Using RVM in Rake Tasks

Last week Micah suggested I create a Rakefile that would run the specs for my Tic Tac Toe, CLI, Rails and Limelight specs. I thought that sounded like a great idea, and much easier than running four groups of specs separately. I wasn’t sure how RVM would behave in a Rake context though, and not surprisingly, RVM does not allow by default the ability to switch versions of Ruby in a Rake task.

But the answer to “why?” wasn’t all that clear to me initially, and I did some digging. I started with the error RVM gave me when I tried making a Ruby Kernel#system call, like so:

system('rvm use default')

error: RVM is not a function, selecting rubies with 'rvm use ...' will not work.

You need to change your terminal emulator preferences to allow login shell.
Sometimes it is required to use `/bin/bash --login` as the command.
Please visit https://rvm.io/integration/gnome-terminal/ for a example.

In my searching I read a lot of the man pages for bash, including its description about what a login shell means:

When bash is invoked as an interactive login shell, or as a non-interactive shell with the –login option, it first reads and executes commands from the file /etc/profile, if that file exists. After reading that file, it looks for ~/.bash_profile, ~/.bash_login, and ~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. (man pages)

This means that Ruby Kernel#system is not executing system commands inside a login shell. So I checked the docs and found out the Kernel#system launches a subshell, and is not an interactive shell (meaning it does not load profiles). I figured that since RVM complains when it’s invoked from a non-login shell it requires access to the various profile files that would normally be read when a login shell process begins. Yup (it’s very dependent on ~/.bash_profile).

So, luckily, bash supports the ability to launch a new login shell with the -i flag like so:

bash -i

And I can pass this to Kernel#system like this:

system('bash -i')

But this isn’t quite enough just yet. When I try to send an RVM command with the previous call, I still don’t get the expected result:

system('bash -i "rvm use default"') # => false

The answer is to use the -c option:

system('bash -i -c "rvm use default"')

It took me awhile to understand why -c is necessary. What I reason is that when we invoke a new bash shell with the -i flag, how is it possible for that shell process to receive input from a string if it’s not told to expect STDIN as a string argument being passed to the shell’s invocation? And so the -c flag is required, with a string argument, so as to allow a string of commands to be passed to a newly invoked shell process. This is what allows us to evaluate rvm use default in the context of this newly invoked login shell, whereas without the -c option it is simply invoked in the existing subshell of Kernel#system (after the newly invoked shell we just created closed because it had nothing to do).

For another read on the -c option, here is the corresponding man page entry:

-c string: If the -c option is present, then commands are read from string. If there are arguments after the string, they are assigned to the positional parameters, starting with $0.

My final solution ended up looking like this:

system "bash -l -c 'cd limelight; rspec spec'"

The reason this works is because I’m relying on limelight/.rvmrc to automatically adjust the version of Ruby and set the required gemset for me when cd limelight occurs.

And happily, RVM is working inside the context of my Rakefile now, and I no longer need to run four separate specs. And a special thank you to my roommate Josh Cheek for working on the Rakefile with me!