NAME

gittutorial - A tutorial introduction to Git

SYNOPSIS

  1. git *

DESCRIPTION

This tutorial explains how to import a new project into Git, makechanges to it, and share changes with other developers.

If you are instead primarily interested in using Git to fetch a project,for example, to test the latest version, you may prefer to start withthe first two chapters of The Git User’s Manual.

First, note that you can get documentation for a command such asgit log —graph with:

  1. $ man git-log

or:

  1. $ git help log

With the latter, you can use the manual viewer of your choice; seegit-help[1] for more information.

It is a good idea to introduce yourself to Git with your name andpublic email address before doing any operation. The easiestway to do so is:

  1. $ git config --global user.name "Your Name Comes Here"
  2. $ git config --global user.email you@yourdomain.example.com

Importing a new project

Assume you have a tarball project.tar.gz with your initial work. Youcan place it under Git revision control as follows.

  1. $ tar xzf project.tar.gz
  2. $ cd project
  3. $ git init

Git will reply

  1. Initialized empty Git repository in .git/

You’ve now initialized the working directory—​you may notice a newdirectory created, named ".git".

Next, tell Git to take a snapshot of the contents of all files under thecurrent directory (note the .), with git add:

  1. $ git add .

This snapshot is now stored in a temporary staging area which Git callsthe "index". You can permanently store the contents of the index in therepository with git commit:

  1. $ git commit

This will prompt you for a commit message. You’ve now stored the firstversion of your project in Git.

Making changes

Modify some files, then add their updated contents to the index:

  1. $ git add file1 file2 file3

You are now ready to commit. You can see what is about to be committedusing git diff with the —cached option:

  1. $ git diff --cached

(Without —cached, git diff will show you any changes thatyou’ve made but not yet added to the index.) You can also get a briefsummary of the situation with git status:

  1. $ git status
  2. On branch master
  3. Changes to be committed:
  4. Your branch is up to date with 'origin/master'.
  5.   (use "git restore --staged <file>..." to unstage)
  6.  
  7.     modified:   file1
  8.     modified:   file2
  9.     modified:   file3

If you need to make any further adjustments, do so now, and then add anynewly modified content to the index. Finally, commit your changes with:

  1. $ git commit

This will again prompt you for a message describing the change, and thenrecord a new version of the project.

Alternatively, instead of running git add beforehand, you can use

  1. $ git commit -a

which will automatically notice any modified (but not new) files, addthem to the index, and commit, all in one step.

A note on commit messages: Though not required, it’s a good idea tobegin the commit message with a single short (less than 50 character)line summarizing the change, followed by a blank line and then a morethorough description. The text up to the first blank line in a commitmessage is treated as the commit title, and that title is usedthroughout Git. For example, git-format-patch[1] turns acommit into email, and it uses the title on the Subject line and therest of the commit in the body.

Git tracks content not files

Many revision control systems provide an add command that tells thesystem to start tracking changes to a new file. Git’s add commanddoes something simpler and more powerful: git add is used both for newand newly modified files, and in both cases it takes a snapshot of thegiven files and stages that content in the index, ready for inclusion inthe next commit.

Viewing project history

At any point you can view the history of your changes using

  1. $ git log

If you also want to see complete diffs at each step, use

  1. $ git log -p

Often the overview of the change is useful to get a feel ofeach step

  1. $ git log --stat --summary

Managing branches

A single Git repository can maintain multiple branches ofdevelopment. To create a new branch named "experimental", use

  1. $ git branch experimental

If you now run

  1. $ git branch

you’ll get a list of all existing branches:

  1.   experimental
  2. * master

The "experimental" branch is the one you just created, and the"master" branch is a default branch that was created for youautomatically. The asterisk marks the branch you are currently on;type

  1. $ git switch experimental

to switch to the experimental branch. Now edit a file, commit thechange, and switch back to the master branch:

  1. (edit file)
  2. $ git commit -a
  3. $ git switch master

Check that the change you made is no longer visible, since it wasmade on the experimental branch and you’re back on the master branch.

You can make a different change on the master branch:

  1. (edit file)
  2. $ git commit -a

at this point the two branches have diverged, with different changesmade in each. To merge the changes made in experimental into master, run

  1. $ git merge experimental

If the changes don’t conflict, you’re done. If there are conflicts,markers will be left in the problematic files showing the conflict;

  1. $ git diff

will show this. Once you’ve edited the files to resolve theconflicts,

  1. $ git commit -a

will commit the result of the merge. Finally,

  1. $ gitk

will show a nice graphical representation of the resulting history.

At this point you could delete the experimental branch with

  1. $ git branch -d experimental

This command ensures that the changes in the experimental branch arealready in the current branch.

If you develop on a branch crazy-idea, then regret it, you can alwaysdelete the branch with

  1. $ git branch -D crazy-idea

Branches are cheap and easy, so this is a good way to try somethingout.

Using Git for collaboration

Suppose that Alice has started a new project with a Git repository in/home/alice/project, and that Bob, who has a home directory on thesame machine, wants to contribute.

Bob begins with:

  1. bob$ git clone /home/alice/project myrepo

This creates a new directory "myrepo" containing a clone of Alice’srepository. The clone is on an equal footing with the originalproject, possessing its own copy of the original project’s history.

Bob then makes some changes and commits them:

  1. (edit files)
  2. bob$ git commit -a
  3. (repeat as necessary)

When he’s ready, he tells Alice to pull changes from the repositoryat /home/bob/myrepo. She does this with:

  1. alice$ cd /home/alice/project
  2. alice$ git pull /home/bob/myrepo master

This merges the changes from Bob’s "master" branch into Alice’scurrent branch. If Alice has made her own changes in the meantime,then she may need to manually fix any conflicts.

The "pull" command thus performs two operations: it fetches changesfrom a remote branch, then merges them into the current branch.

Note that in general, Alice would want her local changes committed beforeinitiating this "pull". If Bob’s work conflicts with what Alice did sincetheir histories forked, Alice will use her working tree and the index toresolve conflicts, and existing local changes will interfere with theconflict resolution process (Git will still perform the fetch but willrefuse to merge —- Alice will have to get rid of her local changes insome way and pull again when this happens).

Alice can peek at what Bob did without merging first, using the "fetch"command; this allows Alice to inspect what Bob did, using a specialsymbol "FETCH_HEAD", in order to determine if he has anything worthpulling, like this:

  1. alice$ git fetch /home/bob/myrepo master
  2. alice$ git log -p HEAD..FETCH_HEAD

This operation is safe even if Alice has uncommitted local changes.The range notation "HEAD..FETCH_HEAD" means "show everything that is reachablefrom the FETCH_HEAD but exclude anything that is reachable from HEAD".Alice already knows everything that leads to her current state (HEAD),and reviews what Bob has in his state (FETCH_HEAD) that she has notseen with this command.

If Alice wants to visualize what Bob did since their histories forkedshe can issue the following command:

  1. $ gitk HEAD..FETCH_HEAD

This uses the same two-dot range notation we saw earlier with git log.

Alice may want to view what both of them did since they forked.She can use three-dot form instead of the two-dot form:

  1. $ gitk HEAD...FETCH_HEAD

This means "show everything that is reachable from either one, butexclude anything that is reachable from both of them".

Please note that these range notation can be used with both gitkand "git log".

After inspecting what Bob did, if there is nothing urgent, Alice maydecide to continue working without pulling from Bob. If Bob’s historydoes have something Alice would immediately need, Alice may choose tostash her work-in-progress first, do a "pull", and then finally unstashher work-in-progress on top of the resulting history.

When you are working in a small closely knit group, it is notunusual to interact with the same repository over and overagain. By defining remote repository shorthand, you can makeit easier:

  1. alice$ git remote add bob /home/bob/myrepo

With this, Alice can perform the first part of the "pull" operationalone using the git fetch command without merging them with her ownbranch, using:

  1. alice$ git fetch bob

Unlike the longhand form, when Alice fetches from Bob using aremote repository shorthand set up with git remote, what wasfetched is stored in a remote-tracking branch, in this casebob/master. So after this:

  1. alice$ git log -p master..bob/master

shows a list of all the changes that Bob made since he branched fromAlice’s master branch.

After examining those changes, Alicecould merge the changes into her master branch:

  1. alice$ git merge bob/master

This merge can also be done by pulling from her own remote-trackingbranch, like this:

  1. alice$ git pull . remotes/bob/master

Note that git pull always merges into the current branch,regardless of what else is given on the command line.

Later, Bob can update his repo with Alice’s latest changes using

  1. bob$ git pull

Note that he doesn’t need to give the path to Alice’s repository;when Bob cloned Alice’s repository, Git stored the location of herrepository in the repository configuration, and that location isused for pulls:

  1. bob$ git config --get remote.origin.url
  2. /home/alice/project

(The complete configuration created by git clone is visible usinggit config -l, and the git-config[1] man pageexplains the meaning of each option.)

Git also keeps a pristine copy of Alice’s master branch under thename "origin/master":

  1. bob$ git branch -r
  2.   origin/master

If Bob later decides to work from a different host, he can stillperform clones and pulls using the ssh protocol:

  1. bob$ git clone alice.org:/home/alice/project myrepo

Alternatively, Git has a native protocol, or can use http;see git-pull[1] for details.

Git can also be used in a CVS-like mode, with a central repositorythat various users push changes to; see git-push[1] andgitcvs-migration[7].

Exploring history

Git history is represented as a series of interrelated commits. Wehave already seen that the git log command can list those commits.Note that first line of each git log entry also gives a name for thecommit:

  1. $ git log
  2. commit c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
  3. Author: Junio C Hamano <junkio@cox.net>
  4. Date:   Tue May 16 17:18:22 2006 -0700
  5.  
  6.     merge-base: Clarify the comments on post processing.

We can give this name to git show to see the details about thiscommit.

  1. $ git show c82a22c39cbc32576f64f5c6b3f24b99ea8149c7

But there are other ways to refer to commits. You can use any initialpart of the name that is long enough to uniquely identify the commit:

  1. $ git show c82a22c39c    # the first few characters of the name are
  2.             # usually enough
  3. $ git show HEAD        # the tip of the current branch
  4. $ git show experimental    # the tip of the "experimental" branch

Every commit usually has one "parent" commitwhich points to the previous state of the project:

  1. $ git show HEAD^  # to see the parent of HEAD
  2. $ git show HEAD^^ # to see the grandparent of HEAD
  3. $ git show HEAD~4 # to see the great-great grandparent of HEAD

Note that merge commits may have more than one parent:

  1. $ git show HEAD^1 # show the first parent of HEAD (same as HEAD^)
  2. $ git show HEAD^2 # show the second parent of HEAD

You can also give commits names of your own; after running

  1. $ git tag v2.5 1b2e1d63ff

you can refer to 1b2e1d63ff by the name "v2.5". If you intend toshare this name with other people (for example, to identify a releaseversion), you should create a "tag" object, and perhaps sign it; seegit-tag[1] for details.

Any Git command that needs to know a commit can take any of thesenames. For example:

  1. $ git diff v2.5 HEAD     # compare the current HEAD to v2.5
  2. $ git branch stable v2.5 # start a new branch named "stable" based
  3.              # at v2.5
  4. $ git reset --hard HEAD^ # reset your current branch and working
  5.              # directory to its state at HEAD^

Be careful with that last command: in addition to losing any changesin the working directory, it will also remove all later commits fromthis branch. If this branch is the only branch containing thosecommits, they will be lost. Also, don’t use git reset on apublicly-visible branch that other developers pull from, as it willforce needless merges on other developers to clean up the history.If you need to undo changes that you have pushed, use _git revert_instead.

The git grep command can search for strings in any version of yourproject, so

  1. $ git grep "hello" v2.5

searches for all occurrences of "hello" in v2.5.

If you leave out the commit name, git grep will search any of thefiles it manages in your current directory. So

  1. $ git grep "hello"

is a quick way to search just the files that are tracked by Git.

Many Git commands also take sets of commits, which can be specifiedin a number of ways. Here are some examples with git log:

  1. $ git log v2.5..v2.6            # commits between v2.5 and v2.6
  2. $ git log v2.5..                # commits since v2.5
  3. $ git log --since="2 weeks ago" # commits from the last 2 weeks
  4. $ git log v2.5.. Makefile       # commits since v2.5 which modify
  5.                 # Makefile

You can also give git log a "range" of commits where the first is notnecessarily an ancestor of the second; for example, if the tips ofthe branches "stable" and "master" diverged from a commoncommit some time ago, then

  1. $ git log stable..master

will list commits made in the master branch but not in thestable branch, while

  1. $ git log master..stable

will show the list of commits made on the stable branch but notthe master branch.

The git log command has a weakness: it must present commits in alist. When the history has lines of development that diverged andthen merged back together, the order in which git log presentsthose commits is meaningless.

Most projects with multiple contributors (such as the Linux kernel,or Git itself) have frequent merges, and gitk does a better job ofvisualizing their history. For example,

  1. $ gitk --since="2 weeks ago" drivers/

allows you to browse any commits from the last 2 weeks of commitsthat modified files under the "drivers" directory. (Note: you canadjust gitk’s fonts by holding down the control key while pressing"-" or "+".)

Finally, most commands that take filenames will optionally allow youto precede any filename by a commit, to specify a particular versionof the file:

  1. $ git diff v2.5:Makefile HEAD:Makefile.in

You can also use git show to see any such file:

  1. $ git show v2.5:Makefile

Next Steps

This tutorial should be enough to perform basic distributed revisioncontrol for your projects. However, to fully understand the depthand power of Git you need to understand two simple ideas on which itis based:

  • The object database is the rather elegant system used tostore the history of your project—​files, directories, andcommits.

  • The index file is a cache of the state of a directory tree,used to create commits, check out working directories, andhold the various trees involved in a merge.

Part two of this tutorial explains the objectdatabase, the index file, and a few other odds and ends that you’llneed to make the most of Git. You can find it at gittutorial-2[7].

If you don’t want to continue with that right away, a few otherdigressions that may be interesting at this point are:

  • git-format-patch[1], git-am[1]: These convertseries of git commits into emailed patches, and vice versa,useful for projects such as the Linux kernel which rely heavilyon emailed patches.

  • git-bisect[1]: When there is a regression in yourproject, one way to track down the bug is by searching throughthe history to find the exact commit that’s to blame. Git bisectcan help you perform a binary search for that commit. It issmart enough to perform a close-to-optimal search even in thecase of complex non-linear history with lots of merged branches.

  • gitworkflows[7]: Gives an overview of recommendedworkflows.

  • giteveryday[7]: Everyday Git with 20 Commands Or So.

  • gitcvs-migration[7]: Git for CVS users.

SEE ALSO

gittutorial-2[7],gitcvs-migration[7],gitcore-tutorial[7],gitglossary[7],git-help[1],gitworkflows[7],giteveryday[7],The Git User’s Manual

GIT

Part of the git[1] suite