Basic Usage

RuboCop has three primary uses:

  1. Code style checker (a.k.a. linter)

  2. A replacement for ruby -w (a subset of its linting capabilities)

  3. Code formatter

In the next sections we’ll briefly cover all of them.

Code style checker

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
    test
    end
end

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

Inspecting 1 file
W

Offenses:

test.rb:1:1: C: Style/FrozenStringLiteralComment: Missing magic comment # frozen_string_literal: true.
def badName
^
test.rb:1:5: C: Naming/MethodName: Use snake_case for method names.
def badName
    ^^^^^^^
test.rb:2:3: C: Style/GuardClause: Use a guard clause instead of wrapping the code inside a conditional expression.
  if something
  ^^
test.rb:2:3: C: Style/IfUnlessModifier: Favor modifier if usage when having a single-line body. Another good alternative is the usage of control flow &&/||.
  if something
  ^^
test.rb:4:5: W: Layout/EndAlignment: end at 4, 4 is not aligned with if at 2, 2.
    end
    ^^^

1 file inspected, 5 offenses detected

Auto-correcting offenses

You can also run RuboCop in an auto-correct mode, where it will try to automatically fix the problems it found in your code:

$ rubocop -a
# or
$ rubocop --auto-correct
See Auto-correct for more details.

Changing what RuboCop considers to be offenses

RuboCop comes with a preconfigured set of rules for each of its cops, based on the Ruby Style Guide. Depending on your project, you may wish to reconfigure a cop, tell to ignore certain files, or disable it altogether.

The most common way to change RuboCop’s behaviour is to create a configuration file named .rubocop.yml in the project’s root directory.

For more information, see Configuration.

RuboCop as a replacement for ruby -w

RuboCop natively implements almost all ruby -w lint warning checks, and then some. If you want you can use RuboCop simply as a replacement for ruby -w:

$ rubocop -l
# or
$ rubocop --lint

RuboCop as a formatter

There’s a handy shortcut to run auto-correction only on code layout (a.k.a. formatting) offenses:

$ rubocop -x
# or
$ rubocop --fix-layout
This option was introduced in RuboCop 0.57.0.

Command-line flags

For more details check the available command-line options:

$ rubocop -h
Command flag Description

-a/--auto-correct

Auto-correct certain offenses. Experimental, use with caution. See Auto-correct.

--auto-gen-config

Generate a configuration file acting as a TODO list.

--[no-]color

Force color output on or off.

-c/--config

Run with specified config file.

-C/--cache

Store and reuse results for faster operation.

-d/--debug

Displays some extra debug output.

--disable-pending-cops

Run without pending cops.

--disable-uncorrectable

Used with --auto-correct to annotate any offenses that do not support autocorrect with rubocop:todo comments.

-D/--[no-]display-cop-names

Displays cop names in offense messages. Default is true.

--display-only-fail-level-offenses

Only output offense messages at the specified --fail-level or above

--enable-pending-cops

Run with pending cops.

--except

Run all cops enabled by configuration except the specified cop(s) and/or departments.

--exclude-limit

Limit how many individual files --auto-gen-config can list in Exclude parameters, default is 15.

-E/--extra-details

Displays extra details in offense messages.

-f/--format

Choose a formatter, see Formatters.

-F/--fail-fast

Inspect files in order of modification time and stops after first file with offenses.

--fail-level

Minimum severity for exit with error code. Full severity name or upper case initial can be given. Normally, auto-corrected offenses are ignored. Use A or autocorrect if you’d like them to trigger failure.

--force-exclusion

Force excluding files specified in the configuration Exclude even if they are explicitly passed as arguments.

--only-recognized-file-types

Inspect files given on the command line only if they are listed in AllCops/Include parameters of user configuration or default configuration.

-h/--help

Print usage information.

--ignore-parent-exclusion

Ignores all Exclude: settings from all .rubocop.yml files present in parent folders. This is useful when you are importing submodules when you want to test them without being affected by the parent module’s rubocop settings.

--init

Generate a .rubocop.yml file in the current directory.

-l/--lint

Run only lint cops.

-L/--list-target-files

List all files RuboCop will inspect.

--no-auto-gen-timestamp

Don’t include the date and time when --auto-gen-config was run in the config file it generates

--no-offense-counts

Don’t show offense counts in config file generated by --auto-gen-config

--only

Run only the specified cop(s) and/or cops in the specified departments.

-o/--out

Write output to a file instead of STDOUT.

--parallel

Use available CPUs to execute inspection in parallel.

-r/--require

Require Ruby file (see Loading Extensions).

--safe

Run only safe cops.

--safe-auto-correct

Omit cops annotated as "not safe". See Auto-correct.

--show-cops

Shows available cops and their configuration.

-s/--stdin

Pipe source from STDIN. This is useful for editor integration. Takes one argument, a path, relative to the root of the project. RuboCop will use this path to determine which cops are enabled (via eg. Include/Exclude), and so that certain cops like Naming/FileName can be checked.

-x/--fix-layout

Auto-correct only code layout (formatting) offenses.

-v/--version

Displays the current version and exits.

-V/--verbose-version

Displays the current version plus the version of Parser and Ruby.

Default command-line options are loaded from .rubocop and RUBOCOP_OPTS and are combined with command-line options that are explicitly passed to rubocop. Thus, the options have the following order of precedence (from highest to lowest):

  1. Explicit command-line options

  2. Options from RUBOCOP_OPTS environment variable

  3. Options from .rubocop file.

Exit codes

RuboCop exits with the following status codes:

  • 0 if no offenses are found or if the severity of all offenses are less than --fail-level. (By default, if you use --auto-correct, offenses which are auto-corrected do not cause RuboCop to fail.)

  • 1 if one or more offenses equal or greater to --fail-level are found. (By default, this is any offense which is not auto-corrected.)

  • 2 if RuboCop terminates abnormally due to invalid configuration, invalid CLI options, or an internal error.