The rubocop binary has a few interesting flags:

• rubocop (using the --parallel default ) scans the current repository for linting issues while using multiple CPU cores
• rubocop -a (or --autocorrect) safely corrects most offenses while doing a sequential scan
• rubocop -A (or --autocorrect-all) also tries to correct unsafe suggestions

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Autocorrection takes significantly longer on large projects because of the sequential nature.
To speed things up, you can use the following alias. It first checks in parallel if any files need to be corrected, then passes them to the autocorrection command:

alias fixcops="bundle exec rubocop -p | grep -P \"\A[^:]+(?=:\d+:\d+: )\" --only-matching | xargs --no-run-if-empty bundle exec rubocop -a"

• bundle exec rubocop -p quickly scans for issues, listing each affected file
• grep -P \"\A[^:]+(?=:\d+:\d+: )\" --only-matching searches for those file names
• xargs --no-run-if-empty bundle exec rubocop -a only runs autocorrection on selected files if there are any