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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