Bash - getopts - Argument Parser (Builtin command)

About

getopts is a command line utility used to parse positional parameters and short option only.

Example

• p, d and f are the options. The script usage is:

script.sh -p valuep -d valued -f

• The implementations

while getopts "p:d:" option
do
   case "$option" in
      p)   VARIABLE_FOR_P="${OPTARG}";;
      d)   VARIABLE_FOR_D="${OPTARG}";;
      ?)   echo -e "Usage";;
      *)   echo -e "option ${OPTARG} unknown. Usage:\n" ; exit 1 ;;
   esac
done

where:

• Bash - Case

• Bonus: Prompt for parameters is some required parameters are missing

if [[ -z "$VARIABLE_FOR_P" ]]; then
	echo "VARIABLE_FOR_P:"
	read VARIABLE_FOR_P
        # Test if null
	[[ "${VARIABLE_FOR_P:?The parameter is null sorry}" ]]
fi

Syntax

getopts optstring name [args]

where:

• optstring contains the option characters to be recognized;
  • If the first character of optstring is a colon, silent error reporting is used.
  • if a character is followed by a colon :, the option is expected to have an argument, which should be separated from it by white space.
  • The colon and question mark characters may not be used as option characters.
• args the positional parameters to parse. if not given parses the argument of the script

Process

Each time it is invoked, getopts set the following variable:

• name. Name refers to the next option (name is initialized if it does not exist)
• OPTIND. Optind refers to a variable containing the index of the next argument to be processed. OPTIND is initialized to 1 each time the shell or a shell script is invoked. The shell does not reset OPTIND automatically; it must be manually reset between multiple calls to getopts within the same shell invocation if a new set of parameters is to be used.
• OPTARG. Optind refers to a variable that contains the argument of the option (if any, see the optstring option)

At the end of the process, OPTIND is set to the index of the first non-option argument, and name is set to ?.

Return value

When the end of options is encountered, getopts exits with a return value greater than zero.

getopts returns:

• true if an option, specified or unspecified, is found.
• false if the end of options is encountered or an error occurs.