Help message for shell scripts

Have you ever thought how good it would be to have a help message for your shell script that you wrote a month ago and already forgot what it is supposed to do?

Yeah, there is always a way to show a message using cat (meow) or a bunch of echo calls.

But there is a neat trick.

Add your message with all the required information on top of your file, just right after the shebang.

#!/bin/bash
###
### my-script — does one thing well
###
### Usage:
###   my-script <input> <output>
###
### Options:
###   <input>   Input file to read.
###   <output>  Output file to write. Use '-' for stdout.
###   -h        Show this message.

Halfway done, now need to get this message in runtime with sed.

help() {
    sed -rn 's/^### ?//;T;p' "$0"
}

$0 means a filename of a file that is being executed.

A bit about the magic that is going here:

• s — stands for substitute the following pattern;
• / — keyword to define start/end of the pattern;
• ^### ? — match a string starting with ### and an optional space after;
• // — there could be something between the slashes, but since nothing is here, replace the pattern with an empty string;
• T — Jumps to the end of sed-script if no s/// has done a successful substitution;
• p — Prints the result of the substitution.

Now just call the help function if an arg -h or no args passed.

if [[ $# == 0 ]] || [[ "$1" == "-h" ]]; then
    help
    exit 1
fi

Hope it helps, cheers!

Full gist is available on GitHub: https://gist.github.com/kovetskiy/a4bb510595b3a6b17bfd1bd9ac8bb4a5

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