RVM: More than Ruby 1.9 and Rails 3

RVM is best known as a tool to help developers upgrade their applications to newer versions of Ruby and Rails (3 specifically). That said, for ruby developers, it has many features which help to make their workflow far simpler. Whilst some have shown with effort and rigorous manual process you can achieve much of what RVM offers, the following is a look into some less often discussed areas of RVM that make it my most favoured tool when it comes to development.

Gemsets and the Gemset Hierarchy

Whilst Bundler solves most of the problems related to isolating gems for me personally, on most projects I develop for there are things that make rvm’s implementation of gemsets (especially when used along with bundler) invaluable to me as a ruby developer.

Isolating Gems

A major strength of RVM is the ability to isolate gems at the system level. This keeps them safely partitioned from any other gems unrelated to my project. The way bundler isolates gems is for the most part at run time – It changes the way rubygems works to solve it’s problem and either installs all gems to a user specified path or to your gem directory.

If you are using the usr specified path option, you must to remember that your custom rubygems bin directory is not contained in your PATH. This means that you must either manually add it to the PATH, or your need to manually prefix each command with bundle exec, or alternatively you can generate the binaries into your applications bin directory and each command with ./bin/

If you take the opposite approach and install your gems into your gem home, you then need to be aware of the fact that each next install will overwrite any previous binaries – the canonical example being Rails 2 and Rails 3. The way that rubygems generates binaries lets you run gem-binary-name _version_ something to call the binary loaded from a specified gem version. Unfortunately, in Rails 3, the rails binary was moved from the rails gem into the railties gem which just so happens to break this feature of rubygems. You can try this for yourself on a new rvm gemset, install rails 2.3.8 and rails 3.0.0 and then try rails _2.3.8_ -v. If you are using the simple solution of installing to gem home, even though you are using bundler it will still overwrite the binary.

One common suggestion is to just create a wrapper binary manually (e.g. rails2) or always prefix your commands with bundle exec. In contrast RVM’s isolation works by explicitly setting rubygems home and path to directories isolated under RVM. RVM takes this even further using gemsets (think subdirectories) to handle more fine grain isolation. For example, I personally use 1 gemset per application, thus avoiding the issues described above as for each application I will have only a single version of rails installed. Even better, switching is shell-local and only persists for a single shell instance. This means that I can have two different applications with two completely different versions of Rails running at the same time in different terminal sessions without issue.

As an added bonus, because RVM isolates at the rubygems system level (using environment variables), your application doesn’t actually need to know or care about conflicts from gems that the application does not use.

Combining RVM and Bundler yields a great deal of flexibility and consisency. This affords you the ‘best of both worlds’, system level isolation with the ability to install gems from git repositories, and in application gem loading based on application environment.

For more information RVM’s gemsets, check out the Gemset’s section on the rvm site. For more information on Bundler, check out Bundler’s documentation.

Gemset Hierarchy

The second feature I use often is the global gemset. Each ruby interpreter installation comes with two gemsets out of the box – the default (blank) gemset (e.g. ree-1.8.7-2010.02) and the global gemset (e.g. ree-1.8.7-2010.02@global). When you use any gemset, rvm not only sets the gem home to a uniq directory for the given gemset (ensuring gems are installed in the correct place) but it also sets GEM_PATH.

Much like PATH, GEM_PATH is a colon separated list of directories that rubygems uses to look up gems when requiring them. By installing a gem to the global gemset for any ruby, e.g. ree-1.8.7-2010.02@global, it will automatically be made available (including it’s binaries) inside both the default gemset and any user-defined gemsets. E.g.,

• ree-1.8.7-2010.02
• ree-1.8.7-2010.02@bighelpmob
• ree-1.8.7-2010.02@tedxperth

Will all have access to gems installed in ree-1.8.7-2010.02@global.

This comes in super handy for things like awesome_print, wirble, bundler, git-up, homesick – Namely, gems you want available in every environment.

Unfortunately, bundler currently does not support multiple items in BUNDLE_PATH. We have hope it will be added in the future but for now gems you want shared between apps using bundler, you’ll get some duplication.

Default Global Gemset Contents

As an added bonus along side the global gemsets, rvm provides a way to declare a file which tells it what gems to install initially. To do this, it uses a ruby string-based directory hierarchy (see the link below) to look for a global.gems file that rvm then imports – For example, my ~/.rvm/gemsets/global.gems (that is imported into the global gemset of all ruby interpreter installs) contains the following simple list:

rake
rdoc
awesome_print
bundler
git-up
ghost
homesick
wirble

For more information on this feature, be sure to read the Automatic Gemset Initialization page on the RVM site.

Set operations

If you are writing gems, one of the most important things you can do as a Ruby developer these days is to ensure they work on all of the major implementations. At the moment, for most gem developers you should generally be testing your code against:

• REE / 1.8.7
• 1.9.2
• Rubinius
• Jruby

And optionally, MagLev and MacRuby. RVM provides tools making it simple to run code and commands against multiple rubies. This includes but is not limited to running Rake tasks and tests / specs. The idea being to make it as frictionless as possible to test your gem against all of the above interpreters. As an example, you can run:

rvm rake test

Which will run rake test against all of your installed ruby interpreters (using default gemset for each). You may need to do some manual setup (e.g. installing test gem dependencies) but rvm tries to make it all as simple as possible and more importantly, heavily integrated into your normal work flow whilst staying out of the way.

Along side rake, rvm also provides wrappers for many common binaries (e.g. ruby, spec and the like) plus the --json and --yaml flags which make it simple to get a summarised view of the program results. Using rvm exec, you can even perform set operations against any arbitrary command.

There is a whole lot more set operations can do and I suggest anyone using or considering using rvm read the set’s section on the rvm site. Go forth and have sets!

Tools for easing dependency-related pains

One of the most commonly encountered problems (since for most people, the ruby interpreter you’d normally use is distributed as a binary install) are related to external dependencies – Namely, things like readline, iconv and openssl.

In an attempt to make life easier (and in particular, work around common problems like a neutered libedit instead of readline on OSX), rvm provides the rvm package set of commands that make it easy to install sandboxed versions of these in the ~/.rvm/usr directory.

As an example, to work around the libedit vs. readline issue on OSX, you can build your ruby against readline 6.0 by running:

rvm package install readline
rvm install <ruby> --with-readline-dir=$HOME/.rvm/usr

And, as an added bonus, to automatically patch older versions of ree and 1.8.7 to respond to Control-C immediately (for example, in irb) instead of when you press enter, you can replace the rvm install command above with:

rvm install <ruby> --with-readline-dir=$HOME/.rvm/usr --patch readline-fix

Simple Ruby Upgrades

Say, for example, a new version of 1.9.2 comes out in the next few weeks and you want to automatically update your rubies to match it correctly. As of approximately rvm 1.0.0, we added an rvm upgrade command that automatically handles doing the following:

• Installing the new ruby
• Moving across gemsets
• Updating wrapper scripts and aliases

The only thing you typically have to update are references in your .rvmrc’s and your passenger configuration. But, with a little forethought (e.g. creating an alias using rvm alias create), it’s entirely possible to make it so the entire upgrade is automatic.

I have personally used this feature to move from the 1.9.2 release candidate to the final patchlevel 0 release – I used rvm upgrade to simplify the process of updating each time it was bumped.

This is currently undocumented on the rvm site but documentation will be added – for the moment, in your rvm install, run rvm help upgrade and/or rvm upgrade help.

Location Matters

One of the underlying decisions related to rvm’s architecture is where it installs your rubies – In most cases, rvm will install your rubies into ~/.rvm/rubies and gems into ~/.rvm/gems. This means that you should almost never have to use sudo for commands (there are a few exceptions, most notably passenger which relies on root permission in order to bind to port 80).

This approach is simple on the scale of things but also brings a lot of freedom – for one, it means you are free to experiment with a Ruby / RVM setup, wiping it away when you’re done with a simple rm -rf ~/.rvm && mv .rvm-original .rvm.

.rvmrc files

All of this is fine and dandy, but when you still have to manually switch ruby interpreters and rubies each time you do something, it can become mildly annoying rather fast. For this exact reason RVM offers project-specific .rvmrc files – If you haven’t encountered them before, project .rvmrc’s are files that are automatically sourced into the current shell when you change (cd) into a directory containing one. These files can contain anything that is valid shell script (and to prevent RVM automatically running code you don’t want, it defaults to asking you to trust it the first time it is run) making things like project bootstrapping and configuration simple and mostly automatic after a quick setup in the project root directory:

rvm --rvmrc --create <ruby>@<gemset>

In my own projects, I use these to automatically install the Ruby if not present, create and use a project-specific gemset and then do the minimum possible required to bootstrap the new environment – in most cases, this is simply a matter of installing a few gems (e.g. for me, I install bundler via rvm gemset import).

In some cases, I even automatically run bundle install (with 1.0, if you have the gems already installed, this is very fast) hence the first time I cd into a project it essentially bootstraps the entire environment for me automatically.

Ultimately, this approach means that the Ruby interpreter and gemset switching are completely transparent to me – No matter which project I’m using, ruby and irb are available and point to the correct executable for the current project I am in. When integrated with some code I wrote for ruby summer of code, it’s even possible to have passenger automatically use this .rvmrc file to automatically change your application’s gemset at run time.

For more information on .rvmrc files, check out the rvmrc page and the Project Workflow page on the rvm site. See RVM’s passenger integration page page for more details on gemsets with passenger.

Conclusion

I hope that you have found at least one new or useful thing about RVM. RVM has been invaluable to me as a developer on multiple open source projects over the last several months. It makes it simple for me to do things that are otherwise harder or less flexible and generally stays out of my way.