Configuration

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

Style/Encoding:
  Enabled: false

Metrics/LineLength:
  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.

Inheritance

RuboCop supports inheriting configuration from one or more supplemental configuration files at runtime. Settings in the file that inherits override settings in the file that's inherited from. Configuration parameter that are hashes, for example PreferredMethods in Style/CollectionMethods are merged with the same parameter in the base configuration, while other parameter, such as AllCops / Include, are simply replaced by the local setting. If arrays were merged, there would be no way to remove elements through overriding them in local configuration. There is a way to have specific array settings merged using the inherit_mode setting.

Inheriting from another configuration file in the project

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:

inherit_from:
  - ../.rubocop.yml
  - ../conf/.rubocop.yml

Inheriting configuration from a remote URL

The optional inherit_from directive can contain a full url to a remote file. This makes it possible to have common project settings stored on a http server and shared between many projects.

The remote config file is cached locally and is only updated if:

The file does not exist.
The file has not been updated in the last 24 hours.
The remote copy has a newer modification time than the local copy.

You can inherit from both remote and local files in the same config and the same inheritance rules apply to remote URLs and inheriting from local files where the first file in the list has the lowest precedence and the last one has the highest. The format for multiple inheritance using URLs is:

inherit_from:
  - http://www.example.com/rubocop.yml
  - ../.rubocop.yml

Inheriting configuration from a dependency gem

The optional inherit_gem directive is used to include configuration from one or more gems external to the current project. This makes it possible to inherit a shared dependency's RuboCop configuration that can be used from multiple disparate projects.

Configurations inherited in this way will be essentially prepended to the inherit_from directive, such that the inherit_gem configurations will be loaded first, then the inherit_from relative file paths will be loaded (overriding the configurations from the gems), and finally the remaining directives in the configuration file will supersede any of the inherited configurations. This means the configurations inherited from one or more gems have the lowest precedence of inheritance.

The directive should be formatted as a YAML Hash using the gem name as the key and the relative path within the gem as the value:

inherit_gem:
  my-shared-gem: .rubocop.yml
  cucumber: conf/rubocop.yml

An array can also be used as the value to include multiple configuration files from a single gem:

inherit_gem:
  my-shared-gem:
    - default.yml
    - strict.yml

Note: If the shared dependency is declared using a Bundler Gemfile and the gem was installed using bundle install, it would be necessary to also invoke RuboCop using Bundler in order to find the dependency's installation path at runtime:

$ bundle exec rubocop <options...>

Merging arrays using inherit_mode

The optional directive inherit_mode is used to specify which configuration keys that have array values should be merged together instead of overriding the inherited value.

This applies to explicit inheritance using inherit_from as well as implicit inheritance from the default configuration.

Given the following config:

# .rubocop.yml
inherit_from:
  - shared.yml

inherit_mode:
  merge:
    - Exclude 

AllCops:
  Exclude:
    - 'generated/**/*.rb'

Style/For:
  Exclude:
    - bar.rb

# .shared.yml
Style/For:
  Exclude:
    - foo.rb

The list of Excludes for the Style/For cop in this example will be ['foo.rb', 'bar.rb']. Similarly, the AllCops:Exclude list will contain all the default patterns plus the generated/**/*.rb entry that was added locally.

The directive can also be used on individual cop configurations to override the global setting.

inherit_from:
  - shared.yml

inherit_mode:
  merge:
    - Exclude 

Style/For:
  inherit_mode:
    override:
      - Exclude
  Exclude:
    - bar.rb

In this example the Exclude would only include bar.rb.

Defaults

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 does a recursive file search starting from the directory it is run in, or directories given as command line arguments. Files that match any pattern listed under AllCops/Include and extensionless files with a hash-bang (#!) declaration containing one of the known ruby interpreters listed under AllCops/RubyInterpreters are inspected, unless the file also matches a pattern in AllCops/Exclude. Hidden directories (i.e., directories whose names start with a dot) are not searched by default. If you'd like RuboCop to check files that are not included by default, you'll need to pass them in on the command line, or to add entries for them under AllCops/Include.

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

AllCops:
  Exclude:
    - 'db/**/*'
    - 'config/**/*'
    - 'script/**/*'
    - !ruby/regexp /old_and_unused\.rb$/

# other configuration
# ...

In .rubocop.yml and any other configuration file beginning with .rubocop, files and directories are specified relative to the directory where the configuration file is. In configuration files that don't begin with .rubocop, e.g. our_company_defaults.yml, paths are relative to the directory where rubocop is run.

Note: Patterns that are just a file name, e.g. Rakefile, will match that file name in any directory, but this pattern style is 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 Include and Exclude parameters are special. They are valid for the directory tree starting where they are defined. They are not shadowed by the setting of Include and 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. This behavior will be overridden if you specify the --ignore-parent-exclusion command line argument.

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.

Rails/HasAndBelongsToMany:
  Include:
    - 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.

Rails/HasAndBelongsToMany:
  Exclude:
    - app/models/problematic.rb

Generic configuration parameters

In addition to Include and Exclude, the following parameters are available for every cop.

Enabled

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

Metrics/LineLength:
  Enabled: false

Most cops are enabled by default. Some cops, configured in config/disabled.yml, are disabled by default. The cop enabling process can be altered by setting DisabledByDefault or EnabledByDefault (but not both) to true.

AllCops:
  DisabledByDefault: true

All cops are then disabled by default, and only cops appearing in user configuration files are enabled. Enabled: true does not have to be set for cops in user configuration. They will be enabled anyway. It is also possible to enable entire departments by adding for example

Style:
  Enabled: true

The exception to the rule is the Rails department, which can not be enabled in its entirety this way. Setting Rails: Enabled: true will have the same effect as running with the --rails command line option, which in the context of DisabledByDefault: true means to make it possible to enable Rails cops individually.

AllCops:
  EnabledByDefault: true

All cops are then enabled by default, and only cops explicitly disabled using Enabled: false in user configuration files are enabled.

Severity

Each cop has a default severity level based on which department it belongs to. The level is normally warning for Lint and convention for all the others, but this can be changed in user configuration. Cops can customize their severity level. Allowed values are refactor, convention, warning, error and fatal.

There is one exception from the general rule above and that is Lint/Syntax, a special cop that checks for syntax errors before the other cops are invoked. It can not be disabled and its severity (fatal) can not be changed in configuration.

Lint:
  Severity: error

Metrics/CyclomaticComplexity:
  Severity: warning

Details

Individual cops can be embellished with extra details in offense messages:

Metrics/LineLength:
  Details: >-
    If lines are too short, text becomes hard to read because you must
    constantly jump from one line to the next while reading. If lines are too
    long, the line jumping becomes too hard because you "lose the line" while
    going back to the start of the next line.  80 characters is a good
    compromise.

AutoCorrect

Cops that support the --auto-correct option can have that support disabled. For example:

Style/PerlBackrefs:
  AutoCorrect: false

Setting the target Ruby version

Some checks are dependent on the version of the Ruby interpreter which the inspected code must run on. For example, enforcing using Ruby 2.3+ safe navigation operator rather than try can help make your code shorter and more consistent... unless it must run on Ruby 2.2.

If .ruby-version exists in the directory RuboCop is invoked in, RuboCop will use the version specified by it. Otherwise, users may let RuboCop know the oldest version of Ruby which your project supports with:

AllCops:
  TargetRubyVersion: 2.2

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 cops that currently detect an offense in the code by excluding the offending files, or disabling the cop altogether once a file count limit has been reached.

By adding the option --exclude-limit COUNT, e.g., rubocop --auto-gen-config --exclude-limit 5, you can change how many files are excluded before the cop is entirely disabled. The default COUNT is 15.

Then you can start removing the entries in the generated .rubocop_todo.yml file one by one as you work through all the offenses in the code.

Updating the configuration file

When you update RuboCop version, sometimes you need to change .rubocop.yml. If you use mry, you can update .rubocop.yml to latest version automatically.

$ gem install mry
# Update to latest version
$ mry .rubocop.yml
# Update to specified version
$ mry --target=0.48.0 .rubocop.yml

See https://github.com/pocke/mry for more information.

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 Metrics/LineLength, Style/StringLiterals
[...]
# rubocop:enable Metrics/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/For