man bundle-exec (1): Execute a command in the context of the bundle

SYNOPSIS

bundle exec command

DESCRIPTION

This command executes the command, making all gems specified in the Gemfile(5) available to require in Ruby programs.

Essentially, if you would normally have run something like rspec spec/my_spec.rb, and you want to use the gems specified in the Gemfile(5) and installed via bundle install(1) bundle-install.1.html, you should run bundle exec rspec spec/my_spec.rb.

Note that bundle exec does not require that an executable is available on your shell's $PATH.

BUNDLE INSTALL --BINSTUBS

If you use the --binstubs flag in bundle install(1) bundle-install.1.html, Bundler will automatically create a directory (which defaults to app_root/bin) containing all of the executables available from gems in the bundle.

After using --binstubs, bin/rspec spec/my_spec.rb is identical to bundle exec rspec spec/my_spec.rb.

ENVIRONMENT MODIFICATIONS

bundle exec makes a number of changes to the shell environment, then executes the command you specify in full.

•: make sure that it's still possible to shell out to bundle from inside a command invoked by bundle exec (using $BUNDLE_BIN_PATH)
•: put the directory containing executables (like rails, rspec, rackup) for your bundle on $PATH
•: make sure that if bundler is invoked in the subshell, it uses the same Gemfile (by setting BUNDLE_GEMFILE)
•: add -rbundler/setup to $RUBYOPT, which makes sure that Ruby programs invoked in the subshell can see the gems in the bundle

It also modifies Rubygems:

•: disallow loading additional gems not in the bundle
•: modify the gem method to be a no-op if a gem matching the requirements is in the bundle, and to raise a Gem::LoadError if it's not
•: Define Gem.refresh to be a no-op, since the source index is always frozen when using bundler, and to prevent gems from the system leaking into the environment
•: Override Gem.bin_path to use the gems in the bundle, making system executables work
•: Add all gems in the bundle into Gem.loaded_specs

Shelling out

When shelling out (using the system or backticks methods, for example), Bundler's environment changes will propogate to the subshell environment. If you desire to shell out without Bundler's environment changes, simply employ the with_clean_env method. It will restore all environment variables to what they were before Bundler was activated. For example:

Bundler.with_clean_env do
  `brew install wget`
end

RUBYGEMS PLUGINS

At present, the Rubygems plugin system requires all files named rubygems_plugin.rb on the load path of any installed gem when any Ruby code requires rubygems.rb. This includes executables installed into the system, like rails, rackup, and rspec.

Since Rubygems plugins can contain arbitrary Ruby code, they commonly end up activating themselves or their dependencies.

For instance, the gemcutter 0.5 gem depended on json_pure. If you had that version of gemcutter installed (even if you also had a newer version without this problem), Rubygems would activate gemcutter 0.5 and json_pure <latest>.

If your Gemfile(5) also contained json_pure (or a gem with a dependency on json_pure), the latest version on your system might conflict with the version in your Gemfile(5), or the snapshot version in your Gemfile.lock.

If this happens, bundler will say:

You have already activated json_pure 1.4.6 but your Gemfile
requires json_pure 1.4.3. Consider using bundle exec.

In this situation, you almost certainly want to remove the underlying gem with the problematic gem plugin. In general, the authors of these plugins (in this case, the gemcutter gem) have released newer versions that are more careful in their plugins.

You can find a list of all the gems containing gem plugins by running

ruby -rubygems -e "puts Gem.find_files('rubygems_plugin.rb')"