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Role models are important.
-- Officer Alex J. Murphy / RoboCop

RuboCop is a Ruby static code analyzer. Out of the box it will enforce many of the guidelines outlined in the community Ruby Style Guide.

Most aspects of its behavior can be tweaked via various configuration options.

Apart from reporting problems in your code, RuboCop can also automatically fix some of the problems for you.


RuboCop's installation is pretty standard:

$ gem install rubocop

If you'd rather install RuboCop using bundler, don't require it in your Gemfile:

gem 'rubocop', require: false

Basic Usage

Running rubocop with no arguments will check all Ruby source files in the current directory:

$ rubocop

Alternatively you can pass rubocop a list of files and directories to check:

$ rubocop app spec lib/something.rb

Here's RuboCop in action. Consider the following Ruby source code:

def badName
  if something

Running RuboCop on it (assuming it's in a file named test.rb) would produce the following report:


test.rb:1:5: C: Use snake_case for methods and variables.
def badName
test.rb:2:3: C: Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||.
  if something
test.rb:4:5: W: end at 4, 4 is not aligned with if at 2, 2

1 file inspected, 3 offenses detected

For more details check the available command-line options:

$ rubocop -h
Command flag Description
-v/--version Displays the current version and exits
-V/--verbose-version Displays the current version plus the version of Parser and Ruby
-F/--fail-fast Inspects in modification time order and stops after first file with offenses
-d/--debug Displays some extra debug output
-D/--display-cop-names Displays cop names in offense messages.
-c/--config Run with specified config file
-f/--format Choose a formatter
-o/--out Write output to a file instead of STDOUT
-r/--require Require Ruby file (see Loading Extensions)
-R/--rails Run extra Rails cops
-l/--lint Run only lint cops
-a/--auto-correct Auto-correct certain offenses Note: Experimental - use with caution
--only Run only the specified cop(s)
--auto-gen-config Generate a configuration file acting as a TODO list
--show-cops Shows available cops and their configuration
--fail-level Minimum severity for exit with error code


In RuboCop lingo the various checks performed on the code are called cops. There are several cop departments.

You can also load custom cops.


Most of the cops in RuboCop are so called style cops that check for stylistics problems in your code. Almost all of the them are based on the Ruby Style Guide. Many of the style cops have configurations options allowing them to support different popular coding conventions.


Lint cops check for possible errors and very bad practices in your code. RuboCop implements in a portable way all built-in MRI lint checks (ruby -wc) and adds a lot of extra lint checks of its own. You can run only the lint cops like this:

$ rubocop -l

The -l/--lint option can be used together with --only to run all the enabled lint cops plus a selection of other cops.

Disabling any of the lint cops is generally a bad idea.


Rails cops are specific to the Ruby on Rails framework. Unlike style and lint cops they are not used by default and you have to request them specifically:

$ rubocop -R


The behavior of RuboCop can be controlled via the .rubocop.yml configuration file. It makes it possible to enable/disable certain cops (checks) and to alter their behavior if they accept any parameters. The file can be placed either in your home directory or in some project directory.

RuboCop will start looking for the configuration file in the directory where the inspected file is and continue its way up to the root directory.

The file has the following format:

inherit_from: ../.rubocop.yml

  Enabled: false

  Max: 99

Note: Qualifying cop name with its type, e.g., Style, is recommended, but not necessary as long as the cop name is unique across all types.


The optional inherit_from directive is used to include configuration from one or more files. This makes it possible to have the common project settings in the .rubocop.yml file at the project root, and then only the deviations from those rules in the subdirectories. The files can be given with absolute paths or paths relative to the file where they are referenced. The settings after an inherit_from directive override any settings in the file(s) inherited from. When multiple files are included, the first file in the list has the lowest precedence and the last one has the highest. The format for multiple inheritance is:

  - ../.rubocop.yml
  - ../conf/.rubocop.yml


The file config/default.yml under the RuboCop home directory contains the default settings that all configurations inherit from. Project and personal .rubocop.yml files need only make settings that are different from the default ones. If there is no .rubocop.yml file in the project or home directory, config/default.yml will be used.

Including/Excluding files

RuboCop checks all files recursively within the directory it is run on. However, it only recognizes files ending with .rb or extensionless files with a #!.*ruby declaration as Ruby files. If you'd like it to check other files you'll need to manually pass them in, or to add entries for them under AllCops/Include. Files and directories can also be ignored through AllCops/Exclude.

Here is an example that might be used for a Rails project:

    - '**/Rakefile'
    - '**/'
    - 'db/**/*'
    - 'config/**/*'
    - 'script/**/*'
    - !ruby/regexp /old_and_unused\.rb$/

# other configuration
# ...

Files and directories are specified relative to the .rubocop.yml file.

Note: Patterns that are just a file name, e.g. Rakefile, will match that file name in any directory, but this pattern style deprecated. The correct way to match the file in any directory, including the current, is **/Rakefile.

Note: The pattern config/** will match any file recursively under config, but this pattern style is deprecated and should be replaced by config/**/*.

Note: The Exclude parameter is special. It is valid for the directory tree starting where it is defined. It is not shadowed by the setting of Exclude in other .rubocop.yml files in subdirectories. This is different from all other parameters, who follow RuboCop's general principle that configuration for an inspected file is taken from the nearest .rubocop.yml, searching upwards.

Cops can be run only on specific sets of files when that's needed (for instance you might want to run some Rails model checks only on files whose paths match app/models/*.rb). All cops support the Include param.

    - app/models/*.rb

Cops can also exclude only specific sets of files when that's needed (for instance you might want to run some cop only on a specific file). All cops support the Exclude param.

    - app/models/problematic.rb

Specific cops can be disabled by setting Enabled to false for that specific cop.

  Enabled: false

Cops can customize their severity level. All cops support the Severity param. Allowed params are refactor, convention, warning, error and fatal.

  Severity: warning

Automatically Generated Configuration

If you have a code base with an overwhelming amount of offenses, it can be a good idea to use rubocop --auto-gen-config and add an inherit_from: .rubocop_todo.yml in your .rubocop.yml. The generated file .rubocop_todo.yml contains configuration to disable all cops that currently detect an offense in the code. Then you can start removing the entries in the generated file one by one as you work through all the offenses in the code.

Disabling Cops within Source Code

One or more individual cops can be disabled locally in a section of a file by adding a comment such as

# rubocop:disable Style/LineLength, Style/StringLiterals
# rubocop:enable Style/LineLength, Style/StringLiterals

You can also disable all cops with

# rubocop:disable all
# rubocop:enable all

One or more cops can be disabled on a single line with an end-of-line comment.

for x in (0..19) # rubocop:disable Style/AvoidFor


You can change the output format of RuboCop by specifying formatters with the -f/--format option. RuboCop ships with several built-in formatters, and also you can create your custom formatter.

Additionaly the output can be redirected to a file instead of $stdout with the -o/--out option.

Some of the built-in formatters produce machine-parsable output and they are considered public APIs. The rest of the formatters are for humans, so parsing their outputs is discouraged.

You can enable multiple formatters at the same time by specifying -f/--format multiple times. The -o/--out option applies to the previously specified -f/--format, or the default progress format if no -f/--format is specified before the -o/--out option.

# Simple format to $stdout.
$ rubocop --format simple

# Progress (default) format to the file result.txt.
$ rubocop --out result.txt

# Both progress and offense count formats to $stdout.
# The offense count formatter outputs only the final summary,
# so you'll mostly see the outputs from the progress formatter,
# and at the end the offense count summary will be outputted.
$ rubocop --format progress --format offenses

# Progress format to $stdout, and JSON format to the file rubocop.json.
$ rubocop --format progress --format json --out rubocop.json
#         ~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~
#                 |               |_______________|
#              $stdout

# Progress format to result.txt, and simple format to $stdout.
$ rubocop --output result.txt --format simple
#         ~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~
#                  |                 |
#           default format        $stdout

You can also load custom formatters.

Progress Formatter (default)

The default progress formatter outputs a character for each inspected file, and at the end it displays all detected offenses in the clang format. A . represents a clean file, and each of the capital letters means the severest offense (convention, warning, error or fatal) found in a file.

$ rubocop
Inspecting 26 files


lib/foo.rb:6:5: C: Missing top-level class documentation comment.
    class Foo


26 files inspected, 46 offenses detected

Clang Style Formatter

The clang formatter displays the offenses in a manner similar to clang:

$ rubocop test.rb
test.rb:1:1: C: Use snake_case for methods and variables.
def badName
test.rb:2:3: C: Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||.
  if something
test.rb:4:5: W: end at 4, 4 is not aligned with if at 2, 2

1 file inspected, 3 offenses detected

Fuubar Style Formatter

The fuubar style formatter displays a progress bar and shows details of offenses in the clang format as soon as they are detected. This is inspired by the Fuubar formatter for RSpec.

$ rubocop --format fuubar
lib/foo.rb.rb:1:1: C: Use snake_case for methods and variables.
def badName
lib/bar.rb:13:14: W: File.exists? is deprecated in favor of File.exist?.
 22/53 files |======== 43 ========>                           |  ETA: 00:00:02

Emacs Style Formatter


The emacs formatter displays the offenses in a format suitable for consumption by Emacs (and possibly other tools).

$ rubocop --format emacs test.rb
/Users/bozhidar/projects/test.rb:1:1: C: Use snake_case for methods and variables.
/Users/bozhidar/projects/test.rb:2:3: C: Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||.
/Users/bozhidar/projects/test.rb:4:5: W: end at 4, 4 is not aligned with if at 2, 2

Simple Formatter

The name of the formatter says it all :-)

$ rubocop --format simple test.rb
== test.rb ==
C:  1:  1: Use snake_case for methods and variables.
C:  2:  3: Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||.
W:  4:  5: end at 4, 4 is not aligned with if at 2, 2

1 file inspected, 3 offenses detected

File List Formatter


Sometimes you might want to just open all files with offenses in your favorite editor. This formatter outputs just the names of the files with offenses in them and makes it possible to do something like:

$ rubocop --format files | xargs vim

JSON Formatter


You can get RuboCop's inspection result in JSON format by passing --format json option in command line. The JSON structure is like the following example:

  "metadata": {
    "rubocop_version": "0.9.0",
    "ruby_engine": "ruby",
    "ruby_version": "2.0.0",
    "ruby_patchlevel": "195",
    "ruby_platform": "x86_64-darwin12.3.0"
  "files": [{
      "path": "lib/foo.rb",
      "offenses": []
    }, {
      "path": "lib/bar.rb",
      "offenses": [{
          "severity": "convention",
          "message": "Line is too long. [81/80]",
          "cop_name": "LineLength",
          "corrected": true,
          "location": {
            "line": 546,
            "column": 80,
            "length": 4
        }, {
          "severity": "warning",
          "message": "Unreachable code detected.",
          "cop_name": "UnreachableCode",
          "corrected": false,
          "location": {
            "line": 15,
            "column": 9,
            "length": 10
  "summary": {
    "offense_count": 2,
    "target_file_count": 2,
    "inspected_file_count": 2

Offense Count Formatter

Sometimes when first applying RuboCop to a codebase, it's nice to be able to see where most of your style cleanup is going to be spent.

With this in mind, you can use the offense count formatter to outline the offended cops and the number of offenses found for each by running:

$ rubocop --format offenses

87   Documentation
12   DotPosition
8    AvoidGlobalVars
7    EmptyLines
6    AssignmentInCondition
4    Blocks
4    CommentAnnotation
3    BlockAlignment
1    IndentationWidth
1    AvoidPerlBackrefs
1    ColonMethodCall
134  Total


RuboCop supports the following Ruby implementations:

  • MRI 1.9.2 (until June 2014)
  • MRI 1.9.3
  • MRI 2.0
  • MRI 2.1
  • JRuby in 1.9 mode
  • Rubinius 2.0+

Editor integration


rubocop.el is a simple Emacs interface for RuboCop. It allows you to run RuboCop inside Emacs and quickly jump between problems in your code.

flycheck > 0.9 also supports RuboCop and uses it by default when available.


The vim-rubocop plugin runs RuboCop and displays the results in Vim.

There's also a RuboCop checker in syntastic.

Sublime Text

If you're a ST user you might find the Sublime RuboCop plugin useful.


The brackets-rubocop extension displays RuboCop results in Brackets. It can be installed via the extension manager in Brackets.


The textmate2-rubocop bundle displays formatted RuboCop results in a new window. Installation instructions can be found here.


The atom-lint package runs RuboCop and highlights the offenses in Atom.

You can also use the linter-rubocop plugin for Atom's linter.


The lt-rubocop plugin provides LightTable integration.

Other Editors

Here's one great opportunity to contribute to RuboCop - implement RuboCop integration for your favorite editor.

Guard integration

If you're fond of Guard you might like guard-rubocop. It allows you to automatically check Ruby code style with RuboCop when files are modified.

Rake integration

To use RuboCop in your Rakefile add the following:

require 'rubocop/rake_task'

The above will use default values

require 'rubocop/rake_task'

desc 'Run RuboCop on the lib directory' do |task|
  task.patterns = ['lib/**/*.rb']
  # only show the files with failures
  task.formatters = ['files']
  # don't abort rake on failure
  task.fail_on_error = false


It's possible to extend RuboCop with custom cops and formatters.

Loading Extensions

Besides the --require command line option you can also specify ruby files that should be loaded with the optional require directive in the .rubocop.yml file:

 - ../my/custom/file.rb
 - rubocop-extension

Note: The pathes are directly passed to Kernel.require. If your extension file is not in $LOAD_PATH, you need to specify the path as relative path prefixed with ./ explicitly, or absolute path.

Custom Cops

You can configure the custom cops in your .rubocop.yml just like any other cop.

Known Custom Cops

Custom Formatters

You can customize RuboCop's output format with custom formatters.

Creating Custom Formatter

To implement a custom formatter, you need to subclass RuboCop::Formatter::BaseFormatter and override some methods, or implement all formatter API methods by duck typing.

Please see the documents below for more formatter API details.

Using Custom Formatter in Command Line

You can tell RuboCop to use your custom formatter with a combination of --format and --require option. For example, when you have defined MyCustomFormatter in ./path/to/my_custom_formatter.rb, you would type this command:

$ rubocop --require ./path/to/my_custom_formatter --format MyCustomFormatter


Here's a list of RuboCop's core developers:


Here's a list of all the people who have contributed to the development of RuboCop.

I'm extremely grateful to each and every one of them!

If you'd like to contribute to RuboCop, please take the time to go through our short contribution guidelines.

Converting more of the Ruby Style Guide into RuboCop cops is our top priority right now. Writing a new cop is a great way to dive into RuboCop!

Of course, bug reports and suggestions for improvements are always welcome. GitHub pull requests are even better! :-)

I'm also accepting financial contributions via gittip.

Support via Gittip

Mailing List

If you're interested in everything regarding RuboCop's development, consider joining its Google Group.


If you're into IRC you can visit the #rubocop channel on Freenode.


RuboCop's changelog is available here.


Copyright (c) 2012-2014 Bozhidar Batsov. See LICENSE.txt for further details.