.gitignore: Telling Git What to Ignore

WHY IT EXISTS: Git is designed to track every file change in a project directory. However, many files are generated during development—like build artifacts, logs, or editor settings—that shouldn't be part of the project's shared history. A .gitignore file provides a mechanism to tell Git to ignore these files from the start.

THE MENTAL MODEL: Think of .gitignore as a set of rules for Git's "do not track" list. When you run git status or git add ., Git first checks these rules. If a file matches a pattern in .gitignore, Git pretends it doesn't exist, keeping your working directory clean and preventing temporary or machine-specific files from being committed.

HOW IT WORKS: Git processes ignore rules from multiple sources in a specific order of precedence. First, patterns from the command line. Second, it looks for .gitignore files in the current directory and all parent directories up to the project root; patterns in a subdirectory's .gitignore override those in a parent. Third, it checks the repository-specific $GIT_DIR/info/exclude file. Finally, it checks the global ignore file defined by your Git configuration. The last matching pattern for a given file wins.

WHEN TO USE IT: Use a project's .gitignore file for patterns that should be shared with everyone who clones the repository. This includes build output directories (/build), dependency caches (node_modules), and log files (.log). For personal rules that apply to all your projects, like editor-specific files (.vscode/, .swp), use a global ignore file. For project-specific but personal rules, use $GIT_DIR/info/exclude.

WHEN NOT TO USE IT: Do not use .gitignore to ignore a file that is already being tracked by Git. It only works on untracked files. To stop tracking a file you've already committed, you must first remove it from the index using git rm --cached <file>. Also, do not rely on .gitignore as a security mechanism to hide secrets; the files still exist on the filesystem.

ONE CANONICAL EXAMPLE: A typical Node.js project's .gitignore would include lines like: node_modules/ to ignore the entire dependency folder; *.log to ignore any file ending in .log; .env to ignore environment variable files containing secrets; and dist/ to ignore the compiled output directory. A line starting with # is a comment, like # Ignore build artifacts.