Using Fish shell's event system to behave like method_missing

Dr Nic’s latest post, hash bang cucumber, reminded me of a piece of hax I whipped up a few weeks ago with Ruby and Fish Shell.

Fish has an event system that allows you to register functions to be auto-run after or during certain events (such as when a particular environment variable is changed). One of this events is called fish_command_not_found. It is triggered whenever you type a non-existant command into the prompt.

With this in mind, you can trigger certain commands to run by matching what fish couldn’t manage to run automatically by catching this event.

For instance:

function __fish_method_missing --on-event fish_command_not_found
  method_missing $argv
end

funcsave method_missing

Given that function is created (simply paste the whole code block into your Fish terminal), I can now create a command called method_missing (or whatever you call inside your __fish_method_missing) and place it somewhere in your $PATH — I like ~/.config/fish/bin.

My method_missing binary is simply something like this:

#!/usr/bin/env ruby

command = ARGV.shift

def run(cmd)
  puts "Running #{cmd.inspect} instead"
  system(cmd)
end

case command
when /^git(@|:\/\/).*\.git$/
  run("git clone #{command.inspect}")
when /^(?:ftp|https?):\/\/.+\.t(?:ar\.)?gz$/
  run("curl #{command.inspect} | tar xzv")
else
  $stderr.puts "No default action defined in #{__FILE__.inspect}"
  abort
end

Each command you want to register just becomes a new when statement. For instance, to implement the functionality Dr Nic was trying to achieve, I simply modify the case block as such:

case command
when /^[a-z0-9_\-\/]+\.feature(:\d+)?$/
  run("cucumber #{command}")
# ...
end

The existing entries I have in my method_missing command will auto-clone the repository of a pasted Git URL and download and expand a URL for a tar file, respectively. Not too shabby, and dead easy to implement.