#!/bin/bash
# rnd.sh: Outputs a 10-digit random number

# Script by Stephane Chazelas.

head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'


# =================================================================== #

# Analysis
# --------

# head:
# -c4 option takes first 4 bytes.

# od:
# -N4 option limits output to 4 bytes.
# -tu4 option selects unsigned decimal format for output.

# sed: 
# -n option, in combination with "p" flag to the "s" command,
# outputs only matched lines.



# The author of this script explains the action of 'sed', as follows.

# head -c4 /dev/urandom | od -N4 -tu4 | sed -ne '1s/.* //p'
# ----------------------------------> |

# Assume output up to "sed" --------> |
# is 0000000 1198195154\n

# sed begins reading characters: 0000000 1198195154\n.
# Here it finds a newline character,
# so it is ready to process the first line (0000000 1198195154).
# It looks at its <range><action>s. The first and only one is

#   range     action
#   1         s/.* //p

# The line number is in the range, so it executes the action:
# tries to substitute the longest string ending with a space in the line
# ("0000000 ") with nothing (//), and if it succeeds, prints the result
# ("p" is a flag to the "s" command here, this is different from the "p" command).

# sed is now ready to continue reading its input. (Note that before
# continuing, if -n option had not been passed, sed would have printed
# the line once again).

# Now, sed reads the remainder of the characters, and finds the end of the file.
# It is now ready to process its 2nd line (which is also numbered '$' as
# it's the last one).
# It sees it is not matched by any <range>, so its job is done.

# In few word this sed commmand means:
# "On the first line only, remove any character up to the right-most space,
# then print it."

# A better way to do this would have been:
#           sed -e 's/.* //;q'

# Here, two <range><action>s (could have been written
#           sed -e 's/.* //' -e q):

#   range                    action
#   nothing (matches line)   s/.* //
#   nothing (matches line)   q (quit)

# Here, sed only reads its first line of input.
# It performs both actions, and prints the line (substituted) before quitting
# (because of the "q" action) since the "-n" option is not passed.

# =================================================================== #

# A simpler altenative to the above 1-line script would be:
#           head -c4 /dev/urandom| od -An -tu4

exit 0