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diff --git a/Documentation/user-manual.txt b/Documentation/user-manual.txt new file mode 100644 index 0000000000..d6caff4248 --- /dev/null +++ b/Documentation/user-manual.txt @@ -0,0 +1,3975 @@ +Git User's Manual (for version 1.5.1 or newer) +______________________________________________ + + +Git is a fast distributed revision control system. + +This manual is designed to be readable by someone with basic unix +command-line skills, but no previous knowledge of git. + +<<repositories-and-branches>> and <<exploring-git-history>> explain how +to fetch and study a project using git--read these chapters to learn how +to build and test a particular version of a software project, search for +regressions, and so on. + +People needing to do actual development will also want to read +<<Developing-with-git>> and <<sharing-development>>. + +Further chapters cover more specialized topics. + +Comprehensive reference documentation is available through the man +pages. For a command such as "git clone", just use + +------------------------------------------------ +$ man git-clone +------------------------------------------------ + +See also <<git-quick-start>> for a brief overview of git commands, +without any explanation. + +Finally, see <<todo>> for ways that you can help make this manual more +complete. + + +[[repositories-and-branches]] +Repositories and Branches +========================= + +[[how-to-get-a-git-repository]] +How to get a git repository +--------------------------- + +It will be useful to have a git repository to experiment with as you +read this manual. + +The best way to get one is by using the gitlink:git-clone[1] command +to download a copy of an existing repository for a project that you +are interested in. If you don't already have a project in mind, here +are some interesting examples: + +------------------------------------------------ + # git itself (approx. 10MB download): +$ git clone git://git.kernel.org/pub/scm/git/git.git + # the linux kernel (approx. 150MB download): +$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git +------------------------------------------------ + +The initial clone may be time-consuming for a large project, but you +will only need to clone once. + +The clone command creates a new directory named after the project +("git" or "linux-2.6" in the examples above). After you cd into this +directory, you will see that it contains a copy of the project files, +together with a special top-level directory named ".git", which +contains all the information about the history of the project. + +In most of the following, examples will be taken from one of the two +repositories above. + +[[how-to-check-out]] +How to check out a different version of a project +------------------------------------------------- + +Git is best thought of as a tool for storing the history of a +collection of files. It stores the history as a compressed +collection of interrelated snapshots (versions) of the project's +contents. + +A single git repository may contain multiple branches. It keeps track +of them by keeping a list of <<def_head,heads>> which reference the +latest version on each branch; the gitlink:git-branch[1] command shows +you the list of branch heads: + +------------------------------------------------ +$ git branch +* master +------------------------------------------------ + +A freshly cloned repository contains a single branch head, by default +named "master", with the working directory initialized to the state of +the project referred to by that branch head. + +Most projects also use <<def_tag,tags>>. Tags, like heads, are +references into the project's history, and can be listed using the +gitlink:git-tag[1] command: + +------------------------------------------------ +$ git tag -l +v2.6.11 +v2.6.11-tree +v2.6.12 +v2.6.12-rc2 +v2.6.12-rc3 +v2.6.12-rc4 +v2.6.12-rc5 +v2.6.12-rc6 +v2.6.13 +... +------------------------------------------------ + +Tags are expected to always point at the same version of a project, +while heads are expected to advance as development progresses. + +Create a new branch head pointing to one of these versions and check it +out using gitlink:git-checkout[1]: + +------------------------------------------------ +$ git checkout -b new v2.6.13 +------------------------------------------------ + +The working directory then reflects the contents that the project had +when it was tagged v2.6.13, and gitlink:git-branch[1] shows two +branches, with an asterisk marking the currently checked-out branch: + +------------------------------------------------ +$ git branch + master +* new +------------------------------------------------ + +If you decide that you'd rather see version 2.6.17, you can modify +the current branch to point at v2.6.17 instead, with + +------------------------------------------------ +$ git reset --hard v2.6.17 +------------------------------------------------ + +Note that if the current branch head was your only reference to a +particular point in history, then resetting that branch may leave you +with no way to find the history it used to point to; so use this command +carefully. + +[[understanding-commits]] +Understanding History: Commits +------------------------------ + +Every change in the history of a project is represented by a commit. +The gitlink:git-show[1] command shows the most recent commit on the +current branch: + +------------------------------------------------ +$ git show +commit 2b5f6dcce5bf94b9b119e9ed8d537098ec61c3d2 +Author: Jamal Hadi Salim <hadi@cyberus.ca> +Date: Sat Dec 2 22:22:25 2006 -0800 + + [XFRM]: Fix aevent structuring to be more complete. + + aevents can not uniquely identify an SA. We break the ABI with this + patch, but consensus is that since it is not yet utilized by any + (known) application then it is fine (better do it now than later). + + Signed-off-by: Jamal Hadi Salim <hadi@cyberus.ca> + Signed-off-by: David S. Miller <davem@davemloft.net> + +diff --git a/Documentation/networking/xfrm_sync.txt b/Documentation/networking/xfrm_sync.txt +index 8be626f..d7aac9d 100644 +--- a/Documentation/networking/xfrm_sync.txt ++++ b/Documentation/networking/xfrm_sync.txt +@@ -47,10 +47,13 @@ aevent_id structure looks like: + + struct xfrm_aevent_id { + struct xfrm_usersa_id sa_id; ++ xfrm_address_t saddr; + __u32 flags; ++ __u32 reqid; + }; +... +------------------------------------------------ + +As you can see, a commit shows who made the latest change, what they +did, and why. + +Every commit has a 40-hexdigit id, sometimes called the "object name" or the +"SHA1 id", shown on the first line of the "git show" output. You can usually +refer to a commit by a shorter name, such as a tag or a branch name, but this +longer name can also be useful. Most importantly, it is a globally unique +name for this commit: so if you tell somebody else the object name (for +example in email), then you are guaranteed that name will refer to the same +commit in their repository that it does in yours (assuming their repository +has that commit at all). Since the object name is computed as a hash over the +contents of the commit, you are guaranteed that the commit can never change +without its name also changing. + +In fact, in <<git-internals>> we shall see that everything stored in git +history, including file data and directory contents, is stored in an object +with a name that is a hash of its contents. + +[[understanding-reachability]] +Understanding history: commits, parents, and reachability +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Every commit (except the very first commit in a project) also has a +parent commit which shows what happened before this commit. +Following the chain of parents will eventually take you back to the +beginning of the project. + +However, the commits do not form a simple list; git allows lines of +development to diverge and then reconverge, and the point where two +lines of development reconverge is called a "merge". The commit +representing a merge can therefore have more than one parent, with +each parent representing the most recent commit on one of the lines +of development leading to that point. + +The best way to see how this works is using the gitlink:gitk[1] +command; running gitk now on a git repository and looking for merge +commits will help understand how the git organizes history. + +In the following, we say that commit X is "reachable" from commit Y +if commit X is an ancestor of commit Y. Equivalently, you could say +that Y is a descendent of X, or that there is a chain of parents +leading from commit Y to commit X. + +[[history-diagrams]] +Understanding history: History diagrams +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +We will sometimes represent git history using diagrams like the one +below. Commits are shown as "o", and the links between them with +lines drawn with - / and \. Time goes left to right: + + +................................................ + o--o--o <-- Branch A + / + o--o--o <-- master + \ + o--o--o <-- Branch B +................................................ + +If we need to talk about a particular commit, the character "o" may +be replaced with another letter or number. + +[[what-is-a-branch]] +Understanding history: What is a branch? +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +When we need to be precise, we will use the word "branch" to mean a line +of development, and "branch head" (or just "head") to mean a reference +to the most recent commit on a branch. In the example above, the branch +head named "A" is a pointer to one particular commit, but we refer to +the line of three commits leading up to that point as all being part of +"branch A". + +However, when no confusion will result, we often just use the term +"branch" both for branches and for branch heads. + +[[manipulating-branches]] +Manipulating branches +--------------------- + +Creating, deleting, and modifying branches is quick and easy; here's +a summary of the commands: + +git branch:: + list all branches +git branch <branch>:: + create a new branch named <branch>, referencing the same + point in history as the current branch +git branch <branch> <start-point>:: + create a new branch named <branch>, referencing + <start-point>, which may be specified any way you like, + including using a branch name or a tag name +git branch -d <branch>:: + delete the branch <branch>; if the branch you are deleting + points to a commit which is not reachable from the current + branch, this command will fail with a warning. +git branch -D <branch>:: + even if the branch points to a commit not reachable + from the current branch, you may know that that commit + is still reachable from some other branch or tag. In that + case it is safe to use this command to force git to delete + the branch. +git checkout <branch>:: + make the current branch <branch>, updating the working + directory to reflect the version referenced by <branch> +git checkout -b <new> <start-point>:: + create a new branch <new> referencing <start-point>, and + check it out. + +The special symbol "HEAD" can always be used to refer to the current +branch. In fact, git uses a file named "HEAD" in the .git directory to +remember which branch is current: + +------------------------------------------------ +$ cat .git/HEAD +ref: refs/heads/master +------------------------------------------------ + +[[detached-head]] +Examining an old version without creating a new branch +------------------------------------------------------ + +The git-checkout command normally expects a branch head, but will also +accept an arbitrary commit; for example, you can check out the commit +referenced by a tag: + +------------------------------------------------ +$ git checkout v2.6.17 +Note: moving to "v2.6.17" which isn't a local branch +If you want to create a new branch from this checkout, you may do so +(now or later) by using -b with the checkout command again. Example: + git checkout -b <new_branch_name> +HEAD is now at 427abfa... Linux v2.6.17 +------------------------------------------------ + +The HEAD then refers to the SHA1 of the commit instead of to a branch, +and git branch shows that you are no longer on a branch: + +------------------------------------------------ +$ cat .git/HEAD +427abfa28afedffadfca9dd8b067eb6d36bac53f +$ git branch +* (no branch) + master +------------------------------------------------ + +In this case we say that the HEAD is "detached". + +This is an easy way to check out a particular version without having to +make up a name for the new branch. You can still create a new branch +(or tag) for this version later if you decide to. + +[[examining-remote-branches]] +Examining branches from a remote repository +------------------------------------------- + +The "master" branch that was created at the time you cloned is a copy +of the HEAD in the repository that you cloned from. That repository +may also have had other branches, though, and your local repository +keeps branches which track each of those remote branches, which you +can view using the "-r" option to gitlink:git-branch[1]: + +------------------------------------------------ +$ git branch -r + origin/HEAD + origin/html + origin/maint + origin/man + origin/master + origin/next + origin/pu + origin/todo +------------------------------------------------ + +You cannot check out these remote-tracking branches, but you can +examine them on a branch of your own, just as you would a tag: + +------------------------------------------------ +$ git checkout -b my-todo-copy origin/todo +------------------------------------------------ + +Note that the name "origin" is just the name that git uses by default +to refer to the repository that you cloned from. + +[[how-git-stores-references]] +Naming branches, tags, and other references +------------------------------------------- + +Branches, remote-tracking branches, and tags are all references to +commits. All references are named with a slash-separated path name +starting with "refs"; the names we've been using so far are actually +shorthand: + + - The branch "test" is short for "refs/heads/test". + - The tag "v2.6.18" is short for "refs/tags/v2.6.18". + - "origin/master" is short for "refs/remotes/origin/master". + +The full name is occasionally useful if, for example, there ever +exists a tag and a branch with the same name. + +As another useful shortcut, the "HEAD" of a repository can be referred +to just using the name of that repository. So, for example, "origin" +is usually a shortcut for the HEAD branch in the repository "origin". + +For the complete list of paths which git checks for references, and +the order it uses to decide which to choose when there are multiple +references with the same shorthand name, see the "SPECIFYING +REVISIONS" section of gitlink:git-rev-parse[1]. + +[[Updating-a-repository-with-git-fetch]] +Updating a repository with git fetch +------------------------------------ + +Eventually the developer cloned from will do additional work in her +repository, creating new commits and advancing the branches to point +at the new commits. + +The command "git fetch", with no arguments, will update all of the +remote-tracking branches to the latest version found in her +repository. It will not touch any of your own branches--not even the +"master" branch that was created for you on clone. + +[[fetching-branches]] +Fetching branches from other repositories +----------------------------------------- + +You can also track branches from repositories other than the one you +cloned from, using gitlink:git-remote[1]: + +------------------------------------------------- +$ git remote add linux-nfs git://linux-nfs.org/pub/nfs-2.6.git +$ git fetch linux-nfs +* refs/remotes/linux-nfs/master: storing branch 'master' ... + commit: bf81b46 +------------------------------------------------- + +New remote-tracking branches will be stored under the shorthand name +that you gave "git remote add", in this case linux-nfs: + +------------------------------------------------- +$ git branch -r +linux-nfs/master +origin/master +------------------------------------------------- + +If you run "git fetch <remote>" later, the tracking branches for the +named <remote> will be updated. + +If you examine the file .git/config, you will see that git has added +a new stanza: + +------------------------------------------------- +$ cat .git/config +... +[remote "linux-nfs"] + url = git://linux-nfs.org/pub/nfs-2.6.git + fetch = +refs/heads/*:refs/remotes/linux-nfs/* +... +------------------------------------------------- + +This is what causes git to track the remote's branches; you may modify +or delete these configuration options by editing .git/config with a +text editor. (See the "CONFIGURATION FILE" section of +gitlink:git-config[1] for details.) + +[[exploring-git-history]] +Exploring git history +===================== + +Git is best thought of as a tool for storing the history of a +collection of files. It does this by storing compressed snapshots of +the contents of a file heirarchy, together with "commits" which show +the relationships between these snapshots. + +Git provides extremely flexible and fast tools for exploring the +history of a project. + +We start with one specialized tool that is useful for finding the +commit that introduced a bug into a project. + +[[using-bisect]] +How to use bisect to find a regression +-------------------------------------- + +Suppose version 2.6.18 of your project worked, but the version at +"master" crashes. Sometimes the best way to find the cause of such a +regression is to perform a brute-force search through the project's +history to find the particular commit that caused the problem. The +gitlink:git-bisect[1] command can help you do this: + +------------------------------------------------- +$ git bisect start +$ git bisect good v2.6.18 +$ git bisect bad master +Bisecting: 3537 revisions left to test after this +[65934a9a028b88e83e2b0f8b36618fe503349f8e] BLOCK: Make USB storage depend on SCSI rather than selecting it [try #6] +------------------------------------------------- + +If you run "git branch" at this point, you'll see that git has +temporarily moved you to a new branch named "bisect". This branch +points to a commit (with commit id 65934...) that is reachable from +v2.6.19 but not from v2.6.18. Compile and test it, and see whether +it crashes. Assume it does crash. Then: + +------------------------------------------------- +$ git bisect bad +Bisecting: 1769 revisions left to test after this +[7eff82c8b1511017ae605f0c99ac275a7e21b867] i2c-core: Drop useless bitmaskings +------------------------------------------------- + +checks out an older version. Continue like this, telling git at each +stage whether the version it gives you is good or bad, and notice +that the number of revisions left to test is cut approximately in +half each time. + +After about 13 tests (in this case), it will output the commit id of +the guilty commit. You can then examine the commit with +gitlink:git-show[1], find out who wrote it, and mail them your bug +report with the commit id. Finally, run + +------------------------------------------------- +$ git bisect reset +------------------------------------------------- + +to return you to the branch you were on before and delete the +temporary "bisect" branch. + +Note that the version which git-bisect checks out for you at each +point is just a suggestion, and you're free to try a different +version if you think it would be a good idea. For example, +occasionally you may land on a commit that broke something unrelated; +run + +------------------------------------------------- +$ git bisect visualize +------------------------------------------------- + +which will run gitk and label the commit it chose with a marker that +says "bisect". Chose a safe-looking commit nearby, note its commit +id, and check it out with: + +------------------------------------------------- +$ git reset --hard fb47ddb2db... +------------------------------------------------- + +then test, run "bisect good" or "bisect bad" as appropriate, and +continue. + +[[naming-commits]] +Naming commits +-------------- + +We have seen several ways of naming commits already: + + - 40-hexdigit object name + - branch name: refers to the commit at the head of the given + branch + - tag name: refers to the commit pointed to by the given tag + (we've seen branches and tags are special cases of + <<how-git-stores-references,references>>). + - HEAD: refers to the head of the current branch + +There are many more; see the "SPECIFYING REVISIONS" section of the +gitlink:git-rev-parse[1] man page for the complete list of ways to +name revisions. Some examples: + +------------------------------------------------- +$ git show fb47ddb2 # the first few characters of the object name + # are usually enough to specify it uniquely +$ git show HEAD^ # the parent of the HEAD commit +$ git show HEAD^^ # the grandparent +$ git show HEAD~4 # the great-great-grandparent +------------------------------------------------- + +Recall that merge commits may have more than one parent; by default, +^ and ~ follow the first parent listed in the commit, but you can +also choose: + +------------------------------------------------- +$ git show HEAD^1 # show the first parent of HEAD +$ git show HEAD^2 # show the second parent of HEAD +------------------------------------------------- + +In addition to HEAD, there are several other special names for +commits: + +Merges (to be discussed later), as well as operations such as +git-reset, which change the currently checked-out commit, generally +set ORIG_HEAD to the value HEAD had before the current operation. + +The git-fetch operation always stores the head of the last fetched +branch in FETCH_HEAD. For example, if you run git fetch without +specifying a local branch as the target of the operation + +------------------------------------------------- +$ git fetch git://example.com/proj.git theirbranch +------------------------------------------------- + +the fetched commits will still be available from FETCH_HEAD. + +When we discuss merges we'll also see the special name MERGE_HEAD, +which refers to the other branch that we're merging in to the current +branch. + +The gitlink:git-rev-parse[1] command is a low-level command that is +occasionally useful for translating some name for a commit to the object +name for that commit: + +------------------------------------------------- +$ git rev-parse origin +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +[[creating-tags]] +Creating tags +------------- + +We can also create a tag to refer to a particular commit; after +running + +------------------------------------------------- +$ git tag stable-1 1b2e1d63ff +------------------------------------------------- + +You can use stable-1 to refer to the commit 1b2e1d63ff. + +This creates a "lightweight" tag. If you would also like to include a +comment with the tag, and possibly sign it cryptographically, then you +should create a tag object instead; see the gitlink:git-tag[1] man page +for details. + +[[browsing-revisions]] +Browsing revisions +------------------ + +The gitlink:git-log[1] command can show lists of commits. On its +own, it shows all commits reachable from the parent commit; but you +can also make more specific requests: + +------------------------------------------------- +$ git log v2.5.. # commits since (not reachable from) v2.5 +$ git log test..master # commits reachable from master but not test +$ git log master..test # ...reachable from test but not master +$ git log master...test # ...reachable from either test or master, + # but not both +$ git log --since="2 weeks ago" # commits from the last 2 weeks +$ git log Makefile # commits which modify Makefile +$ git log fs/ # ... which modify any file under fs/ +$ git log -S'foo()' # commits which add or remove any file data + # matching the string 'foo()' +------------------------------------------------- + +And of course you can combine all of these; the following finds +commits since v2.5 which touch the Makefile or any file under fs: + +------------------------------------------------- +$ git log v2.5.. Makefile fs/ +------------------------------------------------- + +You can also ask git log to show patches: + +------------------------------------------------- +$ git log -p +------------------------------------------------- + +See the "--pretty" option in the gitlink:git-log[1] man page for more +display options. + +Note that git log starts with the most recent commit and works +backwards through the parents; however, since git history can contain +multiple independent lines of development, the particular order that +commits are listed in may be somewhat arbitrary. + +[[generating-diffs]] +Generating diffs +---------------- + +You can generate diffs between any two versions using +gitlink:git-diff[1]: + +------------------------------------------------- +$ git diff master..test +------------------------------------------------- + +Sometimes what you want instead is a set of patches: + +------------------------------------------------- +$ git format-patch master..test +------------------------------------------------- + +will generate a file with a patch for each commit reachable from test +but not from master. Note that if master also has commits which are +not reachable from test, then the combined result of these patches +will not be the same as the diff produced by the git-diff example. + +[[viewing-old-file-versions]] +Viewing old file versions +------------------------- + +You can always view an old version of a file by just checking out the +correct revision first. But sometimes it is more convenient to be +able to view an old version of a single file without checking +anything out; this command does that: + +------------------------------------------------- +$ git show v2.5:fs/locks.c +------------------------------------------------- + +Before the colon may be anything that names a commit, and after it +may be any path to a file tracked by git. + +[[history-examples]] +Examples +-------- + +[[counting-commits-on-a-branch]] +Counting the number of commits on a branch +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Suppose you want to know how many commits you've made on "mybranch" +since it diverged from "origin": + +------------------------------------------------- +$ git log --pretty=oneline origin..mybranch | wc -l +------------------------------------------------- + +Alternatively, you may often see this sort of thing done with the +lower-level command gitlink:git-rev-list[1], which just lists the SHA1's +of all the given commits: + +------------------------------------------------- +$ git rev-list origin..mybranch | wc -l +------------------------------------------------- + +[[checking-for-equal-branches]] +Check whether two branches point at the same history +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Suppose you want to check whether two branches point at the same point +in history. + +------------------------------------------------- +$ git diff origin..master +------------------------------------------------- + +will tell you whether the contents of the project are the same at the +two branches; in theory, however, it's possible that the same project +contents could have been arrived at by two different historical +routes. You could compare the object names: + +------------------------------------------------- +$ git rev-list origin +e05db0fd4f31dde7005f075a84f96b360d05984b +$ git rev-list master +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +Or you could recall that the ... operator selects all commits +contained reachable from either one reference or the other but not +both: so + +------------------------------------------------- +$ git log origin...master +------------------------------------------------- + +will return no commits when the two branches are equal. + +[[finding-tagged-descendants]] +Find first tagged version including a given fix +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Suppose you know that the commit e05db0fd fixed a certain problem. +You'd like to find the earliest tagged release that contains that +fix. + +Of course, there may be more than one answer--if the history branched +after commit e05db0fd, then there could be multiple "earliest" tagged +releases. + +You could just visually inspect the commits since e05db0fd: + +------------------------------------------------- +$ gitk e05db0fd.. +------------------------------------------------- + +Or you can use gitlink:git-name-rev[1], which will give the commit a +name based on any tag it finds pointing to one of the commit's +descendants: + +------------------------------------------------- +$ git name-rev --tags e05db0fd +e05db0fd tags/v1.5.0-rc1^0~23 +------------------------------------------------- + +The gitlink:git-describe[1] command does the opposite, naming the +revision using a tag on which the given commit is based: + +------------------------------------------------- +$ git describe e05db0fd +v1.5.0-rc0-260-ge05db0f +------------------------------------------------- + +but that may sometimes help you guess which tags might come after the +given commit. + +If you just want to verify whether a given tagged version contains a +given commit, you could use gitlink:git-merge-base[1]: + +------------------------------------------------- +$ git merge-base e05db0fd v1.5.0-rc1 +e05db0fd4f31dde7005f075a84f96b360d05984b +------------------------------------------------- + +The merge-base command finds a common ancestor of the given commits, +and always returns one or the other in the case where one is a +descendant of the other; so the above output shows that e05db0fd +actually is an ancestor of v1.5.0-rc1. + +Alternatively, note that + +------------------------------------------------- +$ git log v1.5.0-rc1..e05db0fd +------------------------------------------------- + +will produce empty output if and only if v1.5.0-rc1 includes e05db0fd, +because it outputs only commits that are not reachable from v1.5.0-rc1. + +As yet another alternative, the gitlink:git-show-branch[1] command lists +the commits reachable from its arguments with a display on the left-hand +side that indicates which arguments that commit is reachable from. So, +you can run something like + +------------------------------------------------- +$ git show-branch e05db0fd v1.5.0-rc0 v1.5.0-rc1 v1.5.0-rc2 +! [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if +available + ! [v1.5.0-rc0] GIT v1.5.0 preview + ! [v1.5.0-rc1] GIT v1.5.0-rc1 + ! [v1.5.0-rc2] GIT v1.5.0-rc2 +... +------------------------------------------------- + +then search for a line that looks like + +------------------------------------------------- ++ ++ [e05db0fd] Fix warnings in sha1_file.c - use C99 printf format if +available +------------------------------------------------- + +Which shows that e05db0fd is reachable from itself, from v1.5.0-rc1, and +from v1.5.0-rc2, but not from v1.5.0-rc0. + +[[showing-commits-unique-to-a-branch]] +Showing commits unique to a given branch +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Suppose you would like to see all the commits reachable from the branch +head named "master" but not from any other head in your repository. + +We can list all the heads in this repository with +gitlink:git-show-ref[1]: + +------------------------------------------------- +$ git show-ref --heads +bf62196b5e363d73353a9dcf094c59595f3153b7 refs/heads/core-tutorial +db768d5504c1bb46f63ee9d6e1772bd047e05bf9 refs/heads/maint +a07157ac624b2524a059a3414e99f6f44bebc1e7 refs/heads/master +24dbc180ea14dc1aebe09f14c8ecf32010690627 refs/heads/tutorial-2 +1e87486ae06626c2f31eaa63d26fc0fd646c8af2 refs/heads/tutorial-fixes +------------------------------------------------- + +We can get just the branch-head names, and remove "master", with +the help of the standard utilities cut and grep: + +------------------------------------------------- +$ git show-ref --heads | cut -d' ' -f2 | grep -v '^refs/heads/master' +refs/heads/core-tutorial +refs/heads/maint +refs/heads/tutorial-2 +refs/heads/tutorial-fixes +------------------------------------------------- + +And then we can ask to see all the commits reachable from master +but not from these other heads: + +------------------------------------------------- +$ gitk master --not $( git show-ref --heads | cut -d' ' -f2 | + grep -v '^refs/heads/master' ) +------------------------------------------------- + +Obviously, endless variations are possible; for example, to see all +commits reachable from some head but not from any tag in the repository: + +------------------------------------------------- +$ gitk $( git show-ref --heads ) --not $( git show-ref --tags ) +------------------------------------------------- + +(See gitlink:git-rev-parse[1] for explanations of commit-selecting +syntax such as `--not`.) + +[[making-a-release]] +Creating a changelog and tarball for a software release +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The gitlink:git-archive[1] command can create a tar or zip archive from +any version of a project; for example: + +------------------------------------------------- +$ git archive --format=tar --prefix=project/ HEAD | gzip >latest.tar.gz +------------------------------------------------- + +will use HEAD to produce a tar archive in which each filename is +preceded by "project/". + +If you're releasing a new version of a software project, you may want +to simultaneously make a changelog to include in the release +announcement. + +Linus Torvalds, for example, makes new kernel releases by tagging them, +then running: + +------------------------------------------------- +$ release-script 2.6.12 2.6.13-rc6 2.6.13-rc7 +------------------------------------------------- + +where release-script is a shell script that looks like: + +------------------------------------------------- +#!/bin/sh +stable="$1" +last="$2" +new="$3" +echo "# git tag v$new" +echo "git archive --prefix=linux-$new/ v$new | gzip -9 > ../linux-$new.tar.gz" +echo "git diff v$stable v$new | gzip -9 > ../patch-$new.gz" +echo "git log --no-merges v$new ^v$last > ../ChangeLog-$new" +echo "git shortlog --no-merges v$new ^v$last > ../ShortLog" +echo "git diff --stat --summary -M v$last v$new > ../diffstat-$new" +------------------------------------------------- + +and then he just cut-and-pastes the output commands after verifying that +they look OK. + +[[Finding-comments-with-given-content]] +Finding commits referencing a file with given content +----------------------------------------------------- + +Somebody hands you a copy of a file, and asks which commits modified a +file such that it contained the given content either before or after the +commit. You can find out with this: + +------------------------------------------------- +$ git log --raw -r --abbrev=40 --pretty=oneline -- filename | + grep -B 1 `git hash-object filename` +------------------------------------------------- + +Figuring out why this works is left as an exercise to the (advanced) +student. The gitlink:git-log[1], gitlink:git-diff-tree[1], and +gitlink:git-hash-object[1] man pages may prove helpful. + +[[Developing-with-git]] +Developing with git +=================== + +[[telling-git-your-name]] +Telling git your name +--------------------- + +Before creating any commits, you should introduce yourself to git. The +easiest way to do so is to make sure the following lines appear in a +file named .gitconfig in your home directory: + +------------------------------------------------ +[user] + name = Your Name Comes Here + email = you@yourdomain.example.com +------------------------------------------------ + +(See the "CONFIGURATION FILE" section of gitlink:git-config[1] for +details on the configuration file.) + + +[[creating-a-new-repository]] +Creating a new repository +------------------------- + +Creating a new repository from scratch is very easy: + +------------------------------------------------- +$ mkdir project +$ cd project +$ git init +------------------------------------------------- + +If you have some initial content (say, a tarball): + +------------------------------------------------- +$ tar -xzvf project.tar.gz +$ cd project +$ git init +$ git add . # include everything below ./ in the first commit: +$ git commit +------------------------------------------------- + +[[how-to-make-a-commit]] +How to make a commit +-------------------- + +Creating a new commit takes three steps: + + 1. Making some changes to the working directory using your + favorite editor. + 2. Telling git about your changes. + 3. Creating the commit using the content you told git about + in step 2. + +In practice, you can interleave and repeat steps 1 and 2 as many +times as you want: in order to keep track of what you want committed +at step 3, git maintains a snapshot of the tree's contents in a +special staging area called "the index." + +At the beginning, the content of the index will be identical to +that of the HEAD. The command "git diff --cached", which shows +the difference between the HEAD and the index, should therefore +produce no output at that point. + +Modifying the index is easy: + +To update the index with the new contents of a modified file, use + +------------------------------------------------- +$ git add path/to/file +------------------------------------------------- + +To add the contents of a new file to the index, use + +------------------------------------------------- +$ git add path/to/file +------------------------------------------------- + +To remove a file from the index and from the working tree, + +------------------------------------------------- +$ git rm path/to/file +------------------------------------------------- + +After each step you can verify that + +------------------------------------------------- +$ git diff --cached +------------------------------------------------- + +always shows the difference between the HEAD and the index file--this +is what you'd commit if you created the commit now--and that + +------------------------------------------------- +$ git diff +------------------------------------------------- + +shows the difference between the working tree and the index file. + +Note that "git add" always adds just the current contents of a file +to the index; further changes to the same file will be ignored unless +you run git-add on the file again. + +When you're ready, just run + +------------------------------------------------- +$ git commit +------------------------------------------------- + +and git will prompt you for a commit message and then create the new +commit. Check to make sure it looks like what you expected with + +------------------------------------------------- +$ git show +------------------------------------------------- + +As a special shortcut, + +------------------------------------------------- +$ git commit -a +------------------------------------------------- + +will update the index with any files that you've modified or removed +and create a commit, all in one step. + +A number of commands are useful for keeping track of what you're +about to commit: + +------------------------------------------------- +$ git diff --cached # difference between HEAD and the index; what + # would be commited if you ran "commit" now. +$ git diff # difference between the index file and your + # working directory; changes that would not + # be included if you ran "commit" now. +$ git diff HEAD # difference between HEAD and working tree; what + # would be committed if you ran "commit -a" now. +$ git status # a brief per-file summary of the above. +------------------------------------------------- + +[[creating-good-commit-messages]] +Creating good commit messages +----------------------------- + +Though not required, it's a good idea to begin the commit message +with a single short (less than 50 character) line summarizing the +change, followed by a blank line and then a more thorough +description. Tools that turn commits into email, for example, use +the first line on the Subject line and the rest of the commit in the +body. + +[[ignoring-files]] +Ignoring files +-------------- + +A project will often generate files that you do 'not' want to track with git. +This typically includes files generated by a build process or temporary +backup files made by your editor. Of course, 'not' tracking files with git +is just a matter of 'not' calling "`git add`" on them. But it quickly becomes +annoying to have these untracked files lying around; e.g. they make +"`git add .`" and "`git commit -a`" practically useless, and they keep +showing up in the output of "`git status`", etc. + +Git therefore provides "exclude patterns" for telling git which files to +actively ignore. Exclude patterns are thoroughly explained in the +gitlink:gitignore[5] manual page, but the heart of the concept is simply +a list of files which git should ignore. Entries in the list may contain +globs to specify multiple files, or may be prefixed by "`!`" to +explicitly include (un-ignore) a previously excluded (ignored) file +(i.e. later exclude patterns override earlier ones). The following +example should illustrate such patterns: + +------------------------------------------------- +# Lines starting with '#' are considered comments. +# Ignore foo.txt. +foo.txt +# Ignore (generated) html files, +*.html +# except foo.html which is maintained by hand. +!foo.html +# Ignore objects and archives. +*.[oa] +------------------------------------------------- + +The next question is where to put these exclude patterns so that git can +find them. Git looks for exclude patterns in the following files: + +`.gitignore` files in your working tree::: + You may store multiple `.gitignore` files at various locations in your + working tree. Each `.gitignore` file is applied to the directory where + it's located, including its subdirectories. Furthermore, the + `.gitignore` files can be tracked like any other files in your working + tree; just do a "`git add .gitignore`" and commit. `.gitignore` is + therefore the right place to put exclude patterns that are meant to + be shared between all project participants, such as build output files + (e.g. `\*.o`), etc. +`.git/info/exclude` in your repo::: + Exclude patterns in this file are applied to the working tree as a + whole. Since the file is not located in your working tree, it does + not follow push/pull/clone like `.gitignore` can do. This is therefore + the place to put exclude patterns that are local to your copy of the + repo (i.e. 'not' shared between project participants), such as + temporary backup files made by your editor (e.g. `\*~`), etc. +The file specified by the `core.excludesfile` config directive::: + By setting the `core.excludesfile` config directive you can tell git + where to find more exclude patterns (see gitlink:git-config[1] for + more information on configuration options). This config directive + can be set in the per-repo `.git/config` file, in which case the + exclude patterns will apply to that repo only. Alternatively, you + can set the directive in the global `~/.gitconfig` file to apply + the exclude pattern to all your git repos. As with the above + `.git/info/exclude` (and, indeed, with git config directives in + general), this directive does not follow push/pull/clone, but remain + local to your repo(s). + +[NOTE] +In addition to the above alternatives, there are git commands that can take +exclude patterns directly on the command line. See gitlink:git-ls-files[1] +for an example of this. + +[[how-to-merge]] +How to merge +------------ + +You can rejoin two diverging branches of development using +gitlink:git-merge[1]: + +------------------------------------------------- +$ git merge branchname +------------------------------------------------- + +merges the development in the branch "branchname" into the current +branch. If there are conflicts--for example, if the same file is +modified in two different ways in the remote branch and the local +branch--then you are warned; the output may look something like this: + +------------------------------------------------- +$ git merge next + 100% (4/4) done +Auto-merged file.txt +CONFLICT (content): Merge conflict in file.txt +Automatic merge failed; fix conflicts and then commit the result. +------------------------------------------------- + +Conflict markers are left in the problematic files, and after +you resolve the conflicts manually, you can update the index +with the contents and run git commit, as you normally would when +creating a new file. + +If you examine the resulting commit using gitk, you will see that it +has two parents, one pointing to the top of the current branch, and +one to the top of the other branch. + +[[resolving-a-merge]] +Resolving a merge +----------------- + +When a merge isn't resolved automatically, git leaves the index and +the working tree in a special state that gives you all the +information you need to help resolve the merge. + +Files with conflicts are marked specially in the index, so until you +resolve the problem and update the index, gitlink:git-commit[1] will +fail: + +------------------------------------------------- +$ git commit +file.txt: needs merge +------------------------------------------------- + +Also, gitlink:git-status[1] will list those files as "unmerged", and the +files with conflicts will have conflict markers added, like this: + +------------------------------------------------- +<<<<<<< HEAD:file.txt +Hello world +======= +Goodbye +>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt +------------------------------------------------- + +All you need to do is edit the files to resolve the conflicts, and then + +------------------------------------------------- +$ git add file.txt +$ git commit +------------------------------------------------- + +Note that the commit message will already be filled in for you with +some information about the merge. Normally you can just use this +default message unchanged, but you may add additional commentary of +your own if desired. + +The above is all you need to know to resolve a simple merge. But git +also provides more information to help resolve conflicts: + +[[conflict-resolution]] +Getting conflict-resolution help during a merge +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +All of the changes that git was able to merge automatically are +already added to the index file, so gitlink:git-diff[1] shows only +the conflicts. It uses an unusual syntax: + +------------------------------------------------- +$ git diff +diff --cc file.txt +index 802992c,2b60207..0000000 +--- a/file.txt ++++ b/file.txt +@@@ -1,1 -1,1 +1,5 @@@ +++<<<<<<< HEAD:file.txt + +Hello world +++======= ++ Goodbye +++>>>>>>> 77976da35a11db4580b80ae27e8d65caf5208086:file.txt +------------------------------------------------- + +Recall that the commit which will be commited after we resolve this +conflict will have two parents instead of the usual one: one parent +will be HEAD, the tip of the current branch; the other will be the +tip of the other branch, which is stored temporarily in MERGE_HEAD. + +During the merge, the index holds three versions of each file. Each of +these three "file stages" represents a different version of the file: + +------------------------------------------------- +$ git show :1:file.txt # the file in a common ancestor of both branches +$ git show :2:file.txt # the version from HEAD, but including any + # nonconflicting changes from MERGE_HEAD +$ git show :3:file.txt # the version from MERGE_HEAD, but including any + # nonconflicting changes from HEAD. +------------------------------------------------- + +Since the stage 2 and stage 3 versions have already been updated with +nonconflicting changes, the only remaining differences between them are +the important ones; thus gitlink:git-diff[1] can use the information in +the index to show only those conflicts. + +The diff above shows the differences between the working-tree version of +file.txt and the stage 2 and stage 3 versions. So instead of preceding +each line by a single "+" or "-", it now uses two columns: the first +column is used for differences between the first parent and the working +directory copy, and the second for differences between the second parent +and the working directory copy. (See the "COMBINED DIFF FORMAT" section +of gitlink:git-diff-files[1] for a details of the format.) + +After resolving the conflict in the obvious way (but before updating the +index), the diff will look like: + +------------------------------------------------- +$ git diff +diff --cc file.txt +index 802992c,2b60207..0000000 +--- a/file.txt ++++ b/file.txt +@@@ -1,1 -1,1 +1,1 @@@ +- Hello world + -Goodbye +++Goodbye world +------------------------------------------------- + +This shows that our resolved version deleted "Hello world" from the +first parent, deleted "Goodbye" from the second parent, and added +"Goodbye world", which was previously absent from both. + +Some special diff options allow diffing the working directory against +any of these stages: + +------------------------------------------------- +$ git diff -1 file.txt # diff against stage 1 +$ git diff --base file.txt # same as the above +$ git diff -2 file.txt # diff against stage 2 +$ git diff --ours file.txt # same as the above +$ git diff -3 file.txt # diff against stage 3 +$ git diff --theirs file.txt # same as the above. +------------------------------------------------- + +The gitlink:git-log[1] and gitk[1] commands also provide special help +for merges: + +------------------------------------------------- +$ git log --merge +$ gitk --merge +------------------------------------------------- + +These will display all commits which exist only on HEAD or on +MERGE_HEAD, and which touch an unmerged file. + +You may also use gitlink:git-mergetool[1], which lets you merge the +unmerged files using external tools such as emacs or kdiff3. + +Each time you resolve the conflicts in a file and update the index: + +------------------------------------------------- +$ git add file.txt +------------------------------------------------- + +the different stages of that file will be "collapsed", after which +git-diff will (by default) no longer show diffs for that file. + +[[undoing-a-merge]] +Undoing a merge +--------------- + +If you get stuck and decide to just give up and throw the whole mess +away, you can always return to the pre-merge state with + +------------------------------------------------- +$ git reset --hard HEAD +------------------------------------------------- + +Or, if you've already commited the merge that you want to throw away, + +------------------------------------------------- +$ git reset --hard ORIG_HEAD +------------------------------------------------- + +However, this last command can be dangerous in some cases--never +throw away a commit you have already committed if that commit may +itself have been merged into another branch, as doing so may confuse +further merges. + +[[fast-forwards]] +Fast-forward merges +------------------- + +There is one special case not mentioned above, which is treated +differently. Normally, a merge results in a merge commit, with two +parents, one pointing at each of the two lines of development that +were merged. + +However, if the current branch is a descendant of the other--so every +commit present in the one is already contained in the other--then git +just performs a "fast forward"; the head of the current branch is moved +forward to point at the head of the merged-in branch, without any new +commits being created. + +[[fixing-mistakes]] +Fixing mistakes +--------------- + +If you've messed up the working tree, but haven't yet committed your +mistake, you can return the entire working tree to the last committed +state with + +------------------------------------------------- +$ git reset --hard HEAD +------------------------------------------------- + +If you make a commit that you later wish you hadn't, there are two +fundamentally different ways to fix the problem: + + 1. You can create a new commit that undoes whatever was done + by the previous commit. This is the correct thing if your + mistake has already been made public. + + 2. You can go back and modify the old commit. You should + never do this if you have already made the history public; + git does not normally expect the "history" of a project to + change, and cannot correctly perform repeated merges from + a branch that has had its history changed. + +[[reverting-a-commit]] +Fixing a mistake with a new commit +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Creating a new commit that reverts an earlier change is very easy; +just pass the gitlink:git-revert[1] command a reference to the bad +commit; for example, to revert the most recent commit: + +------------------------------------------------- +$ git revert HEAD +------------------------------------------------- + +This will create a new commit which undoes the change in HEAD. You +will be given a chance to edit the commit message for the new commit. + +You can also revert an earlier change, for example, the next-to-last: + +------------------------------------------------- +$ git revert HEAD^ +------------------------------------------------- + +In this case git will attempt to undo the old change while leaving +intact any changes made since then. If more recent changes overlap +with the changes to be reverted, then you will be asked to fix +conflicts manually, just as in the case of <<resolving-a-merge, +resolving a merge>>. + +[[fixing-a-mistake-by-editing-history]] +Fixing a mistake by editing history +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +If the problematic commit is the most recent commit, and you have not +yet made that commit public, then you may just +<<undoing-a-merge,destroy it using git-reset>>. + +Alternatively, you +can edit the working directory and update the index to fix your +mistake, just as if you were going to <<how-to-make-a-commit,create a +new commit>>, then run + +------------------------------------------------- +$ git commit --amend +------------------------------------------------- + +which will replace the old commit by a new commit incorporating your +changes, giving you a chance to edit the old commit message first. + +Again, you should never do this to a commit that may already have +been merged into another branch; use gitlink:git-revert[1] instead in +that case. + +It is also possible to edit commits further back in the history, but +this is an advanced topic to be left for +<<cleaning-up-history,another chapter>>. + +[[checkout-of-path]] +Checking out an old version of a file +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +In the process of undoing a previous bad change, you may find it +useful to check out an older version of a particular file using +gitlink:git-checkout[1]. We've used git checkout before to switch +branches, but it has quite different behavior if it is given a path +name: the command + +------------------------------------------------- +$ git checkout HEAD^ path/to/file +------------------------------------------------- + +replaces path/to/file by the contents it had in the commit HEAD^, and +also updates the index to match. It does not change branches. + +If you just want to look at an old version of the file, without +modifying the working directory, you can do that with +gitlink:git-show[1]: + +------------------------------------------------- +$ git show HEAD^:path/to/file +------------------------------------------------- + +which will display the given version of the file. + +[[ensuring-good-performance]] +Ensuring good performance +------------------------- + +On large repositories, git depends on compression to keep the history +information from taking up to much space on disk or in memory. + +This compression is not performed automatically. Therefore you +should occasionally run gitlink:git-gc[1]: + +------------------------------------------------- +$ git gc +------------------------------------------------- + +to recompress the archive. This can be very time-consuming, so +you may prefer to run git-gc when you are not doing other work. + + +[[ensuring-reliability]] +Ensuring reliability +-------------------- + +[[checking-for-corruption]] +Checking the repository for corruption +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The gitlink:git-fsck[1] command runs a number of self-consistency checks +on the repository, and reports on any problems. This may take some +time. The most common warning by far is about "dangling" objects: + +------------------------------------------------- +$ git fsck +dangling commit 7281251ddd2a61e38657c827739c57015671a6b3 +dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63 +dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5 +dangling blob 218761f9d90712d37a9c5e36f406f92202db07eb +dangling commit bf093535a34a4d35731aa2bd90fe6b176302f14f +dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e +dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085 +dangling tree b24c2473f1fd3d91352a624795be026d64c8841f +... +------------------------------------------------- + +Dangling objects are not a problem. At worst they may take up a little +extra disk space. They can sometimes provide a last-resort method for +recovering lost work--see <<dangling-objects>> for details. However, if +you wish, you can remove them with gitlink:git-prune[1] or the --prune +option to gitlink:git-gc[1]: + +------------------------------------------------- +$ git gc --prune +------------------------------------------------- + +This may be time-consuming. Unlike most other git operations (including +git-gc when run without any options), it is not safe to prune while +other git operations are in progress in the same repository. + +[[recovering-lost-changes]] +Recovering lost changes +~~~~~~~~~~~~~~~~~~~~~~~ + +[[reflogs]] +Reflogs +^^^^^^^ + +Say you modify a branch with gitlink:git-reset[1] --hard, and then +realize that the branch was the only reference you had to that point in +history. + +Fortunately, git also keeps a log, called a "reflog", of all the +previous values of each branch. So in this case you can still find the +old history using, for example, + +------------------------------------------------- +$ git log master@{1} +------------------------------------------------- + +This lists the commits reachable from the previous version of the head. +This syntax can be used to with any git command that accepts a commit, +not just with git log. Some other examples: + +------------------------------------------------- +$ git show master@{2} # See where the branch pointed 2, +$ git show master@{3} # 3, ... changes ago. +$ gitk master@{yesterday} # See where it pointed yesterday, +$ gitk master@{"1 week ago"} # ... or last week +$ git log --walk-reflogs master # show reflog entries for master +------------------------------------------------- + +A separate reflog is kept for the HEAD, so + +------------------------------------------------- +$ git show HEAD@{"1 week ago"} +------------------------------------------------- + +will show what HEAD pointed to one week ago, not what the current branch +pointed to one week ago. This allows you to see the history of what +you've checked out. + +The reflogs are kept by default for 30 days, after which they may be +pruned. See gitlink:git-reflog[1] and gitlink:git-gc[1] to learn +how to control this pruning, and see the "SPECIFYING REVISIONS" +section of gitlink:git-rev-parse[1] for details. + +Note that the reflog history is very different from normal git history. +While normal history is shared by every repository that works on the +same project, the reflog history is not shared: it tells you only about +how the branches in your local repository have changed over time. + +[[dangling-object-recovery]] +Examining dangling objects +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +In some situations the reflog may not be able to save you. For example, +suppose you delete a branch, then realize you need the history it +contained. The reflog is also deleted; however, if you have not yet +pruned the repository, then you may still be able to find the lost +commits in the dangling objects that git-fsck reports. See +<<dangling-objects>> for the details. + +------------------------------------------------- +$ git fsck +dangling commit 7281251ddd2a61e38657c827739c57015671a6b3 +dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63 +dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5 +... +------------------------------------------------- + +You can examine +one of those dangling commits with, for example, + +------------------------------------------------ +$ gitk 7281251ddd --not --all +------------------------------------------------ + +which does what it sounds like: it says that you want to see the commit +history that is described by the dangling commit(s), but not the +history that is described by all your existing branches and tags. Thus +you get exactly the history reachable from that commit that is lost. +(And notice that it might not be just one commit: we only report the +"tip of the line" as being dangling, but there might be a whole deep +and complex commit history that was dropped.) + +If you decide you want the history back, you can always create a new +reference pointing to it, for example, a new branch: + +------------------------------------------------ +$ git branch recovered-branch 7281251ddd +------------------------------------------------ + +Other types of dangling objects (blobs and trees) are also possible, and +dangling objects can arise in other situations. + + +[[sharing-development]] +Sharing development with others +=============================== + +[[getting-updates-with-git-pull]] +Getting updates with git pull +----------------------------- + +After you clone a repository and make a few changes of your own, you +may wish to check the original repository for updates and merge them +into your own work. + +We have already seen <<Updating-a-repository-with-git-fetch,how to +keep remote tracking branches up to date>> with gitlink:git-fetch[1], +and how to merge two branches. So you can merge in changes from the +original repository's master branch with: + +------------------------------------------------- +$ git fetch +$ git merge origin/master +------------------------------------------------- + +However, the gitlink:git-pull[1] command provides a way to do this in +one step: + +------------------------------------------------- +$ git pull origin master +------------------------------------------------- + +In fact, "origin" is normally the default repository to pull from, +and the default branch is normally the HEAD of the remote repository, +so often you can accomplish the above with just + +------------------------------------------------- +$ git pull +------------------------------------------------- + +See the descriptions of the branch.<name>.remote and branch.<name>.merge +options in gitlink:git-config[1] to learn how to control these defaults +depending on the current branch. Also note that the --track option to +gitlink:git-branch[1] and gitlink:git-checkout[1] can be used to +automatically set the default remote branch to pull from at the time +that a branch is created: + +------------------------------------------------- +$ git checkout --track -b maint origin/maint +------------------------------------------------- + +In addition to saving you keystrokes, "git pull" also helps you by +producing a default commit message documenting the branch and +repository that you pulled from. + +(But note that no such commit will be created in the case of a +<<fast-forwards,fast forward>>; instead, your branch will just be +updated to point to the latest commit from the upstream branch.) + +The git-pull command can also be given "." as the "remote" repository, +in which case it just merges in a branch from the current repository; so +the commands + +------------------------------------------------- +$ git pull . branch +$ git merge branch +------------------------------------------------- + +are roughly equivalent. The former is actually very commonly used. + +[[submitting-patches]] +Submitting patches to a project +------------------------------- + +If you just have a few changes, the simplest way to submit them may +just be to send them as patches in email: + +First, use gitlink:git-format-patch[1]; for example: + +------------------------------------------------- +$ git format-patch origin +------------------------------------------------- + +will produce a numbered series of files in the current directory, one +for each patch in the current branch but not in origin/HEAD. + +You can then import these into your mail client and send them by +hand. However, if you have a lot to send at once, you may prefer to +use the gitlink:git-send-email[1] script to automate the process. +Consult the mailing list for your project first to determine how they +prefer such patches be handled. + +[[importing-patches]] +Importing patches to a project +------------------------------ + +Git also provides a tool called gitlink:git-am[1] (am stands for +"apply mailbox"), for importing such an emailed series of patches. +Just save all of the patch-containing messages, in order, into a +single mailbox file, say "patches.mbox", then run + +------------------------------------------------- +$ git am -3 patches.mbox +------------------------------------------------- + +Git will apply each patch in order; if any conflicts are found, it +will stop, and you can fix the conflicts as described in +"<<resolving-a-merge,Resolving a merge>>". (The "-3" option tells +git to perform a merge; if you would prefer it just to abort and +leave your tree and index untouched, you may omit that option.) + +Once the index is updated with the results of the conflict +resolution, instead of creating a new commit, just run + +------------------------------------------------- +$ git am --resolved +------------------------------------------------- + +and git will create the commit for you and continue applying the +remaining patches from the mailbox. + +The final result will be a series of commits, one for each patch in +the original mailbox, with authorship and commit log message each +taken from the message containing each patch. + +[[public-repositories]] +Public git repositories +----------------------- + +Another way to submit changes to a project is to tell the maintainer of +that project to pull the changes from your repository using git-pull[1]. +In the section "<<getting-updates-with-git-pull, Getting updates with +git pull>>" we described this as a way to get updates from the "main" +repository, but it works just as well in the other direction. + +If you and the maintainer both have accounts on the same machine, then +you can just pull changes from each other's repositories directly; +commands that accept repository URLs as arguments will also accept a +local directory name: + +------------------------------------------------- +$ git clone /path/to/repository +$ git pull /path/to/other/repository +------------------------------------------------- + +or an ssh url: + +------------------------------------------------- +$ git clone ssh://yourhost/~you/repository +------------------------------------------------- + +For projects with few developers, or for synchronizing a few private +repositories, this may be all you need. + +However, the more common way to do this is to maintain a separate public +repository (usually on a different host) for others to pull changes +from. This is usually more convenient, and allows you to cleanly +separate private work in progress from publicly visible work. + +You will continue to do your day-to-day work in your personal +repository, but periodically "push" changes from your personal +repository into your public repository, allowing other developers to +pull from that repository. So the flow of changes, in a situation +where there is one other developer with a public repository, looks +like this: + + you push + your personal repo ------------------> your public repo + ^ | + | | + | you pull | they pull + | | + | | + | they push V + their public repo <------------------- their repo + +We explain how to do this in the following sections. + +[[setting-up-a-public-repository]] +Setting up a public repository +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Assume your personal repository is in the directory ~/proj. We +first create a new clone of the repository and tell git-daemon that it +is meant to be public: + +------------------------------------------------- +$ git clone --bare ~/proj proj.git +$ touch proj.git/git-daemon-export-ok +------------------------------------------------- + +The resulting directory proj.git contains a "bare" git repository--it is +just the contents of the ".git" directory, without any files checked out +around it. + +Next, copy proj.git to the server where you plan to host the +public repository. You can use scp, rsync, or whatever is most +convenient. + +[[exporting-via-git]] +Exporting a git repository via the git protocol +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +This is the preferred method. + +If someone else administers the server, they should tell you what +directory to put the repository in, and what git:// url it will appear +at. You can then skip to the section +"<<pushing-changes-to-a-public-repository,Pushing changes to a public +repository>>", below. + +Otherwise, all you need to do is start gitlink:git-daemon[1]; it will +listen on port 9418. By default, it will allow access to any directory +that looks like a git directory and contains the magic file +git-daemon-export-ok. Passing some directory paths as git-daemon +arguments will further restrict the exports to those paths. + +You can also run git-daemon as an inetd service; see the +gitlink:git-daemon[1] man page for details. (See especially the +examples section.) + +[[exporting-via-http]] +Exporting a git repository via http +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The git protocol gives better performance and reliability, but on a +host with a web server set up, http exports may be simpler to set up. + +All you need to do is place the newly created bare git repository in +a directory that is exported by the web server, and make some +adjustments to give web clients some extra information they need: + +------------------------------------------------- +$ mv proj.git /home/you/public_html/proj.git +$ cd proj.git +$ git --bare update-server-info +$ chmod a+x hooks/post-update +------------------------------------------------- + +(For an explanation of the last two lines, see +gitlink:git-update-server-info[1], and the documentation +link:hooks.html[Hooks used by git].) + +Advertise the url of proj.git. Anybody else should then be able to +clone or pull from that url, for example with a commandline like: + +------------------------------------------------- +$ git clone http://yourserver.com/~you/proj.git +------------------------------------------------- + +(See also +link:howto/setup-git-server-over-http.txt[setup-git-server-over-http] +for a slightly more sophisticated setup using WebDAV which also +allows pushing over http.) + +[[pushing-changes-to-a-public-repository]] +Pushing changes to a public repository +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Note that the two techniques outlined above (exporting via +<<exporting-via-http,http>> or <<exporting-via-git,git>>) allow other +maintainers to fetch your latest changes, but they do not allow write +access, which you will need to update the public repository with the +latest changes created in your private repository. + +The simplest way to do this is using gitlink:git-push[1] and ssh; to +update the remote branch named "master" with the latest state of your +branch named "master", run + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git master:master +------------------------------------------------- + +or just + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git master +------------------------------------------------- + +As with git-fetch, git-push will complain if this does not result in +a <<fast-forwards,fast forward>>. Normally this is a sign of +something wrong. However, if you are sure you know what you're +doing, you may force git-push to perform the update anyway by +proceeding the branch name by a plus sign: + +------------------------------------------------- +$ git push ssh://yourserver.com/~you/proj.git +master +------------------------------------------------- + +Note that the target of a "push" is normally a +<<def_bare_repository,bare>> repository. You can also push to a +repository that has a checked-out working tree, but the working tree +will not be updated by the push. This may lead to unexpected results if +the branch you push to is the currently checked-out branch! + +As with git-fetch, you may also set up configuration options to +save typing; so, for example, after + +------------------------------------------------- +$ cat >>.git/config <<EOF +[remote "public-repo"] + url = ssh://yourserver.com/~you/proj.git +EOF +------------------------------------------------- + +you should be able to perform the above push with just + +------------------------------------------------- +$ git push public-repo master +------------------------------------------------- + +See the explanations of the remote.<name>.url, branch.<name>.remote, +and remote.<name>.push options in gitlink:git-config[1] for +details. + +[[setting-up-a-shared-repository]] +Setting up a shared repository +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +Another way to collaborate is by using a model similar to that +commonly used in CVS, where several developers with special rights +all push to and pull from a single shared repository. See +link:cvs-migration.html[git for CVS users] for instructions on how to +set this up. + +However, while there is nothing wrong with git's support for shared +repositories, this mode of operation is not generally recommended, +simply because the mode of collaboration that git supports--by +exchanging patches and pulling from public repositories--has so many +advantages over the central shared repository: + + - Git's ability to quickly import and merge patches allows a + single maintainer to process incoming changes even at very + high rates. And when that becomes too much, git-pull provides + an easy way for that maintainer to delegate this job to other + maintainers while still allowing optional review of incoming + changes. + - Since every developer's repository has the same complete copy + of the project history, no repository is special, and it is + trivial for another developer to take over maintenance of a + project, either by mutual agreement, or because a maintainer + becomes unresponsive or difficult to work with. + - The lack of a central group of "committers" means there is + less need for formal decisions about who is "in" and who is + "out". + +[[setting-up-gitweb]] +Allowing web browsing of a repository +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +The gitweb cgi script provides users an easy way to browse your +project's files and history without having to install git; see the file +gitweb/INSTALL in the git source tree for instructions on setting it up. + +[[sharing-development-examples]] +Examples +-------- + +[[maintaining-topic-branches]] +Maintaining topic branches for a Linux subsystem maintainer +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ + +This describes how Tony Luck uses git in his role as maintainer of the +IA64 architecture for the Linux kernel. + +He uses two public branches: + + - A "test" tree into which patches are initially placed so that they + can get some exposure when integrated with other ongoing development. + This tree is available to Andrew for pulling into -mm whenever he + wants. + + - A "release" tree into which tested patches are moved for final sanity + checking, and as a vehicle to send them upstream to Linus (by sending + him a "please pull" request.) + +He also uses a set of temporary branches ("topic branches"), each +containing a logical grouping of patches. + +To set this up, first create your work tree by cloning Linus's public +tree: + +------------------------------------------------- +$ git clone git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux-2.6.git work +$ cd work +------------------------------------------------- + +Linus's tree will be stored in the remote branch named origin/master, +and can be updated using gitlink:git-fetch[1]; you can track other +public trees using gitlink:git-remote[1] to set up a "remote" and +git-fetch[1] to keep them up-to-date; see <<repositories-and-branches>>. + +Now create the branches in which you are going to work; these start out +at the current tip of origin/master branch, and should be set up (using +the --track option to gitlink:git-branch[1]) to merge changes in from +Linus by default. + +------------------------------------------------- +$ git branch --track test origin/master +$ git branch --track release origin/master +------------------------------------------------- + +These can be easily kept up to date using gitlink:git-pull[1] + +------------------------------------------------- +$ git checkout test && git pull +$ git checkout release && git pull +------------------------------------------------- + +Important note! If you have any local changes in these branches, then +this merge will create a commit object in the history (with no local +changes git will simply do a "Fast forward" merge). Many people dislike +the "noise" that this creates in the Linux history, so you should avoid +doing this capriciously in the "release" branch, as these noisy commits +will become part of the permanent history when you ask Linus to pull +from the release branch. + +A few configuration variables (see gitlink:git-config[1]) can +make it easy to push both branches to your public tree. (See +<<setting-up-a-public-repository>>.) + +------------------------------------------------- +$ cat >> .git/config <<EOF +[remote "mytree"] + url = master.kernel.org:/pub/scm/linux/kernel/git/aegl/linux-2.6.git + push = release + push = test +EOF +------------------------------------------------- + +Then you can push both the test and release trees using +gitlink:git-push[1]: + +------------------------------------------------- +$ git push mytree +------------------------------------------------- + +or push just one of the test and release branches using: + +------------------------------------------------- +$ git push mytree test +------------------------------------------------- + +or + +------------------------------------------------- +$ git push mytree release +------------------------------------------------- + +Now to apply some patches from the community. Think of a short +snappy name for a branch to hold this patch (or related group of +patches), and create a new branch from the current tip of Linus's +branch: + +------------------------------------------------- +$ git checkout -b speed-up-spinlocks origin +------------------------------------------------- + +Now you apply the patch(es), run some tests, and commit the change(s). If +the patch is a multi-part series, then you should apply each as a separate +commit to this branch. + +------------------------------------------------- +$ ... patch ... test ... commit [ ... patch ... test ... commit ]* +------------------------------------------------- + +When you are happy with the state of this change, you can pull it into the +"test" branch in preparation to make it public: + +------------------------------------------------- +$ git checkout test && git pull . speed-up-spinlocks +------------------------------------------------- + +It is unlikely that you would have any conflicts here ... but you might if you +spent a while on this step and had also pulled new versions from upstream. + +Some time later when enough time has passed and testing done, you can pull the +same branch into the "release" tree ready to go upstream. This is where you +see the value of keeping each patch (or patch series) in its own branch. It +means that the patches can be moved into the "release" tree in any order. + +------------------------------------------------- +$ git checkout release && git pull . speed-up-spinlocks +------------------------------------------------- + +After a while, you will have a number of branches, and despite the +well chosen names you picked for each of them, you may forget what +they are for, or what status they are in. To get a reminder of what +changes are in a specific branch, use: + +------------------------------------------------- +$ git log linux..branchname | git-shortlog +------------------------------------------------- + +To see whether it has already been merged into the test or release branches +use: + +------------------------------------------------- +$ git log test..branchname +------------------------------------------------- + +or + +------------------------------------------------- +$ git log release..branchname +------------------------------------------------- + +(If this branch has not yet been merged you will see some log entries. +If it has been merged, then there will be no output.) + +Once a patch completes the great cycle (moving from test to release, +then pulled by Linus, and finally coming back into your local +"origin/master" branch) the branch for this change is no longer needed. +You detect this when the output from: + +------------------------------------------------- +$ git log origin..branchname +------------------------------------------------- + +is empty. At this point the branch can be deleted: + +------------------------------------------------- +$ git branch -d branchname +------------------------------------------------- + +Some changes are so trivial that it is not necessary to create a separate +branch and then merge into each of the test and release branches. For +these changes, just apply directly to the "release" branch, and then +merge that into the "test" branch. + +To create diffstat and shortlog summaries of changes to include in a "please +pull" request to Linus you can use: + +------------------------------------------------- +$ git diff --stat origin..release +------------------------------------------------- + +and + +------------------------------------------------- +$ git log -p origin..release | git shortlog +------------------------------------------------- + +Here are some of the scripts that simplify all this even further. + +------------------------------------------------- +==== update script ==== +# Update a branch in my GIT tree. If the branch to be updated +# is origin, then pull from kernel.org. Otherwise merge +# origin/master branch into test|release branch + +case "$1" in +test|release) + git checkout $1 && git pull . origin + ;; +origin) + before=$(cat .git/refs/remotes/origin/master) + git fetch origin + after=$(cat .git/refs/remotes/origin/master) + if [ $before != $after ] + then + git log $before..$after | git shortlog + fi + ;; +*) + echo "Usage: $0 origin|test|release" 1>&2 + exit 1 + ;; +esac +------------------------------------------------- + +------------------------------------------------- +==== merge script ==== +# Merge a branch into either the test or release branch + +pname=$0 + +usage() +{ + echo "Usage: $pname branch test|release" 1>&2 + exit 1 +} + +if [ ! -f .git/refs/heads/"$1" ] +then + echo "Can't see branch <$1>" 1>&2 + usage +fi + +case "$2" in +test|release) + if [ $(git log $2..$1 | wc -c) -eq 0 ] + then + echo $1 already merged into $2 1>&2 + exit 1 + fi + git checkout $2 && git pull . $1 + ;; +*) + usage + ;; +esac +------------------------------------------------- + +------------------------------------------------- +==== status script ==== +# report on status of my ia64 GIT tree + +gb=$(tput setab 2) +rb=$(tput setab 1) +restore=$(tput setab 9) + +if [ `git rev-list test..release | wc -c` -gt 0 ] +then + echo $rb Warning: commits in release that are not in test $restore + git log test..release +fi + +for branch in `ls .git/refs/heads` +do + if [ $branch = test -o $branch = release ] + then + continue + fi + + echo -n $gb ======= $branch ====== $restore " " + status= + for ref in test release origin/master + do + if [ `git rev-list $ref..$branch | wc -c` -gt 0 ] + then + status=$status${ref:0:1} + fi + done + case $status in + trl) + echo $rb Need to pull into test $restore + ;; + rl) + echo "In test" + ;; + l) + echo "Waiting for linus" + ;; + "") + echo $rb All done $restore + ;; + *) + echo $rb "<$status>" $restore + ;; + esac + git log origin/master..$branch | git shortlog +done +------------------------------------------------- + + +[[cleaning-up-history]] +Rewriting history and maintaining patch series +============================================== + +Normally commits are only added to a project, never taken away or +replaced. Git is designed with this assumption, and violating it will +cause git's merge machinery (for example) to do the wrong thing. + +However, there is a situation in which it can be useful to violate this +assumption. + +[[patch-series]] +Creating the perfect patch series +--------------------------------- + +Suppose you are a contributor to a large project, and you want to add a +complicated feature, and to present it to the other developers in a way +that makes it easy for them to read your changes, verify that they are +correct, and understand why you made each change. + +If you present all of your changes as a single patch (or commit), they +may find that it is too much to digest all at once. + +If you present them with the entire history of your work, complete with +mistakes, corrections, and dead ends, they may be overwhelmed. + +So the ideal is usually to produce a series of patches such that: + + 1. Each patch can be applied in order. + + 2. Each patch includes a single logical change, together with a + message explaining the change. + + 3. No patch introduces a regression: after applying any initial + part of the series, the resulting project still compiles and + works, and has no bugs that it didn't have before. + + 4. The complete series produces the same end result as your own + (probably much messier!) development process did. + +We will introduce some tools that can help you do this, explain how to +use them, and then explain some of the problems that can arise because +you are rewriting history. + +[[using-git-rebase]] +Keeping a patch series up to date using git-rebase +-------------------------------------------------- + +Suppose that you create a branch "mywork" on a remote-tracking branch +"origin", and create some commits on top of it: + +------------------------------------------------- +$ git checkout -b mywork origin +$ vi file.txt +$ git commit +$ vi otherfile.txt +$ git commit +... +------------------------------------------------- + +You have performed no merges into mywork, so it is just a simple linear +sequence of patches on top of "origin": + +................................................ + o--o--o <-- origin + \ + o--o--o <-- mywork +................................................ + +Some more interesting work has been done in the upstream project, and +"origin" has advanced: + +................................................ + o--o--O--o--o--o <-- origin + \ + a--b--c <-- mywork +................................................ + +At this point, you could use "pull" to merge your changes back in; +the result would create a new merge commit, like this: + +................................................ + o--o--O--o--o--o <-- origin + \ \ + a--b--c--m <-- mywork +................................................ + +However, if you prefer to keep the history in mywork a simple series of +commits without any merges, you may instead choose to use +gitlink:git-rebase[1]: + +------------------------------------------------- +$ git checkout mywork +$ git rebase origin +------------------------------------------------- + +This will remove each of your commits from mywork, temporarily saving +them as patches (in a directory named ".dotest"), update mywork to +point at the latest version of origin, then apply each of the saved +patches to the new mywork. The result will look like: + + +................................................ + o--o--O--o--o--o <-- origin + \ + a'--b'--c' <-- mywork +................................................ + +In the process, it may discover conflicts. In that case it will stop +and allow you to fix the conflicts; after fixing conflicts, use "git +add" to update the index with those contents, and then, instead of +running git-commit, just run + +------------------------------------------------- +$ git rebase --continue +------------------------------------------------- + +and git will continue applying the rest of the patches. + +At any point you may use the --abort option to abort this process and +return mywork to the state it had before you started the rebase: + +------------------------------------------------- +$ git rebase --abort +------------------------------------------------- + +[[modifying-one-commit]] +Modifying a single commit +------------------------- + +We saw in <<fixing-a-mistake-by-editing-history>> that you can replace the +most recent commit using + +------------------------------------------------- +$ git commit --amend +------------------------------------------------- + +which will replace the old commit by a new commit incorporating your +changes, giving you a chance to edit the old commit message first. + +You can also use a combination of this and gitlink:git-rebase[1] to edit +commits further back in your history. First, tag the problematic commit with + +------------------------------------------------- +$ git tag bad mywork~5 +------------------------------------------------- + +(Either gitk or git-log may be useful for finding the commit.) + +Then check out that commit, edit it, and rebase the rest of the series +on top of it (note that we could check out the commit on a temporary +branch, but instead we're using a <<detached-head,detached head>>): + +------------------------------------------------- +$ git checkout bad +$ # make changes here and update the index +$ git commit --amend +$ git rebase --onto HEAD bad mywork +------------------------------------------------- + +When you're done, you'll be left with mywork checked out, with the top +patches on mywork reapplied on top of your modified commit. You can +then clean up with + +------------------------------------------------- +$ git tag -d bad +------------------------------------------------- + +Note that the immutable nature of git history means that you haven't really +"modified" existing commits; instead, you have replaced the old commits with +new commits having new object names. + +[[reordering-patch-series]] +Reordering or selecting from a patch series +------------------------------------------- + +Given one existing commit, the gitlink:git-cherry-pick[1] command +allows you to apply the change introduced by that commit and create a +new commit that records it. So, for example, if "mywork" points to a +series of patches on top of "origin", you might do something like: + +------------------------------------------------- +$ git checkout -b mywork-new origin +$ gitk origin..mywork & +------------------------------------------------- + +And browse through the list of patches in the mywork branch using gitk, +applying them (possibly in a different order) to mywork-new using +cherry-pick, and possibly modifying them as you go using commit +--amend. + +Another technique is to use git-format-patch to create a series of +patches, then reset the state to before the patches: + +------------------------------------------------- +$ git format-patch origin +$ git reset --hard origin +------------------------------------------------- + +Then modify, reorder, or eliminate patches as preferred before applying +them again with gitlink:git-am[1]. + +[[patch-series-tools]] +Other tools +----------- + +There are numerous other tools, such as stgit, which exist for the +purpose of maintaining a patch series. These are outside of the scope of +this manual. + +[[problems-with-rewriting-history]] +Problems with rewriting history +------------------------------- + +The primary problem with rewriting the history of a branch has to do +with merging. Suppose somebody fetches your branch and merges it into +their branch, with a result something like this: + +................................................ + o--o--O--o--o--o <-- origin + \ \ + t--t--t--m <-- their branch: +................................................ + +Then suppose you modify the last three commits: + +................................................ + o--o--o <-- new head of origin + / + o--o--O--o--o--o <-- old head of origin +................................................ + +If we examined all this history together in one repository, it will +look like: + +................................................ + o--o--o <-- new head of origin + / + o--o--O--o--o--o <-- old head of origin + \ \ + t--t--t--m <-- their branch: +................................................ + +Git has no way of knowing that the new head is an updated version of +the old head; it treats this situation exactly the same as it would if +two developers had independently done the work on the old and new heads +in parallel. At this point, if someone attempts to merge the new head +in to their branch, git will attempt to merge together the two (old and +new) lines of development, instead of trying to replace the old by the +new. The results are likely to be unexpected. + +You may still choose to publish branches whose history is rewritten, +and it may be useful for others to be able to fetch those branches in +order to examine or test them, but they should not attempt to pull such +branches into their own work. + +For true distributed development that supports proper merging, +published branches should never be rewritten. + +[[advanced-branch-management]] +Advanced branch management +========================== + +[[fetching-individual-branches]] +Fetching individual branches +---------------------------- + +Instead of using gitlink:git-remote[1], you can also choose just +to update one branch at a time, and to store it locally under an +arbitrary name: + +------------------------------------------------- +$ git fetch origin todo:my-todo-work +------------------------------------------------- + +The first argument, "origin", just tells git to fetch from the +repository you originally cloned from. The second argument tells git +to fetch the branch named "todo" from the remote repository, and to +store it locally under the name refs/heads/my-todo-work. + +You can also fetch branches from other repositories; so + +------------------------------------------------- +$ git fetch git://example.com/proj.git master:example-master +------------------------------------------------- + +will create a new branch named "example-master" and store in it the +branch named "master" from the repository at the given URL. If you +already have a branch named example-master, it will attempt to +<<fast-forwards,fast-forward>> to the commit given by example.com's +master branch. In more detail: + +[[fetch-fast-forwards]] +git fetch and fast-forwards +--------------------------- + +In the previous example, when updating an existing branch, "git +fetch" checks to make sure that the most recent commit on the remote +branch is a descendant of the most recent commit on your copy of the +branch before updating your copy of the branch to point at the new +commit. Git calls this process a <<fast-forwards,fast forward>>. + +A fast forward looks something like this: + +................................................ + o--o--o--o <-- old head of the branch + \ + o--o--o <-- new head of the branch +................................................ + + +In some cases it is possible that the new head will *not* actually be +a descendant of the old head. For example, the developer may have +realized she made a serious mistake, and decided to backtrack, +resulting in a situation like: + +................................................ + o--o--o--o--a--b <-- old head of the branch + \ + o--o--o <-- new head of the branch +................................................ + +In this case, "git fetch" will fail, and print out a warning. + +In that case, you can still force git to update to the new head, as +described in the following section. However, note that in the +situation above this may mean losing the commits labeled "a" and "b", +unless you've already created a reference of your own pointing to +them. + +[[forcing-fetch]] +Forcing git fetch to do non-fast-forward updates +------------------------------------------------ + +If git fetch fails because the new head of a branch is not a +descendant of the old head, you may force the update with: + +------------------------------------------------- +$ git fetch git://example.com/proj.git +master:refs/remotes/example/master +------------------------------------------------- + +Note the addition of the "+" sign. Alternatively, you can use the "-f" +flag to force updates of all the fetched branches, as in: + +------------------------------------------------- +$ git fetch -f origin +------------------------------------------------- + +Be aware that commits that the old version of example/master pointed at +may be lost, as we saw in the previous section. + +[[remote-branch-configuration]] +Configuring remote branches +--------------------------- + +We saw above that "origin" is just a shortcut to refer to the +repository that you originally cloned from. This information is +stored in git configuration variables, which you can see using +gitlink:git-config[1]: + +------------------------------------------------- +$ git config -l +core.repositoryformatversion=0 +core.filemode=true +core.logallrefupdates=true +remote.origin.url=git://git.kernel.org/pub/scm/git/git.git +remote.origin.fetch=+refs/heads/*:refs/remotes/origin/* +branch.master.remote=origin +branch.master.merge=refs/heads/master +------------------------------------------------- + +If there are other repositories that you also use frequently, you can +create similar configuration options to save typing; for example, +after + +------------------------------------------------- +$ git config remote.example.url git://example.com/proj.git +------------------------------------------------- + +then the following two commands will do the same thing: + +------------------------------------------------- +$ git fetch git://example.com/proj.git master:refs/remotes/example/master +$ git fetch example master:refs/remotes/example/master +------------------------------------------------- + +Even better, if you add one more option: + +------------------------------------------------- +$ git config remote.example.fetch master:refs/remotes/example/master +------------------------------------------------- + +then the following commands will all do the same thing: + +------------------------------------------------- +$ git fetch git://example.com/proj.git master:refs/remotes/example/master +$ git fetch example master:refs/remotes/example/master +$ git fetch example +------------------------------------------------- + +You can also add a "+" to force the update each time: + +------------------------------------------------- +$ git config remote.example.fetch +master:ref/remotes/example/master +------------------------------------------------- + +Don't do this unless you're sure you won't mind "git fetch" possibly +throwing away commits on mybranch. + +Also note that all of the above configuration can be performed by +directly editing the file .git/config instead of using +gitlink:git-config[1]. + +See gitlink:git-config[1] for more details on the configuration +options mentioned above. + + +[[git-internals]] +Git internals +============= + +Git depends on two fundamental abstractions: the "object database", and +the "current directory cache" aka "index". + +[[the-object-database]] +The Object Database +------------------- + +The object database is literally just a content-addressable collection +of objects. All objects are named by their content, which is +approximated by the SHA1 hash of the object itself. Objects may refer +to other objects (by referencing their SHA1 hash), and so you can +build up a hierarchy of objects. + +All objects have a statically determined "type" which is +determined at object creation time, and which identifies the format of +the object (i.e. how it is used, and how it can refer to other +objects). There are currently four different object types: "blob", +"tree", "commit", and "tag". + +A <<def_blob_object,"blob" object>> cannot refer to any other object, +and is, as the name implies, a pure storage object containing some +user data. It is used to actually store the file data, i.e. a blob +object is associated with some particular version of some file. + +A <<def_tree_object,"tree" object>> is an object that ties one or more +"blob" objects into a directory structure. In addition, a tree object +can refer to other tree objects, thus creating a directory hierarchy. + +A <<def_commit_object,"commit" object>> ties such directory hierarchies +together into a <<def_DAG,directed acyclic graph>> of revisions - each +"commit" is associated with exactly one tree (the directory hierarchy at +the time of the commit). In addition, a "commit" refers to one or more +"parent" commit objects that describe the history of how we arrived at +that directory hierarchy. + +As a special case, a commit object with no parents is called the "root" +commit, and is the point of an initial project commit. Each project +must have at least one root, and while you can tie several different +root objects together into one project by creating a commit object which +has two or more separate roots as its ultimate parents, that's probably +just going to confuse people. So aim for the notion of "one root object +per project", even if git itself does not enforce that. + +A <<def_tag_object,"tag" object>> symbolically identifies and can be +used to sign other objects. It contains the identifier and type of +another object, a symbolic name (of course!) and, optionally, a +signature. + +Regardless of object type, all objects share the following +characteristics: they are all deflated with zlib, and have a header +that not only specifies their type, but also provides size information +about the data in the object. It's worth noting that the SHA1 hash +that is used to name the object is the hash of the original data +plus this header, so `sha1sum` 'file' does not match the object name +for 'file'. +(Historical note: in the dawn of the age of git the hash +was the sha1 of the 'compressed' object.) + +As a result, the general consistency of an object can always be tested +independently of the contents or the type of the object: all objects can +be validated by verifying that (a) their hashes match the content of the +file and (b) the object successfully inflates to a stream of bytes that +forms a sequence of <ascii type without space> {plus} <space> {plus} <ascii decimal +size> {plus} <byte\0> {plus} <binary object data>. + +The structured objects can further have their structure and +connectivity to other objects verified. This is generally done with +the `git-fsck` program, which generates a full dependency graph +of all objects, and verifies their internal consistency (in addition +to just verifying their superficial consistency through the hash). + +The object types in some more detail: + +[[blob-object]] +Blob Object +----------- + +A "blob" object is nothing but a binary blob of data, and doesn't +refer to anything else. There is no signature or any other +verification of the data, so while the object is consistent (it 'is' +indexed by its sha1 hash, so the data itself is certainly correct), it +has absolutely no other attributes. No name associations, no +permissions. It is purely a blob of data (i.e. normally "file +contents"). + +In particular, since the blob is entirely defined by its data, if two +files in a directory tree (or in multiple different versions of the +repository) have the same contents, they will share the same blob +object. The object is totally independent of its location in the +directory tree, and renaming a file does not change the object that +file is associated with in any way. + +A blob is typically created when gitlink:git-update-index[1] +is run, and its data can be accessed by gitlink:git-cat-file[1]. + +[[tree-object]] +Tree Object +----------- + +The next hierarchical object type is the "tree" object. A tree object +is a list of mode/name/blob data, sorted by name. Alternatively, the +mode data may specify a directory mode, in which case instead of +naming a blob, that name is associated with another TREE object. + +Like the "blob" object, a tree object is uniquely determined by the +set contents, and so two separate but identical trees will always +share the exact same object. This is true at all levels, i.e. it's +true for a "leaf" tree (which does not refer to any other trees, only +blobs) as well as for a whole subdirectory. + +For that reason a "tree" object is just a pure data abstraction: it +has no history, no signatures, no verification of validity, except +that since the contents are again protected by the hash itself, we can +trust that the tree is immutable and its contents never change. + +So you can trust the contents of a tree to be valid, the same way you +can trust the contents of a blob, but you don't know where those +contents 'came' from. + +Side note on trees: since a "tree" object is a sorted list of +"filename+content", you can create a diff between two trees without +actually having to unpack two trees. Just ignore all common parts, +and your diff will look right. In other words, you can effectively +(and efficiently) tell the difference between any two random trees by +O(n) where "n" is the size of the difference, rather than the size of +the tree. + +Side note 2 on trees: since the name of a "blob" depends entirely and +exclusively on its contents (i.e. there are no names or permissions +involved), you can see trivial renames or permission changes by +noticing that the blob stayed the same. However, renames with data +changes need a smarter "diff" implementation. + +A tree is created with gitlink:git-write-tree[1] and +its data can be accessed by gitlink:git-ls-tree[1]. +Two trees can be compared with gitlink:git-diff-tree[1]. + +[[commit-object]] +Commit Object +------------- + +The "commit" object is an object that introduces the notion of +history into the picture. In contrast to the other objects, it +doesn't just describe the physical state of a tree, it describes how +we got there, and why. + +A "commit" is defined by the tree-object that it results in, the +parent commits (zero, one or more) that led up to that point, and a +comment on what happened. Again, a commit is not trusted per se: +the contents are well-defined and "safe" due to the cryptographically +strong signatures at all levels, but there is no reason to believe +that the tree is "good" or that the merge information makes sense. +The parents do not have to actually have any relationship with the +result, for example. + +Note on commits: unlike some SCM's, commits do not contain +rename information or file mode change information. All of that is +implicit in the trees involved (the result tree, and the result trees +of the parents), and describing that makes no sense in this idiotic +file manager. + +A commit is created with gitlink:git-commit-tree[1] and +its data can be accessed by gitlink:git-cat-file[1]. + +[[trust]] +Trust +----- + +An aside on the notion of "trust". Trust is really outside the scope +of "git", but it's worth noting a few things. First off, since +everything is hashed with SHA1, you 'can' trust that an object is +intact and has not been messed with by external sources. So the name +of an object uniquely identifies a known state - just not a state that +you may want to trust. + +Furthermore, since the SHA1 signature of a commit refers to the +SHA1 signatures of the tree it is associated with and the signatures +of the parent, a single named commit specifies uniquely a whole set +of history, with full contents. You can't later fake any step of the +way once you have the name of a commit. + +So to introduce some real trust in the system, the only thing you need +to do is to digitally sign just 'one' special note, which includes the +name of a top-level commit. Your digital signature shows others +that you trust that commit, and the immutability of the history of +commits tells others that they can trust the whole history. + +In other words, you can easily validate a whole archive by just +sending out a single email that tells the people the name (SHA1 hash) +of the top commit, and digitally sign that email using something +like GPG/PGP. + +To assist in this, git also provides the tag object... + +[[tag-object]] +Tag Object +---------- + +Git provides the "tag" object to simplify creating, managing and +exchanging symbolic and signed tokens. The "tag" object at its +simplest simply symbolically identifies another object by containing +the sha1, type and symbolic name. + +However it can optionally contain additional signature information +(which git doesn't care about as long as there's less than 8k of +it). This can then be verified externally to git. + +Note that despite the tag features, "git" itself only handles content +integrity; the trust framework (and signature provision and +verification) has to come from outside. + +A tag is created with gitlink:git-mktag[1], +its data can be accessed by gitlink:git-cat-file[1], +and the signature can be verified by +gitlink:git-verify-tag[1]. + + +[[the-index]] +The "index" aka "Current Directory Cache" +----------------------------------------- + +The index is a simple binary file, which contains an efficient +representation of the contents of a virtual directory. It +does so by a simple array that associates a set of names, dates, +permissions and content (aka "blob") objects together. The cache is +always kept ordered by name, and names are unique (with a few very +specific rules) at any point in time, but the cache has no long-term +meaning, and can be partially updated at any time. + +In particular, the index certainly does not need to be consistent with +the current directory contents (in fact, most operations will depend on +different ways to make the index 'not' be consistent with the directory +hierarchy), but it has three very important attributes: + +'(a) it can re-generate the full state it caches (not just the +directory structure: it contains pointers to the "blob" objects so +that it can regenerate the data too)' + +As a special case, there is a clear and unambiguous one-way mapping +from a current directory cache to a "tree object", which can be +efficiently created from just the current directory cache without +actually looking at any other data. So a directory cache at any one +time uniquely specifies one and only one "tree" object (but has +additional data to make it easy to match up that tree object with what +has happened in the directory) + +'(b) it has efficient methods for finding inconsistencies between that +cached state ("tree object waiting to be instantiated") and the +current state.' + +'(c) it can additionally efficiently represent information about merge +conflicts between different tree objects, allowing each pathname to be +associated with sufficient information about the trees involved that +you can create a three-way merge between them.' + +Those are the ONLY three things that the directory cache does. It's a +cache, and the normal operation is to re-generate it completely from a +known tree object, or update/compare it with a live tree that is being +developed. If you blow the directory cache away entirely, you generally +haven't lost any information as long as you have the name of the tree +that it described. + +At the same time, the index is at the same time also the +staging area for creating new trees, and creating a new tree always +involves a controlled modification of the index file. In particular, +the index file can have the representation of an intermediate tree that +has not yet been instantiated. So the index can be thought of as a +write-back cache, which can contain dirty information that has not yet +been written back to the backing store. + + + +[[the-workflow]] +The Workflow +------------ + +Generally, all "git" operations work on the index file. Some operations +work *purely* on the index file (showing the current state of the +index), but most operations move data to and from the index file. Either +from the database or from the working directory. Thus there are four +main combinations: + +[[working-directory-to-index]] +working directory -> index +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You update the index with information from the working directory with +the gitlink:git-update-index[1] command. You +generally update the index information by just specifying the filename +you want to update, like so: + +------------------------------------------------- +$ git-update-index filename +------------------------------------------------- + +but to avoid common mistakes with filename globbing etc, the command +will not normally add totally new entries or remove old entries, +i.e. it will normally just update existing cache entries. + +To tell git that yes, you really do realize that certain files no +longer exist, or that new files should be added, you +should use the `--remove` and `--add` flags respectively. + +NOTE! A `--remove` flag does 'not' mean that subsequent filenames will +necessarily be removed: if the files still exist in your directory +structure, the index will be updated with their new status, not +removed. The only thing `--remove` means is that update-cache will be +considering a removed file to be a valid thing, and if the file really +does not exist any more, it will update the index accordingly. + +As a special case, you can also do `git-update-index --refresh`, which +will refresh the "stat" information of each index to match the current +stat information. It will 'not' update the object status itself, and +it will only update the fields that are used to quickly test whether +an object still matches its old backing store object. + +[[index-to-object-database]] +index -> object database +~~~~~~~~~~~~~~~~~~~~~~~~ + +You write your current index file to a "tree" object with the program + +------------------------------------------------- +$ git-write-tree +------------------------------------------------- + +that doesn't come with any options - it will just write out the +current index into the set of tree objects that describe that state, +and it will return the name of the resulting top-level tree. You can +use that tree to re-generate the index at any time by going in the +other direction: + +[[object-database-to-index]] +object database -> index +~~~~~~~~~~~~~~~~~~~~~~~~ + +You read a "tree" file from the object database, and use that to +populate (and overwrite - don't do this if your index contains any +unsaved state that you might want to restore later!) your current +index. Normal operation is just + +------------------------------------------------- +$ git-read-tree <sha1 of tree> +------------------------------------------------- + +and your index file will now be equivalent to the tree that you saved +earlier. However, that is only your 'index' file: your working +directory contents have not been modified. + +[[index-to-working-directory]] +index -> working directory +~~~~~~~~~~~~~~~~~~~~~~~~~~ + +You update your working directory from the index by "checking out" +files. This is not a very common operation, since normally you'd just +keep your files updated, and rather than write to your working +directory, you'd tell the index files about the changes in your +working directory (i.e. `git-update-index`). + +However, if you decide to jump to a new version, or check out somebody +else's version, or just restore a previous tree, you'd populate your +index file with read-tree, and then you need to check out the result +with + +------------------------------------------------- +$ git-checkout-index filename +------------------------------------------------- + +or, if you want to check out all of the index, use `-a`. + +NOTE! git-checkout-index normally refuses to overwrite old files, so +if you have an old version of the tree already checked out, you will +need to use the "-f" flag ('before' the "-a" flag or the filename) to +'force' the checkout. + + +Finally, there are a few odds and ends which are not purely moving +from one representation to the other: + +[[tying-it-all-together]] +Tying it all together +~~~~~~~~~~~~~~~~~~~~~ + +To commit a tree you have instantiated with "git-write-tree", you'd +create a "commit" object that refers to that tree and the history +behind it - most notably the "parent" commits that preceded it in +history. + +Normally a "commit" has one parent: the previous state of the tree +before a certain change was made. However, sometimes it can have two +or more parent commits, in which case we call it a "merge", due to the +fact that such a commit brings together ("merges") two or more +previous states represented by other commits. + +In other words, while a "tree" represents a particular directory state +of a working directory, a "commit" represents that state in "time", +and explains how we got there. + +You create a commit object by giving it the tree that describes the +state at the time of the commit, and a list of parents: + +------------------------------------------------- +$ git-commit-tree <tree> -p <parent> [-p <parent2> ..] +------------------------------------------------- + +and then giving the reason for the commit on stdin (either through +redirection from a pipe or file, or by just typing it at the tty). + +git-commit-tree will return the name of the object that represents +that commit, and you should save it away for later use. Normally, +you'd commit a new `HEAD` state, and while git doesn't care where you +save the note about that state, in practice we tend to just write the +result to the file pointed at by `.git/HEAD`, so that we can always see +what the last committed state was. + +Here is an ASCII art by Jon Loeliger that illustrates how +various pieces fit together. + +------------ + + commit-tree + commit obj + +----+ + | | + | | + V V + +-----------+ + | Object DB | + | Backing | + | Store | + +-----------+ + ^ + write-tree | | + tree obj | | + | | read-tree + | | tree obj + V + +-----------+ + | Index | + | "cache" | + +-----------+ + update-index ^ + blob obj | | + | | + checkout-index -u | | checkout-index + stat | | blob obj + V + +-----------+ + | Working | + | Directory | + +-----------+ + +------------ + + +[[examining-the-data]] +Examining the data +------------------ + +You can examine the data represented in the object database and the +index with various helper tools. For every object, you can use +gitlink:git-cat-file[1] to examine details about the +object: + +------------------------------------------------- +$ git-cat-file -t <objectname> +------------------------------------------------- + +shows the type of the object, and once you have the type (which is +usually implicit in where you find the object), you can use + +------------------------------------------------- +$ git-cat-file blob|tree|commit|tag <objectname> +------------------------------------------------- + +to show its contents. NOTE! Trees have binary content, and as a result +there is a special helper for showing that content, called +`git-ls-tree`, which turns the binary content into a more easily +readable form. + +It's especially instructive to look at "commit" objects, since those +tend to be small and fairly self-explanatory. In particular, if you +follow the convention of having the top commit name in `.git/HEAD`, +you can do + +------------------------------------------------- +$ git-cat-file commit HEAD +------------------------------------------------- + +to see what the top commit was. + +[[merging-multiple-trees]] +Merging multiple trees +---------------------- + +Git helps you do a three-way merge, which you can expand to n-way by +repeating the merge procedure arbitrary times until you finally +"commit" the state. The normal situation is that you'd only do one +three-way merge (two parents), and commit it, but if you like to, you +can do multiple parents in one go. + +To do a three-way merge, you need the two sets of "commit" objects +that you want to merge, use those to find the closest common parent (a +third "commit" object), and then use those commit objects to find the +state of the directory ("tree" object) at these points. + +To get the "base" for the merge, you first look up the common parent +of two commits with + +------------------------------------------------- +$ git-merge-base <commit1> <commit2> +------------------------------------------------- + +which will return you the commit they are both based on. You should +now look up the "tree" objects of those commits, which you can easily +do with (for example) + +------------------------------------------------- +$ git-cat-file commit <commitname> | head -1 +------------------------------------------------- + +since the tree object information is always the first line in a commit +object. + +Once you know the three trees you are going to merge (the one "original" +tree, aka the common tree, and the two "result" trees, aka the branches +you want to merge), you do a "merge" read into the index. This will +complain if it has to throw away your old index contents, so you should +make sure that you've committed those - in fact you would normally +always do a merge against your last commit (which should thus match what +you have in your current index anyway). + +To do the merge, do + +------------------------------------------------- +$ git-read-tree -m -u <origtree> <yourtree> <targettree> +------------------------------------------------- + +which will do all trivial merge operations for you directly in the +index file, and you can just write the result out with +`git-write-tree`. + + +[[merging-multiple-trees-2]] +Merging multiple trees, continued +--------------------------------- + +Sadly, many merges aren't trivial. If there are files that have +been added.moved or removed, or if both branches have modified the +same file, you will be left with an index tree that contains "merge +entries" in it. Such an index tree can 'NOT' be written out to a tree +object, and you will have to resolve any such merge clashes using +other tools before you can write out the result. + +You can examine such index state with `git-ls-files --unmerged` +command. An example: + +------------------------------------------------ +$ git-read-tree -m $orig HEAD $target +$ git-ls-files --unmerged +100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello.c +100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2 hello.c +100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello.c +------------------------------------------------ + +Each line of the `git-ls-files --unmerged` output begins with +the blob mode bits, blob SHA1, 'stage number', and the +filename. The 'stage number' is git's way to say which tree it +came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD` +tree, and stage3 `$target` tree. + +Earlier we said that trivial merges are done inside +`git-read-tree -m`. For example, if the file did not change +from `$orig` to `HEAD` nor `$target`, or if the file changed +from `$orig` to `HEAD` and `$orig` to `$target` the same way, +obviously the final outcome is what is in `HEAD`. What the +above example shows is that file `hello.c` was changed from +`$orig` to `HEAD` and `$orig` to `$target` in a different way. +You could resolve this by running your favorite 3-way merge +program, e.g. `diff3`, `merge`, or git's own merge-file, on +the blob objects from these three stages yourself, like this: + +------------------------------------------------ +$ git-cat-file blob 263414f... >hello.c~1 +$ git-cat-file blob 06fa6a2... >hello.c~2 +$ git-cat-file blob cc44c73... >hello.c~3 +$ git merge-file hello.c~2 hello.c~1 hello.c~3 +------------------------------------------------ + +This would leave the merge result in `hello.c~2` file, along +with conflict markers if there are conflicts. After verifying +the merge result makes sense, you can tell git what the final +merge result for this file is by: + +------------------------------------------------- +$ mv -f hello.c~2 hello.c +$ git-update-index hello.c +------------------------------------------------- + +When a path is in unmerged state, running `git-update-index` for +that path tells git to mark the path resolved. + +The above is the description of a git merge at the lowest level, +to help you understand what conceptually happens under the hood. +In practice, nobody, not even git itself, uses three `git-cat-file` +for this. There is `git-merge-index` program that extracts the +stages to temporary files and calls a "merge" script on it: + +------------------------------------------------- +$ git-merge-index git-merge-one-file hello.c +------------------------------------------------- + +and that is what higher level `git merge -s resolve` is implemented with. + +[[pack-files]] +How git stores objects efficiently: pack files +---------------------------------------------- + +We've seen how git stores each object in a file named after the +object's SHA1 hash. + +Unfortunately this system becomes inefficient once a project has a +lot of objects. Try this on an old project: + +------------------------------------------------ +$ git count-objects +6930 objects, 47620 kilobytes +------------------------------------------------ + +The first number is the number of objects which are kept in +individual files. The second is the amount of space taken up by +those "loose" objects. + +You can save space and make git faster by moving these loose objects in +to a "pack file", which stores a group of objects in an efficient +compressed format; the details of how pack files are formatted can be +found in link:technical/pack-format.txt[technical/pack-format.txt]. + +To put the loose objects into a pack, just run git repack: + +------------------------------------------------ +$ git repack +Generating pack... +Done counting 6020 objects. +Deltifying 6020 objects. + 100% (6020/6020) done +Writing 6020 objects. + 100% (6020/6020) done +Total 6020, written 6020 (delta 4070), reused 0 (delta 0) +Pack pack-3e54ad29d5b2e05838c75df582c65257b8d08e1c created. +------------------------------------------------ + +You can then run + +------------------------------------------------ +$ git prune +------------------------------------------------ + +to remove any of the "loose" objects that are now contained in the +pack. This will also remove any unreferenced objects (which may be +created when, for example, you use "git reset" to remove a commit). +You can verify that the loose objects are gone by looking at the +.git/objects directory or by running + +------------------------------------------------ +$ git count-objects +0 objects, 0 kilobytes +------------------------------------------------ + +Although the object files are gone, any commands that refer to those +objects will work exactly as they did before. + +The gitlink:git-gc[1] command performs packing, pruning, and more for +you, so is normally the only high-level command you need. + +[[dangling-objects]] +Dangling objects +---------------- + +The gitlink:git-fsck[1] command will sometimes complain about dangling +objects. They are not a problem. + +The most common cause of dangling objects is that you've rebased a +branch, or you have pulled from somebody else who rebased a branch--see +<<cleaning-up-history>>. In that case, the old head of the original +branch still exists, as does everything it pointed to. The branch +pointer itself just doesn't, since you replaced it with another one. + +There are also other situations that cause dangling objects. For +example, a "dangling blob" may arise because you did a "git add" of a +file, but then, before you actually committed it and made it part of the +bigger picture, you changed something else in that file and committed +that *updated* thing - the old state that you added originally ends up +not being pointed to by any commit or tree, so it's now a dangling blob +object. + +Similarly, when the "recursive" merge strategy runs, and finds that +there are criss-cross merges and thus more than one merge base (which is +fairly unusual, but it does happen), it will generate one temporary +midway tree (or possibly even more, if you had lots of criss-crossing +merges and more than two merge bases) as a temporary internal merge +base, and again, those are real objects, but the end result will not end +up pointing to them, so they end up "dangling" in your repository. + +Generally, dangling objects aren't anything to worry about. They can +even be very useful: if you screw something up, the dangling objects can +be how you recover your old tree (say, you did a rebase, and realized +that you really didn't want to - you can look at what dangling objects +you have, and decide to reset your head to some old dangling state). + +For commits, you can just use: + +------------------------------------------------ +$ gitk <dangling-commit-sha-goes-here> --not --all +------------------------------------------------ + +This asks for all the history reachable from the given commit but not +from any branch, tag, or other reference. If you decide it's something +you want, you can always create a new reference to it, e.g., + +------------------------------------------------ +$ git branch recovered-branch <dangling-commit-sha-goes-here> +------------------------------------------------ + +For blobs and trees, you can't do the same, but you can still examine +them. You can just do + +------------------------------------------------ +$ git show <dangling-blob/tree-sha-goes-here> +------------------------------------------------ + +to show what the contents of the blob were (or, for a tree, basically +what the "ls" for that directory was), and that may give you some idea +of what the operation was that left that dangling object. + +Usually, dangling blobs and trees aren't very interesting. They're +almost always the result of either being a half-way mergebase (the blob +will often even have the conflict markers from a merge in it, if you +have had conflicting merges that you fixed up by hand), or simply +because you interrupted a "git fetch" with ^C or something like that, +leaving _some_ of the new objects in the object database, but just +dangling and useless. + +Anyway, once you are sure that you're not interested in any dangling +state, you can just prune all unreachable objects: + +------------------------------------------------ +$ git prune +------------------------------------------------ + +and they'll be gone. But you should only run "git prune" on a quiescent +repository - it's kind of like doing a filesystem fsck recovery: you +don't want to do that while the filesystem is mounted. + +(The same is true of "git-fsck" itself, btw - but since +git-fsck never actually *changes* the repository, it just reports +on what it found, git-fsck itself is never "dangerous" to run. +Running it while somebody is actually changing the repository can cause +confusing and scary messages, but it won't actually do anything bad. In +contrast, running "git prune" while somebody is actively changing the +repository is a *BAD* idea). + +[[birdview-on-the-source-code]] +A birds-eye view of Git's source code +------------------------------------- + +It is not always easy for new developers to find their way through Git's +source code. This section gives you a little guidance to show where to +start. + +A good place to start is with the contents of the initial commit, with: + +---------------------------------------------------- +$ git checkout e83c5163 +---------------------------------------------------- + +The initial revision lays the foundation for almost everything git has +today, but is small enough to read in one sitting. + +Note that terminology has changed since that revision. For example, the +README in that revision uses the word "changeset" to describe what we +now call a <<def_commit_object,commit>>. + +Also, we do not call it "cache" any more, but "index", however, the +file is still called `cache.h`. Remark: Not much reason to change it now, +especially since there is no good single name for it anyway, because it is +basically _the_ header file which is included by _all_ of Git's C sources. + +If you grasp the ideas in that initial commit, you should check out a +more recent version and skim `cache.h`, `object.h` and `commit.h`. + +In the early days, Git (in the tradition of UNIX) was a bunch of programs +which were extremely simple, and which you used in scripts, piping the +output of one into another. This turned out to be good for initial +development, since it was easier to test new things. However, recently +many of these parts have become builtins, and some of the core has been +"libified", i.e. put into libgit.a for performance, portability reasons, +and to avoid code duplication. + +By now, you know what the index is (and find the corresponding data +structures in `cache.h`), and that there are just a couple of object types +(blobs, trees, commits and tags) which inherit their common structure from +`struct object`, which is their first member (and thus, you can cast e.g. +`(struct object *)commit` to achieve the _same_ as `&commit->object`, i.e. +get at the object name and flags). + +Now is a good point to take a break to let this information sink in. + +Next step: get familiar with the object naming. Read <<naming-commits>>. +There are quite a few ways to name an object (and not only revisions!). +All of these are handled in `sha1_name.c`. Just have a quick look at +the function `get_sha1()`. A lot of the special handling is done by +functions like `get_sha1_basic()` or the likes. + +This is just to get you into the groove for the most libified part of Git: +the revision walker. + +Basically, the initial version of `git log` was a shell script: + +---------------------------------------------------------------- +$ git-rev-list --pretty $(git-rev-parse --default HEAD "$@") | \ + LESS=-S ${PAGER:-less} +---------------------------------------------------------------- + +What does this mean? + +`git-rev-list` is the original version of the revision walker, which +_always_ printed a list of revisions to stdout. It is still functional, +and needs to, since most new Git programs start out as scripts using +`git-rev-list`. + +`git-rev-parse` is not as important any more; it was only used to filter out +options that were relevant for the different plumbing commands that were +called by the script. + +Most of what `git-rev-list` did is contained in `revision.c` and +`revision.h`. It wraps the options in a struct named `rev_info`, which +controls how and what revisions are walked, and more. + +The original job of `git-rev-parse` is now taken by the function +`setup_revisions()`, which parses the revisions and the common command line +options for the revision walker. This information is stored in the struct +`rev_info` for later consumption. You can do your own command line option +parsing after calling `setup_revisions()`. After that, you have to call +`prepare_revision_walk()` for initialization, and then you can get the +commits one by one with the function `get_revision()`. + +If you are interested in more details of the revision walking process, +just have a look at the first implementation of `cmd_log()`; call +`git-show v1.3.0~155^2~4` and scroll down to that function (note that you +no longer need to call `setup_pager()` directly). + +Nowadays, `git log` is a builtin, which means that it is _contained_ in the +command `git`. The source side of a builtin is + +- a function called `cmd_<bla>`, typically defined in `builtin-<bla>.c`, + and declared in `builtin.h`, + +- an entry in the `commands[]` array in `git.c`, and + +- an entry in `BUILTIN_OBJECTS` in the `Makefile`. + +Sometimes, more than one builtin is contained in one source file. For +example, `cmd_whatchanged()` and `cmd_log()` both reside in `builtin-log.c`, +since they share quite a bit of code. In that case, the commands which are +_not_ named like the `.c` file in which they live have to be listed in +`BUILT_INS` in the `Makefile`. + +`git log` looks more complicated in C than it does in the original script, +but that allows for a much greater flexibility and performance. + +Here again it is a good point to take a pause. + +Lesson three is: study the code. Really, it is the best way to learn about +the organization of Git (after you know the basic concepts). + +So, think about something which you are interested in, say, "how can I +access a blob just knowing the object name of it?". The first step is to +find a Git command with which you can do it. In this example, it is either +`git show` or `git cat-file`. + +For the sake of clarity, let's stay with `git cat-file`, because it + +- is plumbing, and + +- was around even in the initial commit (it literally went only through + some 20 revisions as `cat-file.c`, was renamed to `builtin-cat-file.c` + when made a builtin, and then saw less than 10 versions). + +So, look into `builtin-cat-file.c`, search for `cmd_cat_file()` and look what +it does. + +------------------------------------------------------------------ + git_config(git_default_config); + if (argc != 3) + usage("git-cat-file [-t|-s|-e|-p|<type>] <sha1>"); + if (get_sha1(argv[2], sha1)) + die("Not a valid object name %s", argv[2]); +------------------------------------------------------------------ + +Let's skip over the obvious details; the only really interesting part +here is the call to `get_sha1()`. It tries to interpret `argv[2]` as an +object name, and if it refers to an object which is present in the current +repository, it writes the resulting SHA-1 into the variable `sha1`. + +Two things are interesting here: + +- `get_sha1()` returns 0 on _success_. This might surprise some new + Git hackers, but there is a long tradition in UNIX to return different + negative numbers in case of different errors -- and 0 on success. + +- the variable `sha1` in the function signature of `get_sha1()` is `unsigned + char \*`, but is actually expected to be a pointer to `unsigned + char[20]`. This variable will contain the 160-bit SHA-1 of the given + commit. Note that whenever a SHA-1 is passed as `unsigned char \*`, it + is the binary representation, as opposed to the ASCII representation in + hex characters, which is passed as `char *`. + +You will see both of these things throughout the code. + +Now, for the meat: + +----------------------------------------------------------------------------- + case 0: + buf = read_object_with_reference(sha1, argv[1], &size, NULL); +----------------------------------------------------------------------------- + +This is how you read a blob (actually, not only a blob, but any type of +object). To know how the function `read_object_with_reference()` actually +works, find the source code for it (something like `git grep +read_object_with | grep ":[a-z]"` in the git repository), and read +the source. + +To find out how the result can be used, just read on in `cmd_cat_file()`: + +----------------------------------- + write_or_die(1, buf, size); +----------------------------------- + +Sometimes, you do not know where to look for a feature. In many such cases, +it helps to search through the output of `git log`, and then `git show` the +corresponding commit. + +Example: If you know that there was some test case for `git bundle`, but +do not remember where it was (yes, you _could_ `git grep bundle t/`, but that +does not illustrate the point!): + +------------------------ +$ git log --no-merges t/ +------------------------ + +In the pager (`less`), just search for "bundle", go a few lines back, +and see that it is in commit 18449ab0... Now just copy this object name, +and paste it into the command line + +------------------- +$ git show 18449ab0 +------------------- + +Voila. + +Another example: Find out what to do in order to make some script a +builtin: + +------------------------------------------------- +$ git log --no-merges --diff-filter=A builtin-*.c +------------------------------------------------- + +You see, Git is actually the best tool to find out about the source of Git +itself! + +[[glossary]] +include::glossary.txt[] + +[[git-quick-start]] +Appendix A: Git Quick Reference +=============================== + +This is a quick summary of the major commands; the previous chapters +explain how these work in more detail. + +[[quick-creating-a-new-repository]] +Creating a new repository +------------------------- + +From a tarball: + +----------------------------------------------- +$ tar xzf project.tar.gz +$ cd project +$ git init +Initialized empty Git repository in .git/ +$ git add . +$ git commit +----------------------------------------------- + +From a remote repository: + +----------------------------------------------- +$ git clone git://example.com/pub/project.git +$ cd project +----------------------------------------------- + +[[managing-branches]] +Managing branches +----------------- + +----------------------------------------------- +$ git branch # list all local branches in this repo +$ git checkout test # switch working directory to branch "test" +$ git branch new # create branch "new" starting at current HEAD +$ git branch -d new # delete branch "new" +----------------------------------------------- + +Instead of basing new branch on current HEAD (the default), use: + +----------------------------------------------- +$ git branch new test # branch named "test" +$ git branch new v2.6.15 # tag named v2.6.15 +$ git branch new HEAD^ # commit before the most recent +$ git branch new HEAD^^ # commit before that +$ git branch new test~10 # ten commits before tip of branch "test" +----------------------------------------------- + +Create and switch to a new branch at the same time: + +----------------------------------------------- +$ git checkout -b new v2.6.15 +----------------------------------------------- + +Update and examine branches from the repository you cloned from: + +----------------------------------------------- +$ git fetch # update +$ git branch -r # list + origin/master + origin/next + ... +$ git checkout -b masterwork origin/master +----------------------------------------------- + +Fetch a branch from a different repository, and give it a new +name in your repository: + +----------------------------------------------- +$ git fetch git://example.com/project.git theirbranch:mybranch +$ git fetch git://example.com/project.git v2.6.15:mybranch +----------------------------------------------- + +Keep a list of repositories you work with regularly: + +----------------------------------------------- +$ git remote add example git://example.com/project.git +$ git remote # list remote repositories +example +origin +$ git remote show example # get details +* remote example + URL: git://example.com/project.git + Tracked remote branches + master next ... +$ git fetch example # update branches from example +$ git branch -r # list all remote branches +----------------------------------------------- + + +[[exploring-history]] +Exploring history +----------------- + +----------------------------------------------- +$ gitk # visualize and browse history +$ git log # list all commits +$ git log src/ # ...modifying src/ +$ git log v2.6.15..v2.6.16 # ...in v2.6.16, not in v2.6.15 +$ git log master..test # ...in branch test, not in branch master +$ git log test..master # ...in branch master, but not in test +$ git log test...master # ...in one branch, not in both +$ git log -S'foo()' # ...where difference contain "foo()" +$ git log --since="2 weeks ago" +$ git log -p # show patches as well +$ git show # most recent commit +$ git diff v2.6.15..v2.6.16 # diff between two tagged versions +$ git diff v2.6.15..HEAD # diff with current head +$ git grep "foo()" # search working directory for "foo()" +$ git grep v2.6.15 "foo()" # search old tree for "foo()" +$ git show v2.6.15:a.txt # look at old version of a.txt +----------------------------------------------- + +Search for regressions: + +----------------------------------------------- +$ git bisect start +$ git bisect bad # current version is bad +$ git bisect good v2.6.13-rc2 # last known good revision +Bisecting: 675 revisions left to test after this + # test here, then: +$ git bisect good # if this revision is good, or +$ git bisect bad # if this revision is bad. + # repeat until done. +----------------------------------------------- + +[[making-changes]] +Making changes +-------------- + +Make sure git knows who to blame: + +------------------------------------------------ +$ cat >>~/.gitconfig <<\EOF +[user] + name = Your Name Comes Here + email = you@yourdomain.example.com +EOF +------------------------------------------------ + +Select file contents to include in the next commit, then make the +commit: + +----------------------------------------------- +$ git add a.txt # updated file +$ git add b.txt # new file +$ git rm c.txt # old file +$ git commit +----------------------------------------------- + +Or, prepare and create the commit in one step: + +----------------------------------------------- +$ git commit d.txt # use latest content only of d.txt +$ git commit -a # use latest content of all tracked files +----------------------------------------------- + +[[merging]] +Merging +------- + +----------------------------------------------- +$ git merge test # merge branch "test" into the current branch +$ git pull git://example.com/project.git master + # fetch and merge in remote branch +$ git pull . test # equivalent to git merge test +----------------------------------------------- + +[[sharing-your-changes]] +Sharing your changes +-------------------- + +Importing or exporting patches: + +----------------------------------------------- +$ git format-patch origin..HEAD # format a patch for each commit + # in HEAD but not in origin +$ git am mbox # import patches from the mailbox "mbox" +----------------------------------------------- + +Fetch a branch in a different git repository, then merge into the +current branch: + +----------------------------------------------- +$ git pull git://example.com/project.git theirbranch +----------------------------------------------- + +Store the fetched branch into a local branch before merging into the +current branch: + +----------------------------------------------- +$ git pull git://example.com/project.git theirbranch:mybranch +----------------------------------------------- + +After creating commits on a local branch, update the remote +branch with your commits: + +----------------------------------------------- +$ git push ssh://example.com/project.git mybranch:theirbranch +----------------------------------------------- + +When remote and local branch are both named "test": + +----------------------------------------------- +$ git push ssh://example.com/project.git test +----------------------------------------------- + +Shortcut version for a frequently used remote repository: + +----------------------------------------------- +$ git remote add example ssh://example.com/project.git +$ git push example test +----------------------------------------------- + +[[repository-maintenance]] +Repository maintenance +---------------------- + +Check for corruption: + +----------------------------------------------- +$ git fsck +----------------------------------------------- + +Recompress, remove unused cruft: + +----------------------------------------------- +$ git gc +----------------------------------------------- + + +[[todo]] +Appendix B: Notes and todo list for this manual +=============================================== + +This is a work in progress. + +The basic requirements: + - It must be readable in order, from beginning to end, by + someone intelligent with a basic grasp of the unix + commandline, but without any special knowledge of git. If + necessary, any other prerequisites should be specifically + mentioned as they arise. + - Whenever possible, section headings should clearly describe + the task they explain how to do, in language that requires + no more knowledge than necessary: for example, "importing + patches into a project" rather than "the git-am command" + +Think about how to create a clear chapter dependency graph that will +allow people to get to important topics without necessarily reading +everything in between. + +Scan Documentation/ for other stuff left out; in particular: + howto's + some of technical/? + hooks + list of commands in gitlink:git[1] + +Scan email archives for other stuff left out + +Scan man pages to see if any assume more background than this manual +provides. + +Simplify beginning by suggesting disconnected head instead of +temporary branch creation? + +Add more good examples. Entire sections of just cookbook examples +might be a good idea; maybe make an "advanced examples" section a +standard end-of-chapter section? + +Include cross-references to the glossary, where appropriate. + +Document shallow clones? See draft 1.5.0 release notes for some +documentation. + +Add a section on working with other version control systems, including +CVS, Subversion, and just imports of series of release tarballs. + +More details on gitweb? + +Write a chapter on using plumbing and writing scripts. + +Alternates, clone -reference, etc. + +git unpack-objects -r for recovery |