In auto-correct mode, RuboCop will try to automatically fix offenses:

$ rubocop -A
# or
$ rubocop --auto-correct-all

There are a couple of things to keep in mind about auto-correct:

  • For some offenses, it is not possible to implement automatic correction.

  • Some automatic corrections that are possible have not been implemented yet.

  • Some automatic corrections might change (slightly) the semantics of the code, meaning they’d produce code that’s mostly equivalent to the original code, but not 100% equivalent. We call such auto-correct behaviour "unsafe".

You should always run your test suite after using the auto-correct functionality.

Safe auto-correct

$ rubocop -a
# or
$ rubocop --auto-correct

In RuboCop 0.60, we began to annotate cops as Safe or not safe. The definition of safety is that the cop doesn’t generate false positives. On top of that there’s SafeAutoCorrect that might be set to false in cases where only the auto-correct performed by a cop is unsafe, but that the offense detection logic is safe. To sum it up:

  • Safe (true/false) - indicates whether the cop can yield false positives (by design) or not.

  • SafeAutoCorrect (true/false) - indicates whether the auto-correct a cop does is safe (equivalent) by design. If a cop is unsafe its auto-correct automatically becomes unsafe as well.

If a cop or its auto-correct is annotated as "not safe", it will be omitted when using --auto-correct.

Currently there might still be cops that aren’t marked as unsafe or with unsafe auto-correct. Eventually, the safety of each cop will be specified in the default configuration.

Example of Unsafe Cop

class Miner
  def dig(how_deep)
    # ...
end # => Style/SingleArgumentDig
                  # => Use[] 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.rb:
str = 'hello' # => Missing magic comment `# frozen_string_literal: true`
str << 'world'

# auto-corrects 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 auto-correction is not; some string literals need to be prefixed with + to avoid having them frozen.

To run all auto-corrections (safe and unsafe):

$ rubocop -A
# or
$ rubocop --auto-correct-all

It is recommended to be even more vigilant when using this option and review carefully the changes.

Generating comments

$ rubocop --auto-correct --disable-uncorrectable


$ rubocop --auto-correct-all --disable-uncorrectable

You can add the flag --disable-uncorrectable, which will generate # rubocop:todo comments in the code to stop the reporting of offenses that could not be corrected automatically.