bash Cookbook

Parsing Arguments for Your Shell Script

Problem

You want to have some options on your shell script, some flags that you can use to alter its behavior. You could do the parsing directly, using ${#} to tell you how many arguments have been supplied, and testing ${1:0:1} to test the first character of the first argument to see if it is a minus sign. You would need some if/then or case logic to identify which option it is and whether it takes an argument. What if the user doesn’t supply a required argument? What if the user calls your script with two options combined (e.g., -ab)? Will you also parse for that? The need to parse options for a shell script is a common situation. Lots of scripts have options. Isn’t there a more standard way to do this?

Solution

Use bash’s built-in getopts command to help parse options.

Here is an example, based largely on the example in the manpage for getopts:

#!/usr/bin/env bash
# cookbook filename: getopts_example
#
# using getopts
#
aflag=
bflag=
while getopts 'ab:' OPTION
do
    case $OPTION in
        a) aflag=1
           ;;
        b) bflag=1
           bval="$OPTARG"
           ;;
        ?) printf "Usage: %s: [-a] [-b value] args\n" $(basename $0) >&2
           exit 2
           ;;
    esac
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

There are two kinds of options supported here. The first and simpler kind is an option that stands alone. It typically represents a flag to modify ...