Why are gems installed in a directory with a different Ruby version than I’m running?

Question

When I install a gem, it gets installed in a directory named 1.9.1, despite that not being the version of Ruby I have installed:

$ ruby -v
ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12.2.0]

$ gem which rails
.../ruby/gems/1.9.1/gems/railties-3.2.9/lib/rails.rb

Why does this happen? I have no other Ruby versions installed (and certainly not v1.9.1).

Answer 1

Note that the following is also for all later Ruby versions as of this writing, not just 1.9.2.

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Per the 1.9.2 release announcement:

Standard library is installed in /usr/local/lib/ruby/1.9.1

This version is a "library compatible version." Ruby 1.9.2 is almost 1.9.1 compatible, so the library is installed in the 1.9.1 directory.

Even though it is installed in a differently-numbered directory, it is using 1.9.2. RubyGems can show all the directories it’s using via gem env.

This ensures that a set of installed gems is only used by versions that they can actually run with (especially due to compiled C extensions), and that when upgrading to a newer, but “library compatible”, version, one doesn’t have to reinstall all gems.

Answer 2

I believe it's because they share the same standard library.

There were some significant upgrades in the 1.9.2 core, but I don't think anything in the standard library was changed, so they share the same path. It's nothing to worry about, though — as you said, everything is working fine.