Handling Command Line Options (getopts)

Most Unix command line utilities we have looked at provide multiple command line options. Some of them, such as tar allow you to flexibly combine command line options to modify command behavior in multiple ways at once.

The shell (bash) provides the getopts construct that allows you to add flexible command line option handling to shell scripts.

Sample script #1 that uses getopts:

#!/bin/bash

# getopts_test1
# This script has 3 command line options, one of which takes an argument.

function usage {
      echo "script usage: $(basename $0) [-l] [-m] [-o somevalue]" >&2
      exit 1
}

while getopts 'lmo:' OPTION; do
  case "$OPTION" in
    l)
      echo "option l selected"
      ;;

    m)
      echo "option m selected"
      ;;

    o)
      oarg="$OPTARG"
      echo "The o option's argument was $oarg"
      ;;
    ?)
      usage
      ;;
  esac
done

shift "$(($OPTIND - 1))"

• The getopts command is used as the condition of a while loop.
  • The string after getopts, 'lmo:', contains the single letter options that the script accepts.
    • Here, they are l, m and o.
    • 'lmo:' is called an optstring.
  • Within the loop, in a case structure, define what to do with the options that the script will accept.
    • Notice the colon after the last option in 'lmo:'. It is the way we tell getopts that the option o requires an argument.
    • When the shell script is running, each parsed option will be stored inside the $OPTION variable, while an argument, when present, will become the value of the $OPTARG variable.
    • Each option is managed inside the case statement, with a final ? case that will be matched whenever an invalid option is found. In that case we will just echo the user the correct script usage and exit with an error code.
• When the shell script is running, getopts looks for a dash (-) and a letter or a series of letters that follow the -. It parses the letters as command line options.
  • Valid ways to call the above script:

./getopts_test1 -l -m
./getopts_test1 -l -m -o 3
./getopts_test1 -lm
./getopts_test1 -lm -o 3
./getopts_test1 -lmo 3
./getopts_test1 -lmo3
./getopts_test1 -o 3 -l -m
./getopts_test1 -o3 -lm
./getopts_test1 -o3 -m
(and more)

• The $OPTIND variable contains the number of command line option arguments that getopts parsed.

./getopts_test1 -o 3 -l -m  # $OPTIND = 4
./getopts_test1 -o3 -lm     # $OPTIND = 4
./getopts_test1 -l -m       # $OPTIND = 2
./getopts_test1 -lm         # $OPTIND = 2

• shift "$(($OPTIND - 1))"
  • The shift command is a shell built-in command which moves the positional parameters of the script down a specified number of positions, discarding the related arguments.
  • Running shift "$(($OPTIND - 1))" discards the command line options; this is a standard thing to do in a shell script using getopts, when the command line options are no longer needed.

Sample script #2 that uses getopts:

#!/bin/bash

# getopts_test2:
# This script has 3 command line options and
# takes an additional script argument ($1)

while getopts dmy OPTION
do
        case $OPTION in
          d) dopt=1;;
          m) mopt=1;;
          y) yopt=1;;
          ?) echo "Unrecognized or invalid option"
             exit 1;;
        esac
done

if [ ! -z $dopt ]; then
    day=$(date '+%d')
fi

if [ ! -z $mopt ]; then
    mon=$(date '+%b')
fi

if [ ! -z $yopt ]; then
    year=$(date '+%Y')
fi

shift $(($OPTIND - 1))

echo "The date is: $day $mon $year"

if [ ! -z $1 ]; then
   echo "Welcome, $1."
fi

• To display day, month, year and a “Welcome” message, run the script in one of the following ways:

./getopts_test2 -d -m -y Dave
./getopts_test2 -dmy Dave

• Without doing 'shift $(($OPTIND -1 ))' to discard the command line options, the script would not be able to properly process the additional command line argument ($1).

Test a version of the text2png script that uses getopts for command line options handling: text2png_getopts