How can I configure gem to only ever use system gems?

I want to configure gem to only use system gems - never those in $HOME/.gem. This is because I'm writing a script that will access Gem.path and I don't want it to return the path to gems in my home directory.

I'm pretty sure I haven't explicitly set GEM_HOME or anything like that in my .bashrc, .bash_login etc.

But Gem.path returns my homedir gems first:

irb
> Gem.path
=> ["/home/nfm/.gem/ruby/1.9.1", "/usr/local/lib/ruby/gems/1.9.1"]

Can I stop this from happening? Where is it configured? Or is it just the default to look in homedir first?

If I can't configure this, can I return the system path for gems with regexp hackery?

More details:

which ruby
/usr/local/bin/ruby
ruby --version
ruby 1.9.2p0 (2010-08-18 revision 29036) [x86_64-linux]

gem env
  - RUBYGEMS VERSION: 1.3.7
  - RUBY VERSION: 1.9.2 (2010-08-18 patchlevel 0) [x86_64-linux]
  - INSTALLATION DIRECTORY: /usr/local/lib/ruby/gems/1.9.1
  - RUBY EXECUTABLE: /usr/local/bin/ruby
  - EXECUTABLE DIRECTORY: /usr/local/bin
  - RUBYGEMS PLATFORMS:
    - ruby
    - x86_64-linux
  - GEM PATHS:
     - /usr/local/lib/ruby/gems/1.9.1
     - /home/nfm/.gem/ruby/1.9.1
  - GEM CONFIGURATION:
     - :update_sources => true
     - :verbose => true
     - :benchmark => false
     - :backtrace => false
     - :bulk_threshold => 1000
  - REMOTE SOURCES:
     - http://rubygems.org/

Update:

So apparently this can be configured in ~/.gemrc:

# Note the use of a symbol before the colon - the string version doesn't work!
:gempath:
  - /usr/local/lib/ruby/gems/1.9.1

However, this does not seem to take effect if you fire up irb开发者_StackOverflow. This has something to do with the fact that the config file is YAML, and apparently yaml isn't loaded when irb starts (not sure on this one!):

$ irb
> Gem.path
=> ["/home/nfm/.gem/ruby/1.9.1", "/usr/local/lib/ruby/gems/1.9.1"]
> Gem.configuration.path
=> ["/usr/local/lib/ruby/gems/1.9.1"]
# Ready for a WTF moment?
> Gem.path
=> ["/usr/local/lib/ruby/gems/1.9.1"]

So, the answer below seems like the only consistent way to get the correct behavior, even though you'd assume that ~/.gemrc would work and would be a nicer way to wrap you config up.

However, setting :gempath: in my ~/.gemrc worked in the context of the script being in my Rakefile in a Rails app, presumably because yaml is explicitly loaded.

Not sure exactly what's going on with yaml, but this explanation seems consistent with what I'm seeing here.

Mod up! :P

overwrite it here:

#in ~/.bashrc
export GEM_PATH=/usr/local/lib/ruby/gems/1.9.1