Update the gems specified (all gems, if none are specified), ignoring the previously installed gems specified in the Gemfile.lock. In general, you should use bundle install to install the same exact gems and versions across machines.

You would use bundle update to explicitly update the version of a gem.

If you run bundle update with no parameters, bundler will ignore any previously installed gems and resolve all dependencies again based on the latest versions of all gems available in the sources.

Consider the following Gemfile:

source 'https://rubygems.org'
gem 'rails', '3.0.0.rc'
gem 'nokogiri'

When you run bundle install the first time, bundler will resolve all of the dependencies, all the way down, and install what you need:

Fetching source index for https://rubygems.org/
Installing rake (0.8.7)
Installing abstract (1.0.0)
Installing activesupport (3.0.0.rc)
Installing builder (2.1.2)
Installing i18n (0.4.1)
Installing activemodel (3.0.0.rc)
Installing erubis (2.6.6)
Installing rack (1.2.1)
Installing rack-mount (0.6.9)
Installing rack-test (0.5.4)
Installing tzinfo (0.3.22)
Installing actionpack (3.0.0.rc)
Installing mime-types (1.16)
Installing polyglot (0.3.1)
Installing treetop (1.4.8)
Installing mail (2.2.5)
Installing actionmailer (3.0.0.rc)
Installing arel (0.4.0)
Installing activerecord (3.0.0.rc)
Installing activeresource (3.0.0.rc)
Installing bundler (1.0.0.rc.3)
Installing nokogiri (1.4.3.1) with native extensions
Installing thor (0.14.0)
Installing railties (3.0.0.rc)
Installing rails (3.0.0.rc)

Your bundle is complete! Use `bundle show [gemname]` to see where a
bundled gem is installed.

As you can see, even though you have just two gems in the Gemfile, your application actually needs 25 different gems in order to run. Bundler remembers the exact versions it installed in Gemfile.lock. The next time you run bundle install, bundler skips the dependency resolution and installs the same gems as it installed last time.

After checking in the Gemfile.lock into version control and cloning it on another machine, running bundle install will _still_ install the gems that you installed last time. You don't need to worry that a new release of erubis or mail changes the gems you use.

However, from time to time, you might want to update the gems you are using to the newest versions that still match the gems in your Gemfile.

To do this, run bundle update, which will ignore the Gemfile.lock, and resolve all the dependencies again. Keep in mind that this process can result in a significantly different set of the 25 gems, based on the requirements of new gems that the gem authors released since the last time you ran bundle update.

Sometimes, you want to update a single gem in the Gemfile, and leave the rest of the gems that you specified locked to the versions in the Gemfile.lock.

For instance, in the scenario above, imagine that nokogiri releases version 1.4.4, and you want to update it _without_ updating Rails and all of its dependencies. To do this, run

bundle update nokogiri

Bundler will update nokogiri and any of its dependencies, but leave alone Rails and its dependencies.

Sometimes, multiple gems declared in your Gemfile are satisfied by the same second-level dependency. For instance, consider the case of thin and rack-perftools-profiler.

source 'https://rubygems.org'

gem 'thin'
gem 'rack-perftools-profiler'

The thin gem depends on rack >= 1.0, while rack-perftools-profiler depends on rack ~> 1.0. If you run bundle install, you get:

Fetching source index for https://rubygems.org/
Installing daemons (1.1.0)
Installing eventmachine (0.12.10) with native extensions
Installing open4 (1.0.1)
Installing perftools.rb (0.4.7) with native extensions
Installing rack (1.2.1)
Installing rack-perftools_profiler (0.0.2)
Installing thin (1.2.7) with native extensions
Using bundler (1.0.0.rc.3)

In this case, the two gems have their own set of dependencies, but they share rack in common. If you run bundle update thin, bundler will update daemons, eventmachine and rack, which are dependencies of thin, but not open4 or perftools.rb, which are dependencies of rack-perftools_profiler. Note that bundle update thin will update rack even though it's _also_ a dependency of rack-perftools_profiler.

In short, when you update a gem using bundle update, bundler will update all dependencies of that gem, including those that are also dependencies of another gem.

In this scenario, updating the thin version manually in the Gemfile, and then running bundle install will only update daemons and eventmachine, but not rack.