Overview
On newer v6+ platforms with support for multiple Ruby interpreters, installing a gem may fail resulting in a similar error message:
[user@sol ~]$ gem install --no-rdoc --no-ri passenger rails Fetching: passenger-5.0.6.gem (100%) ERROR: While executing gem ... (Gem::FilePermissionError) You don't have write permissions for the /.socket/ruby/gems/ruby-2.1.2 directory.
Cause
The environment variable GEM_HOME is not configured until rvm use is executed Rubygems attempts to install to the system default directory, which must be reconfigured, at run-time with rvm.
Solution
Select which Ruby version to use with rvm use. This will install a rvm shim necessary to set GEM_HOME.
cd /var/www # rvm shim is installed under /var/www rvm use 2.2.2 # rvm will confirm this version is selected gem install --no-rdoc --no-ri rails # Rails installs now without incident