How to write a command line tool using bash

So far as I know, "long options", like --help and --version are not POSIX standard, but GNU standard. For command-line utilities the POSIX standard says:

The arguments that consist of hyphens and single letters or digits, such as 'a', are known as "options" (or, historically, "flags").

To support POSIX short options options it is worth getting to know getopts (there are tutorials on the web), but it does not support GNU long options.

For long options you have to roll your own:

filename=default
while (( $# > 0 ))
do
    opt="$1"
    shift

    case $opt in
    --help)
        helpfunc
        exit 0
        ;;
    --version)
        echo "$0 version $version"
        exit 0
        ;;
    --file)  # Example with an operand
        filename="$1"
        shift
        ;;
    --*)
        echo "Invalid option: '$opt'" >&2
        exit 1
        ;;
    *)
        # end of long options
        break;
        ;;
   esac

done

You can use the 'getopts' builtin, like so:

#!/bin/bash

# Parse arguments
usage() {
    echo "Usage: $0 [-h] [-v] [-f FILE]"
    echo "  -h  Help. Display this message and quit.
    echo "  -v  Version. Print version number and quit.
    echo "  -f  Specify configuration file FILE."
    exit
}

optspec="hvf:"
while getopts "$optspec" optchar
do
    case "${optchar}" in
        h)
            usage
            ;;
        v)
            version
            ;;
        f)
            file=${OPTARG}
            ;;
        *)
            usage
            ;;
    esac
done

This only works for single character options, not for long options like -help or --help. In practice, I've never found that this is a significant restriction; any script which is complex enough to require long options is probably something that I would write in a different language.

There is probably a better way to do this, but here is what I find useful: Each argument is represented by a variable in BASH. The first argument is $1. The second is $2, and so on. Match an option string with the first argument, and if it matches run some code accordingly.

Example:

#!/bin/bash
if [ $1 == "--hello" ] 
    then
        echo "Hello"
    else
        echo "Goodbye"
fi

If you code in C or C++, then use the **argv variable. **argv is a double pointer that holds a list of all arguments passed to the program (with argv[0] being the program name).