Shell Programming and Scripting

You should avoid cat of a single file. Grep can read the file just fine, you don't need cat to feed it.

Code:

#!/bin/sh

case $# in 0) echo foo: need an argument >&2; exit 1;; esac

for i in "$@"; do
  grep "$i" /etc/passwd | cut -d: -f6
done

Pay attention to the quoting; you want to avoid throwing an error if somebody calls your script with a string with spaces in it, or (worse yet) an actual shell command which gets executed and has dire side effects (think `rm -rf $HOME` in backticks).

You should probably actually grep "^$i:" to avoid accidentally printing the wrong information because of a coincidental substring match. (True story: grep era /etc/passwd would find "System Operator" and print root's home directory on BSD.)