SwiftLint

A tool to enforce Swift style and conventions, loosely based on the now archived GitHub Swift Style Guide. SwiftLint enforces the style guide rules that are generally accepted by the Swift community. These rules are well described in popular style guides like Kodeco’s Swift Style Guide.

SwiftLint hooks into Clang and SourceKit to use the AST representation of your source files for more accurate results.

This project adheres to the Contributor Covenant Code of Conduct. By participating, you are expected to uphold this code. Please report unacceptable behavior to info@realm.io.

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Video Introduction

To get a high-level overview of SwiftLint, we encourage you to watch this presentation recorded January 9th, 2017 by JP Simard (transcript provided):

Swift Package Manager

SwiftLint can be used as a command plugin or a build tool plugin.

Add

.package(url: "https://github.com/SimplyDanny/SwiftLintPlugins", from: "<version>")

to your Package.swift file to consume the latest release of SwiftLint automatically or pin the dependency to a specific version:

.package(url: "https://github.com/SimplyDanny/SwiftLintPlugins", exact: "<version>")

Therein, replace <version> with the desired minimum or exact version.

[!NOTE] Consuming the plugins directly from the SwiftLint repository comes with several drawbacks. To avoid them and reduce the overhead imposed, it’s highly recommended to consume the plugins from the dedicated SwiftLintPlugins repository, even though plugins from the SwiftLint repository are also absolutely functional. If the plugins from SwiftLint are preferred, just use the URL https://github.com/realm/SwiftLint in the package declarations above.

However, SwiftLintPlugins facilitates plugin adoption massively. It lists some of the reasons that drive the plugins as provided by SwiftLint itself very troublesome. Since the plugin code and the releases are kept in sync, there is no difference in functionality between the two, but you spare yourself a lot of time and trouble using the dedicated plugins repository.

This document assumes you’re relying on SwiftLintPlugins.

Xcode Package Dependency

Use the following link to add SwiftLint as a Package Dependency to an Xcode project:

https://github.com/SimplyDanny/SwiftLintPlugins

Homebrew

brew install swiftlint

CocoaPods

Add the following to your Podfile:

pod 'SwiftLint'

This will download the SwiftLint binaries and dependencies in Pods/ during your next pod install execution and will allow you to invoke it via ${PODS_ROOT}/SwiftLint/swiftlint in your Script Build Phases.

Installing via Cocoapods also enables pinning to a specific version of SwiftLint rather than simply the latest (which is the case with Homebrew).

Note that this will add the SwiftLint binaries, its dependencies’ binaries, and the Swift binary library distribution to the Pods/ directory, so checking in this directory to SCM such as Git is discouraged.

Mint

mint install realm/SwiftLint

Bazel

Put this in your MODULE.bazel:

bazel_dep(name = "swiftlint", version = "0.52.4", repo_name = "SwiftLint")

Or put this in your WORKSPACE:

WORKSPACE

```bzl load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive") http_archive( name = "build_bazel_rules_apple", sha256 = "390841dd5f8a85fc25776684f4793d56e21b098dfd7243cd145b9831e6ef8be6", url = "https://github.com/bazelbuild/rules_apple/releases/download/2.4.1/rules_apple.2.4.1.tar.gz", ) load( "@build_bazel_rules_apple//apple:repositories.bzl", "apple_rules_dependencies", ) apple_rules_dependencies() load( "@build_bazel_rules_swift//swift:repositories.bzl", "swift_rules_dependencies", ) swift_rules_dependencies() load( "@build_bazel_rules_swift//swift:extras.bzl", "swift_rules_extra_dependencies", ) swift_rules_extra_dependencies() http_archive( name = "SwiftLint", sha256 = "c6ea58b9c72082cdc1ada4a2d48273ecc355896ed72204cedcc586b6ccb8aca6", url = "https://github.com/realm/SwiftLint/releases/download/0.52.4/bazel.tar.gz", ) load("@SwiftLint//bazel:repos.bzl", "swiftlint_repos") swiftlint_repos() load("@SwiftLint//bazel:deps.bzl", "swiftlint_deps") swiftlint_deps() ```

Then you can run SwiftLint in the current directory with this command:

bazel run -c opt @SwiftLint//:swiftlint

Pre-Built Package

Download SwiftLint.pkg from the latest GitHub release and run it.

From Source

Make sure the build tool Bazel and a recent Swift toolchain are installed and all tools are discoverable in your PATH.

To build SwiftLint, clone this repository and run make install.

Setup

[!IMPORTANT] While it may seem intuitive to run SwiftLint before compiling Swift source files to exit a build early when there are lint violations, it is important to understand that SwiftLint is designed to analyze valid source code that is compilable. Non-compiling code can very easily lead to unexpected and confusing results, especially when executing with --fix/--autocorrect command line arguments.

Build Tool Plugins

SwiftLint can be used as a build tool plugin for both Swift Package projects and Xcode projects.

The build tool plugin determines the SwiftLint working directory by locating the topmost config file within the package/project directory. If a config file is not found therein, the package/project directory is used as the working directory.

The plugin throws an error when it is unable to resolve the SwiftLint working directory. For example, this will occur in Xcode projects where the target’s Swift files are not located within the project directory.

To maximize compatibility with the plugin, avoid project structures that require the use of the --config option.

Swift Package Projects

[!NOTE] Requires installing via Swift Package Manager.

Build tool plugins run when building each target. When a project has multiple targets, the plugin must be added to the desired targets individually.

To do this, add the plugin to the target(s) to be linted as follows:

.target(
    ...
    plugins: [.plugin(name: "SwiftLintBuildToolPlugin", package: "SwiftLintPlugins")]
),

Swift Package Command Plugin

[!NOTE] Requires installing via Swift Package Manager.

The command plugin enables running SwiftLint from the command line as follows:

swift package plugin swiftlint

Xcode Projects

[!NOTE] Requires installing via Xcode Package Dependency.

Build tool plugins run as a build phase of each target. When a project has multiple targets, the plugin must be added to the desired targets individually.

To do this, add the SwiftLintBuildToolPlugin to the Run Build Tool Plug-ins phase of the Build Phases for the target(s) to be linted.

[!TIP] When using the plugin for the first time, be sure to trust and enable it when prompted. If a macros build warning exists, select it to trust and enable the macros as well.

For unattended use (e.g. on CI), package plugin and macro validations can be disabled with either of the following:

• Using xcodebuild options:

  -skipPackagePluginValidation
  -skipMacroValidation

• Setting Xcode defaults:

  defaults write com.apple.dt.Xcode IDESkipPackagePluginFingerprintValidatation -bool YES
  defaults write com.apple.dt.Xcode IDESkipMacroFingerprintValidation -bool YES

[!IMPORTANT] The unattended use options bypass Xcode’s validation dialogs and implicitly trust all plugins and macros, which has security implications.

Consideration for Xcode 15.0

Xcode 15 made a significant change by setting the default value of the ENABLE_USER_SCRIPT_SANDBOXING build setting from NO to YES. As a result, SwiftLint encounters an error related to missing file permissions, which typically manifests as error: Sandbox: swiftlint(19427) deny(1) file-read-data.

To resolve this issue, it is necessary to manually set the ENABLE_USER_SCRIPT_SANDBOXING setting to NO for the specific target that SwiftLint is being configured for.

Additional Considerations

If you wish to fix violations as well, your script could run swiftlint --fix && swiftlint instead of just swiftlint. This will mean that all correctable violations are fixed while ensuring warnings show up in your project for remaining violations.

If you’ve installed SwiftLint via CocoaPods the script should look like this:

"${PODS_ROOT}/SwiftLint/swiftlint"

Visual Studio Code

To integrate SwiftLint with Visual Studio Code, install the vscode-swiftlint extension from the marketplace.

Fastlane

You can use the official swiftlint fastlane action to run SwiftLint as part of your fastlane process.

swiftlint(
    mode: :lint,                            # SwiftLint mode: :lint (default) or :autocorrect
    executable: "Pods/SwiftLint/swiftlint", # The SwiftLint binary path (optional). Important if you've installed it via CocoaPods
    path: "/path/to/lint",                  # Specify path to lint (optional)
    output_file: "swiftlint.result.json",   # The path of the output file (optional)
    reporter: "json",                       # The custom reporter to use (optional)
    config_file: ".swiftlint-ci.yml",       # The path of the configuration file (optional)
    files: [                                # List of files to process (optional)
        "AppDelegate.swift",
        "path/to/project/Model.swift"
    ],
    ignore_exit_status: true,               # Allow fastlane to continue even if SwiftLint returns a non-zero exit status (Default: false)
    quiet: true,                            # Don't print status logs like 'Linting ' & 'Done linting' (Default: false)
    strict: true                            # Fail on warnings? (Default: false)
)

Working With Multiple Swift Versions

SwiftLint hooks into SourceKit so it continues working even as Swift evolves!

This also keeps SwiftLint lean, as it doesn’t need to ship with a full Swift compiler, it just communicates with the official one you already have installed on your machine.

You should always run SwiftLint with the same toolchain you use to compile your code.

You may want to override SwiftLint’s default Swift toolchain if you have multiple toolchains or Xcodes installed.

Here’s the order in which SwiftLint determines which Swift toolchain to use:

• $XCODE_DEFAULT_TOOLCHAIN_OVERRIDE
• $TOOLCHAIN_DIR or $TOOLCHAINS
• xcrun -find swift
• /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain
• /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain
• ~/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain
• ~/Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain

sourcekitd.framework is expected to be found in the usr/lib/ subdirectory of the value passed in the paths above.

You may also set the TOOLCHAINS environment variable to the reverse-DNS notation that identifies a Swift toolchain version:

TOOLCHAINS=com.apple.dt.toolchain.Swift_2_3 swiftlint --fix

On Linux, SourceKit is expected to be located in /usr/lib/libsourcekitdInProc.so or specified by the LINUX_SOURCEKIT_LIB_PATH environment variable.

Git pre-commit Hook

SwiftLint can be run as a pre-commit hook. Once installed, add this to the .pre-commit-config.yaml in the root of your repository:

repos:
  - repo: https://github.com/realm/SwiftLint
    rev: 0.50.3
    hooks:
      - id: swiftlint

Adjust rev to the SwiftLint version of your choice. pre-commit autoupdate can be used to update to the current version.

SwiftLint can be configured using entry to apply fixes and fail on errors:

-   repo: https://github.com/realm/SwiftLint
    rev: 0.50.3
    hooks:
    -   id: swiftlint
        entry: swiftlint --fix --strict

Opt-In Rules

opt_in_rules are disabled by default (i.e., you have to explicitly enable them in your configuration file).

Guidelines on when to mark a rule as opt-in:

• A rule that can have many false positives (e.g. empty_count)
• A rule that is too slow
• A rule that is not general consensus or is only useful in some cases (e.g. force_unwrapping)

Disable rules in code

Rules can be disabled with a comment inside a source file with the following format:

// swiftlint:disable <rule1> [<rule2> <rule3>...]

The rules will be disabled until the end of the file or until the linter sees a matching enable comment:

// swiftlint:enable <rule1> [<rule2> <rule3>...]

For example:

// swiftlint:disable colon
let noWarning :String = "" // No warning about colons immediately after variable names!
// swiftlint:enable colon
let hasWarning :String = "" // Warning generated about colons immediately after variable names

Including the all keyword will disable all rules until the linter sees a matching enable comment:

// swiftlint:disable all // swiftlint:enable all

For example:

// swiftlint:disable all
let noWarning :String = "" // No warning about colons immediately after variable names!
let i = "" // Also no warning about short identifier names
// swiftlint:enable all
let hasWarning :String = "" // Warning generated about colons immediately after variable names
let y = "" // Warning generated about short identifier names

It’s also possible to modify a disable or enable command by appending :previous, :this or :next for only applying the command to the previous, this (current) or next line respectively.

For example:

// swiftlint:disable:next force_cast
let noWarning = NSNumber() as! Int
let hasWarning = NSNumber() as! Int
let noWarning2 = NSNumber() as! Int // swiftlint:disable:this force_cast
let noWarning3 = NSNumber() as! Int
// swiftlint:disable:previous force_cast

Run swiftlint rules to print a list of all available rules and their identifiers.

Configuration

Configure SwiftLint by adding a .swiftlint.yml file from the directory you’ll run SwiftLint from. The following parameters can be configured:

Rule inclusion:

• disabled_rules: Disable rules from the default enabled set.
• opt_in_rules: Enable rules that are not part of the default set. The special all identifier will enable all opt in linter rules, except the ones listed in disabled_rules.
• only_rules: Only the rules specified in this list will be enabled. Cannot be specified alongside disabled_rules or opt_in_rules.
• analyzer_rules: This is an entirely separate list of rules that are only run by the analyze command. All analyzer rules are opt-in, so this is the only configurable rule list, there are no equivalents for disabled_rules and only_rules. The special all identifier can also be used here to enable all analyzer rules, except the ones listed in disabled_rules.

# By default, SwiftLint uses a set of sensible default rules you can adjust:
disabled_rules: # rule identifiers turned on by default to exclude from running
  - colon
  - comma
  - control_statement
opt_in_rules: # some rules are turned off by default, so you need to opt-in
  - empty_count # find all the available rules by running: `swiftlint rules`

# Alternatively, specify all rules explicitly by uncommenting this option:
# only_rules: # delete `disabled_rules` & `opt_in_rules` if using this
#   - empty_parameters
#   - vertical_whitespace

analyzer_rules: # rules run by `swiftlint analyze`
  - explicit_self

# Case-sensitive paths to include during linting. Directory paths supplied on the
# command line will be ignored.
included: 
  - Sources
excluded: # case-sensitive paths to ignore during linting. Takes precedence over `included`
  - Carthage
  - Pods
  - Sources/ExcludedFolder
  - Sources/ExcludedFile.swift
  - Sources/*/ExcludedFile.swift # exclude files with a wildcard

# If true, SwiftLint will not fail if no lintable files are found.
allow_zero_lintable_files: false

# If true, SwiftLint will treat all warnings as errors.
strict: false

# The path to a baseline file, which will be used to filter out detected violations.
baseline: Baseline.json

# The path to save detected violations to as a new baseline.
write_baseline: Baseline.json

# configurable rules can be customized from this configuration file
# binary rules can set their severity level
force_cast: warning # implicitly
force_try:
  severity: warning # explicitly
# rules that have both warning and error levels, can set just the warning level
# implicitly
line_length: 110
# they can set both implicitly with an array
type_body_length:
  - 300 # warning
  - 400 # error
# or they can set both explicitly
file_length:
  warning: 500
  error: 1200
# naming rules can set warnings/errors for min_length and max_length
# additionally they can set excluded names
type_name:
  min_length: 4 # only warning
  max_length: # warning and error
    warning: 40
    error: 50
  excluded: iPhone # excluded via string
  allowed_symbols: ["_"] # these are allowed in type names
identifier_name:
  min_length: # only min_length
    error: 4 # only error
  excluded: # excluded via string array
    - id
    - URL
    - GlobalAPIKey
reporter: "xcode" # reporter type (xcode, json, csv, checkstyle, codeclimate, junit, html, emoji, sonarqube, markdown, github-actions-logging, summary)

You can also use environment variables in your configuration file, by using ${SOME_VARIABLE} in a string.

Defining Custom Rules

In addition to the rules that the main SwiftLint project ships with, SwiftLint can also run two types of custom rules that you can define yourself in your own projects:

1. Swift Custom Rules

These rules are written the same way as the Swift-based rules that ship with SwiftLint so they’re fast, accurate, can leverage SwiftSyntax, can be unit tested, and more.

Using these requires building SwiftLint with Bazel as described in this video or its associated code in github.com/jpsim/swiftlint-bazel-example.

2. Regex Custom Rules

You can define custom regex-based rules in your configuration file using the following syntax:

custom_rules:
  pirates_beat_ninjas: # rule identifier
    included:
      - ".*\\.swift" # regex that defines paths to include during linting. optional.
    excluded:
      - ".*Test\\.swift" # regex that defines paths to exclude during linting. optional
    name: "Pirates Beat Ninjas" # rule name. optional.
    regex: "([nN]inja)" # matching pattern
    capture_group: 0 # number of regex capture group to highlight the rule violation at. optional.
    match_kinds: # SyntaxKinds to match. optional.
      - comment
      - identifier
    message: "Pirates are better than ninjas." # violation message. optional.
    severity: error # violation severity. optional.
  no_hiding_in_strings:
    regex: "([nN]inja)"
    match_kinds: string

This is what the output would look like:

It is important to note that the regular expression pattern is used with the flags s and m enabled, that is . matches newlines and ^/$ match the start and end of lines, respectively. If you do not want to have . match newlines, for example, the regex can be prepended by (?-s).

You can filter the matches by providing one or more match_kinds, which will reject matches that include syntax kinds that are not present in this list. Here are all the possible syntax kinds:

• argument
• attribute.builtin
• attribute.id
• buildconfig.id
• buildconfig.keyword
• comment
• comment.mark
• comment.url
• doccomment
• doccomment.field
• identifier
• keyword
• number
• objectliteral
• parameter
• placeholder
• string
• string_interpolation_anchor
• typeidentifier

All syntax kinds used in a snippet of Swift code can be extracted asking SourceKitten. For example, sourcekitten syntax --text "struct S {}" delivers

• source.lang.swift.syntaxtype.keyword for the struct keyword and
• source.lang.swift.syntaxtype.identifier for its name S

which match to keyword and identifier in the above list.

If using custom rules in combination with only_rules, make sure to add custom_rules as an item under only_rules.

Unlike Swift custom rules, you can use official SwiftLint builds (e.g. from Homebrew) to run regex custom rules.

Auto-correct

SwiftLint can automatically correct certain violations. Files on disk are overwritten with a corrected version.

Please make sure to have backups of these files before running swiftlint --fix, otherwise important data may be lost.

Standard linting is disabled while correcting because of the high likelihood of violations (or their offsets) being incorrect after modifying a file while applying corrections.

Analyze

The swiftlint analyze command can lint Swift files using the full type-checked AST. The compiler log path containing the clean swiftc build command invocation (incremental builds will fail) must be passed to analyze via the --compiler-log-path flag. e.g. --compiler-log-path /path/to/xcodebuild.log

This can be obtained by

1. Cleaning DerivedData (incremental builds won’t work with analyze)
2. Running xcodebuild -workspace {WORKSPACE}.xcworkspace -scheme {SCHEME} > xcodebuild.log
3. Running swiftlint analyze --compiler-log-path xcodebuild.log

Analyzer rules tend to be considerably slower than lint rules.

Using Multiple Configuration Files

SwiftLint offers a variety of ways to include multiple configuration files. Multiple configuration files get merged into one single configuration that is then applied just as a single configuration file would get applied.

There are quite a lot of use cases where using multiple configuration files could be helpful:

For instance, one could use a team-wide shared SwiftLint configuration while allowing overrides in each project via a child configuration file.

Team-Wide Configuration:

disabled_rules:
- force_cast

Project-Specific Configuration:

opt_in_rules:
- force_cast

Child / Parent Configs (Locally)

You can specify a child_config and / or a parent_config reference within a configuration file. These references should be local paths relative to the folder of the configuration file they are specified in. This even works recursively, as long as there are no cycles and no ambiguities.

A child config is treated as a refinement and thus has a higher priority, while a parent config is considered a base with lower priority in case of conflicts.

Here’s an example, assuming you have the following file structure:

ProjectRoot
    |_ .swiftlint.yml
    |_ .swiftlint_refinement.yml
    |_ Base
        |_ .swiftlint_base.yml

To include both the refinement and the base file, your .swiftlint.yml should look like this:

child_config: .swiftlint_refinement.yml
parent_config: Base/.swiftlint_base.yml

When merging parent and child configs, included and excluded configurations are processed carefully to account for differences in the directory location of the containing configuration files.

Child / Parent Configs (Remote)

Just as you can provide local child_config / parent_config references, instead of referencing local paths, you can just put urls that lead to configuration files. In order for SwiftLint to detect these remote references, they must start with http:// or https://.

The referenced remote configuration files may even recursively reference other remote configuration files, but aren’t allowed to include local references.

Using a remote reference, your .swiftlint.yml could look like this:

parent_config: https://myteamserver.com/our-base-swiftlint-config.yml

Every time you run SwiftLint and have an Internet connection, SwiftLint tries to get a new version of every remote configuration that is referenced. If this request times out, a cached version is used if available. If there is no cached version available, SwiftLint fails – but no worries, a cached version should be there once SwiftLint has run successfully at least once.

If needed, the timeouts for the remote configuration fetching can be specified manually via the configuration file(s) using the remote_timeout / remote_timeout_if_cached specifiers. These values default to 2 / 1 second(s).

Command Line

Instead of just providing one configuration file when running SwiftLint via the command line, you can also pass a hierarchy, where the first configuration is treated as a parent, while the last one is treated as the highest-priority child.

A simple example including just two configuration files looks like this:

swiftlint --config .swiftlint.yml --config .swiftlint_child.yml

Nested Configurations

In addition to a main configuration (the .swiftlint.yml file in the root folder), you can put other configuration files named .swiftlint.yml into the directory structure that then get merged as a child config, but only with an effect for those files that are within the same directory as the config or in a deeper directory where there isn’t another configuration file. In other words: Nested configurations don’t work recursively – there’s a maximum number of one nested configuration per file that may be applied in addition to the main configuration.

.swiftlint.yml files are only considered as a nested configuration if they have not been used to build the main configuration already (e. g. by having been referenced via something like child_config: Folder/.swiftlint.yml). Also, parent_config / child_config specifications of nested configurations are getting ignored because there’s no sense to that.

If one (or more) SwiftLint file(s) are explicitly specified via the --config parameter, that configuration will be treated as an override, no matter whether there exist other .swiftlint.yml files somewhere within the directory. So if you want to use nested configurations, you can’t use the --config parameter.

About

SwiftLint is maintained and funded by Realm Inc. The names and logos for Realm are trademarks of Realm Inc.

We :heart: open source software! See our other open source projects, read our blog, or say hi on twitter (@realm).

Our thanks to MacStadium for providing a Mac Mini to run our performance tests.