How to identify untracked files that Git should intentionally ignore; files already tracked by Git are not affected.

Some links:

• Sample gitignore files

Each line in a gitignore/exclude file specifies a pattern. When deciding whether to ignore a path, Git normally checks gitignore patterns from multiple sources, with the following order of precedence, from highest to lowest (within one level of precedence, the last matching pattern decides the outcome):

• Patterns read from the command line for those commands that support them,
• Patterns read from a .gitignore file in the same directory as the path, or in any parent directory, with patterns in the higher level files (up to the toplevel of the work tree) being overridden by those in lower level files down to the directory containing the file. These patterns match relative to the location of the .gitignore file. A project normally includes such .gitignore files in its repository, containing patterns for files generated as part of the project build,
• Patterns read from (repository-specific) $GIT_DIR/info/exclude,
• Patterns read from the file specified by the configuration variable core.excludesFile (possibly ~/.my_ignorefile).

• Patterns which should be version-controlled and distributed to other repositories via clone (i.e., files that all developers will want to ignore) should go into a .gitignore file that comes with the repository.
• Patterns which are both specific to a particular repository and specific to an individual user’s workflow should go into the $GIT_DIR/info/exclude file for that repository.
• Patterns which a user wants Git to ignore in all situations (e.g., backup or temporary files generated by the user’s editor of choice) generally go into a file specified by core.excludesFile in the user’s ~/.gitconfig.

$ man git-check-ignore