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+bundle-exec(1) -- Execute a command in the context of the bundle
+================================================================
+
+## SYNOPSIS
+
+`bundle exec` [--keep-file-descriptors] <command>
+
+## DESCRIPTION
+
+This command executes the command, making all gems specified in the
+[`Gemfile(5)`][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)`][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 `$PATH`.
+
+## OPTIONS
+
+* `--keep-file-descriptors`:
+  Exec in Ruby 2.0 began discarding non-standard file descriptors. When this
+  flag is passed, exec will revert to the 1.9 behaviour of passing all file
+  descriptors to the new process.
+
+## BUNDLE INSTALL --BINSTUBS
+
+If you use the `--binstubs` flag in [bundle install(1)](bundle-install.1.html), Bundler will
+automatically create a directory (which defaults to `app_root/bin`)
+containing all of the executables available from gems in the bundle.
+
+After using `--binstubs`, `bin/rspec spec/my_spec.rb` is identical
+to `bundle exec rspec spec/my_spec.rb`.
+
+## ENVIRONMENT MODIFICATIONS
+
+`bundle exec` makes a number of changes to the shell environment,
+then executes the command you specify in full.
+
+* make sure that it's still possible to shell out to `bundle`
+  from inside a command invoked by `bundle exec` (using
+  `$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
+
+Finally, `bundle exec` also implicitly modifies `Gemfile.lock` if the lockfile
+and the Gemfile do not match. Bundler needs the Gemfile to determine things
+such as a gem's groups, `autorequire`, and platforms, etc., and that
+information isn't stored in the lockfile. The Gemfile and lockfile must be
+synced in order to `bundle exec` successfully, so `bundle exec`
+updates the lockfile beforehand.
+
+### Loading
+
+By default, when attempting to `bundle exec` to a file with a ruby shebang,
+Bundler will `Kernel.load` that file instead of using `Kernel.exec`. For the
+vast majority of cases, this is a performance improvement. In a rare few cases,
+this could cause some subtle side-effects (such as dependence on the exact
+contents of `$0` or `__FILE__`) and the optimization can be disabled by enabling
+the `disable_exec_load` setting.
+
+### Shelling out
+
+Any Ruby code that opens a subshell (like `system`, backticks, or `%x{}`) will
+automatically use the current Bundler environment. If you need to shell out to
+a Ruby command that is not part of your current bundle, use the
+`with_clean_env` method with a block. Any subshells created inside the block
+will be given the environment present before Bundler was activated. For
+example, Homebrew commands run Ruby, but don't work inside a bundle:
+
+    Bundler.with_clean_env do
+      `brew install wget`
+    end
+
+Using `with_clean_env` is also necessary if you are shelling out to a different
+bundle. Any Bundler commands run in a subshell will inherit the current
+Gemfile, so commands that need to run in the context of a different bundle also
+need to use `with_clean_env`.
+
+    Bundler.with_clean_env do
+      Dir.chdir "/other/bundler/project" do
+        `bundle exec ./script`
+      end
+    end
+
+Bundler provides convenience helpers that wrap `system` and `exec`, and they
+can be used like this:
+
+    Bundler.clean_system('brew install wget')
+    Bundler.clean_exec('brew install wget')
+
+
+## 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 -rrubygems -e "puts Gem.find_files('rubygems_plugin.rb')"
+
+At the very least, you should remove all but the newest
+version of each gem plugin, and also remove all gem plugins
+that you aren't using (`gem uninstall gem_name`).