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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.

1. 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

See Auto-correct.

2. 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

3. RuboCop as a formatter

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

$ rubocop -x

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.
-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
--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 Inspects in modification time order 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.
-h/--help Print usage information.
--ignore-parent-exlusion 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).
-R/--rails Run extra Rails 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.