getopts - Linux

Overview

getopts is a utility in Linux that enables the handling of command-line options in shell scripts. It simplifies the processing of options and their associated arguments in a secure and efficient manner. This command is primarily useful in scenario scripts where options may vary; it helps script writers to easily parse and handle complex command line inputs.

Syntax

The typical syntax for getopts in a shell script is:

getopts optstring name [arg...]
  • optstring contains the option letters to be recognized; if a letter is followed by a colon, the option is expected to have an argument, which should be separated from it by whitespace.
  • name is the name of the variable that will be used to store the option being processed.
  • args are optional and specify additional arguments that may be processed by the command.

Options/Flags

There are no traditional flags like -h or --help as getopts is a shell builtin, but rather it uses an optstring where each character represents a single option. A colon : following an option indicates that the option requires an argument. Here’s how specific characters in optstring affect its behavior:

  • : (colon at the start) – Changes the error handling for cases where options without required arguments are given an argument.

Examples

1. Basic Example: Parsing options a and b, where b requires an argument.

while getopts "ab:" opt; do
  case $opt in
    a) echo "-a was triggered!" ;;
    b) echo "-b was triggered, Argument: $OPTARG" ;;
    *) echo "Invalid option: -$OPTARG" ;;
  esac
done

2. Advanced Example: Using getopts to parse long, combined options which are typical in scripts.

while getopts ":a:b:c" opt; do
  case $opt in
    a) echo "Option a, argument '$OPTARG'";;
    b) echo "Option b, argument '$OPTARG'";;
    c) echo "Option c provided";;
    \?) echo "Invalid option: -$OPTARG";;
    :) echo "Option -$OPTARG requires an argument.";;
  esac
done

Common Issues

1. Forgetting Colon: If you forget to put a colon after an option in optstring that requires an argument, getopts will not treat it correctly, and $OPTARG will not be populated.

2. Using Unsupported Long Options: getopts does not naturally support long options (like --help). Workarounds like using getopt command or manual parsing are needed for those.

Solution: Always double-check your optstring and consider alternative tools if your script requires long option support.

Integration

getopts can be integrated with other shell commands and utilities to create robust and flexible scripts. A common scenario is deploying scripts with various operational parameters, which can then be piped to tools like grep, awk, etc.

Example of script integration:

while getopts "f:v" opt; do
  case $opt in
    f) cat $OPTARG | grep 'error' ;;
    v) verbose=1 ;;
  esac
done
  • getopt – An external utility that supports long options and more complex command-line parsing.
  • case – Often used with getopts to handle different options.
  • shift – Used to iterate over additional arguments once options have been parsed.

For further reading and more advanced use cases, refer to the BASH man page where getopts is documented under shell builtins.