Git is fairly logical once you have learned the ideas behind it, so this articles has quite a lot on concepts, and much more abbreviated notes on the important commands.
The essential component of version control is the repository, a database that holds information about changes to a set of files, known as the working tree. The changes and other information that make up a set of files and directories are all stored as objects in the repository database. When you request a version of a working tree or a particular file the relevant objects are rapidly combined, and the result is written to disk.
Each time that the user requests a commit, Git stores the state of the entire working tree as a snapshot, along with the contents of those files that the user specifies and have changed. This means that Git only records those changes to the tree that the user chooses to register.
Distributed Version Control
Git is a distributed system, so the concept of a master repository is not really appropriate. Instead, you start with one repository, and then make extra copies for distribution and backup purposes. Once created, copies may diverge from the original, but you can synchronize them at any time. You can treat one particular repository as the canonical copy if you wish, but Git has no such notion.
Similarly, clients and servers no longer have the same meaning with a distributed version control system. You can copy to and from any repository that you have access to, either on the same computer, or over a network.
The Git software package does include a network service for sharing repositories, but this is only necessary to protect a common repository from locking issues caused by multiple simultaneous requests, or to enable controlled remote access without exposing the rest of the system to the repository users.
Normally, you create the original repository on your desktop computer, and then use the clone facility of git to create copies of it. These clones can then put on remote servers, removable drives, or anywhere that you like. You could equally create the original repository on a server, and then create a clone on your own computer.
To conveniently interact with another copy of a repository, you register it with your local copy as a remote. Each remote has an alias, which can have any name that you wish. If you clone an existing repository then the source repository is automatically registered in the clone as a remote with the name origin.
The Two Forms of Repository
Git repositories are not binary files, unlike most databases. Instead, a Git repository is a directory that contains a set of files and subdirectories within it. These files and subdirectories each have a specific function. It is not necessary to enter or directly edit any of the pieces of a repository.
A standard Git repository is linked to a copy of the working tree. By default, the repository is a subdirectory within a copy of the working tree, and has the name .git. You may configure a repository to manage a working tree in another location, but this is outside of the scope of these notes.
A bare repository is just a copy of the repository itself, and is not linked to a copy of the working tree. By convention, a directory that holds a bare repository has the suffix .git attached to it’s name, e.g. my-project.git. Other repositories can submit or collect information to and from bare repositories.
Bare repositories provide collection and distribution points for changes that were recorded by standard repositories elsewhere. Usually, each developer working on a project has a standard repository copy on their own computer, and all of the developers have access to a bare repository that resides on a central server.
Both forms of repository use the same storage format. The repository format for Git is stable, which means that you can upgrade installed copies of Git without modifying your repositories.
Git is a Content Tracker
Git actually works with the contents of files, and is flexible about where the content currently resides. The advantage of this approach is that you can rename and move files without much issue. The disadvantages are that Git does not maintain file permissions, nor does it track completely empty directories.
Unfortunately, Git must treat binary files in the tree as indivisible blobs. This means it must make a copy of the entire binary file every time that a change is registered, and it cannot tell you what a change actually did. For these reasons, put your data and source code into version control, along with any scripts needed to compile binaries, but do not store the generated binaries.
The Index (or Staging Area)
Whenever you register that one or more files in the working tree have changed, the details are noted in the index. The commit command permanently saves all of the pending changes from the index to the object database for the repository, and then flushes the index.
Although other version control systems have a similar concept to the index, only Git exposes it to the user so directly. Some documentation refers to the index as the staging area, which is a much more appropriate name.
The stash command saves the current states of both the working tree and the index, without making commits.
Branches and Tags
Every Git repository has one or more branches and tags, which identify alternative versions of the working tree. Each branch and tag may provide a completely different directory structure and set of files. Tags and branches are actually just types of pointers, or refs, that identify the commit that is the latest in the sequence. This means they take up very little space, and there is no limit on the number that can be held in a repository.
A tag identifies the state of the working tree at a particular point in time, such as a release. It may be digitally signed, so that others can verify copies later.
A branch indicates a version of the working tree that may continue to be changed. At any time, one branch is the current branch. This is the branch that will be targeted by commands such as merge.
The branch that Git automatically creates when it initializes a completely new repository is named master. This first branch is the current branch, until you explicitly change to another branch.
Git provides two forms of synchronization between branches. A merge attempts to reconcile two or more branches. A rebase makes the target branch identical to the source branch, and then reapplies all of the committed changes. This enables outside developers to track the main line of development whilst working on customizations.
By default, a repository database only actually holds the objects for those branches that were created locally. Each remote tracking branch is simply a reference to a branch in a remote repository. You may add remote branches to a repository from any other repository. Once a remote branch is registered in your repository, you may synchronize a local branch with it at any time.
To work on the content from a remote branch, create a copy as a local branch, and make your changes on this local branch. Once a change is complete, reconcile the local and remote versions.
If you clone a repository then the master branch is fully copied to the clone, and this local branch becomes the current branch. All of the other branches in the source repository are created as remote branches in the clone. These inherited branches have a name prefixed by origin/.
Identifiers for Commits
Git uniquely identifies each change with an SHA1 checksum, which looks like this:
The mathematics behind SHA1 mean that every checksum that is generated is truly, globally unique, and will never be repeated by any other system in the world. This means that all of the changes registered in separate copies of a repository are guaranteed to have unique identifiers, and can be compared and reconciled without error at any time.
To enable users to refer to commits without needing to quote the entire checksum, Git supports a number of alternate ways to specify a commit. A short form identifier is referred to as a treeish in the documentation. By far the most common is the partial checksum:
Git automatically resolves other identifiers to the nearest matching checksum. For example, this identifier will be resolved to the commit that was made yesterday:
Use the tilde identifiers to ask Git to find a commit, relative to the commit with the specified checksum. This identifier specifies the commit that was two commits before the commit that has a partial checksum of c34a14:
To find a series of commits, specify the first commit before the start of the range, and the last commit that is within the range, separated by two dots:
To specify one of the parent commits, use the caret:
This is mainly useful for the results of a merge, where the specified commit may have two or more parents, rather than just one.
If Git cannot resolve an identifier, it produces an error.
HEAD: The pointer HEAD always refers to the most recent commit on the current branch.
Refer to the Git Community Book for more on the available identifiers.
Common Working Styles
In practice there are three standard to arrange sets of Git repositories:
- A copy on a desktop as the canonical repository, with remote clones
- A canonical repository on a remote system, with a local clone as your personal working copy
- Forking a repository on GitHub or Gitorious, and then working on a clone
The third use case actually involves three repositories: the source repository, a clone on the hosting service that is private to you, and a local clone of your clone. This means that you have to register the first source repository as a remote branch in your local clone - it will not automatically be registered.
Whichever operating system you use, remember to configure Git before you create a repository.
Installing Git on Linux
To install Git on Linux, simply use the package management system built into the operating system. The package is often named git-core, to differentiate it from another Open Source product that is also called git. For example, to install the Git version control system on Debian or Ubuntu systems, run this command:
sudo apt-get install git-core
Debian and Ubuntu provide other supporting software, such as the gitweb interface, but put these in separate packages.
Installing Git on Microsoft Windows
Install one of the following:
- Git for Windows - Package for command-line Git on Windows, including SSH
- Git Extensions - Provides Git for Windows, as well as a graphical utility, and a Visual Studio plugin for Git
Installing Git on macOS
Go to the Git Web site and follow the link for Other Download Options, to obtain a macOS disk image. Use the disk image as normal.
Once you have installed Git on a system, always set your details before you create or clone a repository. This requires two commands:
git config —global user.name "Your Name" git config —global user.email "email@example.com"
The —global option means that the setting will apply to every repository that you work with in the current user account.
To enable colors in the output, which can be very helpful, enter this command:
git config --global color.ui auto
You may want to ensure that some files never appear in any commit that you do in any repository. Specify a global ignore file with the core.excludesfile setting. You must give the full path of your exclusions file, or the feature will silently fail. For example:
git config --global core.excludesfile /home/you/.gitexclusions.txt
Your exclusions file uses the same format as .gitignore files.
Finally, you will probably to create short aliases for the commands that you use often. Use the keyword alias, followed by the alias, like this:
git config —global alias.br branch git config —global alias.co checkout git config —global alias.ci commit git config —global alias.df diff git config —global alias.lg log -p git config —global alias.st status
You may define aliases for any Git command.
Creating a New Repository
To turn a directory into a Git repository, simply enter these commands:
git init touch .gitignore git add . git commit -m “Initial commit”
Add the appropriate entries to the .gitignore file, as explained below, and then commit the change.
You should immediately create a branch and switch to it. This command creates a branch called spike, and makes it the current branch:
git checkout -b spike
You can then makes changes and selectively merge them to the master branch.
In addition to the global exclusions for your user account, you can specify exclusions for a repository in either the exclude file, or .gitignore files. Each .gitignore file defines a set of exclusions for the directory that it resides in, and subdirectories. These files are tracked by Git in the usual way, and so they apply to every copy of that repository. The listings in the repository exclusion file .git/info/exclude apply to the current copy of the entire repository, but this file is not replicated between copies of a repository.
Always exclude these files, which are automatically generated by operating systems:
Other exclusions depend upon the type of project. As a rule, you should exclude files that are compiled or generated from the source code. For example, these exclusions cover the files that Ruby on Rails projects generate:
If you use RVM then exclude .rvmrc files from the project repository. These files are system-specific.
The Most Commonly Used Commands
These are the standard commands for working with files in a Git repository.
Remember that the man page for each Git command is prefixed with git-, so to view the man page for git commit, type this command:
Adding Changes to the Index: git add
Use git add to add changes to the index. All of the changes made on the specified files are registered. If you run git add again with the same file before you commit, the information in the indexed is automatically updated.
git add my-file.txt git add some/directory/* git add some/other/directory/*.txt
Use the -p option to add just some of the changes to a file:
git add -p my-file.txt
Git then lets you interactively choose which of the changes to the file to register in the index.
Removing Changes to the Index: git reset
Use git rm —cached or git reset to remove files from the index, without actually deleting the copies in the working tree:
git reset myfile.txt
The reset command accepts patterns as well as file names.
Committing the Index: git commit
The commit message should have a one line summary, a blank line, then details.
git commit -m "Committed minor change to Blah module. This now does blah blah instead."
To amend the previous commit, use the —amend option. For example, to amend the message on a commit:
git commit --amend -m "New commit message" c34a14
Listing Changes: git status and git log
For a list of changes, whether they are staged or unstaged:
This also shows any untracked files that are in the working tree.
To see a list of committed changes for all branches, use git log:
Use the -p option to show the content of each commit:
git log -p
The git log accepts many filters and options. One of the most useful changes the format to show one commit per line:
git log --pretty=oneline
Reviewing the Details of Changes: git show and git diff
To see a specific commit, use git show:
git show c34a14
To see the details of changes between particular revisions, use git diff. If you do not specify revisions, or give any options, git diff shows that changes between the index and the working files.
Use the —staged option to see the differences between the HEAD and the index:
git diff --staged
If you specify revisions, git diff compares them. Just specify HEAD to see all of the differences between the repository and the working files:
git diff HEAD
To compare particular commits, specify the identifiers:
git diff c34a14 b9c38e
You may specify tag or branch names, or other identifiers.
To narrow the scope of any diff to a single directory or file, append the name of the target to the command:
git diff c34a14 b9c38e myfile.txt
To compare a set of changes, specify a range:
git diff c34a14...b9c38e myfile.txt
Reverting Changes: git reset and git revert
To get a previous version of an individual file, you can use git checkout:
git checkout HEAD myfile.txt
To go back to a previous version of the working tree, use git reset. For example:
git reset --hard HEAD
These commands to revert changes do not destroy files in the working tree.
To remove all untracked files from the working tree, use git clean:
git clean -f
The git revert command undoes the result of the specified commit, and creates a new commit to register the changes. For example, to undo the specific changes made by the commit c34a140, run this command:
git revert c34a14
Deleting Files: git rm
Use git rm to delete a file or directory from the working tree. This also marks it as deleted in the index, so that the next commit will register the change in the repository.
git rm my-file.txt
Moving or Renaming Files
As Git tracks content, you do not need to use any special commands to safely move or rename files. Simply copy, move, or rename the file, and then use git add to register the resulting file in the staging area.
Cloning an Existing Repository: git clone
To create a new copy of an existing repository, get the URL of the repository, and use git clone:
git clone http://server.domain.com/a-project.git git clone git+ssh://server.domain.com/a-project.git git clone git://server.domain.com/a-project.git
By default, this creates a working tree that matches the HEAD of the master branch. To create the clone as a bare repository, add the —bare option to the command.
Remember to create a local branch before you make any changes to the new clone.
Managing Linked Remote Repositories: git remote
To register a remote with your repository, use the remote command:
git remote add other-repo
Similarly, to remove a remote, use the rm option of git remote:
git remote rm other-repo
You can also safely add or remove remotes by editing the .git/config file directly, if you wish.
Synchronizing with Another Repository: git pull and git fetch
To fully synchronize a repository with another, Git has to do three things. Firstly, it has to get all of the objects that are stored within the source repository that the target does not have. Secondly, it has to reset HEAD on the target to point to the latest commit. Thirdly, it has to update the working tree of the target to match the new HEAD.
To perform all of these operations with one command, use git pull. By default, this merges the master branch from the remote repository that is registered as origin into the current local branch.
In many cases you do not want all of the steps to happen immediately. To just copy new objects from the remote repository to the local repository database without updating the HEAD pointer or your copy of the working tree, use git fetch:
To reset the HEAD pointer and the working tree, use either the merge or rebase facilities to apply the outstanding changes. This command merge the master branch from the origin repository with the current branch:
git merge origin/master
Exporting from a Repository: git archive and git checkout-index
By default, git archive streams the specified version of a working tree in tar archive format to the terminal STDOUT, so that you can pipe or redirect the data to any command or location of your choice. Use git archive with the option —format=zip to export the tree in compressed zip format. For example, this command exports the latest version of the tree as a zip archive and saves it to a file named my-archive.zip:
git archive HEAD --format=zip > my-archive.zip
To export a tree without compressing it, we must use git checkout-index. Use the prefix option to specify the destination for the exported files:
git checkout-index --prefix=/path/to/destination/ -a
Refer to the Git Ready article for more on exporting repositories.
Working with Branches: git branch and git checkout
Creating a New Branch
You can create a branch at any time. Remember that this creates an alternate version of the entire working tree. If you do not specify a commit, Git creates a branch that is copy of HEAD:
git branch new-branch
This does not actually switch branches.
To create a new branch and switch to it immediately, use git checkout -b:
git checkout -b new-branch
If you create a branch to work on content from another branch that was created elsewhere, name the new branch the same as the original, with a prefix of your initials followed by a forward slash. For example, J S Bach’s local copy of the useful-feature branch should be named jsb/useful-feature. This is purely a helpful convention.
Listing Available Branches
To see the local branches, use git branch -l:
git branch -l
The current branch has an asterisk next to it.
To see the remote branches, use git branch -r:
git branch -r
To change the current branch, use git checkout. Note that you must use the -f option to actually change the working tree to match:
git checkout -f new-branch
Transferring Changes Between Branches
To merge the content of another branch into the current branch, use git merge or git rebase. Normally you use merge to apply just the differences:
git merge other-branch
The rebase function is more aggressive. It makes the current branch the same as the specified branch, and then applies all of the changes between the current branch and the common ancestor of the two branches.
git rebase other-branch
To copy a single commit from one branch to another, switch to the target branch, and then use cherry-pick to import the specified commit:
git cherry-pick f28e67
Publishing a Branch to Another Repository
To publish a branch to another repository that you have write access to, use git push. This adds the objects to the database of the remote repository, so that others with access to the repository can reproduce your branch if they wish.
If the remote repository has been specified as a mirror, then every push will automatically transfer all new changes from all of the branches in the repository.
Deleting a Branch
To delete a local branch, use git branch with the -d option:
git branch -d unwanted-branch
To delete a remote branch, just push to the repository, with a colon before the name of the branch that you want to remove:
git push origin :unwanted-branch
Working with Tags: git tag
Listing Available Tags
To see the tags, use git tag -l:
git tag -l
Creating a New Tag
To create a new tag from the current branch:
git tag -a -m "This is a tag" tag-name
The submodule facility enables you to attach another complete repository to a directory within the main repository. Commands within a submodule work as if the submodule was a separate clone.
Some experts advise using the newer subtree option instead of submodules.
Important: Submodule Names
Git commands that use the submodule require the name of the submodule, not the local directory that it is attached to. By default, the local directory has the same name, which may confuse.
This specifies the name, which is correct:
git add submodule-name
This is a file path, which is wrong:
git add submodule-name/
Creating a Submodule
To create a submodule within a project, use the add option of the git submodule:
git submodule add url local-name git commit -m “Added submodule”
This creates a directory called local-name, and clones the repository plus the working tree under it. The submodule is registered to a specific commit.
Next, run the init function to activate all of the registered submodules:
git submodule init
Updating a Submodule
To update a submodule to match the latest remote version:
cd submodule-path git pull
This updates the working tree inside the submodule.
To update the commit that the parent repository thinks the submodule should link to:
cd parent git add submodule-name git commit -m “Updated submodule”
Ensure that you use the name of the submodule, not the path of the local directory that it is attached to.
Assuming that the original is called my-project, and the remote server has a directory called /srv/repo-mirrors, these commands make a clone bare repository, and transfer it to the server with SSH:
git clone —bare my-project my-project.git scp -r my-project.git my-server.my-domain.com/srv/repo-mirrors
Once there is a clone of the original on a remote server, these commands register the remote clone as a mirror, with the alias replicant, and carry out a test push:
cd master-repo git remote add replicant ssh://my-server.my-domain.com/srv/repo-mirrors/master-repo.git —mirror git push replicant
The mirror setting means that data for all branches will be transferred to the remote every time that you use git push.
To synchronize the remote clone with the local copy, simply run git push on the local copy. You could write a script to do this and schedule it to run periodically, or add a post-commit hook script to the local copy that pushes every change to the clone immediately after it is made.
Backing Up Your Repositories
There are no special tools required to backup a copy of a Git repository - it is just a collection of files and directories. If you are sharing the repository through a service, then you should create a clone which is automatically updated, and back that up, simply because the shared repository may be changing at any time.
Resources and Documentation
- Git for Designers is actually a very lucid introduction to the concepts of Git that is suitable for anyone.
- Git Reference is actually a fairly short and well-written tutorial
- Effective Project Management with Git (MP4 download) - An excellent video presentation
- Git SVN Crash Course for those coming from Subversion.
Advanced Use of Git
- git ready
- Understanding Git Submodules
- Git Submodules - Adding Using, Removing, and Updating
- Git From The Bottom Up - Explains the internal design of Git