How to use the command 'rubocop' (with examples)

RuboCop is a tool that analyzes Ruby code and enforces a standard set of rules known as cops. These cops help to ensure code quality, maintainability, and adherence to coding best practices. In this article, we will explore various use cases of the rubocop command.

Use case 1: Check all files in the current directory (including subdirectories)

Code:

rubocop

Motivation: By running rubocop without any arguments, it will check all files in the current directory (including subdirectories) and provide feedback on any violations of the configured linting rules. This is useful when you want to ensure code consistency across your entire project.

Explanation:

• rubocop: Executes the RuboCop command.

Example output:

Inspecting 50 files

..C.C.C..C...C..................

Offenses:

file1.rb:2:17: C: Layout/IndentationConsistency: Inconsistent indentation detected.
file3.rb:8:5: C: Naming/MethodName: Use snake_case for method names.

50 files inspected, 2 offenses detected

Use case 2: Check one or more specific files or directories

Code:

rubocop path/to/file path/to/directory

Motivation: Sometimes, you may only want to run RuboCop on specific files or directories rather than the entire project. This can be useful when you want to focus on a particular area of the codebase.

Explanation:

• rubocop: Executes the RuboCop command.
• path/to/file: Specifies the path to a specific file to be checked.
• path/to/directory: Specifies the path to a specific directory to be checked.

Example output:

Inspecting 2 files

...

Offenses:

path/to/file1.rb:12:3: C: Style/GuardClause: Use a guard clause instead of wrapping the code inside a conditional expression.
path/to/file2.rb:5:15: C: Naming/VariableName: Use snake_case for variable names.

2 files inspected, 2 offenses detected

Use case 3: Write output to file

Code:

rubocop --out path/to/file

Motivation: Sometimes, you may want to save the RuboCop output to a file for further reference or analysis. This can be useful when you need to review the violations at a later time or share them with your team.

Explanation:

• rubocop: Executes the RuboCop command.
• --out path/to/file: Specifies the path to the output file.

Example output:

Inspecting 3 files

...

Offenses:

file1.rb:2:17: C: Layout/IndentationConsistency: Inconsistent indentation detected.
file2.rb:5:15: C: Naming/VariableName: Use snake_case for variable names.

3 files inspected, 2 offenses detected

A file named path/to/file will be created with the RuboCop output.

Use case 4: View list of cops (linter rules)

Code:

rubocop --show-cops

Motivation: Understanding the available cops (linter rules) and their configurations is helpful when customizing your RuboCop setup. By running --show-cops, you can see a complete list of the available cops and their associated information.

Explanation:

• rubocop: Executes the RuboCop command.
• --show-cops: Shows a list of all the cops.

Example output:

Layout/AlignArray:
  Description: Checks that array elements are aligned.
  StyleGuide: '[RuboCop - Layout/AlignArray](https://docs.rubocop.org/rubocop/cops_layout.html#layoutalignarray)'
  Enabled: true
  SafeAutoCorrect: false
  ...

Metrics/MethodLength:
  Description: Checks the length of methods.
  StyleGuide: '[RuboCop - Metrics/MethodLength](https://docs.rubocop.org/rubocop-cops_metrics.html#metricsmethodlength)'
  Enabled: true
  SafeAutoCorrect: true

203 cops found

This shows a comprehensive list of cops along with their descriptions, style guides, and other relevant information.

Use case 5: Exclude a cop

Code:

rubocop --except cop_1 cop_2

Motivation: There may be instances where you want to exclude specific cops (linter rules) from being enforced by RuboCop. This can be useful if you disagree with a particular rule or for special cases where certain rules do not apply.

Explanation:

• rubocop: Executes the RuboCop command.
• --except cop_1 cop_2: Excludes the specified cops from the linting process.

Example output:

Inspecting 2 files

..

Offenses:

file.rb:24:1: W: Layout/IndentationWidth: Use 2 (not 4) spaces for indentation.
file.rb:32:5: W: Style/FrozenStringLiteralComment: Missing frozen string literal comment.

2 files inspected, 0 offenses detected

In the example above, the Layout/IndentationWidth and Style/FrozenStringLiteralComment cops were excluded, resulting in no offenses being detected for those rules.

Use case 6: Run only specified cops

Code:

rubocop --only cop_1 cop_2

Motivation: You might want to focus on specific cops during the linting process instead of checking all cops. This can be useful when you want to ensure adherence to only a select set of rules or when you need to prioritize certain rules.

Explanation:

• rubocop: Executes the RuboCop command.
• --only cop_1 cop_2: Runs only the specified cops during the linting process.

Example output:

Inspecting 3 files

...

Offenses:

file1.rb:12:3: C: Style/GuardClause: Use a guard clause instead of wrapping the code inside a conditional expression.
file2.rb:5:15: C: Naming/VariableName: Use snake_case for variable names.

3 files inspected, 2 offenses detected

In this example, only the Style/GuardClause and Naming/VariableName cops were checked, resulting in offenses being detected specifically for those rules.

Use case 7: Auto-correct files (experimental)

Code:

rubocop --auto-correct

Motivation: RuboCop provides an experimental feature called auto-correction, which can automatically fix some linter offenses. However, it’s important to note that auto-correction may not handle all cases perfectly and should be used with caution. This feature can save time and effort by automatically correcting violations where possible.

Explanation:

• rubocop: Executes the RuboCop command.
• --auto-correct: Enables the experimental auto-correction feature.

Example output:

Inspecting 3 files

...

Offenses:

file1.rb:2:17: C: Layout/IndentationConsistency: Inconsistent indentation detected.
file2.rb:5:15: C: Naming/VariableName: Use snake_case for variable names.

3 files inspected, 2 offenses detected, 2 offenses corrected

In this example, RuboCop identified two offenses and automatically corrected them. The output indicates that two offenses were corrected. It’s worth noting that not all offenses can be auto-corrected, so manual intervention may still be required.

Conclusion:

The rubocop command is a powerful tool for linting Ruby code. It helps maintain consistent code quality and adherence to best practices. By utilizing its various options and arguments, you can customize the linting process to suit your specific needs. Whether it’s checking all files, focusing on specific rules, or even auto-correcting some offenses, RuboCop empowers developers with the ability to improve code quality throughout their projects.