File: Basic Usage — RuboCop master

RuboCop has three primary uses:

Code style checker (a.k.a. linter)
A replacement for ruby -w (a subset of its linting capabilities)
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

Autocorrecting offenses

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

$ rubocop -a
# or
$ rubocop --autocorrect

Tip	See Autocorrect 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 behavior 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 autocorrection only on code layout (a.k.a. formatting) offenses:

$ rubocop -x
# or
$ rubocop --fix-layout

Note	This option was introduced in RuboCop 0.57.0.

Command-line flags

For more details check the available command-line options:

$ rubocop -h

To specify multiple cops for one flag, separate cops with commas and no spaces:

$ rubocop --only Rails/Blank,Layout/HeredocIndentation,Naming/FileName

Command flag Description

Command flag	Description
`-a/--autocorrect`	Autocorrect offenses (only when it’s safe). See Autocorrect.
`--auto-correct`	Deprecated alias of `-a/--autocorrect`.
`-A/--autocorrect-all`	Autocorrect offenses (safe and unsafe). See Autocorrect.
`--auto-correct-all`	Deprecated alias of `-A/--autocorrect-all`.
`--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 --autocorrect 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-time`	Display elapsed time in seconds.
`--display-only-fail-level-offenses`	Only output offense messages at the specified `--fail-level` or above.
`--display-only-correctable`	Only output correctable offense messages.
`--display-only-safe-correctable`	Only output safe correctable offense messages.
`--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, autocorrected offenses are ignored. Use `A` or `autocorrect` if you’d like any autocorrectable offense to trigger failure, regardless of severity.
`--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.
`--ignore-unrecognized-cops`	Ignore unrecognized cops or departments in the config.
`--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-only-exclude`	Generate only `Exclude` parameters and not `Max` when running `--auto-gen-config`, except if the number of files with offenses is bigger than `exclude-limit`. Default is false
`--[no-]auto-gen-timestamp`	Include the date and time when `--auto-gen-config` was run in the config file it generates. Default is true.
`--[no-]offense-counts`	Show offense counts in config file generated by `--auto-gen-config`. Default is true.
`--only`	Run only the specified cop(s) and/or cops in the specified departments.
`-o/--out`	Write output to a file instead of STDOUT.
`--[no-]parallel`	Use available CPUs to execute inspection in parallel. Default is parallel.
`--raise-cop-error`	Raise cop-related errors with cause and location. This is used to prevent cops from failing silently. Default is false.
`-r/--require`	Require Ruby file (see Loading Extensions).
`--regenerate-todo`	Regenerate the TODO list using the same options as the last time it was generated with `--auto-gen-config` (generation options can be overridden).
`--safe`	Run only safe cops.
`--safe-auto-correct`	Deprecated alias of `-a/--autocorrect`.
`--show-cops`	Shows available cops and their configuration.
`--show-docs-url`	Shows urls for documentation pages of supplied cops.
`--stderr`	Write all output to stderr except for the autocorrected source. This is especially useful when combined with `--autocorrect` and `--stdin`.
`-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.
`--editor-mode`	Optimize real-time feedback in editors, adjusting behaviors for editing experience. Editors that run RuboCop directly (e.g., by shelling out) encounter the same issues as with `--lsp`. This option is designed for such editors.
`-S/--display-style-guide`	Display style guide URLs in offense messages.
`-x/--fix-layout`	Autocorrect 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.

-a/--autocorrect

Autocorrect offenses (only when it’s safe). See Autocorrect.

--auto-correct

Deprecated alias of -a/--autocorrect.

-A/--autocorrect-all

Autocorrect offenses (safe and unsafe). See Autocorrect.

--auto-correct-all

Deprecated alias of -A/--autocorrect-all.

--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 --autocorrect 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-time

Display elapsed time in seconds.

--display-only-fail-level-offenses

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

--display-only-correctable

Only output correctable offense messages.

--display-only-safe-correctable

Only output safe correctable offense messages.

--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, autocorrected offenses are ignored. Use A or autocorrect if you’d like any autocorrectable offense to trigger failure, regardless of severity.

--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.

--ignore-unrecognized-cops

Ignore unrecognized cops or departments in the config.

--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-only-exclude

Generate only Exclude parameters and not Max when running --auto-gen-config, except if the number of files with offenses is bigger than exclude-limit. Default is false

--[no-]auto-gen-timestamp

Include the date and time when --auto-gen-config was run in the config file it generates. Default is true.

--[no-]offense-counts

Show offense counts in config file generated by --auto-gen-config. Default is true.

--only

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

-o/--out

Write output to a file instead of STDOUT.

--[no-]parallel

Use available CPUs to execute inspection in parallel. Default is parallel.

--raise-cop-error

Raise cop-related errors with cause and location. This is used to prevent cops from failing silently. Default is false.

-r/--require

Require Ruby file (see Loading Extensions).

--regenerate-todo

Regenerate the TODO list using the same options as the last time it was generated with --auto-gen-config (generation options can be overridden).

--safe

Run only safe cops.

--safe-auto-correct

Deprecated alias of -a/--autocorrect.

--show-cops

Shows available cops and their configuration.

--show-docs-url

Shows urls for documentation pages of supplied cops.

--stderr

Write all output to stderr except for the autocorrected source. This is especially useful when combined with --autocorrect and --stdin.

-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.

--editor-mode

Optimize real-time feedback in editors, adjusting behaviors for editing experience. Editors that run RuboCop directly (e.g., by shelling out) encounter the same issues as with --lsp. This option is designed for such editors.

-S/--display-style-guide

Display style guide URLs in offense messages.

-x/--fix-layout

Autocorrect 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):

Explicit command-line options
Options from RUBOCOP_OPTS environment variable
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 --autocorrect, offenses which are autocorrected 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 autocorrected.)
2 if RuboCop terminates abnormally due to invalid configuration, invalid CLI options, or an internal error.

Parallel Processing

RuboCop enables the --parallel option by default. This option allows for parallel processing using the number of available CPUs based on Etc.nprocessors. If the default number of parallel processes causes excessive consumption of CPU and memory, you can adjust the number of parallel processes using the $PARALLEL_PROCESSOR_COUNT environment variable. Here is how to set the parallel processing to use two CPUs:

$ PARALLEL_PROCESSOR_COUNT=2 bundle exec rubocop

Alternatively, specifying --no-parallel will disable parallel processing.