===== Overview =====

How to identify untracked files that Git should intentionally ignore when running numerous commands, such as ''git add -u'', ''git status'', ''git clean'' and many more.

Some links:

  * [[https://github.com/github/gitignore|Sample gitignore files]]
  * [[https://www.gitignore.io|Generate a gitignore file]]

===== Introductory .gitignore =====

==== Basic .gitignore file ====

Excerpts from the Linux kernel source top-level ''.gitignore'' file, which forces ignoring of all these file patterns //recursively//:

<code>
.*                               [all hidden files/directories]
*.a
*.asn1.[ch]
*.bz2
*.c.[012]*.*
*.dt.yaml
*.dtb
*.dtb.S
*.dwo
*.elf
*.gz
*.ko
...
Module.symvers                    [specific filename]
...
#
# Top-level generic files
#
/tags
/TAGS
/linux
/vmlinux
/vmlinux.32
/vmlinux-gdb.py
...
/debian/                           [top-level directory only]
...
#
# Exceptions: git files that we don't want to ignore even if they are dot-files
#
!.gitignore
!.mailmap
!.cocciconfig
!.clang-format
...
#
# Generated include files
#
include/config
include/generated
include/ksym
arch/*/include/generated
...
</code>

==== Recursive .gitignore ====

Subdirectories can have more specific ''.gitignore'' files that take precedence in those subdirectories. For the Linux kernel source, the top-level directory ''scripts/'' has the ''.gitignore'' file:

<code>
#
# Generated files
#
bin2c
conmakehash
kallsyms
pnmtologo
unifdef
recordmcount
sortextable
asn1_compiler
extract-cert
sign-file
insert-sys-cert
</code>

The Linux kernel source repo has over 200 ''.gitignore'' files.
===== Advanced ignoring =====

==== Four levels of ignoring 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 (see ''man git-clean''),
  * 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'').

==== Rationale for different levels of gitignore files ====

  * 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 after the user has cloned it.
  * 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''.

<code>
$ git config --global core.excludesfile ~/.my_ignorefile
</code>

==== Debugging gitignore files ====

<code>
$ man git-check-ignore
</code>

===== gitignore gotchas =====

  * Since ''.gitignore'' is a versioned file, checking out a different commit might result in a different ''.gitignore'' file, causing unexpected behaviour.