# Git

Related articles

- Super Quick Git Guide
- Gitweb
- Cgit
- Subversion
- Concurrent Versions System

Git is the version control system (VCS) coded by Linus Torvalds (the creator of the Linux kernel) after being criticized for using the proprietary BitKeeper with the Linux kernel. Git is now used to maintain sources for the Linux kernel as well as thousands of other projects, including a number of Arch Linux projects.

There is extensive documentation, including guides and tutorials, available from the official web site.

## Installation

git can be installed with pacman from the official repositories. If you care about using Git with other VCS software, mail servers, or using Git's GUI, pay close attention to the optional dependencies.

Bash completion (e.g. hitting `Tab` to complete commands you are typing) should work if you add this line to your `~/.bashrc` file:

    source /usr/share/git/completion/git-completion.bash

Alternatively, you can install the bash-completion package to load the completions automatically for new shells.

If you want to use Git's built-in GUI (e.g. `gitk` or `git gui`) you should install the tk package, or you will get a rather cryptic message:

    /usr/bin/gitk: line 3: exec: wish: not found.

In addition, if you're trying to the GUI on a headless server (i.e. a system on which you haven't done a full-blown install of some desktop environment) you may want to make sure you have the gsfonts package installed, or you might encounter this problem when trying to run gitk:

    Segmentation fault (core dumped)

If you want to use the Git SVN bridge(`git svn`) and receive the following error:

    Can't locate Term/ReadKey.pm in @INC (you may need to install the Term::ReadKey module)

you need to install perl-term-readkey.

## Configuration

Git reads its configuration from a few INI type configuration files. In each git repository `.git/config` is used for configuration options specific to that repository. Per-user ("global") configuration in `$HOME/.gitconfig` is used as a fall-back from the repository configuration. You can edit the files directly but the preferred method is to use the git-config utility. For example,

    $ git config --global core.editor "nano -w"

adds `editor = nano -w` to the `[core]` section of your `~/.gitconfig` file.

The man page for the git-config utility has a fairly long list of variables which can be set.

The two settings you should set before using Git are your name and email. These are used to sign commits you make.

    $ git config --global user.name "Firstname Lastname"
    $ git config --global user.email "your_email@youremail.com"

### Colors in Git prompt

`color.ui` is also a very useful option to set because it enables colors for all Git output.

    $ git config --global color.ui true

## Basic usage

### Cloning a repository

    $ git clone repository_location dir

will clone a Git repository in a new directory inside your current directory. Leaving out `<dir>` will cause it to name the folder after the Git repository. For example,

    $ git clone git://github.com/torvalds/linux.git

clones GitHub's mirror of the Linux kernel into a directory named `linux` in the terminal's current directory.

### Committing files

Git's commit process involves two steps:

1. Add new files, add changes for existing files (both with `git add <files>`), and/or remove files (with `git rm`). These changes are put in a staging area called the index.
2. Call `git commit` to commit the changes.

Git commit will open up a text editor to provide a commit message. You can set this editor to whatever you want by changing the `core.editor` option with `git config`.

Alternatively, you can use `git commit -m <message>` to supply the commit message without opening the text editor.

Other useful commit tricks:

- `git commit -a` lets you commit changes you have made to files already under Git control without having to take the step of adding the changes to the index. You still have to add new files with `git add`.

- `git commit -s` adds the following line to the end of the commit message, which is useful when submitting patches to mailing lists or contributing to open-source software projects.

        Signed-off-by: <firstname> <lastname> <email>

- It is common for users to regularly contribute to a various mixture of private repositories, that users typically do not use "Signed-off-by" lines with, and open-source software projects that typically require patches to contain a "Signed-off-by" line. If you want to make sure you do not forget to add a "Signed-off-by" line when contributing to open-source software projects, set the following Git configuration setting to automatically append a "Signed-off-by" line when using `git format-patch <commit>`:

        $ git config --local format.signoff true

- `git add -p` lets you commit specific parts of files you have changed. This is useful if you have made a large number of changes that you think would be best split into several commits.

### Pushing your changes

To push your changes up to a server (such as GitHub), use

    $ git push server_name branch

Adding `-u` will make this server the default one to push to for this branch.

If you have cloned the repository as described above, the server will default to the location you cloned the repository from (named "origin"), and the branch will default to the "master" branch. In other words, if you have followed this guide's instructions in cloning, `git push` will suffice.

You can set up Git to push to multiple servers if you want, but that is a more advanced topic.

Branches will be discussed later in this guide.

### Pulling from the server

If you are working on multiple machines and want to update your local repository to what the server has, change directory into your local repository and use

    $ git pull server_name branch

Similarly to push, the server name and branch should have sane defaults, so `git pull` should suffice. Git pull is actually shorthand for doing two things:

1. Calling `git fetch`, which updates the local copy of what the server has. Such branches are called "remote" branches because they are mirroring remote servers.
2. Calling `git merge`, which merges what the remote branch has with what you have. If your commit history is the same as the server's commit history, you will be automatically fast-forwarded to the latest commit on the server. If your history does not match (maybe someone else has pushed commits since you last synced), the two histories will be merged.

It is not a bad idea to get into the practice of using these two commands instead of `git pull`. This way you can check to make sure that the server contains what you would expect before merging.

### Examining history

The command `git log` shows the history of your current branch. Note that each commit is identified by a SHA-1 hash. The author, commit date, and commit message follow. A more useful command is

    $ git log --graph --oneline --decorate

which provides a display similar to TortoiseGit's log window. It shows the following:

- The first 7 digits of each commit's SHA-1 hash (enough to be unique)
- The `--graph` option shows how any branches (if there are others) fork off from the current branch.
- The `--oneline` option shows only the first line of each commit message
- The `--decorate` option shows all commit labels (branches and tags)

It may be convenient to alias this command as `git graph` by doing the following:

    $ git config --global alias.graph 'log --graph --oneline --decorate'

Now typing `git graph` will run `git log --graph --oneline --decorate`.

`git graph` and `git log` may be given the `--all` flag in order to view all branches instead of just the current one. Adding `--stat` to one of these commands is also useful because it shows which files each commit changed and how many lines were changed in each file.

### Dealing with merges

Merges happen when you pull, as a result of a rebase operation, and when you merge one branch into another. Like other version control tools, when Git cannot automatically merge a commit, it turns to you. See this section of the Git Book for an explanation on how to resolve merge conflicts. If you screw up and would like to back out of the merge, you can usually abort the merge using the `--abort` flag with whatever command started the merge (e.g. `git merge --abort`, `git pull --abort`, `git rebase --abort`).

## Taking advantage of DVCS

The above commands only provide the basics. The real power and convenience in Git (and other distributed version control systems) come from leveraging its local commits and fast branching. A typical Git workflow looks like this:

1. Create and check out a branch to add a feature.
2. Make as many commits as you would like on that branch while developing that feature.
3. Squash, rearrange, and edit your commits until you are satisfied with the commits enough to push them to the central server and make them public.
4. Merge your branch back into the main branch.
5. Delete your branch, if you desire.
6. Push your changes to the central server.

### Creating a branch

    $ git branch branch_name

can be used to create a branch that will branch off the current commit. After it has been created, you should switch to it using

    $ git checkout branch_name

A simpler method is to do both in one step with

    $ git checkout -b branch_name

To see a list of branches, and which branch is currently checked out, use

    $ git branch

### A word on commits

Many of the following commands take commits as arguments. A commit can be identified by any of the following:

- Its 40-digit SHA-1 hash (the first 7 digits are usually sufficient to identify it uniquely)
- Any commit label such as a branch or tag name
- The label `HEAD` always refers to the currently checked-out commit (usually the head of the branch, unless you used `git checkout` to jump back in history to an old commit)
- Any of the above plus `~` to refer to previous commits. For example, `HEAD~` refers to one commit before `HEAD` and `HEAD~5` refers to five commits before `HEAD`.

### Commits as checkpoints

In Subversion and other older, centralized version control systems, commits are permanent - once you make them, they are there on the server for everyone to see. In Git, your commits are local and you can combine, rearrange, and edit them before pushing them to the server. This gives you more flexibility and lets you use commits as checkpoints. Commit early and commit often.

### Editing the previous commit

    $ git commit --amend

allows you to modify the previous commit. The contents of the index will be applied to it, allowing you to add more files or changes you forgot to put in. You can also use it to edit the commit message, if you would like.

### Squashing, rearranging, and changing history

    $ git rebase -i commit

will bring up a list of all commits between `<commit>` and the present, including `HEAD` but excluding `<commit>`. This command allows you rewrite history. To the left of each commit, a command is specified. Your options are as follows:

- The "pick" command (the default) uses that commit in the rewritten history.
- The "reword" command lets you change a commit message without changing the commit's contents.
- The "edit" command will cause Git to pause during the history rewrite at this commit. You can then modify it with `git commit --amend` or insert new commits.
- The "squash" command will cause a commit to be folded into the previous one. You will be prompted to enter a message for the combined commit.
- The "fixup" command works like squash, but discards the message of the commit being squashed instead of prompting for a new message.
- Commits can be erased from history by deleting them from the list of commits
- Commits can be re-ordered by re-ordering them in the list. When you are done modifying the list, Git will prompt you to resolve any resulting merge problems (after doing so, continue rebasing with `git rebase --continue`)

When you are done modifying the list, Git will perform the desired actions. If Git stops at a commit (due to merge conflicts caused by re-ordering the commits or due to the "edit" command), use `git rebase --continue` to resume. You can always back out of the rebase operation with `git rebase --abort`.

Warning: Only use `git rebase -i` on local commits that have not yet been pushed to anybody else. Modifying commits that are on the central server will cause merge problems for obvious reasons.

## Git prompt

The Git package comes with a prompt script. To enable the prompt addition, you will need to source the `git-prompt.sh` script and add `$(__git_ps1 " (%s)")` to your PS1 variable.

- Add the following line to your `~/.bashrc`/`~/.zshrc`:

    source /usr/share/git/completion/git-prompt.sh

- For Bash:

    PS1='[\u@\h \W$(__git_ps1 " (%s)")]\$ '

Note: For information about coloring your Bash prompt, see Color_Bash_Prompt
Note: For information about coloring your Git prompt, see #Colors in Git Prompt

- For zsh:

    PS1='[%n@%m %c$(__git_ps1 " (%s)")]\$ '

The `%s` is replaced by the current branch name. Git information is displayed only if you are navigating in a Git repository. You can enable extra information by setting and exporting certain variables to a non-empty value as shown in the following table:

Variable | Information
--- | ---
GIT_PS1_SHOWDIRTYSTATE | * for unstaged and + for staged changes
GIT_PS1_SHOWSTASHSTATE | $ if something is stashed
GIT_PS1_SHOWUNTRACKEDFILES | % if there are untracked files

In addition you can set the `GIT_PS1_SHOWUPSTREAM` variable to `"auto"` in order to see `<` if you are behind upstream, `>` if you are ahead and `<>` if you have diverged.

Note: If you experience that `$(__git_ps1)` returns `((unknown))`, then there's a `.git` folder in your current directory which does not contain any repository, and therefore Git does not recognize it. This can, for example, happen if you mistake Git's configuration file to be `~/.git/config` instead of `~/.gitconfig`.

## Transfer protocols

### Smart HTTP

Git is able to use the HTTP(S) protocol as efficiently as the SSH or Git protocols, by utilizing the git-http-backend. Furthermore it is not only possible to clone or pull from repositories, but also to push into repositories over HTTP(S).

The setup for this is rather simple as all you need to have installed is the Apache web server (apache, with `mod_cgi`, `mod_alias`, and `mod_env` enabled) and of course, git.

Once you have your basic setup running, add the following to your Apache configuration file, which is usually located at `/etc/httpd/conf/httpd.conf`:

    <Directory "/usr/lib/git-core*">
        Require all granted
    </Directory>

    SetEnv GIT_PROJECT_ROOT /srv/git
    SetEnv GIT_HTTP_EXPORT_ALL
    ScriptAlias /git/ /usr/lib/git-core/git-http-backend/

The above example configuration assumes that your Git repositories are located at `/srv/git` and that you want to access them via something like http(s)://your_address.tld/git/your_repo.git. Feel free to customize this to your needs.

Note: Of course, you have to make sure that Apache can read and write (if you want to enable push access) to your Git repositories.

For more detailed documentation, visit the following links:

- http://progit.org/2010/03/04/smart-http.html
- https://www.kernel.org/pub/software/scm/git/docs/v1.7.10.1/git-http-backend.html

### Git SSH

You first need to have a public SSH key. For that follow the guide at SSH keys. To set up SSH itself, you need to follow the SSH guide. This assumes you have a public SSH key now and that your SSH is working. Open your SSH key in your favorite editor (default public key name is `~/.ssh/id_rsa.pub`), and copy its content. Now go to your user where you have made your Git repository, since we now need to allow that SSH key to log in on that user to access the Git repository. Open `~/.ssh/authorized_keys` in your favorite editor, and paste the contents of `~/.ssh/id_rsa.pub` in it. Be sure it is all on one line! That is important! It should look somewhat like this:

Warning: Do not copy the line below! It is an example! It will not work if you use that line!

`ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQCboOH6AotCh4OcwJgsB4AtXzDo9Gzhl+BAHuEvnDRHNSYIURqGN4CrP+b5Bx/iLrRFOBv58TcZz1jyJ2PaGwT74kvVOe9JCCdgw4nSMBV44cy+6cTJiv6f1tw8pHRS2H6nHC9SCSAWkMX4rpiSQ0wkhjug+GtBWOXDaotIzrFwLw== username@hostname` Now you can checkout your Git repository this way (change where needed. Here it is using the username `git` and the host `localhost`):

    git clone git@localhost:my_repository.git

You should now get an SSH yes/no question, if you have the SSH client setting `StrictHostKeyChecking` set to `ask` (the default). Type `yes` followed by `Enter`. Then you should have your repository checked out. Because this is with SSH, you also have commit rights now. To modify an existing repository to use SSH, the URL for the origin branch can simply be changed.

    git remote set-url origin git@localhost:my_repository.git

For more information, read the Super Quick Git Guide.

#### Specifying a non-standard port

Connecting on a port other than 22 can be configured on a per-host basis in `/etc/ssh/ssh_config` or `~/.ssh/config`. To set up ports for a repository, specify the path in `.git/config` using the port number `N` and the absolute path `/PATH/TO/REPO`:

    ssh://user@example.org:N/PATH/TO/REPO

Typically the repository resides in the home directory of the user which allows you to use tilde-expansion. Thus to connect on port N=443,

    url = git@example.org:repo.git

becomes:

    url = ssh://git@example.org:443/~git/repo.git

### Git daemon

Note: The Git daemon only allows read access. For write access, refer to #Git SSH.

This will allow URLs like `git clone git://localhost/my_repository.git`.

Start `git-daemon.socket` with root privileges.

    # systemctl start git-daemon.socket

To run the git-daemon every time at boot, enable the systemd unit `git-daemon.socket`.

The daemon is started with the following parameters which are placed in the systemd service file.

    ExecStart=-/usr/lib/git-core/git-daemon --inetd --export-all --base-path=/srv/git

You have to place your repositories in `/srv/git/` to be able to clone them with the Git daemon.

Clients can now simply use the following:

    $ git clone git://localhost/my_repository.git

### Git repositories rights

To restrict read and/or write access, you can simply use Unix permissions. Refer to http://sitaramc.github.com/gitolite/doc/overkill.html[dead link 2013-11-06] for more information.

For fine-grained access management, refer to gitolite and gitosis.

## Tunneling Git through HTTP proxies

Restrictive corporate firewalls typically block the port that git uses. However, git can be made to tunnel through HTTP proxies using utilities such as corkscrew. When git sees the environment variable `GIT_PROXY_COMMAND` set, it will run the command in `$GIT_PROXY_COMMAND` and use that program's stdin and stdout, instead of a network socket.

Create a script file `corkscrewtunnel.sh`

    #! /bin/bash

    corkscrew proxyhost proxyport $*

Set `GIT_PROXY_COMMAND`

    export GIT_PROXY_COMMAND=path-to-corkscrewtunnel.sh

Now, git should be able to tunnel successfully through the HTTP proxy.

## See also

- Pro Git book
- Git Reference
- https://www.kernel.org/pub/software/scm/git/docs/
- https://help.github.com/
- https://wiki.videolan.org/Git

Category:

- Version Control System