What Are Linters? (+ Why Your Team Should Use Them)

A linter is a tool that analyzes your source code to catch errors, style violations, and potential bugs before you ever run it. If that sounds simple, it's because the core idea hasn't changed much since 1978.

The name comes from the original "lint" tool, created by computer scientist Stephen C. Johnson at Bell Labs. It analyzed C source code to catch problems, much like a clothes dryer's lint trap catches the tiny bits of fiber shed during the drying process. (And yes, it is related to the lint roller you use on cat hair.)

Linters are the most common form of static code analysis: the practice of analyzing code before you execute it, catching mistakes in the early stages of development rather than after deployment.

By automating that analysis, linters save developers time and help make sure code defects don't slip through to later development cycles, where they become far more expensive and time-consuming to fix.

In this article:

• What is a linter?
• How does a linter work?
• The benefits of linting
• Potential pitfalls of using linters
• Popular linters to consider
• How to add linters to your workflow
• Frequently Asked Questions

What is a linter?

Linters are the most widely adopted type of static code analysis tools, with options available for virtually every major language. Linters help developers analyze their source code to uncover potential issues, like coding errors, stylistic inconsistencies, bugs, violations of coding standards, and potential security vulnerabilities.

And while Johnson’s original tool only checked C code, you can find a linter today for just about any programming language. Modern linters also detect a much more comprehensive range of issues than their predecessors, extending beyond error detection to help improve code readability, maintainability, and overall consistency across a codebase.

Early linters were created to optimize the compiling process, transforming human-readable source code into computer-executable machine code. Today, linters are arguably even more valuable for analyzing interpreted (or dynamic) languages (like Ruby, Python, and JavaScript) that don’t have a compiler to detect errors during development.

How does a linter work?

Like most static code analysis tools, a linter compares the code you’ve created with a predefined set of coding rules. If your code deviates from these rules in any way, the linter notifies you of these issues.

Linters often integrate with other developer tools, like text editors and integrated development environments (IDEs), to streamline development workflows.

The rules your code is being checked against depend on the linter you’re using and can often be customized and tailored to fit the specifications of your project.

1. Parsing: The tool breaks source code into individual tokens (keywords, operators, etc.).
2. Modeling: It builds an Abstract Syntax Tree (AST) to map code structures and hierarchies.
3. Analysis: The linter compares the AST against defined rules and standards.
4. Reporting: It identifies violations, providing the specific line number and the nature of the rule broken.

Some linters will even offer recommendations on how to fix detected issues.

What do linters check for?

Modern linters go well beyond compiler support, checking for syntax errors, insecure patterns, style violations, and code smells across dozens of rules.

• Syntax errors:When working with interpreted languages like JavaScript, linters enable you to verify syntax errors early in the development cycle. Teams often use pre-commit hooks with linters to prevent developers from pushing their code if syntax errors are detected.
• Coding standards: Linters help enforce the coding rules and conventions that are important to your team, providing code style consistency. Consistent style makes your code easier to understand. Some linters come with predefined rules, while others are highly flexible and enable you to customize what coding standards and styles your code should adhere to.

• Code smells: Linters uncover signs that point to potential problems with your code. When you’re able to automate the process of detecting code smells, you can focus your attention on investigating them, finding the issue, and providing a resolution for it.
• Security issues: Data breaches resulting from undetected security problems can be catastrophic for software companies financially and in terms of reputation. Linters can flag secrets, insecure dependencies, and other insecure patterns in your code.
• Dead code: Linters can identify unused variables, unreachable code paths, and redundant logic that adds complexity without adding value. Removing dead code keeps your codebase leaner and easier to maintain over time.
• License compliance: Some linters and analysis tools can flag license incompatibilities in your dependencies, helping teams catch compliance issues before they become legal or audit problems.

The benefits of linting

Linters help you automate and simplify the process of maintaining code quality, providing the following potential benefits and advantages to development teams:

• Decreasing the number of errors that make it to production. When you’re able to diagnose and remedy code issues early in the development cycle, the chances of deploying problematic code are reduced considerably.
• Creating consistent, readable, and maintainable code. Linters enforce the coding rules and standards that provide the pillars and coding principles that lead to cleaner code that’s easier to work with and build upon.
• Measuring code quality objectively. When there’s complete transparency regarding what code rules and standards are essential to your team, determining the quality of code produced is no longer a subjective process. Linters provide the criteria needed for measurable code assessment.
• Improving security. Linters can be set up to uphold specific security standards related to your industry and other regulatory requirements that are important to your organization, like OWASP Top 10. When your team can catch and fix security issues on time, you’re decreasing the chances of your code being infiltrated by malicious attacks like SQL injection, cross-site scripting, insecure authentication, XML external entities, buffer overflows, and more.
• Educating developers. Linters can be used to help less-experienced members of your team learn and understand coding best practices. Explaining concerns and issues in code helps developers grasp specific language complexities and nuances.
• Saving money. Linters are central to the shift-left principle: catch issues at the point of writing, not after they've propagated through the stack. When issues and errors are caught early, teams reduce the impact of errors and time needed to debug later. Catching security vulnerabilities at the linting stage is significantly cheaper than remediating them post-breach.
• Enhancing team collaboration. Deciding on what linters to use and what types of issues to check for allows your team to define code quality expectations. When code quality expectations and objectives are documented, communicated, and fully transparent, collaboration throughout the entire development team becomes easier.

Potential pitfalls of using linters

The case for linting is straightforward: catching errors early costs less than fixing them later.

Constant notifications can also result in warning fatigue, which could turn into a habit of ignoring what the linter is telling you entirely.

Some developers also worry about false negatives—code segments being flagged as problematic when they really aren’t. This can lead to developers wasting time chasing down and evaluating problems that don’t actually exist.

As already mentioned, linting tools are better suited for developers using interpreted languages. If you are coding in a compiled language, linters might be less helpful since you’re already relying on a compiler to spot potential issues.

Popular linters to consider

Some commonly used linters, organized by language and use case:

Linters for Code Formatting and Conventions

• JavaScript: ESLint, StandardJS, JSLint, and Prettier
• Ruby: Rubocop
• C#: StyleCop
• CSS: CSS Lint
• Python: Black, PyLint
• Go: Gofmt
• Swift: Swiftlint
• Rust: rustfmt
• Elixir: Credo

Linters for Security

• Semgrep for several languages (C, C#, Go, JavaScript, Java, Python, TypeScript, Ruby, and more)
• Go: Gosec
• Python: Bandit
• Ruby: Brakeman

Analysis Tools is a great resource for researching and finding the perfect linters for your company that lists hundreds of tools and allows you to filter them by programming language, license, pricing, and more.

How to add linters to your workflow

In a well-established software development workflow, it's customary to leverage multiple linters to guarantee a thorough examination of potential issues within the codebase.

Codacy runs more than 50 different linters under one platform, identifying code defects, security vulnerabilities, code duplication, and complexity concerns through static code analysis. Instead of stitching together individual tools, you get a single dashboard with unified results across your repositories.

To get started with Codacy, sign up for a free trial today or book a demo to see how consolidating your linting and code quality toolchain works in practice.

FAQs

What Is the Purpose of a Linter?

A linter's core purpose is to catch code problems before they reach production. It scans your source code against a defined set of rules and flags errors, style violations, security issues, and potential bugs. Catching these problems early is faster and cheaper than fixing them later in the development cycle.

Do You Need a Linter?

For most engineering teams, yes. If you work with interpreted languages like JavaScript, Python, or Ruby, a linter is especially useful because there is no compiler to catch errors during development. Even for compiled languages, linters catch style inconsistencies and security issues that compilers miss. The main consideration is configuring your linter well so it surfaces real problems without overwhelming your team with false positives.

What Is a Linter in Python?

A Python linter is a tool that analyzes your Python code for errors, style problems, and potential bugs before you run it. Common options include:

PyLint checks for coding errors and enforces style standards across your codebase. Black focuses on automatic code formatting so your team's Python code stays consistent. Bandit scans for common security vulnerabilities specific to Python projects.

Most Python projects use at least one of these tools as part of their standard development workflow.