Ubuntu Manpage: gittutorial - A tutorial introduction to Git

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NAME

       gittutorial - A tutorial introduction to Git

SYNOPSIS

       git *

DESCRIPTION

       This tutorial explains how to import a new project into Git, make changes 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 with the first two chapters of The Git
       User’s Manual[1].

       First, note that you can get documentation for a command such as git log --graph with:

           $ man git-log

       or:

           $ git help log

       With the latter, you can use the manual viewer of your choice; see git-help(1) for more
       information.

       It is a good idea to introduce yourself to Git with your name and public email address
       before doing any operation. The easiest way to do so is:

           $ git config --global user.name "Your Name Comes Here"
           $ 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. You can place it under
       Git revision control as follows.

           $ tar xzf project.tar.gz
           $ cd project
           $ git init

       Git will reply

           Initialized empty Git repository in .git/

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

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

           $ git add .

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

           $ git commit

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

MAKING CHANGES

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

           $ git add file1 file2 file3

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

           $ git diff --cached

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

           $ git status
           On branch master
           Changes to be committed:
             (use "git restore --staged <file>..." to unstage)

                   modified:   file1
                   modified:   file2
                   modified:   file3

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

           $ git commit

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

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

           $ git commit -a

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

       A note on commit messages: Though not required, it’s a good idea to begin the commit
       message with a single short (no more than 50 characters) line summarizing the change,
       followed by a blank line and then a more thorough description. The text up to the first
       blank line in a commit message is treated as the commit title, and that title is used
       throughout Git. For example, git-format-patch(1) turns a commit into email, and it uses
       the title on the Subject line and the rest of the commit in the body.

GIT TRACKS CONTENT NOT FILES

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

VIEWING PROJECT HISTORY

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

           $ git log

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

           $ git log -p

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

           $ git log --stat --summary

MANAGING BRANCHES

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

$ git branch experimental

If you now run

$ git branch

you’ll get a list of all existing branches:

experimental
* master

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

$ git switch experimental

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

(edit file)
$ git commit -a
$ git switch master

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

You can make a different change on the master branch:

(edit file)
$ git commit -a

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

$ 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;

$ git diff

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

$ git commit -a

will commit the result of the merge. Finally,

$ gitk

will show a nice graphical representation of the resulting history.

At this point you could delete the experimental branch with

$ git branch -d experimental

This command ensures that the changes in the experimental branch are already in the
current branch.

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

$ git branch -D crazy-idea

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

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 the same machine, wants to contribute.

       Bob begins with:

           bob$ git clone /home/alice/project myrepo

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

       Bob then makes some changes and commits them:

           (edit files)
           bob$ git commit -a
           (repeat as necessary)

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

           alice$ cd /home/alice/project
           alice$ git pull /home/bob/myrepo master

       This merges the changes from Bob’s master branch into Alice’s current 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 changes from a remote branch,
       then merges them into the current branch.

       Note that in general, Alice would want her local changes committed before initiating this
       pull. If Bob’s work conflicts with what Alice did since their histories forked, Alice will
       use her working tree and the index to resolve conflicts, and existing local changes will
       interfere with the conflict resolution process (Git will still perform the fetch but will
       refuse to merge — Alice will have to get rid of her local changes in some 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 special symbol FETCH_HEAD, in order to determine if
       he has anything worth pulling, like this:

           alice$ git fetch /home/bob/myrepo master
           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 reachable from 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 not
       seen with this command.

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

           $ 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:

           $ gitk HEAD...FETCH_HEAD

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

       Please note that these range notation can be used with both gitk and git log.

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

       When you are working in a small closely knit group, it is not unusual to interact with the
       same repository over and over again. By defining remote repository shorthand, you can make
       it easier:

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

       With this, Alice can perform the first part of the pull operation alone using the git
       fetch command without merging them with her own branch, using:

           alice$ git fetch bob

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

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

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

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

           alice$ git merge bob/master

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

           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

           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 her repository in the repository configuration, and
       that location is used for pulls:

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

       (The complete configuration created by git clone is visible using git config -l, and the
       git-config(1) man page explains the meaning of each option.)

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

           bob$ git branch -r
             origin/master

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

           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 repository that various users push
       changes to; see git-push(1) and gitcvs-migration(7).

EXPLORING HISTORY

       Git history is represented as a series of interrelated commits. We have 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 the commit:

           $ git log
           commit c82a22c39cbc32576f64f5c6b3f24b99ea8149c7
           Author: Junio C Hamano <junkio@cox.net>
           Date:   Tue May 16 17:18:22 2006 -0700

               merge-base: Clarify the comments on post processing.

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

           $ git show c82a22c39cbc32576f64f5c6b3f24b99ea8149c7

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

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

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

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

       Note that merge commits may have more than one parent:

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

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

           $ git tag v2.5 1b2e1d63ff

       you can refer to 1b2e1d63ff by the name v2.5. If you intend to share this name with other
       people (for example, to identify a release version), you should create a "tag" object, and
       perhaps sign it; see git-tag(1) for details.

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

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

       Be careful with that last command: in addition to losing any changes in the working
       directory, it will also remove all later commits from this branch. If this branch is the
       only branch containing those commits, they will be lost. Also, don’t use git reset on a
       publicly-visible branch that other developers pull from, as it will force 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 your project, so

           $ 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 the files it manages in your
       current directory. So

           $ 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 specified in a number of ways.
       Here are some examples with git log:

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

       You can also give git log a "range" of commits where the first is not necessarily an
       ancestor of the second; for example, if the tips of the branches stable and master
       diverged from a common commit some time ago, then

           $ git log stable..master

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

           $ git log master..stable

       will show the list of commits made on the stable branch but not the master branch.

       The git log command has a weakness: it must present commits in a list. When the history
       has lines of development that diverged and then merged back together, the order in which
       git log presents those 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 of visualizing their history. For example,

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

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

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

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

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

           $ git show v2.5:Makefile

NEXT STEPS

This tutorial should be enough to perform basic distributed revision control for your
projects. However, to fully understand the depth and power of Git you need to understand
two simple ideas on which it is based:

• The object database is the rather elegant system used to store the history of your
project—files, directories, and commits.

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

Part two of this tutorial explains the object database, the index file, and a few other
odds and ends that you’ll need 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 other digressions that may be
interesting at this point are:

• git-format-patch(1), git-am(1): These convert series of git commits into emailed
patches, and vice versa, useful for projects such as the Linux kernel which rely
heavily on emailed patches.

• git-bisect(1): When there is a regression in your project, one way to track down the
bug is by searching through the history to find the exact commit that’s to blame. git
bisect can help you perform a binary search for that commit. It is smart enough to
perform a close-to-optimal search even in the case of complex non-linear history with
lots of merged branches.

• gitworkflows(7): Gives an overview of recommended workflows.

• giteveryday(7): Everyday Git with 20 Commands Or So.

• gitcvs-migration(7): Git for CVS users.

GIT

       Part of the git(1) suite

NOTES

        1. The Git User’s Manual
           file:///usr/share/doc/git/html/user-manual.html