Blake Smith

create. code. learn.


gem executables loading the relative path

In a common gem project structure, a simple layout might look like this:

`-> lib
  `-> my_gem.rb
  `-> my_gem
    `-> runner.rb
 -> bin
  `-> my_executable

With an executable file in the bin folder, a common ruby idiom might follow this pattern:

#!/usr/bin/env ruby

require 'rubygems'
require 'my_gem'!(ARGV)

This is great, we’ve deffered all of our run logic to a class inside of our lib folder (in this case, runner.rb) which makes it easier to unit test our code.

The dilema that this situation creates, is that by default Ruby will load the my_gem classes from the default rubygems path instead of in the folder relative to the executable. This means that everytime we update our code, we have to rebuild the gem and reinstall it to smoke-test our executable. Otherwise if we only change the code in our project directory, when we run the executable that’s located in our project directory, it will still be loading the stale code in our rubygems path.

A great solution is to add this line before you require your gem:

$LOAD_PATH.unshift File.join(File.dirname(__FILE__), *%w[.. lib])

You can also use the shorthand variable $: as an alias for $LOAD_PATH.

So the final code would look like this:

#!/usr/bin/env ruby
$LOAD_PATH.unshift File.join(File.dirname(__FILE__), *%w[.. lib])

require 'rubygems'
require 'my_gem'!(ARGV)

A Ruby master would laugh at me for just realizing this pattern, but what this means is that each time your executable is run, even from your local project directory, it will always load the code that is relative to the executable, which is exactly what you want.

Just a little tidbit I thought I’d pass on to ease the friction of those developing executables in their gem projects.

about the author

Blake Smith is a Principal Software Engineer at Sprout Social.