Autocorrect

In autocorrect mode, RuboCop will try to automatically fix offenses.

There are three autocorrect modes:

Flag Description

-a / --autocorrect

Safe corrections only — won’t change code semantics.

-A / --autocorrect-all

All corrections, including unsafe ones that may change semantics.

-x / --fix-layout

Layout (formatting) corrections only — never changes logic.
Always review the diff and run your test suite after autocorrecting, especially when using -A.

Safe vs. unsafe

RuboCop distinguishes between safe and unsafe cops and autocorrections:

  • Safe (true/false) — indicates whether the cop can yield false positives by design.

  • SafeAutoCorrect (true/false) — indicates whether the autocorrection preserves code semantics. If a cop itself is unsafe, its autocorrect is automatically considered unsafe as well.

When you run rubocop -a, cops and autocorrections marked as unsafe are skipped. When you run rubocop -A, everything is applied.

Some cops may not yet be annotated for safety. Eventually, the safety of each cop will be specified in the default configuration.

Example: unsafe cop

class Miner
  def dig(how_deep)
    # ...
  end
end


Miner.new.dig(42) # => Style/SingleArgumentDig
                  # => Use Miner.new[] instead of dig

This is the wrong diagnostic; this (contrived) use of dig is not an issue, and there might not be an alternative. This cop is marked as Safe: false.

Example: safe cop with unsafe autocorrect

# example.rb:
str = 'hello' # => Missing magic comment `# frozen_string_literal: true`
str << 'world'

# autocorrects to:
# frozen_string_literal: true

str = 'hello'
str << 'world' # => now fails because `str` is frozen

# must be manually corrected to:
# frozen_string_literal: true

str = +'hello' # => We want an unfrozen string literal here...
str << 'world' # => ok

This diagnostic is valid since the magic comment is indeed missing (thus Safe: true), but the autocorrection is not; some string literals need to be prefixed with + to avoid having them frozen.

Disabling uncorrectable offenses

$ rubocop --autocorrect --disable-uncorrectable

The --disable-uncorrectable flag generates # rubocop:todo comments in the code to suppress reporting of offenses that could not be corrected automatically. This is useful for gradually adopting new cops.

Configuring autocorrect per cop

Individual cops can have their autocorrect behavior configured in .rubocop.yml. See AutoCorrect for the available settings: always, contextual, and disabled.