The Git project started with Linus Torvalds scratching the very serious itch of needing a fast, efficient and massively distributed source code management system for Linux kernel development.
The kernel team had moved from a patch emailing system to the proprietary BitKeeper SCM in 2002. That ended in April 2005 when BitMover stopped providing a free version of its tool to the open source community because they felt some developers had reverse engineered it in violation of the license.
Since Linus had (and still has) a passionate dislike of just about all existing source code management systems, he decided to write his own. Thus, in April of 2005, Git was born. A few months later, in July,maintenance was turned over to Junio Hamano, who has maintained the project ever since.
“I’m an egotistical bastard, and I name all my projects after myself. First Linux, now git.” –Linus
The first commit message for the Git project was
‘initial version of “git”, the information manager from hell’ – Linus, 4/7/05
Git is a stupid content tracker, Git tracks content – files and directories.
It is at its heart a collection of simple tools that implement a tree history storage and directory content management system.
It is simply used as an SCM (Software Configuration Management), not really
designed as one.
“In many ways you can just see git as a filesystem — it’s content- addressable, and it has a notion of versioning, but I really really designed it coming at the problem from the viewpoint of a filesystem person (hey, kernels is what I do), and I actually have absolutely zero interest in creating a traditional SCM system.” – Linus (http://marc.info/?l=linux-kernel&m=111314792424707)
When most SCMs store a new version of a project, they store the code delta or diff. When Git stores a new version of a project, it stores a new tree – a bunch of blobs of content and a collection of point- ers that can be expanded back out into a full directory of files and subdirectories. If you want a diff between two versions, it doesn’t add up all the deltas, it simply looks at the two trees and runs a new diff on them.
This is what fundamentally allows the system to be easily distributed
– it doesn’t have issues figuring out how to apply a complex series of
deltas, it simply transfers all the directories and content that one user
has and another does not have but is requesting.
It is efficient about
it – it only stores identical files and directories once and it can com-
press and transfer its content using delta-compressed packfiles – but
in concept, it is a very simple beast. Git is at it’s heart very stupid-
simple.
Git is optimized for cheap and efficient branching and merging.
It is built to be worked on simultaneously by many people, having mul-
tiple branches developed by individual developers, being merged,
branched and re-merged constantly.
Because of this, branching is
incredibly cheap and merging is incredibly easy
Git is built to make distributed development simple.
No repository is special or central in Git – each clone is basically equal and could
generally replace any other one at any time.
It works completely offline or with hundreds of remote repositories that can push to and/
or fetch from each other over several simple and standard protocols.
Git is very efficient. Compared to many popular SCM systems, it seems downright unbelievably fast. Most operations are local, which reduces unnecessary network overhead. Repositories are generally packed very efficiently, which often leads to surprisingly small repo sizes.
The Ruby on Rails Git repository download, which includes the full history of the project – every version of every file, weighs in at around 13M, which is not even twice the size of a single checkout of the project (~9M). The Subversion server repository for the same project is about 115M.
Git also is efficient in its network operations – the common Git transfer protocols transfer only packed versions of only the objects that have changed. It also won’t try to transfer content twice, so if you have the same file under two different names, it will only transfer the content once.
Each Git repository contains 4 components:
- The working directory
- The staging area
- Commit history
- Development branches
where you actually edits files, compile code, and otherwise develop your project. For all intents and purposes, you can treat the working directory as a normal folder. Except, you now have access to all sorts of commands that can record, alter, and transfer the contents of that folder.
An intermediary between the working directory and the
project history.
Instead of forcing you to commit all of your changes at once, Git
lets you group them into related changesets. Staged changes are not yet part of
the project history.
Once you’ve configured your changes in the staging area, you can commit it to the project history where it will remain as a “safe” revision. Commits are “safe” in the sense that Git will never change them on its own, although it is possible for you to manually rewrite project history.
So far, we’re still only able to create a linear project history, adding one commit on top of another. Branches make it possible to develop multiple unrelated features in parallel by forking the project history.
Download the installer from git-scm.com
sudo apt install gitsudo dnf install git sudo pacman -S gityou can set options with the git config command.
or by manually editing the ~/.gitconfig file, in your home directory.
The first thing you want to do when you install git is to introduce yourself to it.
git records this information with your commits, and third-party services like GitHub use this information to identify you.
git config --global user.name "John Doe"
git config --global user.email "John@example.com"the --global flag records options in the ~/.gitconfig file, which applies to all projects on your system.\
By default, git will use your system’s default text editor.
You can change this by setting the core.editor option.
git config --global core.editor "code"By default, Git doesn’t come with any shortcuts, but you can add your own by aliasing commands. If you’re coming from an SVN background, you’ll appreciate the following bindings:
git config --global alias.st status
git config --global alias.ci commit
git config --global alias.co checkout
git config --global alias.br branchLearn more by running the git help config in your Git Bash prompt.
Git will sometimes need credentials from the user in order to perform operations; for example, it may need to ask for a username and password in order to access a remote repository over HTTP.
To avoid inputting these credentials repeatedly:
- Make Git store the username and password, and it will never ask for them :
git config --global credential.helper store - Save the username and password for a session (cache it) :
git config --global credential.helper cache - You can also set a timeout for the above setting :
git config --global credential.helper 'cache --timeout=600'
If you have an existing project directory that you want to track with Git, you can go into the directory and type:
git initThis will create a new subdirectory named .git that contains all of your necessary repository files – a Git repository skeleton. At this point, nothing in your project is tracked yet.
If you want to get a copy of an existing Git repository – for example, a project you’d like to contribute to – the command you need is git clone.
git cloneis the process of moving changes from the working directory to the staged snapshot.
To add new or modified files from working directory to staging area, use
git add <file>To add all new or modified files from working directory to staging area, use
git add .
git add -A
git add --allTo add files with a specific extension from working directory to staging area, use
git add *.<extension>to remove files from staging area, use
git rm --cached <file>Inspect the contents of the staging area with
git status
# or
git status -s
# or git status --shortoutput will look like this:
# on branch <branch-name>
# Changes to be committed:
#...
# Changes not staged for commit:
#...
# Untracked files:
#...for more details, run
git diff --cachedCommits represent every saved version of a project, which makes them the atomic unit of Git-based version control. Each commit contains a snapshot of the project, your user information, the date, a commit message, and an SHA-1 checksum of its entire contents:
commit b650e3bd831aba05fa62d6f6d064e7ca02b5ee1b
Author: john <john@example.com>
Date:
Wed Jan 11 00:45:10 2012 -0600
Some commit messageTo commit the staged snapshot and add it to the history of the current branch, execute the following:
git commit -m "commit message"note: the commit message should be short and descriptive.
Inspecting Commits
git log
Tags are simple pointers to commits, and they are incredibly useful for bookmarking important revisions like public releases. The git tag command can be used to create a new tag:
git tag -a v1.0 -m "Stable release"The -a option tells Git to create an annotated tag, which lets you record a
message along with it (specified with -m).
Running the same command without arguments will list your existing tags:
git tag.
You can undo changes in:
- Working directory
- staging area
- entire commit
To undo changes in working directory, use
git checkout reset --hard HEAD
git clean -fThis configuration of git reset makes the working directory and the stage match the files in the most recent commit (also called HEAD), effectively obliterating all uncommitted changes in tracked files. To get rid of untracked files, you have to use the git clean command. Git is very careful about removing code, so you must also supply the -f option to force the deletion of these files.
for individual files, use
git checkout HEAD <file>This command doesn’t change the project history at all, so you can safely replace HEAD with a commit ID, branch,or tag to make the file match the version in that commit. But, do not try this with git reset, as it will change your history.
to undo changes in staging area, use
git reset HEAD <file>Omitting the --hard flag tells Git to leave the working directory alone (opposed to git reset --hard HEAD, which resets every file in both the working directory and the stage). The staged version of the file matches HEAD, and the working directory retains the modified version.
There are two ways to undo a commit using Git:
- You can either reset it by simply removing it from the project history,
- or you can revert it by generating a new commit that gets rid of the changes introduced in the original. Undoing by introducing another commit may seem excessive, but rewriting history by completely removing commits can have dire consequences in multi-user workflows.
git reset --hard HEAD~1
The HEAD~1 syntax parameter specifies the commit that occurs immediately before HEAD.
you can replace HEAD~1 with a commit ID.
git revert <commit-id>This takes the changes in the specified commit, figures out how to undo them, and creates a new commit with the resulting changeset.
In addition to completely undoing commits, you can also amend the most recent commit by staging changes as usual, then running:
git commit –amendThis replaces the previous commit instead of creating a new one, which is very useful if you forgot to add a file or two. For your convenience, the commit editor is seeded with the old commit’s message. Again, you must be careful when using the --amend flag, since it rewrites history much like git reset.
you can also use git commit --amend --no-edit to amend the most recent commit without changing the commit message.
Sometimes you need to temporarily put aside changes you’ve made to your project. This is especially useful when you’re working on a bug fix or a feature that depends on a change that you haven’t finished yet. You can do this by stashing your changes, which saves them away and reverts them from your working directory.
git stash
# or -m to add a message
git stash -m "message" # or git stash save "message"to inspect the stash, use
git stash listoutput will look like this:
stash@{0}: WIP on master: 2a2b3c4 message
stash@{1}: WIP on master: 1a2b3c4 message
...
stash@{n}: WIP on master: 1a2b3c4 messageyou can do multiple things with the stashed changes:
- apply them
git stash applystash isn’t removed from the list - pop them
git stash popstash is removed from the list and applied - drop them
git stash dropstash is removed from the list without applying without adding any arguments, that would apply the most recent stash. if you want to apply a specific stash, you can usegit stash apply stash@{<stash-number>}
Branches multiply the basic functionality offered by commits by allowing users to fork their history.
Creating a new branch is akin to requesting a new development environment, complete with an isolated working directory, staging area, and project history.
Branches enable a non-linear workflow—the ability to develop unrelated features in parallel.
In contrast, Git branches are simply a pointer to a commit. Since they work on the commit level instead of directly on the file level, Git branches make it much easier to merge diverging histories. This has a dramatic impact on branching workflows.
git branchwill output something like this:
* master
feature-a
quick-bug-fix
# asterisk indicates the current branchgit branch <branch-name>This creates a pointer to the current HEAD, but does not switch to the new branch (you’ll need git checkout for that). Immediately after requesting a new branch, your repository will look something like the following.
if we switched branch to be some-feature, your history would look like the following after committing a snapshot.
note: red commit is the current commit.
note: before switching branches, you should commit your changes or stash them or you will lose them.
When you checkout a commit, you are in a detached HEAD state. This means that you are not on any branch, and any commits you make will not be tied to any branch.
You're just viewing the commit, and any changes you make will be lost when you move to another commit or branch.
if you want to work upon that commit without losing your changes, you can create a new branch from that commit and work on that branch.
git checkout -b <branch-name>that would create a new branch from the commit you checked out and switch to that branch.
Merging is the process of pulling commits from one branch into another.
There're two common types of merges:
- fast-forward merge
- 3-way merge
they both use the same command:
git checkout <branch-to-merge-in>
git merge <target-branch-name>but the method is automatically determined based on the structure of your history, but in each case the branch you want to merge in is the one you’re currently on.
the target branch will remain unchanged, and the current branch will be updated to point to the new commit.
We created a branch to develop some new feature, added two commits, and now it’s ready to be integrated into the main code base. Instead of rewriting the two commits missing from master, Git can “fast-forward” the master branch’s pointer to match the location of some-feature.
Of course, we could have made the two commits directly on the master branch; however, using a dedicated feature branch gave us a safe environment to experiment with new code. If it didn’t turn out quite right, we could have simply deleted the branch (opposed to resetting/reverting).
But, not all situations are simple enough for a fast-forward commit. Remember, the main advantage of branches is the ability to explore many independent lines of development simultaneously. As a result, you’ll often encounter a scenario that looks like the following:
Git can’t fast-forward the master pointer to some-feature without backtracking. Instead, it generates a new merge commit that represents the combined snapshot of both branches. Note that this new commit has two parent commits, giving it access to both histories
If you try to combine two branches that make different changes to the same portion of code, Git won’t know which version to use. This is called a merge conflict. Obviously, this can never happen during a fast-forward merge. WhenGit encounters a merge conflict, you’ll see the following message:
Auto-merging index.html
CONFLICT (content): Merge conflict in <file>
Automatic merge failed; fix conflicts and then commit
the result.if run git status, you’ll see the following:
# On branch master
# You have unmerged paths.
# (fix conflicts and run "git commit")
# both modified: <file>Every file with a conflict is stored under the “Unmerged paths” section. Git annotates these files to show you the content from both versions:
<<<<<<< HEAD
This content is from the current branch.
=======
This is a conflicting change from another branch.
>>>>>>> some-featureTo resolve the conflict, get rid of the <<<<<<, =======, and >>>>>>> notation,
and change the code to whatever you want to keep. Then, tell Git you’re done
resolving the conflict with the git add command:
git add <file>That’s right; all you have to do is stage the conflicted file to mark it as resolved. Finally, complete the 3-way merge by generating the merge commit:
git commit -m "Merged some-feature into master"is the process of moving a branch to a new base. Git’s rebasing
capabilities make branches even more flexible by allowing users to manually
organize their branches. Like merging, git rebase requires the branch to be
checked out and takes the new base as an argument:
git checkout some-feature
git rebase masterThis moves the entire some-feature branch onto the tip of master:
Compare this linear history with a merge of master into some-feature, which results in the exact same code base in the final snapshot:
golden rule of rebasing: never rebase a branch that has been pushed to a public repository.
Simply put, a remote repository is one that is not your own. It could be on a central server, another developer’s personal computer, or even your file system. As long as you can access it from some kind of network protocol, Git makes it incredibly easy to share contributions with other repositories.
You can view your existing remotes by calling the git remote command with no arguments:
git remoteTo add a new remote, you can use the git remote add command. This command takes two arguments: a name for the remote and the URL of the remote repository:
git remote add <remote-name> <remote-url>To remove a remote, you can use the git remote rm command. This command takes a single argument: the name of the remote you want to remove:
git remote rm <remote-name>The act of downloading branches from another repository is called fetching.
To fetch a remote, you can use the git fetch command. This command takes a
single argument: the name of the remote you want to fetch from:
git fetch <remote-name>Since the fetch/merge sequence is such a common occurrence in distributed
development, Git provides a pull command as a convenient shortcut:
git pull origin/masterThis fetches the origin’s master branch, and then merges it into the current
branch in one step. You can also pass the --rebase option to use git rebase
instead of git merge.
To complement the git fetch command, Git also provides a push command.
Pushing is almost the opposite of fetching, in that fetching imports branches,
while pushing exports branches to another repository.
git push <remote> <branch>The above command sends the local <branch> to the specified remote
repository. Except, instead of a remote branch, git push creates a local
branch.
Every collaboration model involves at least one public repository that serves as a point-of-entry for multiple developers. Public repositories have the unique constraint of being bare—they must not have a working directory. This prevents developers from accidentally overwriting each others’ work with git push. You can create a bare repository by passing the --bare option to git init:
git init --bare <path>Public repositories should only function as storage facilities—not development environments
There is a single repository that all developers push to and pull from.
This is a highly hierarchical model where one individual has com- mit rights to a blessed repository that everyone else fetches from. Changes are fetched from developers by lieutenants responsible for specific subsystems and merged and tested. Lieutenant branches are then fetched by the dictator and merged and pushed into the blessed repository, where the cycle starts over again.
This is where each developer has a public repository, but one is considered the ‘official’ repository – it is used to create the packages and binaries. A person or core team has commit rights to it, but many other developers have public forks of that repository. When they have changes, they issue a pull request to an integration man- ager, who adds them as a remote if they haven’t already – then merges, tests, accepts and pushes.
This is largely how community-based git repositories like GitHub were built to work and how many smaller open source projects operate.
conclusion:
In the end, there is really no single right way to do it – being a decentralized system, you can have a model with all of these aspects to it, or any combination you can think of.
In the end, you and your team will have to think about what will work best for you.