Extract and display block of usage info

usage.sh

#!/usr/bin/env bash
##
##  Usage: usage [command|path]
##    Assuming that a command script file starts with a block of usage info
##    where each line in the block starts with two '##' characters. This script
##    extracts and displays that block of usage info.
##
##  Example:
##    Given a file a_command with the following contents:
##
##    #!/usr/bin/env bash
##    ##
##    ##  Usage: a_command <args>
##    ##    This is a_command
##    ##
##
##    > usage ~/bin/a_command
##
##      Usage: a_command <args>
##        This is a_command
##      ...
##
##    If the command a_command is in PATH
##    > usage a_command
##
##      Usage: a_command <args>
##        This is a_command
##      ...
##

if [ $# -ne 0 ]; then
  grep "^##" "$(which "$1")" | sed "s/^##//"
else
  exit 1
fi