What does bundle exec rake mean?

Question

What does bundle exec rake db:migrate mean? Or just bundle exec rake <command> in general?

I understand that bundle takes care of maintaining things in the Gemfile. I know what the word "exec" means. I understand that rake maintains all the different scripty things you can do, and I know that db:migrate is one of those. I just don't know what all these words are doing together. Why should bundle be used to execute rake to execute a database migrate?

Answer 1

bundle exec is a Bundler command to execute a script in the context of the current bundle (the one from your directory's Gemfile). rake db:migrate is the script where db is the namespace and migrate is the task name defined.

So bundle exec rake db:migrate executes the rake script with the command db:migrate in the context of the current bundle.

As to the "why?" I'll quote from the bundler page:

In some cases, running executables without bundle exec may work, if the executable happens to be installed in your system and does not pull in any gems that conflict with your bundle.

However, this is unreliable and is the source of considerable pain. Even if it looks like it works, it may not work in the future or on another machine.