file -z test.tar.gz test.tar.gz: GNU tar archive (gzip compressed data, deflated, last modified: Sun Sep 16 13:34:51 2001, os: Unix) &stripc; which which command which which command-xxx gives the full path to command-xxx. This is useful for finding out whether a particular command or utility is installed on the system. $bash which rm /usr/bin/rm whereis whereis command whereis Similar to which, above, whereis command-xxx gives the full path to command-xxx, but also to its manpage. $bash whereis rm rm: /bin/rm /usr/share/man/man1/rm.1.bz2 whatis whatis command whatis whatis filexxx looks up filexxx in the whatis database. This is useful for identifying system commands and important configuration files. Consider it a simplified man command. $bash whatis whatis whatis (1) - search the whatis database for complete words &what; See also . vdir vdir command ls Show a detailed directory listing. The effect is similar to ls -l. This is one of the GNU fileutils. bash$ vdir total 10 -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo bash ls -l total 10 -rw-r--r-- 1 bozo bozo 4034 Jul 18 22:04 data1.xrolo -rw-r--r-- 1 bozo bozo 4602 May 25 13:58 data1.xrolo.bak -rw-r--r-- 1 bozo bozo 877 Dec 17 2000 employment.xrolo locate slocate locate command locate slocate command slocate The locate command searches for files using a database stored for just that purpose. The slocate command is the secure version of locate (which may be aliased to slocate). $bash locate hickson /usr/lib/xephem/catalogs/hickson.edb readlink readlink command link Disclose the file that a symbolic link points to. bash$ readlink /usr/bin/awk ../../bin/gawk strings strings command strings Use the strings command to find printable strings in a binary or data file. It will list sequences of printable characters found in the target file. This might be handy for a quick 'n dirty examination of a core dump or for looking at an unknown graphic image file (strings image-file | more might show something like JFIF, which would identify the file as a jpeg graphic). In a script, you would probably parse the output of strings with grep or sed. See and . &wstrings; diff patch diff command diff patch command patch diff: flexible file comparison utility. It compares the target files line-by-line sequentially. In some applications, such as comparing word dictionaries, it may be helpful to filter the files through sort and uniq before piping them to diff. diff file-1 file-2 outputs the lines in the files that differ, with carets showing which file each particular line belongs to. The --side-by-side option to diff outputs each compared file, line by line, in separate columns, with non-matching lines marked. The -c and -u options likewise make the output of the command easier to interpret. There are available various fancy frontends for diff, such as spiff, wdiff, xdiff, and mgdiff. The diff command returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use of diff in a test construct within a shell script (see below). A common use for diff is generating difference files to be used with patch The -e option outputs files suitable for ed or ex scripts. patch: flexible versioning utility. Given a difference file generated by diff, patch can upgrade a previous version of a package to a newer version. It is much more convenient to distribute a relatively small diff file than the entire body of a newly revised package. Kernel patches have become the preferred method of distributing the frequent releases of the Linux kernel. patch -p1 <patch-file # Takes all the changes listed in 'patch-file' # and applies them to the files referenced therein. # This upgrades to a newer version of the package. Patching the kernel: cd /usr/src gzip -cd patchXX.gz | patch -p0 # Upgrading kernel source using 'patch'. # From the Linux kernel docs "README", # by anonymous author (Alan Cox?). The diff command can also recursively compare directories (for the filenames present). bash$ diff -r ~/notes1 ~/notes2 Only in /home/bozo/notes1: file02 Only in /home/bozo/notes1: file03 Only in /home/bozo/notes2: file04 Use zdiff to compare gzipped files. diff3 diff3 command diff3 An extended version of diff that compares three files at a time. This command returns an exit value of 0 upon successful execution, but unfortunately this gives no information about the results of the comparison. bash$ diff3 file-1 file-2 file-3 ==== 1:1c This is line 1 of "file-1". 2:1c This is line 1 of "file-2". 3:1c This is line 1 of "file-3" sdiff sdiff command sdiff Compare and/or edit two files in order to merge them into an output file. Because of its interactive nature, this command would find little use in a script. cmp cmp command cmp The cmp command is a simpler version of diff, above. Whereas diff reports the differences between two files, cmp merely shows at what point they differ. Like diff, cmp returns an exit status of 0 if the compared files are identical, and 1 if they differ. This permits use in a test construct within a shell script. &filecomp; Use zcmp on gzipped files. comm comm command comm Versatile file comparison utility. The files must be sorted for this to be useful. comm -options first-file second-file comm file-1 file-2 outputs three columns: column 1 = lines unique to file-1 column 2 = lines unique to file-2 column 3 = lines common to both. The options allow suppressing output of one or more columns. -1 suppresses column 1 -2 suppresses column 2 -3 suppresses column 3 -12 suppresses both columns 1 and 2, etc. basename basename command basename Strips the path information from a file name, printing only the file name. The construction basename $0 lets the script know its name, that is, the name it was invoked by. This can be used for usage messages if, for example a script is called with missing arguments: echo "Usage: `basename $0` arg1 arg2 ... argn" dirname dirname command dirname Strips the basename from a filename, printing only the path information. basename and dirname can operate on any arbitrary string. The argument does not need to refer to an existing file, or even be a filename for that matter (see ). &ex35; split split command split Utility for splitting a file into smaller chunks. Usually used for splitting up large files in order to back them up on floppies or preparatory to e-mailing or uploading them. sum cksum md5sum sum command sum cksum command cksum md5sum command md5sum These are utilities for generating checksums. A checksum is a number mathematically calculated from the contents of a file, for the purpose of checking its integrity. A script might refer to a list of checksums for security purposes, such as ensuring that the contents of key system files have not been altered or corrupted. For security applications, use the 128-bit md5sum (message digest checksum) command. bash$ cksum /boot/vmlinuz 1670054224 804083 /boot/vmlinuz bash$ md5sum /boot/vmlinuz 0f43eccea8f09e0a0b2b5cf1dcf333ba /boot/vmlinuz Note that cksum also shows the size, in bytes, of the target file. &fileintegrity; See also for a creative use of the md5sum command. shred shred command secure delete Securely erase a file by overwriting it multiple times with random bit patterns before deleting it. This command has the same effect as , but does it in a more thorough and elegant manner. This is one of the GNU fileutils. Advanced forensic technology may still be able to recover the contents of a file, even after application of shred. uuencode uuencode command uuencode This utility encodes binary files into ASCII characters, making them suitable for transmission in the body of an e-mail message or in a newsgroup posting. uudecode uudecode command uudecode This reverses the encoding, decoding uuencoded files back into the original binaries. &ex52; The fold -s command may be useful (possibly in a pipe) to process long uudecoded text messages downloaded from Usenet newsgroups. mimencode mmencode mimencode command mime mmencode command encode The mimencode and mmencode commands process multimedia-encoded e-mail attachments. Although mail user agents (such as pine or kmail) normally handle this automatically, these particular utilities permit manipulating such attachments manually from the command line or in a batch by means of a shell script. crypt crypt command crypt At one time, this was the standard UNIX file encryption utility. This is a symmetric block cipher, used to encrypt files on a single system or local network, as opposed to the public key cipher class, of which pgp is a well-known example. Politically motivated government regulations prohibiting the export of encryption software resulted in the disappearance of crypt from much of the UNIX world, and it is still missing from most Linux distributions. Fortunately, programmers have come up with a number of decent alternatives to it, among them the author's very own cruft (see ). mktemp temporary command filename Create a temporary file with a unique filename. PREFIX=filename tempfile=`mktemp $PREFIX.XXXXXX` # ^^^^^^ Need at least 6 placeholders #+ in the filename template. echo "tempfile name = $tempfile" # tempfile name = filename.QA2ZpY # or something similar... make make command Makefile Utility for building and compiling binary packages. This can also be used for any set of operations that is triggered by incremental changes in source files. The make command checks a Makefile, a list of file dependencies and operations to be carried out. install install command install Special purpose file copying command, similar to cp, but capable of setting permissions and attributes of the copied files. This command seems tailormade for installing software packages, and as such it shows up frequently in Makefiles (in the make install : section). It could likewise find use in installation scripts. dos2unix dos2unix command file converter This utility, written by Benjamin Lin and collaborators, converts DOS-formatted text files (lines terminated by CR-LF) to UNIX format (lines terminated by LF only), and vice-versa. ptx ptx command index The ptx [targetfile] command outputs a permuted index (cross-reference list) of the targetfile. This may be further filtered and formatted in a pipe, if necessary. more less more command more less command less Pagers that display a text file or stream to stdout, one screenful at a time. These may be used to filter the output of a script. Certain of the following commands find use in chasing spammers, as well as in network data transfer and analysis. host host command host Searches for information about an Internet host by name or IP address, using DNS. bash$ host surfacemail.com surfacemail.com. has address 202.92.42.236 ipcalc ipcalc command ipcalc Carries out IP address lookups. With the -h option, ipcalc does a reverse DNS lookup, finding the name of the host (server) from the IP address. bash$ ipcalc -h 202.92.42.236 HOSTNAME=surfacemail.com nslookup nslookup command name server lookup Do an Internet name server lookup on a host by IP address. This is essentially equivalent to ipcalc -h or dig -x . The command may be run either interactively or noninteractively, i.e., from within a script. The nslookup command has allegedly been deprecated, but it still has its uses. bash$ nslookup -sil 66.97.104.180 nslookup kuhleersparnis.ch Server: 135.116.137.2 Address: 135.116.137.2#53 Non-authoritative answer: Name: kuhleersparnis.ch dig dig command domain information groper Similar to nslookup, do an Internet name server lookup on a host. May be run either interactively or noninteractively, i.e., from within a script. Compare the output of dig -x with ipcalc -h and nslookup. bash$ dig -x 81.9.6.2 ;; Got answer: ;; ->>HEADER<<- opcode: QUERY, status: NXDOMAIN, id: 11649 ;; flags: qr rd ra; QUERY: 1, ANSWER: 0, AUTHORITY: 1, ADDITIONAL: 0 ;; QUESTION SECTION: ;2.6.9.81.in-addr.arpa. IN PTR ;; AUTHORITY SECTION: 6.9.81.in-addr.arpa. 3600 IN SOA ns.eltel.net. noc.eltel.net. 2002031705 900 600 86400 3600 ;; Query time: 537 msec ;; SERVER: 135.116.137.2#53(135.116.137.2) ;; WHEN: Wed Jun 26 08:35:24 2002 ;; MSG SIZE rcvd: 91 traceroute traceroute command traceroute Trace the route taken by packets sent to a remote host. This command works within a LAN, WAN, or over the Internet. The remote host may be specified by an IP address. The output of this command may be filtered by grep or sed in a pipe. bash$ traceroute 81.9.6.2 traceroute to 81.9.6.2 (81.9.6.2), 30 hops max, 38 byte packets 1 tc43.xjbnnbrb.com (136.30.178.8) 191.303 ms 179.400 ms 179.767 ms 2 or0.xjbnnbrb.com (136.30.178.1) 179.536 ms 179.534 ms 169.685 ms 3 192.168.11.101 (192.168.11.101) 189.471 ms 189.556 ms * ... ping ping command ping Broadcast an ICMP ECHO_REQUEST packet to other machines, either on a local or remote network. This is a diagnostic tool for testing network connections, and it should be used with caution. A successful ping returns an exit status of 0. This can be tested for in a script. bash$ ping localhost PING localhost.localdomain (127.0.0.1) from 127.0.0.1 : 56(84) bytes of data. Warning: time of day goes back, taking countermeasures. 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=0 ttl=255 time=709 usec 64 bytes from localhost.localdomain (127.0.0.1): icmp_seq=1 ttl=255 time=286 usec --- localhost.localdomain ping statistics --- 2 packets transmitted, 2 packets received, 0% packet loss round-trip min/avg/max/mdev = 0.286/0.497/0.709/0.212 ms whois whois command domain name server Perform a DNS (Domain Name System) lookup. The -h option permits specifying which whois server to query. See . finger finger command finger Retrieve information about users on a network. Optionally, this command can display a user's ~/.plan, ~/.project, and ~/.forward files, if present. bash$ finger Login Name Tty Idle Login Time Office Office Phone bozo Bozo Bozeman tty1 8 Jun 25 16:59 bozo Bozo Bozeman ttyp0 Jun 25 16:59 bozo Bozo Bozeman ttyp1 Jun 25 17:07 bash$ finger bozo Login: bozo Name: Bozo Bozeman Directory: /home/bozo Shell: /bin/bash On since Fri Aug 31 20:13 (MST) on tty1 1 hour 38 minutes idle On since Fri Aug 31 20:13 (MST) on pts/0 12 seconds idle On since Fri Aug 31 20:13 (MST) on pts/1 On since Fri Aug 31 20:31 (MST) on pts/2 1 hour 16 minutes idle No mail. No Plan. Out of security considerations, many networks disable finger and its associated daemon. A daemon is a background process not attached to a terminal session. Daemons perform designated services either at specified times or explicitly triggered by certain events. The word daemon means ghost in Greek, and there is certainly something mysterious, almost supernatural, about the way UNIX daemons silently wander about behind the scenes, carrying out their appointed tasks. vrfy vrfy command vrfy Verify an Internet e-mail address. sx rx sx command sx rx command rx The sx and rx command set serves to transfer files to and from a remote host using the xmodem protocol. These are generally part of a communications package, such as minicom. sz rz sz command sz rz command rz The sz and rz command set serves to transfer files to and from a remote host using the zmodem protocol. Zmodem has certain advantages over xmodem, such as faster transmission rate and resumption of interrupted file transfers. Like sx and rx, these are generally part of a communications package. ftp ftp command file transfer Utility and protocol for uploading / downloading files to or from a remote host. An ftp session can be automated in a script (see , , and ). uucp uucp command uucp UNIX to UNIX copy. This is a communications package for transferring files between UNIX servers. A shell script is an effective way to handle a uucp command sequence. Since the advent of the Internet and e-mail, uucp seems to have faded into obscurity, but it still exists and remains perfectly workable in situations where an Internet connection is not available or appropriate. cu cu command call up Call Up a remote system and connect as a simple terminal. This command is part of the uucp package. It is a sort of dumbed-down version of telnet. telnet telnet command telnet Utility and protocol for connecting to a remote host. The telnet protocol contains security holes and should therefore probably be avoided. wget wget command download The wget utility non-interactively retrieves or downloads files from a Web or ftp site. It works well in a script. wget -p http://www.xyz23.com/file01.html wget -r ftp://ftp.xyz24.net/~bozo/project_files/ -o $SAVEFILE lynx lynx command browser The lynx Web and file browser can be used inside a script (with the -dump option) to retrieve a file from a Web or ftp site non-interactively. lynx -dump http://www.xyz23.com/file01.html >$SAVEFILE rlogin rlogin command remote login Remote login, initates a session on a remote host. This command has security issues, so use ssh instead. rsh rsh command remote shell Remote shell, executes command(s) on a remote host. This has security issues, so use ssh instead. rcp rcp command remote copy Remote copy, copies files between two different networked machines. Using rcp and similar utilities with security implications in a shell script may not be advisable. Consider, instead, using ssh or an expect script. ssh ssh command secure shell Secure shell, logs onto a remote host and executes commands there. This secure replacement for telnet, rlogin, rcp, and rsh uses identity authentication and encryption. See its manpage for details. write write command write This is a utility for terminal-to-terminal communication. It allows sending lines from your terminal (console or xterm) to that of another user. The mesg command may, of course, be used to disable write access to a terminal Since write is interactive, it would not normally find use in a script. mail mail command mail Send or read e-mail messages. This stripped-down command-line mail client works fine as a command embedded in a script. &selfmailer; mailto mailto command MIME mail Similar to the mail command, mailto sends e-mail messages from the command line or in a script. However, mailto also permits sending MIME (multimedia) messages. vacation vacation command mail This utility automatically replies to e-mails that the intended recipient is on vacation and temporarily unavailable. This runs on a network, in conjunction with sendmail, and is not applicable to a dial-up POPmail account. tput tput command terminal Initialize terminal and/or fetch information about it from terminfo data. Various options permit certain terminal operations. tput clear is the equivalent of clear, below. tput reset is the equivalent of reset, below. tput sgr0 also resets the terminal, but without clearing the screen. bash$ tput longname xterm terminal emulator (XFree86 4.0 Window System) Issuing a tput cup X Y moves the cursor to the (X,Y) coordinates in the current terminal. A clear to erase the terminal screen would normally precede this. Note that stty offers a more powerful command set for controlling a terminal. infocmp infocmp command terminal This command prints out extensive information about the current terminal. It references the terminfo database. bash$ infocmp # Reconstructed via infocmp from file: /usr/share/terminfo/r/rxvt rxvt|rxvt terminal emulator (X Window System), am, bce, eo, km, mir, msgr, xenl, xon, colors#8, cols#80, it#8, lines#24, pairs#64, acsc=``aaffggjjkkllmmnnooppqqrrssttuuvvwwxxyyzz{{||}}~~, bel=^G, blink=\E[5m, bold=\E[1m, civis=\E[?25l, clear=\E[H\E[2J, cnorm=\E[?25h, cr=^M, ... reset reset command reset Reset terminal parameters and clear text screen. As with clear, the cursor and prompt reappear in the upper lefthand corner of the terminal. clear clear command clear The clear command simply clears the text screen at the console or in an xterm. The prompt and cursor reappear at the upper lefthand corner of the screen or xterm window. This command may be used either at the command line or in a script. See . script script command script This utility records (saves to a file) all the user keystrokes at the command line in a console or an xterm window. This, in effect, creates a record of a session. factor factor command factor Decompose an integer into prime factors. bash$ factor 27417 27417: 3 13 19 37 bc bc command bc Bash can't handle floating point calculations, and it lacks operators for certain important mathematical functions. Fortunately, bc comes to the rescue. Not just a versatile, arbitrary precision calculation utility, bc offers many of the facilities of a programming language. bc has a syntax vaguely resembling C. Since it is a fairly well-behaved UNIX utility, and may therefore be used in a pipe, bc comes in handy in scripts. Here is a simple template for using bc to calculate a script variable. This uses command substitution. variable=$(echo "OPTIONS; OPERATIONS" | bc) &monthlypmt; &base; An alternate method of invoking bc involves using a here document embedded within a command substitution block. This is especially appropriate when a script needs to pass a list of options and commands to bc. variable=`bc << LIMIT_STRING options statements operations LIMIT_STRING ` ...or... variable=$(bc << LIMIT_STRING options statements operations LIMIT_STRING ) &altbc; &cannon; dc dc command dc The dc (desk calculator) utility is stack-oriented and uses RPN (Reverse Polish Notation). Like bc, it has much of the power of a programming language. Most persons avoid dc, since it requires non-intuitive RPN input. Yet it has its uses. &hexconvert; Studying the info page for dc gives some insight into its intricacies. However, there seems to be a small, select group of dc wizards who delight in showing off their mastery of this powerful, but arcane utility. &factr; awk awk command math Yet another way of doing floating point math in a script is using awk's built-in math functions in a shell wrapper. &hypot; jot seq jot command jot seq command seq loop arguments These utilities emit a sequence of integers, with a user-selected increment. The normal separator character between each integer is a newline, but this can be changed with the -s option. bash$ seq 5 1 2 3 4 5 bash$ seq -s : 5 1:2:3:4:5 Both jot and seq come in handy in a for loop. &ex53; getopt getopt command option The getopt command parses command-line options preceded by a dash. This external command corresponds to the getopts Bash builtin, but it is not nearly as versatile. &ex33a; run-parts run-parts command run-parts The run-parts command This is actually a script adapted from the Debian Linux distribution. executes all the scripts in a target directory, sequentially in ASCII-sorted filename order. Of course, the scripts need to have execute permission. The crond daemon invokes run-parts to run the scripts in the /etc/cron.* directories. yes yes command yes In its default behavior the yes command feeds a continuous string of the character y followed by a line feed to stdout. A controlc terminates the run. A different output string may be specified, as in yes different string, which would continually output different string to stdout. One might well ask the purpose of this. From the command line or in a script, the output of yes can be redirected or piped into a program expecting user input. In effect, this becomes a sort of poor man's version of expect. yes | fsck /dev/hda1 runs fsck non-interactively (careful!). yes | rm -r dirname has same effect as rm -rf dirname (careful!). Caution advised when piping yes to a potentially dangerous system command, such as fsck or fdisk. It may have unintended side-effects. banner banner command banner Prints arguments as a large vertical banner to stdout, using an ASCII character (default '#'). This may be redirected to a printer for hardcopy. printenv printenv command environment Show all the environmental variables set for a particular user. bash$ printenv | grep HOME HOME=/home/bozo lp lp command lpr The lp and lpr commands send file(s) to the print queue, to be printed as hard copy. The print queue is the group of jobs waiting in line to be printed. These commands trace the origin of their names to the line printers of another era. bash$ lp file1.txt or bash lp <file1.txt It is often useful to pipe the formatted output from pr to lp. bash$ pr -options file1.txt | lp Formatting packages, such as groff and Ghostscript may send their output directly to lp. bash$ groff -Tascii file.tr | lp bash$ gs -options | lp file.ps Related commands are lpq, for viewing the print queue, and lprm, for removing jobs from the print queue. tee tee command tee [UNIX borrows an idea here from the plumbing trade.] This is a redirection operator, but with a difference. Like the plumber's tee, it permits siponing off to a file the output of a command or commands within a pipe, but without affecting the result. This is useful for printing an ongoing process to a file or paper, perhaps to keep track of it for debugging purposes. tee |------> to file | ===============|=============== command--->----|-operator-->---> result of command(s) =============================== cat listfile* | sort | tee check.file | uniq > result.file (The file check.file contains the concatenated sorted listfiles, before the duplicate lines are removed by uniq.) mkfifo mkfifo command mkfifo This obscure command creates a named pipe, a temporary first-in-first-out buffer for transferring data between processes. For an excellent overview of this topic, see Andy Vaught's article, Introduction to Named Pipes, in the September, 1997 issue of Linux Journal. Typically, one process writes to the FIFO, and the other reads from it. See . pathchk pathchk command pathchk This command checks the validity of a filename. If the filename exceeds the maximum allowable length (255 characters) or one or more of the directories in its path is not searchable, then an error message results. Unfortunately, pathchk does not return a recognizable error code, and it is therefore pretty much useless in a script. Consider instead the file test operators. dd dd command dd This is the somewhat obscure and much feared data duplicator command. Originally a utility for exchanging data on magnetic tapes between UNIX minicomputers and IBM mainframes, this command still has its uses. The dd command simply copies a file (or stdin/stdout), but with conversions. Possible conversions are ASCII/EBCDIC, EBCDIC (pronounced ebb-sid-ic) is an acronym for Extended Binary Coded Decimal Interchange Code. This is an IBM data format no longer in much use. A bizarre application of the conv=ebcdic option of dd is as a quick 'n easy, but not very secure text file encoder. cat $file | dd conv=swab,ebcdic > $file_encrypted # Encode (looks like gibberish). # Might as well switch bytes (swab), too, for a little extra obscurity. cat $file_encrypted | dd conv=swab,ascii > $file_plaintext # Decode. upper/lower case, swapping of byte pairs between input and output, and skipping and/or truncating the head or tail of the input file. A dd --help lists the conversion and other options that this powerful utility takes. # Exercising 'dd'. n=3 p=5 input_file=project.txt output_file=log.txt dd if=$input_file of=$output_file bs=1 skip=$((n-1)) count=$((p-n+1)) 2> /dev/null # Extracts characters n to p from file $input_file. echo -n "hello world" | dd cbs=1 conv=unblock 2> /dev/null # Echoes "hello world" vertically. # Thanks, S.C. To demonstrate just how versatile dd is, let's use it to capture keystrokes. &ddkeypress; The dd command can do random access on a data stream. echo -n . | dd bs=1 seek=4 of=file conv=notrunc # The "conv=notrunc" option means that the output file will not be truncated. # Thanks, S.C. The dd command can copy raw data and disk images to and from devices, such as floppies and tape drives (). A common use is creating boot floppies. dd if=kernel-image of=/dev/fd0H1440 Similarly, dd can copy the entire contents of a floppy, even one formatted with a foreign OS, to the hard drive as an image file. dd if=/dev/fd0 of=/home/bozo/projects/floppy.img Other applications of dd include initializing temporary swap files () and ramdisks (). It can even do a low-level copy of an entire hard drive partition, although this is not necessarily recommended. People (with presumably nothing better to do with their time) are constantly thinking of interesting applications of dd. &blotout; od od command od The od, or octal dump filter converts input (or files) to octal (base-8) or other bases. This is useful for viewing or processing binary data files or otherwise unreadable system device files, such as /dev/urandom, and as a filter for binary data. See and . hexdump hexdump command hexadecimal Performs a hexadecimal, octal, decimal, or ASCII dump of a binary file. This command is the rough equivalent of od, above, but not nearly as useful. objdump objdump command object binary dump Displays an object file or binary executable in either hexadecimal form or as a disassembled listing (with the -d option). bash$ objdump -d /bin/ls /bin/ls: file format elf32-i386 Disassembly of section .init: 080490bc <.init>: 80490bc: 55 push %ebp 80490bd: 89 e5 mov %esp,%ebp . . . mcookie magic command cookie This command generates a magic cookie, a 128-bit (32-character) pseudorandom hexadecimal number, normally used as an authorization signature by the X server. This also available for use in a script as a quick 'n dirty random number. random000=`mcookie | sed -e '2p'` # Uses 'sed' to strip off extraneous characters. Of course, a script could use md5 for the same purpose. # Generate md5 checksum on the script itself. random001=`md5sum $0 | awk '{print $1}'` # Uses 'awk' to strip off the filename. The mcookie command gives yet another way to generate a unique filename. &tempfilename; units units command conversion This utility converts between different units of measure. While normally invoked in interactive mode, units may find use in a script. &unitconversion; m4 m4 command macro A hidden treasure, m4 is a powerful macro processing filter, A macro is a symbolic constant that expands into a command string or a set of operations on parameters. virtually a complete language. Although originally written as a pre-processor for RatFor, m4 turned out to be useful as a stand-alone utility. In fact, m4 combines some of the functionality of eval, tr, and awk, in addition to its extensive macro expansion facilities. The April, 2002 issue of Linux Journal has a very nice article on m4 and its uses. &m4; doexec doexec command executable arg list The doexec command enables passing an arbitrary list of arguments to a binary executable. In particular, passing argv[0] (which corresponds to $0 in a script) lets the executable be invoked by various names, and it can then carry out different sets of actions, according to the name by which it was called. What this amounts to is roundabout way of passing options to an executable. For example, the /usr/local/bin directory might contain a binary called aaa. Invoking doexec /usr/local/bin/aaa list would list all those files in the current working directory beginning with an a, while invoking (the same executable with) doexec /usr/local/bin/aaa delete would delete those files. The various behaviors of the executable must be defined within the code of the executable itself, analogous to something like the following in a shell script: case `basename $0` in "name1" ) do_something;; "name2" ) do_something_else;; "name3" ) do_yet_another_thing;; * ) bail_out;; esac dialog dialog command dialog The dialog family of tools provide a method of calling interactive dialog boxes from a script. The more elaborate variations of dialog -- gdialog, Xdialog, and kdialog -- actually invoke X-Windows widgets. See . The startup and shutdown scripts in /etc/rc.d illustrate the uses (and usefulness) of many of these comands. These are usually invoked by root and used for system maintenance or emergency filesystem repairs. Use with caution, as some of these commands may damage your system if misused. users users command users Show all logged on users. This is the approximate equivalent of who -q. groups groups command groups Lists the current user and the groups she belongs to. This corresponds to the $GROUPS internal variable, but gives the group names, rather than the numbers. bash$ groups bozita cdrom cdwriter audio xgrp bash$ echo $GROUPS 501 chown chgrp chown command chown chgrp command chgrp The chown command changes the ownership of a file or files. This command is a useful method that root can use to shift file ownership from one user to another. An ordinary user may not change the ownership of files, not even her own files. This is the case on a Linux machine or a UNIX system with disk quotas. root# chown bozo *.txt The chgrp command changes the group ownership of a file or files. You must be owner of the file(s) as well as a member of the destination group (or root) to use this operation. chgrp --recursive dunderheads *.data # The "dunderheads" group will now own all the "*.data" files #+ all the way down the $PWD directory tree (that's what "recursive" means). useradd userdel useradd command useradd userdel command userdel The useradd administrative command adds a user account to the system and creates a home directory for that particular user, if so specified. The corresponding userdel command removes a user account from the system The userdel command will fail if the particular user being deleted is still logged on. and deletes associated files. The adduser command is a synonym for useradd and is usually a symbolic link to it. id id command id The id command lists the real and effective user IDs and the group IDs of the current user. This is the counterpart to the $UID, $EUID, and $GROUPS internal Bash variables. bash$ id uid=501(bozo) gid=501(bozo) groups=501(bozo),22(cdrom),80(cdwriter),81(audio) bash$ echo $UID 501 Also see . who who command whoami Show all users logged on to the system. bash$ who bozo tty1 Apr 27 17:45 bozo pts/0 Apr 27 17:46 bozo pts/1 Apr 27 17:47 bozo pts/2 Apr 27 17:49 The -m gives detailed information about only the current user. Passing any two arguments to who is the equivalent of who -m, as in who am i or who The Man. bash$ who -m localhost.localdomain!bozo pts/2 Apr 27 17:49 whoami is similar to who -m, but only lists the user name. bash$ whoami bozo w w command w Show all logged on users and the processes belonging to them. This is an extended version of who. The output of w may be piped to grep to find a specific user and/or process. bash$ w | grep startx bozo tty1 - 4:22pm 6:41 4.47s 0.45s startx logname logname command logname Show current user's login name (as found in /var/run/utmp). This is a near-equivalent to whoami, above. bash$ logname bozo bash$ whoami bozo However... bash$ su Password: ...... bash# whoami root bash# logname bozo su su command su Runs a program or script as a substitute user. su rjones starts a shell as user rjones. A naked su defaults to root. See . sudo sudo command sudo Runs a command as root (or another user). This may be used in a script, thus permitting a regular user to run the script. #!/bin/bash # Some commands. sudo cp /root/secretfile /home/bozo/secret # Some more commands. The file /etc/sudoers holds the names of users permitted to invoke sudo. passwd passwd command password Sets or changes a user's password. The passwd can be used in a script, but should not be. #!/bin/bash # set-new-password.sh: Not a good idea. # This script must be run as root, #+ or better yet, not run at all. ROOT_UID=0 # Root has $UID 0. E_WRONG_USER=65 # Not root? if [ "$UID" -ne "$ROOT_UID" ] then echo; echo "Only root can run this script."; echo exit $E_WRONG_USER else echo; echo "You should know better than to run this script, root." fi username=bozo NEWPASSWORD=security_violation echo "$NEWPASSWORD" | passwd --stdin "$username" # The '--stdin' option to 'passwd' permits #+ getting new password from stdin (or a pipe). echo; echo "User $username's password changed!" # Using the 'passwd' command in a script is dangerous. exit 0 ac ac command accounting Show users' logged in time, as read from /var/log/wtmp. This is one of the GNU accounting utilities. bash$ ac total 68.08 last last command logged in List last logged in users, as read from /var/log/wtmp. This command can also show remote logins. newgrp newgrp command group Change user's group ID without logging out. This permits access to the new group's files. Since users may be members of multiple groups simultaneously, this command finds little use. tty tty command tty Echoes the name of the current user's terminal. Note that each separate xterm window counts as a different terminal. bash$ tty /dev/pts/1 stty stty command stty Shows and/or changes terminal settings. This complex command, used in a script, can control terminal behavior and the way output displays. See the info page, and study it carefully. &erase; &secretpw; A creative use of stty is detecting a user keypress (without hitting ENTER). &keypress; Also see . Normally, a terminal works in the canonical mode. When a user hits a key, the resulting character does not immediately go to the program actually running in this terminal. A buffer local to the terminal stores keystrokes. When the user hits the ENTER key, this sends all the stored keystrokes to the program running. There is even a basic line editor inside the terminal. bash$ stty -a speed 9600 baud; rows 36; columns 96; line = 0; intr = ^C; quit = ^\; erase = ^H; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; flush = ^O; ... isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt Using canonical mode, it is possible to redefine the special keys for the local terminal line editor. bash$ cat > filexxx wha<ctl-W>I<ctl-H>foo bar<ctl-U>hello world<ENTER> <ctl-D> bash$ cat filexxx hello world bash$ bash$ wc -c < file 13 The process controlling the terminal receives only 13 characters (12 alphabetic ones, plus a newline), although the user hit 26 keys. In non-canonical (raw) mode, every key hit (including special editing keys such as ctl-H) sends a character immediately to the controlling process. The Bash prompt disables both icanon and echo, since it replaces the basic terminal line editor with its own more elaborate one. For example, when you hit ctl-A at the Bash prompt, there's no ^A echoed by the terminal, but Bash gets a \1 character, interprets it, and moves the cursor to the begining of the line. Stephane Chazelas tset tset command tset Show or initialize terminal settings. This is a less capable version of stty. bash$ tset -r Terminal type is xterm-xfree86. Kill is control-U (^U). Interrupt is control-C (^C). setserial setserial command serial Set or display serial port parameters. This command must be run by root user and is usually found in a system setup script. # From /etc/pcmcia/serial script: IRQ=`setserial /dev/$DEVICE | sed -e 's/.*IRQ: //'` setserial /dev/$DEVICE irq 0 ; setserial /dev/$DEVICE irq $IRQ getty agetty getty command getty agetty command agetty The initialization process for a terminal uses getty or agetty to set it up for login by a user. These commands are not used within user shell scripts. Their scripting counterpart is stty. mesg mesg command mesg Enables or disables write access to the current user's terminal. Disabling access would prevent another user on the network to write to the terminal. It can be very annoying to have a message about ordering pizza suddenly appear in the middle of the text file you are editing. On a multi-user network, you might therefore wish to disable write access to your terminal when you need to avoid interruptions. wall wall command wall This is an acronym for write all, i.e., sending a message to all users at every terminal logged into the network. It is primarily a system administrator's tool, useful, for example, when warning everyone that the system will shortly go down due to a problem (see ). bash$ wall System going down for maintenance in 5 minutes! Broadcast message from bozo (pts/1) Sun Jul 8 13:53:27 2001... System going down for maintenance in 5 minutes! If write access to a particular terminal has been disabled with mesg, then wall cannot send a message to it. dmesg dmesg command dmesg Lists all system bootup messages to stdout. Handy for debugging and ascertaining which device drivers were installed and which system interrupts in use. The output of dmesg may, of course, be parsed with grep, sed, or awk from within a script. bash$ dmesg | grep hda Kernel command line: ro root=/dev/hda2 hda: IBM-DLGA-23080, ATA DISK drive hda: 6015744 sectors (3080 MB) w/96KiB Cache, CHS=746/128/63 hda: hda1 hda2 hda3 < hda5 hda6 hda7 > hda4 uname uname command uname Output system specifications (OS, kernel version, etc.) to stdout. Invoked with the -a option, gives verbose system info (see ). The -s option shows only the OS type. bash$ uname -a Linux localhost.localdomain 2.2.15-2.5.0 #1 Sat Feb 5 00:13:43 EST 2000 i686 unknown bash$ uname -s Linux arch arch command arch Show system architecture. Equivalent to uname -m. See . bash$ arch i686 bash$ uname -m i686 lastcomm lastcomm command last Gives information about previous commands, as stored in the /var/account/pacct file. Command name and user name can be specified by options. This is one of the GNU accounting utilities. lastlog lastlog command last List the last login time of all system users. This references the /var/log/lastlog file. bash$ lastlog root tty1 Fri Dec 7 18:43:21 -0700 2001 bin **Never logged in** daemon **Never logged in** ... bozo tty1 Sat Dec 8 21:14:29 -0700 2001 bash$ lastlog | grep root root tty1 Fri Dec 7 18:43:21 -0700 2001 This command will fail if the user invoking it does not have read permission for the /var/log/lastlog file. lsof lsof command lsof List open files. This command outputs a detailed table of all currently open files and gives information about their owner, size, the processes associated with them, and more. Of course, lsof may be piped to grep and/or awk to parse and analyze its results. bash$ lsof COMMAND PID USER FD TYPE DEVICE SIZE NODE NAME init 1 root mem REG 3,5 30748 30303 /sbin/init init 1 root mem REG 3,5 73120 8069 /lib/ld-2.1.3.so init 1 root mem REG 3,5 931668 8075 /lib/libc-2.1.3.so cardmgr 213 root mem REG 3,5 36956 30357 /sbin/cardmgr ... strace strace command strace Diagnostic and debugging tool for tracing system calls and signals. The simplest way of invoking it is strace COMMAND. bash$ strace df execve("/bin/df", ["df"], [/* 45 vars */]) = 0 uname({sys="Linux", node="bozo.localdomain", ...}) = 0 brk(0) = 0x804f5e4 ... This is the Linux equivalent of truss. nmap nmap command port scan Network port scanner. This command scans a server to locate open ports and the services associated with those ports. It is an important security tool for locking down a network against hacking attempts. #!/bin/bash SERVER=$HOST # localhost.localdomain (127.0.0.1). PORT_NUMBER=25 # SMTP port. nmap $SERVER | grep -w "$PORT_NUMBER" # Is that particular port open? # grep -w matches whole words only, #+ so this wouldn't match port 1025, for example. exit 0 # 25/tcp open smtp free free command free Shows memory and cache usage in tabular form. The output of this command lends itself to parsing, using grep, awk or Perl. The procinfo command shows all the information that free does, and much more. bash$ free total used free shared buffers cached Mem: 30504 28624 1880 15820 1608 16376 -/+ buffers/cache: 10640 19864 Swap: 68540 3128 65412 To show unused RAM memory: bash$ free | grep Mem | awk '{ print $4 }' 1880 procinfo procinfo command procinfo Extract and list information and statistics from the /proc pseudo-filesystem. This gives a very extensive and detailed listing. bash$ procinfo | grep Bootup Bootup: Wed Mar 21 15:15:50 2001 Load average: 0.04 0.21 0.34 3/47 6829 lsdev lsdev command device List devices, that is, show installed hardware. bash$ lsdev Device DMA IRQ I/O Ports ------------------------------------------------ cascade 4 2 dma 0080-008f dma1 0000-001f dma2 00c0-00df fpu 00f0-00ff ide0 14 01f0-01f7 03f6-03f6 ... du du command du Show (disk) file usage, recursively. Defaults to current working directory, unless otherwise specified. bash$ du -ach 1.0k ./wi.sh 1.0k ./tst.sh 1.0k ./random.file 6.0k . 6.0k total df df command df Shows filesystem usage in tabular form. bash$ df Filesystem 1k-blocks Used Available Use% Mounted on /dev/hda5 273262 92607 166547 36% / /dev/hda8 222525 123951 87085 59% /home /dev/hda7 1408796 1075744 261488 80% /usr stat stat command stat Gives detailed and verbose statistics on a given file (even a directory or device file) or set of files. bash$ stat test.cru File: "test.cru" Size: 49970 Allocated Blocks: 100 Filetype: Regular File Mode: (0664/-rw-rw-r--) Uid: ( 501/ bozo) Gid: ( 501/ bozo) Device: 3,8 Inode: 18185 Links: 1 Access: Sat Jun 2 16:40:24 2001 Modify: Sat Jun 2 16:40:24 2001 Change: Sat Jun 2 16:40:24 2001 If the target file does not exist, stat returns an error message. bash$ stat nonexistent-file nonexistent-file: No such file or directory vmstat vmstat command virtual memory Display virtual memory statistics. bash$ vmstat procs memory swap io system cpu r b w swpd free buff cache si so bi bo in cs us sy id 0 0 0 0 11040 2636 38952 0 0 33 7 271 88 8 3 89 netstat netstat command netstat Show current network statistics and information, such as routing tables and active connections. This utility accesses information in /proc/net (). See . netstat -r is equivalent to route. uptime uptime command uptime Shows how long the system has been running, along with associated statistics. bash$ uptime 10:28pm up 1:57, 3 users, load average: 0.17, 0.34, 0.27 hostname hostname command hostname Lists the system's host name. This command sets the host name in an /etc/rc.d setup script (/etc/rc.d/rc.sysinit or similar). It is equivalent to uname -n, and a counterpart to the $HOSTNAME internal variable. bash$ hostname localhost.localdomain bash$ echo $HOSTNAME localhost.localdomain hostid hostid command host id Echo a 32-bit hexadecimal numerical identifier for the host machine. bash$ hostid 7f0100 This command allegedly fetches a unique serial number for a particular system. Certain product registration procedures use this number to brand a particular user license. Unfortunately, hostid only returns the machine network address in hexadecimal, with pairs of bytes transposed. The network address of a typical non-networked Linux machine, is found in /etc/hosts. bash$ cat /etc/hosts 127.0.0.1 localhost.localdomain localhost As it happens, transposing the bytes of 127.0.0.1, we get 0.127.1.0, which translates in hex to 007f0100, the exact equivalent of what hostid returns, above. There exist only a few million other Linux machines with this identical hostid. sar sar command system activity report Invoking sar (System Activity Reporter) gives a very detailed rundown on system statistics. The Santa Cruz Operation (SCO) released sar as Open Source in June, 1999. This command is not part of the base Linux distribution, but may be obtained as part of the sysstat utilities package, written by Sebastien Godard. bash$ sar Linux 2.4.7-10 (localhost.localdomain) 12/31/2001 10:30:01 AM CPU %user %nice %system %idle 10:40:00 AM all 1.39 0.00 0.77 97.84 10:50:00 AM all 76.83 0.00 1.45 21.72 11:00:00 AM all 1.32 0.00 0.69 97.99 11:10:00 AM all 1.17 0.00 0.30 98.53 11:20:00 AM all 0.51 0.00 0.30 99.19 06:30:00 PM all 100.00 0.00 100.01 0.00 Average: all 1.39 0.00 0.66 97.95 readelf elf command<