samizdat-shell-help.bash

#!/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.

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

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

echo Hello World

My version, that doesn't require multiple # (but allows more than one if you want it for stylistic reasons):

#!/bin/bash
# Usage:
#   test_usage.sh ...

print_usage_and_exit() {
  # The sed script below:
  # - Deletes the first line (shebang).
  # - Quits on the first line that doesn't start with #.
  # - Removes the '#+ ?' prefix from the string.
  sed -En '1d; /^[^#]/ q; s/^#+ ?//p;' < "$0" >&2
  exit 1
}