How to Parse Options in Bash Script: Simple Guide

To parse options in a bash script, use the getopts built-in command which processes short options one by one. Define option letters and handle them in a while loop with a case statement to respond to each option.

Syntax

The basic syntax for parsing options with getopts is:

• getopts optstring variable: Reads options from the script's arguments.
• optstring: A string of option letters to recognize; a colon : after a letter means that option requires an argument.
• variable: The name of the variable that will hold the current option letter.
• Use a while loop to process all options.
• Use a case statement inside the loop to handle each option.

bash

while getopts "ab:c" opt; do
  case $opt in
    a)
      echo "Option a triggered" ;;
    b)
      echo "Option b triggered with argument: $OPTARG" ;;
    c)
      echo "Option c triggered" ;;
    *)
      echo "Invalid option: -$opt" ;;
  esac
done

Example

This example script parses options -a, -b with an argument, and -c. It prints messages based on the options given.

bash

#!/bin/bash

while getopts "ab:c" opt; do
  case $opt in
    a)
      echo "Option a triggered" ;;
    b)
      echo "Option b triggered with argument: $OPTARG" ;;
    c)
      echo "Option c triggered" ;;
    *)
      echo "Invalid option: -$opt" ;;
  esac
done

Output

Option a triggered
Option b triggered with argument: value
Option c triggered

Common Pitfalls

Common mistakes when parsing options in bash scripts include:

• Not using a colon : after option letters that require arguments in optstring.
• Forgetting to handle the case when an option requires an argument but none is provided.
• Not shifting the positional parameters after getopts processing, which can cause issues if you want to process non-option arguments later.
• Using $OPTARG incorrectly outside the getopts loop.

Example of wrong and right usage:

bash

# Wrong: Missing colon for option b which needs an argument
while getopts "ab c" opt; do
  case $opt in
    b)
      echo "Option b argument: $OPTARG" ;; # OPTARG will be empty
  esac
done

# Right: Colon after b to expect argument
while getopts "ab:c" opt; do
  case $opt in
    b)
      echo "Option b argument: $OPTARG" ;; # OPTARG holds the argument
  esac
done

Quick Reference

Symbol	Meaning
a	Option letter 'a' without argument
b:	Option letter 'b' requires an argument
OPTARG	Variable holding the argument value for options that require one
getopts	Built-in command to parse options
$opt	Variable holding the current option letter
while getopts ...	Loop to process all options
case $opt in ...	Handle each option inside the loop

Key Takeaways

Use getopts with a while loop and case statement to parse options in bash scripts.

Add a colon after option letters in optstring to indicate they require arguments.

Access option arguments with the OPTARG variable inside the getopts loop.

Shift positional parameters if you need to handle non-option arguments after parsing.

Always handle invalid options to provide helpful error messages.