Why doesn't "rails s" work from the app directory?

Learn why doesn't "rails s" work from the app directory? with practical examples, diagrams, and best practices. Covers ruby-on-rails, ruby, gemfile development techniques with visual explanations.
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Why 'rails s' Doesn't Work from Your Rails Application Root

Why 'rails s' Doesn't Work from Your Rails Application Root

Unraveling the mystery of why the rails s command, meant to start your Rails server, fails when executed directly from your application's root directory and how to correctly run it.

Many new Ruby on Rails developers encounter a common head-scratcher: attempting to start their Rails server with rails s from the application's root directory, only to be met with an error like No such file or directory - rails or rails: command not found. This article delves into the underlying reasons for this behavior, explaining how RubyGems, Bundler, and the Rails executable path interact, and provides the correct methods to launch your development server.

The Role of RubyGems and Executables

When you install Rails, you're actually installing a RubyGem that includes the Rails framework and its associated executables. These executables, such as rails, rake, and bundle, are typically placed in a directory like /usr/local/bin or ~/.rbenv/shims, which is part of your system's PATH environment variable. When you type rails s in your terminal, your shell searches these PATH directories for an executable named rails.

echo $PATH

This command displays the directories your shell searches for executables.

Bundler and the Gemfile.lock

A Rails application relies on specific versions of gems defined in its Gemfile. Bundler is a gem manager that ensures your application uses the exact gem versions specified. When you run bundle install, Bundler creates a Gemfile.lock file, which records these versions. Crucially, Bundler also provides an executable wrapper mechanism. When you prefix a command with bundle exec, Bundler intercepts the command and ensures it's run within the context of your application's Gemfile, using the correct gem versions. This is the recommended way to run Rails commands within your application.

A flowchart diagram illustrating the command execution flow. 'User types command' -> 'Shell searches PATH' -> 'If rails found in PATH, executes Global Rails' -> 'If bundle exec rails is used, Bundler intercepts' -> 'Bundler reads Gemfile.lock' -> 'Bundler loads application-specific Rails executable'. Use blue rectangles for actions, green diamonds for decisions, and arrows for flow.

Command Execution Flow with and without Bundler

Why the Application Root is Special

The application root directory itself is not typically included in your system's PATH. Therefore, when you're inside your Rails app's root and type rails s, the shell looks for rails in its standard PATH directories, not in your current working directory. The rails executable that your application needs to use is often managed by Bundler to ensure version consistency. Directly calling rails s without bundle exec might either call a globally installed Rails (which might be a different version) or fail if no rails executable is found in the PATH that satisfies the application's dependencies.

1. Step 1

Navigate to your Rails application's root directory in your terminal.

2. Step 2

Ensure all necessary gems are installed by running bundle install.

3. Step 3

Start the Rails server using bundle exec rails s.

By understanding the roles of the system's PATH, RubyGems, and Bundler, you can avoid common pitfalls and ensure your Rails applications run smoothly with their intended gem versions. Always remember to use bundle exec when running Rails-specific commands within your application's context.