#!/bin/bash

# From username, gets "real name" from /etc/passwd.

ARGCOUNT=1  # Expect one arg.
E_WRONGARGS=65

file=/etc/passwd
pattern=$1

if [ $# -ne "$ARGCOUNT" ]
then
  echo "Usage: `basename $0` USERNAME"
  exit $E_WRONGARGS
fi  

file_excerpt ()  # Scan file for pattern, the print relevant portion of line.
{
while read line  # while does not necessarily need "[ condition]"
do
  echo "$line" | grep $1 | awk -F":" '{ print $5 }'  # Have awk use ":" delimiter.
done
} <$file  # Redirect into function's stdin.

file_excerpt $pattern

# Yes, this entire script could be reduced to
#       grep PATTERN /etc/passwd | awk -F":" '{ print $5 }'
# or
#       awk -F: '/PATTERN/ {print $5}'
# or
#       awk -F: '($1 == "username") { print $5 }' # real name from username
# However, it might not be as instructive.

exit 0