Formatters

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.

Additionally 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 --out 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
..W.C....C..CWCW.C...WC.CC

Offenses:

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

...

26 files inspected, 46 offenses detected

Auto Gen Formatter

Behaves like Progress Formatter except that it will not show any offenses.

$ rubocop --format autogenconf
Inspecting 26 files
..W.C....C..CWCW.C...WC.CC

26 files inspected, 46 offenses detected

Clang Style Formatter

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

$ rubocop --format clang test.rb
Inspecting 1 file
W

Offenses:

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/DefEndAlignment: end at 4, 4 is not aligned with if at 2, 2
    end
    ^^^

1 file inspected, 4 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: Naming/MethodName: Use snake_case for method names.
def badName
    ^^^^^^^
lib/bar.rb:13:14: W: Lint/DeprecatedClassMethods: File.exists? is deprecated in favor of File.exist?.
        File.exists?(path)
             ^^^^^^^
 22/53 files |======== 43 ========>                           |  ETA: 00:00:02

Pacman Style Formatter

The pacman style formatter prints a PACDOT per every file to be analyzed. Pacman will "eat" one PACDOT per file when no offense is detected. Otherwise it will print a Ghost. This is inspired by the Pacman formatter for RSpec.

$ rubocop --format pacman
Eating 31 files
src/foo.rb:1:1: C: Style/FrozenStringLiteralComment: Missing magic comment # frozen_string_literal: true.
src/bar.rb:14:15: C: Style/MutableConstant: Freeze mutable objects assigned to constants.
      GHOST = 'ᗣ'
              ^^^
....ᗣ...ᗣ...ᗧ••••••••••••••••••
31 examples, 2 failures

Emacs Style Formatter

Machine-parsable

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: Naming/MethodName: Use snake_case for method names.
/Users/bozhidar/projects/test.rb:2:3: C: Style/IfUnlessModifier: 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: Layout/DefEndAlignment: 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:  5: Naming/MethodName: Use snake_case for method names.
C:  2:  3: Style/GuardClause: Use a guard clause instead of wrapping the code inside a conditional expression.
C:  2:  3: Style/IfUnlessModifier: Favor modifier if usage when having a single-line body. Another good alternative is the usage of control flow &&/||.
W:  4:  5: Layout/DefEndAlignment: end at 4, 4 is not aligned with if at 2, 2

1 file inspected, 4 offenses detected

Quiet Formatter

Behaves like Simple Formatter if there are offenses. Completely quiet otherwise:

$ rubocop --format quiet

File List Formatter

Machine-parsable

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

Machine-parsable

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": "1.12.0",
    "ruby_engine": "ruby",
    "ruby_version": "3.0.0",
    "ruby_patchlevel": "0",
    "ruby_platform": "x86_64-darwin19"
  },
  "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
  }
}

JUnit Style Formatter

Machine-parsable

The junit style formatter provides the JUnit formatting. This formatter is based on the rubocop-junit-formatter gem.

$ rubocop --format junit
<?xml version='1.0'?>
<testsuites>
  <testsuite name='rubocop'>
    <testcase classname='example' name='Style/FrozenStringLiteralComment'>
      <failure type='Style/FrozenStringLiteralComment' message='Style/FrozenStringLiteralComment: Missing frozen string literal comment.'>
        /tmp/src/example.rb:1:1
      </failure>
    </testcase>
    <testcase classname='example' name='Naming/MethodName'>
      <failure type='Naming/MethodName' message='Naming/MethodName: Use snake_case for method names.'>
        /tmp/src/example.rb:1:5
      </failure>
    </testcase>
    <testcase classname='example' name='Lint/DeprecatedClassMethods'>
      <failure type='Lint/DeprecatedClassMethods' message='Lint/DeprecatedClassMethods: `File.exists?` is deprecated in favor of `File.exist?`.'>
        /tmp/src/example.rb:2:8
      </failure>
    </testcase>
  </testsuite>
</testsuites>

The junit style formatter is very useful for continuous integration systems such as Jenkins, most of which support junit formatting when parsing test results. A typical invocation in this type of scenario might look like:

$ rubocop --format junit --out test-reports/junit.xml

Since there is one XML node for each cop for each file, the size of the resulting XML can get quite large. If it is too large for you, you can restrict the output to just failures by adding the --display-only-failed option.

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

36   Layout/LineLength
18   Style/StringLiterals
13   Style/Documentation
10   Style/ExpandPathArguments
8    Style/EmptyMethod
6    Layout/IndentationConsistency
4    Lint/SuppressedException
3    Layout/EmptyLinesAroundAccessModifier
2    Layout/ExtraSpacing
1    Layout/AccessModifierIndentation
1    Style/ClassAndModuleChildren
--
102  Total

Worst Offenders Formatter

Similar to the Offense Count formatter, but lists the files which need the most attention:

$ rubocop --format worst

89  this/file/is/really/bad.rb
2   much/better.rb
--
91  Total

HTML Formatter

Useful for CI environments. It will create an HTML report like this.

$ rubocop --format html -o rubocop.html

TAP Formatter

Machine-parsable

Useful for CI environments, it will format report following the Test Anything Protocol.

$ rubocop --format tap
1..3
not ok 1 - lib/rubocop.rb
# lib/rubocop.rb:2:3: C: foo
# This is line 2.
#   ^
ok 2 - spec/spec_helper.rb
not ok 3 - exe/rubocop
# exe/rubocop:5:2: E: bar
# This is line 5.
#  ^
# exe/rubocop:6:1: C: foo
# This is line 6.
# ^

3 files inspected, 3 offenses detected

GitHub Actions Formatter

Useful for GitHub Actions. Formats offenses as workflow commands to create annotations in GitHub UI.

The formatter uses fail_level to determine which GitHub level to use for each annotation. By default, all RuboCop severities are errors. If fail level is set and severity is below fail level, a warning will be created instead.

$ rubocop --format github

::error file=lib/foo.rb,line=6,col=5::Style/Documentation: Missing top-level class documentation comment.