bash Cookbook

Parsing Arguments with Your Own Error Messages

Problem

You are using getopts to parse your options for your shell script. But you don’t like the error messages that it writes when it encounters bad input. Can you still use getopts but write your own error handling?

Solution

If you just want getopts to be quiet and not report any errors at all, just assign $OPTERR=0 before you begin parsing. But if you want getopts to give you more information without the error messages, then just begin the option list with a colon. (The v--- in the comments below is meant to be an arrow pointing to a particular place in the line below it, to show that special colon.)

#!/usr/bin/env bash
# cookbook filename: getopts_custom
#
# using getopts - with custom error messages
#
aflag=
bflag=
# since we don't want getopts to generate error
# messages, but want this script to issue its
# own messages, we will put, in the option list, a
# leading ':' v---here to silence getopts.
while getopts :ab: FOUND
do
    case $FOUND in
        a)  aflag=1
            ;;
        b)  bflag=1
            bval="$OPTARG"
            ;;
        \:) printf "argument missing from -%s option\n" $OPTARG
            printf "Usage: %s: [-a] [-b value] args\n" $(basename $0)
            exit 2
            ;;
        \?) printf "unknown option: -%s\n" $OPTARG
            printf "Usage: %s: [-a] [-b value] args\n" $(basename $0)
            exit 2
            ;;
        esac >&2
    done
shift $(($OPTIND - 1))

if [ "$aflag" ]
then
    printf "Option -a specified\n"
fi
if [ "$bflag" ]
then
    printf 'Option -b "%s" specified\n' "$bval"
fi
printf "Remaining arguments are: %s\n" "$*"

Discussion

The script ...