Introduced in Rails 4, the ./bin directory contains your app's "binstubs." Binstubs are wrappers around gem executables, like rails or bundle, which ensures a gem executable is run inside the correct environment for your Rails app.

Binstubs can be used in lieu of bundle exec to run a gem's executable inside your app's environment. For example, instead of typing bundle exec rails scaffold products you can type bin/rails scaffold products. Using binstubs is more flexible than bundle exec, because you don't have to cd to the app's root directory and type bundle exec before everything.

By default, bundle, rails, rake, setup, spring binstubs are created automatically for new rails projects. To make a binstub for a gem executable, just type bundle binstubs name_of_gem_executable. You'll find the new binstub in your ./bin directory.

Some suggest putting ./bin in your shell's search $PATH, so that you don't have to type bin/rails and you can just type rails. This is risky because it depends on ./bin coming before the gem executable's path in $PATH; if you happen to forget this ordering and adjust $PATH such that the gem's executable is found before the binstub wrapper, you could easily invoke the gem's executable -- sans the environmental pretext -- without realizing it.