getopts

Understanding getopts

getopts is designed to process options provided to your script. It’s particularly useful when dealing with scripts that accept multiple options, some of which may require arguments. Unlike manually parsing $1, $2, etc., getopts provides a structured and error-handling way to handle options and their arguments.

The basic syntax is:

getopts optstring variable

• optstring: A string specifying the valid options your script accepts. Options starting with a colon (:) indicate that they require an argument.
• variable: The name of a variable where getopts will store information about the processed option.

Example 1: Simple Option Parsing

Let’s create a script that takes a -n option (for name) and a -a option (for age):

#!/bin/bash

while getopts ":n:a:" opt; do
  case $opt in
    n)
      name="$OPTARG"
      ;;
    a)
      age="$OPTARG"
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done

echo "Name: $name"
echo "Age: $age"

This script:

1. Uses a while loop to iterate through the options.
2. getopts ":n:a:" opt defines that -n and -a are valid options, both requiring arguments (due to the colon).
3. The case statement handles each option:
   • n) and a) assign the argument to $OPTARG.
   • \?) handles invalid options, printing an error message.
   • :) handles missing arguments for options that require them.
4. Finally, it prints the name and age.

To run this:

./my_script.sh -n John -a 30

Example 2: Option with a Default Value

We can provide a default value if an option is not specified:

#!/bin/bash

name="Unknown"
age=0

while getopts ":n:a:" opt; do
  case $opt in
    n)
      name="$OPTARG"
      ;;
    a)
      age="$OPTARG"
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done

echo "Name: $name"
echo "Age: $age"

Now, if you run the script without -n or -a, the default values will be used.

Example 3: Boolean Options

Boolean options don’t require arguments. Let’s add a -v (verbose) option:

#!/bin/bash

verbose=false

while getopts ":n:a:v" opt; do
  case $opt in
    n)
      name="$OPTARG"
      ;;
    a)
      age="$OPTARG"
      ;;
    v)
      verbose=true
      ;;
    \?)
      echo "Invalid option: -$OPTARG" >&2
      exit 1
      ;;
    :)
      echo "Option -$OPTARG requires an argument." >&2
      exit 1
      ;;
  esac
done

if $verbose; then
  echo "Verbose mode enabled."
fi

echo "Name: $name"
echo "Age: $age"

Here, -v sets the verbose variable to true.

Handling Long Options

While getopts primarily handles short options, you can achieve long option support using getopt (a separate command, not a shell built-in) which is more complex but offers greater flexibility. This warrants a separate discussion.

These examples demonstrate the versatility of getopts for creating complex command-line interfaces for your shell scripts. By mastering its usage, you can improve the usability and robustness of your programs.