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Chapter 1 Repositories and Branches

1. REVISION HISTORY NUMBER DATE DESCRIPTION NAME Contents I Git User s Manual for version 1 5 3 or newer 1 1 Repositories and Branches 3 Ll HOw to peta Cal Pepositery coe ra ee ORE EER EEG EAA ee ee PASE HG 3 1 2 How to check out a different version of a project oo c e co we A ee eo 3 1 3 Understanding History Commits o 6 4 4 Soe wea bee ee ER a ee EE ewe eee he ae 4 1 3 1 Understanding history commits parents and reachability 5 1 3 2 Understanding history History diagrams 2 2 0 e 5 1 33 Understanding history What is abranch lt o oco e eso 56854803 e 60 bee een eee ees 5 14 Wieland DROS io eG eR a AO RE ER REE SOE EEE Oe e A 5 1 5 Examining an old version without creating anew branch e 6 L6 Examining branches froma remote repository ss cs ek cedri RE ERE RE Ew RE EG 6 1 7 Naming branches Tags and other references ee eR RS ee ee a ea 7 Le Updating a repository with git fete sao ee Ra eee Oe Re ee Ee YY De eee ee e 7 LY Fetching branches irom other repositories pa vena ade enae Se a Pee eee eG Gs 8 2 Exploring Git history 9 2 1 Howto use bisectiotindaresression 2 we ee ee ee ee ee 9 Le Namin COMMING coe oe a ae ea RG a ea ee Ea ae eR SER Ee ee hea ee 10 25 o oe Se aE SEER RBS REED ER Rae EE RAR Rae SE ee ask 11 2A BROWSING revisions oo eee Se Eo wee eee e a 11 pis Enere ANE Ss o
2. git checkout release amp amp 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 1s 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 After pushing your work to myt ree you can use git request pull 1 to prepare a please pull request message to send to Linus git push mytree git request pull origin mytree release Here are some of the scripts that simplify al
3. 22 70 3 10 2 Fixing a mistake by rewriting history If the problematic commit is the most recent commit and you have not yet made that commit public then you may just 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 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 git revert 1 instead in that case It is also possible to replace commits further back in the history but this is an advanced topic to be left for another chapter 3 10 3 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 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 git show 1 git show HEAD path to file which will
4. 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 7 1 8 Recovering from repository corruption By design Git treats data trusted to it with caution However even in the absence of bugs in Git itself it is still possible that hardware or operating system errors could corrupt data The first defense against such problems is backups You can back up a Git directory using clone or just using cp tar or any other backup mechanism 46 70 As a last resort you can search for the corrupted objects and attempt to replace them by hand Back up your repository before attempting this in case you corrupt things even more in the process We ll assume that the problem is a single missing or corrupted blob which is sometimes a solvable problem Recovering missing trees and especially commits is much harder Before starting verify that there is corruption and figure out where it is with git fsck 1 this may be time consuming Assume the output looks like this git fsck full no dangling broken link from tree 2d9263c6dqd23595e7cb2a2le5ebbb53655278dqdff8 to blob 469458b378
5. changes ago gitk master yesterday See where it pointed yesterday gitk master l 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 git reflog 1 and git gc 1 to learn how to control this pruning and see the SPECIFYING REVISIONS section of gitrevisions 7 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 24 70 3 12 2 2 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 Section 7 1 7 for the details git fsck dangling commit 7281251ddd2a61e38657c827739c57015671a6b3 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21
6. You can rejoin two diverging branches of development using git merge 1 git merge branchname merges the development in the branch branchname into the current branch A merge is made by combining the changes made in branchname and the changes made up to the latest commit in your current branch since their histories forked The work tree is overwritten by the result of the merge when this combining is done cleanly or overwritten by a half merged results when this combining results in conflicts Therefore if you have uncommitted changes touching the same files as the ones impacted by the merge Git will refuse to proceed Most of the time you will want to commit your changes before you can merge and if you don t then git stash 1 can take these changes away while you re doing the merge and reapply them afterwards If the changes are independent enough Git will automatically complete the merge and commit the result or reuse an existing commit in case of fast forward see below On the other hand 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 19 70 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
7. A single Git repository can track development on multiple branches It does this by keeping a list of heads which reference the latest commit on each branch the 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 tags Tags like heads are references into the project s history and can be listed using the git tag 1 command git tag l v2 6 11 v2 6 11 tree 4 70 v2 0 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 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 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 bran
8. commit 2be7fcb4764f2dbcee52635b91fedbl1lb3dcf7ab4 tree fb3a8bdd0ceddd019615af4d57a53f43d8cee2bFf parent 257a84d9d02e90447b149af58b271c19405edb6a author Dave Watson lt dwatson mimvista com gt 1187576872 0400 committer Junio C Hamano lt gitster pobox com gt 1187591163 0700 Fix misspelling of suppress in docs Signed off by Junio C Hamano lt gitster pobox com gt As you can see a commit is defined by e atree The SHA 1 name of a tree object as defined below representing the contents of a directory at a certain point in time e parent s The SHA 1 name s of some number of commits which represent the immediately previous step s in the history of the project The example above has one parent merge commits may have more than one A commit with no parents is called a root commit and represents the initial revision of a project Each project must have at least one root A project can also have multiple roots though that isn t common or necessarily a good idea e an author The name of the person responsible for this change together with its date acommitter The name of the person who actually created the commit with the date it was done This may be different from the author for example if the author was someone who wrote a patch and emailed it to the person who used it to create the commit e a comment describing this commit Note that a commit does not itself contain any information about what actually change
9. However if you prefer to keep the history in mywork a simple series of commits without any merges you may instead choose to use 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 git rebase apply 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 0 O 0 0 o x origin a b c lt 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 If you need to reorder or edit a number of commits in a branch it may be easier to use git rebase i which allows you to reorder and squash commits as well as marking them for individual editing during the rebase See Section 5 5 for details and Section 5 4 for alternatives 5 3 Rewriting a single commit We saw in Section 3 10 2 that you can replace the most recent commit using git commit amend which will replace the old commit by a new commit
10. 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 old 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 3 10 1 Fixing a mistake with a new commit Creating a new commit that reverts an earlier change is very easy just pass the 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
11. commit As a noun A single point in the Git history the entire history of a project is represented as a set of interrelated commits The word commit is often used by Git in the same places other revision control systems use the words revision or version Also used as a short hand for commit object As a verb The action of storing a new snapshot of the project s state in the Git history by creating a new commit representing the current state of the index and advancing HEAD to point at the new commit commit object An object which contains the information about a particular revision such as parents committer author date and the tree object which corresponds to the top directory of the stored revision core Git Fundamental data structures and utilities of Git Exposes only limited source code management tools DAG Directed acyclic graph The commit objects form a directed acyclic graph because they have parents directed and the graph of commit objects is acyclic there is no chain which begins and ends with the same object dangling object An unreachable object which is not reachable even from other unreachable objects a dangling object has no references to it from any reference or object in the repository detached HEAD Normally the HEAD stores the name of a branch However Git also allows you to check out an arbitrary commit that isn t necessarily the tip of any particular branch In this case HEAD is said to be detache
12. if Sbranch test o Sbranch release then continue fa echo n gb branch Srestore status for ref in test release origin master do not in test Srestore en agt 0 if git rev list S ref branch we then status Sstatus ref 0 1 fi done case Sstatus in trl echo rb Need to pull into test Srestore rr rl echo In test ri 1 echo Waiting for linus ri ate echo rb All done Srestore rr echo rb lt Sstatus gt Srestore rr esac git log origin master Sbranch git shortlog 34 70 Chapter 5 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 5 1 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 o
13. MERGE_HEAD and which touch an unmerged file You may also use 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 21 70 3 8 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 committed 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 3 9 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 3 10 Fixing mistakes
14. Note that the two techniques outlined above exporting via http or 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 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 forward see the following section for details on handling this case Note that the target of a push is normally a bare repository You can also push to a repository that has a checked out working tree but a push to update the currently checked out branch is denied by default to prevent confusion See the description of the receive denyCurrentBranch option in git config 1 for details As with git fetch you may also set up configuration options to save typing so for example git remote add public repo ssh yourserver com you proj git adds the following to git config remote public repo url yourserver com proj git fetch refs heads x refs remotes example x which lets you do the same push with just git push public repo master See the explanations of the remote lt name gt url branch lt
15. This may take some time git fsck dangling commit 7281251ddd2a61e38657c827739c57015671a6 b3 dangling commit 2706a059f258c6b245f298dc4ff2ccd30ec21a63 dangling commit 13472b7c4b80851a1bc551779171dcb03655e9b5 dangling blob 218761 9d90712d37a9c5e36 406f 92202db07eb dangling commit bf093535a34a4d35731laa2bd90fe6b176302f14f dangling commit 8e4bec7f2ddaa268bef999853c25755452100f8e dangling tree d50bb86186bf27b681d25af89d3b5b68382e4085 dangling tree b24c2473f1fd3d91352a624795be026d64c8841f You will see informational messages on dangling objects They are objects that still exist in the repository but are no longer refer enced by any of your branches and can and will be removed after a while with gc You can run git fsck no dangling to suppress these messages and still view real errors 3 12 2 Recovering lost changes 3 12 2 1 Reflogs Say you modify a branch with git reset 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 master branch head This syntax can be used 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
16. also contain a second and even a third version of a working tree which are used when merging index entry The information regarding a particular file stored in the index An index entry can be unmerged if a merge was started but not yet finished i e if the index contains multiple versions of that file master The default development branch Whenever you create a Git repository a branch named master is created and becomes the active branch In most cases this contains the local development though that is purely by convention and is not required merge As a verb To bring the contents of another branch possibly from an external repository into the current branch In the case where the merged in branch is from a different repository this is done by first fetching the remote branch and then merging the result into the current branch This combination of fetch and merge operations is called a pull Merging is performed by an automatic process that identifies changes made since the branches diverged and then applies all those changes together In cases where changes conflict manual intervention may be required to complete the merge As a noun unless it is a fast forward a successful merge results in the creation of a new commit representing the result of the merge and having as parents the tips of the merged branches This commit is referred to as a merge commit or sometimes just a merge object The unit of storage in Git It
17. 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 x commit to achieve the same as amp commit gt object ie 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 Section 2 2 There are quite a few ways to name an object and not only revisions All of these are handled in shal_name c Just have a quick look at the function get_shal A lot of the special handling is done by functions like get_shal_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 pars default HEAD SQ A LESS S PAGER less What does this mean git rev 1list 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 commands start out as scripts using git rev list git rev parselsnot as important any more
18. and todo list for this manual This is a work in progress The basic requirements e It must be readable in order from beginning to end by someone intelligent with a basic grasp of the UNIX command line but without any special knowledge of Git If necessary any other prerequisites should be specifically mentioned as they arise e 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 e howto s e some of technical e hooks e list of commands in 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 S
19. commits you can just use gitk lt dangling commit sha goes here gt 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 lt dangling commit sha goes here gt For blobs and trees you can t do the same but you can still examine them You can just do git show lt dangling blob tr sha goes here gt to show what the contents of the blob were or for a tree basically what the 1s 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 1l 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
20. committed state was Here is an ASCII art by Jon Loeliger that illustrates how various pieces fit together commit tree commit obj ho V V Object DB Backing Store write tree tree obj read tree tree obj Y Index cache update index blob obj checkout index u checkout index stat blob obj V Working Directory 9 3 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 git cat file 1 to examine details about the object git cat file t lt objectname gt 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 lt objectname gt 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 55 70 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 9 4 Merging multiple trees Git helps you do a three way merge which you can expand to n way by rep
21. e woe i ee Ae A tea eee BATE SP hed se Bee oF 12 20 Viewing Old Tle Versions a cok po RRR RARE EAE AGO SEO ee ee e 12 Ze ERPE 4 5 sas Foe oe edd bee a ee eects amp Ace a ah ed Se Sei we eR Se hae eG E 12 2 7 1 Counting the number of commits on a branch 5 ce ee ee EE eS 12 2 7 2 Check whether two branches point at the same history oaoa o 12 27 3 Pind first tagged versi n including a given K o lt creac o ers rss E ee ee 13 274 Showing commits unique to a given branch 2 5 6 40055883 Fo ea be e eee es 14 2 7 5 Creating a changelog and tarball for a software release o o 14 2 7 6 Finding commits referencing a file with given conteMt 15 3 Developing with Git 341 Tellme Oi veut Name pe eh ee eB a Be wR ee A e ek eae a Je Creamm a New Me en oe ee EEE Bah CES he eed See be Bee ee Behe o 32 Howie WERE OOO i 4 663 24 E RR Poe eS ee ed ew e 34 Creating good commit Messages 2 2 kk ee ee e ee ee So IBAS lt a fe aoe dat A AR BME Oat he ewes See EA b 4 Si HOW TS MILLER A a eas Oe EEE Cha ee Set RESPINIACAMERES 6 ee RR Mee ee ES e o ee da 37 1 Getting conflict resolution help during amerse sec c ees a bee bee eee eee ees 3 6 Undoing aie c eek eee bd eae Dab eas Oe PEA Cha eee 39 Fastforward merges kk ee eS eG ROR a A ea eh Be ee SND TOE ES e A amp ee SES ADA VES ess oe wae wae Ss 310 1 Fixing a mistake with a new commit lt lt o s estores
22. files to look for changes 3 It can 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 We saw in Section 3 7 1 that during a merge the index can store multiple versions of a single file called stages The third column in the git ls files 1 output above is the stage number and will take on values other than 0 for files with merge conflicts The index is thus a sort of temporary staging area which is filled with a tree which you are in the process of working on If you blow the index away entirely you generally haven t lost any information as long as you have the name of the tree that it described 48 70 Chapter 8 Submodules Large projects are often composed of smaller self contained modules For example an embedded Linux distribution s source tree would include every piece of software in the distribution with some local modifications a movie player might need to build against a specific known working version of a decompression library several independent programs might all share the same build scripts With centralized revision control systems this is often accomplished by including every module in one single repository Devel opers can check out all modules or only the modules they need to work with They can even modify files across se
23. 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 betwe
24. incorporating your changes giving you a chance to edit the old commit message first This is useful for fixing typos in your last commit or for adjusting the patch contents of a poorly staged commit If you need to amend commits from deeper in your history you can use interactive rebase s edit instruction 36 70 5 4 Reordering or selecting from a patch series Sometimes you want to edit a commit deeper in your history One approach is to use git format patch to create a series of patches and then reset the state to before the patches git format patch origin git reset hard origin Then modify reorder or eliminate patches as needed before applying them again with git am 1 git am patch 5 5 Using interactive rebases You can also edit a patch series with an interactive rebase This is the same as reordering a patch series using format patch so use whichever interface you like best Rebase your current HEAD on the last commit you want to retain as is For example if you want to reorder the last 5 commits use git rebase i HEAD 5 This will open your editor with a list of steps to be taken to perform your rebase pick deadb The oneline of this commit pick falafel The oneline of the next commit Rebase cOff deadb onto cOffeee Commands p pick use commit r reword use commit but edit the commit message edit use commit but stop for amending s squash use commit but
25. index at any time by going in the other direction 9 2 3 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 tr lt SHA 1 of tree gt 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 9 2 4 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 filenam 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 m
26. is uniquely identified by the SHA1 of its contents Consequently an object can not be changed object database Stores a set of objects and an individual object is identified by its object name The objects usually live in SGIT_DIR object object identifier Synonym for object name 63 70 object name The unique identifier of an object The hash of the object s contents using the Secure Hash Algorithm 1 and usually represented by the 40 character hexadecimal encoding of the hash of the object object type One of the identifiers commit tree tag or blob describing the type of an object octopus To merge more than two branches Also denotes an intelligent predator origin The default upstream repository Most projects have at least one upstream project which they track By default origin is used for that purpose New upstream updates will be fetched into remote remote tracking branches named origin name of upstream branch which you can see using git branch r pack A set of objects which have been compressed into one file to save space or to transmit them efficiently pack index The list of identifiers and other information of the objects in a pack to assist in efficiently accessing the contents of a pack pathspec Pattern used to specify paths wow n n Pathspecs are used on the command line of git Is files git ls tree git add git grep git diff git checkout and many other comman
27. 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 con sumption 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 1557 2 4 and scroll down to that function note that you no longer need to call setup_pager directly Nowadays git log isa builtin which means that it is contained in the command git The source side of a builtin is e a function called cmd_ lt bla gt typically defined in builtin lt bla gt c and declared in builtin h e 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
28. make this easy by breaking their changes into small self contained commits That won t help in the case above however because the problem isn t obvious from examination of any single commit instead a global view of the development is required To make matters worse the change in semantics in the problematic function may be just one small part of the changes in the upper line of development On the other hand if instead of merging at C you had rebased the history between Z to B on top of A you would have gotten this linear history Z O X isa o A O o Y x Ka O Bx Dx Bisecting between Z and D would hit a single culprit commit Y and understanding why Y was broken would probably be easier Partly for this reason many experienced Git users even when working on an otherwise merge heavy project keep the history linear by rebasing against the latest upstream version before publishing 39 70 Chapter 6 Advanced branch management 6 1 Fetching individual branches Instead of using 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 t odo from the remote repository and to store it locally under the name refs heads my todo work You can also fetch branches from
29. 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 4 4 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 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 27 70 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 c
30. meld into previous commit f fixup like squash but discard this commit s log message x exec run command the rest of the line using shell These lines can be re ordered they ar xecuted from top to bottom If you remove a line here THAT COMMIT WILL BE LOST However if you remove everything the rebase will be aborted Note that empty commits are commented out As explained in the comments you can reorder commits squash them together edit commit messages etc by editing the list Once you are satisfied save the list and close your editor and the rebase will begin The rebase will stop where pick has been replaced with edit or when a step in the list fails to mechanically resolve conflicts and needs your help When you are done editing and or resolving conflicts you can continue with git rebase continue If you decide that things are getting too hairy you can always bail out with git rebase abort Even after the rebase is complete you can still recover the original branch by using the reflog For a more detailed discussion of the procedure and additional tips see the INTERACTIVE MODE section of git rebase 1 37 70 5 6 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 5 7 Problems with rewriting history The primary problem with rewriting the history of a branch has to do
31. 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 forward to the commit given by example com s master branch In more detail 6 2 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 forward A fast forward looks something like this O 0 0 0 lt old head of the branch O 0 0 lt 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 0 0 a b lt old head of the branch O 0 0 lt 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 referen
32. refers to the SHA 1 of the commit instead of to a branch and git branch shows that you are no longer on a branch cat git HEAD 427abfa28afedffadfca9 dd8b067eb6d36bbac53f 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 1 6 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 called remote tracking branches which you can view using the r option to git branch 1 7 70 git branch r origin HEAD origin html origin maint origin man origin master origin next origin pu origin todo In this example origin is called a remote repository or remote for short The branches of this repository are called remote branches from our point of view The remote tracking branches listed above were created based on the remote branches at clone time and will be updated by git fetch hence git pull and git push See Section 1 8 for details You might want to build on one of these remote tracking branches on a branch of your own just as you would for a ta
33. tracking branch A regular Git branch that is used to follow changes from another repository A remote tracking branch should not contain direct modifications or have local commits made to it A remote tracking branch can usually be identified as the right hand side ref in a Pull refspec repository A collection of refs together with an object database containing all objects which are reachable from the refs possibly accompanied by meta data from one or more porcelains A repository can share an object database with other repositories via alternates mechanism resolve The action of fixing up manually what a failed automatic merge left behind revision A particular state of files and directories which was stored in the object database It is referenced by a commit object rewind To throw away part of the development i e to assign the head to an earlier revision SCM Source code management tool SHA1 Synonym for object name 65 70 shallow repository A shallow repository has an incomplete history some of whose commits have parents cauterized away in other words Git 1s told to pretend that these commits do not have the parents even though they are recorded in the commit object This is sometimes useful when you are interested only in the recent history of a project even though the real history recorded in the upstream is much larger A shallow repository is created by giving the depth option to git clone 1 and its histor
34. with merging Suppose somebody fetches your branch and merges it into their branch with a result something like this 0 0 0 0 6 0 lt origin t t t m lt their branch Then suppose you modify the last three commits O 0 0 lt new head of origin O 0 O 0 0 o lt old head of origin If we examined all this history together in one repository it will look like O 0 0 lt new head of origin O 0 O 0 0 o lt old head of origin t t t m lt 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 5 8 Why bisecting merge commits can be harder than bisecting linear history The git bisect 1 command correctly handles hi
35. 05f075a84f96b360d05984b 13 70 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 2 7 3 Find first tagged version including a given fix Suppose you know that the commit eO5dbOfd 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 eO5dbOfd then there could be multiple earliest tagged releases You could just visually inspect the commits since e05db0fd gitk e05db0fd Or you can use 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 rel 0 23 The 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 git merge base 1 git merge base e05db0fd v1 5 0 rcl e05db0fd4f31dde7005f075a84f96b360d05984b The merge base command finds a common ancestor of the given commits and always returns one or the other in the case wh
36. 266b98 26ldfac 160000 Minn Mm a a b a ee 1 1 ee Subproject commit d266b9873ad50488163457f025db7cdd9683d88b Subproject commit 261dfac35cb991380eb966e102c1197139f7fa24 git add a git commit m Updated submodule a git push You have to run git submodule update after git pull if you want to update submodules too 8 1 Pitfalls with submodules Always publish the submodule change before publishing the change to the superproject that references it If you forget to publish the submodule change others won t be able to clone the repository cd git super a echo i added another line to this file gt gt a txt git commit a m doing it wrong this time Svcd ku git add a git commit m Updated submodule a again git push cd git cloned 51 70 git pull git submodule update error pathspec 261dfac35cb99d380eb966e102c1197139f7 a24 did not match any file s known to git Did you forget to git add Unable to checkout 261dfac35cb99 d380eb966e102c1197139f7fa24 in submodule path a r In older Git versions it could be easily forgotten to commit new or modified files in a submodule which silently leads to similar problems as not pushing the submodule changes Starting with Git 1 7 0 both git status andgit diff in the superproject show submodules as modified when they contain new or modified files to protect against accidentally committing such a state git diff
37. 457f025db7cdd9683d88b a e81d457da1530 b4fef4249aba9b50187999670d b c1536a972b affea0f16e0680ba87332dc059146 c d96249ff5d571de5bde093e6bbaff edaafa5276a74 d Note The commit object names shown above would be different for you but they should match the HEAD commit object names of your repositories You can check it by running git ls remote a Pulling down the submodules is a two step process First run git submodule init to add the submodule repository URLs to git config git submodule init 50 70 Now use git submodule update to clone the repositories and check out the commits specified in the superproject git submodule update cd a ls a 9git a txt One major difference between git submodule update and git submodule addis that git submodule update checks out a specific commit rather than the tip of a branch It s like checking out a tag the head is detached so you re not working on a branch git branch no branch master If you want to make a change within a submodule and you have a detached head then you should create or checkout a branch make your changes publish the change within the submodule and then update the superproject to reference the new commit git checkout master or git checkout b fix up then echo adding a line again gt gt a txt git commit a m Updated the submodule from within the superproject git push cd ds git diff diff git a a b a index d
38. 4a05 a3414e99f6f44bebc1le7 refs heads master 24dbc180eal4dclaebe09f14c8ecf32010690627 refs heads tutorial 2 1e87486ae06626c2f3leaa63d26fc0fd646c8af2 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 gitrevisions 7 for explanations of commit selecting syntax such as not 2 7 5 Creating a changelog and tarball for a software release The git archive 1 command can create a tar or zip archive from any version of a project for example git archive o latest tar gz prefix project HEAD will use HEAD to produce a gzipped tar archive in which each filename is preceded by project The output file format is inferred from the output file extension if possible see git archive 1 for details Versions of Git older than 1 7 7 don t know about the tar gz format
39. 6228369c63936db65827de3cc06200 missing blob 4b9458b3786228369c63936db65827de3cc06200 Now you know that blob 4b9458b3 is missing and that the tree 2d9263c6 points to it If you could find just one copy of that missing blob object possibly in some other repository you could move it into git objects 4b 9458b3 and be done Suppose you can t You can still examine the tree that pointed to it with git ls tree 1 which might output something like git ls tree 2d9263c6d23595e7cb2a21le5ebbb53655278dff8 100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8 gitignore 100644 blob ebf9bf84da0aab5ed9442 64a5db2a65fe3a3e883 mailmap 100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c COPYING 100644 blob 4b9458b3786228369c63936db65827de3cc06200 myfile So now you know that the missing blob was the data for a file named myfile And chances are you can also identify the directory let s say it s in somedirectory If you re lucky the missing copy might be the same as the copy you have checked out in your working tree at somedirectory myfile you can test whether that s right with git hash object 1 git hash object w somedirectory myfile which will create and store a blob object with the contents of somedirectory myfile and output the SHA 1 of that object if you re extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200 in which case you ve guessed right and the corruption is fixed Otherwise you need more in
40. a branch see Chapter 5 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
41. a63 dangling commit 13472b7c4b80851a1lbc551779171dcb03655e9b5 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 25 70 Chapter 4 Sharing development with others 4 1 Getting updates with git pull After you clone a repository and commit 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 how to keep remote tracking branches up to date with 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 git pull 1 command provides a way to do this i
42. ae ed ee ea eae eu 3102 Fixing a mistake by rewnting history c ce eo earr t pey Ok ee Po YE eee eG 3103 Checking outa oll version of a ME ent ee eae eR Eee A oH 3 10 4 Temporarily setting aside work in progress oosa ee 301 Bnsurine good Remrormance e cca E SRK ERAS EK E Oe Re E a SY 2 12 Ensure relay cc ap a AG REGS OEE SE ER EE BE a Se RA Sb eek 3 42 1 Checking the repository for compton lt lt lt s corso ed a a dotes 3122 Recoyenne Wisi CRANES c e pe He ae E Oe ee Pa Y Bee ee eS Dildo Relek ioc aces 8S ANS REA Rawk SLAG EE ES be RG eee ek 3122 2 Examining dangling objects lt cs e ee a ee bed we ee eae we Sharing development with others 4 1 eis updates with sit pull ook se ee pe Ree ee ee A eR OS 4 2 Submilling patches toa project e cs sespe eR RR ee ee ew ee eee ae a Ls Importing patches tg apraject lt eqe oca e mee Se ie Hee Dee Se ewe a Pee ee ened be 44 Publie Gut repostones lt A eee 44 1 Setting upapublic repository ne eR we 442 Exporting a Git repository via the Git protocol lt s ee ee eRe eee ED EES 443 Exporting a gitrepository viaHTTP lt os co oe etones ee ee ee eae 4 4 4 Pushing changes to a public repository 0 656 be eR ee RO we ee 245 Whattovdo where push ims cc 266 Sed 4 ewe ede eee ES Serle a ox 44 6 Setting upa shared repository conca ee ee ee ee eee 44 7 Allowing web browsing ofarepository 0 2 0 eee eee ee eee ds ERIN sas A Gat A Dak a eee AREAS A oe 4 5 1 Maintaining top
43. amed origin master and can be updated using git fetch 1 you can track other public trees using git remote 1 to set up a remote and git fetch 1 to keep them up to date see Chapter 1 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 t rack option to 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 git pull 1 git checkout test amp amp git pull git checkout release amp amp 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 git config 1 can make it easy to push both branches to your public tree See Section 4 4 1 cat gt gt git config lt lt 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 git push 1 git p
44. ash of the top commit and digitally sign that email using something like GPG PGP To assist in this Git also provides the tag object 7 1 5 Tag Object A tag object contains an object object type tag name the name of the person tagger who created the tag and a message which may contain a signature as can be seen using git cat file 1 44 70 git cat file tag v1 5 0 object 437b1b20df4b356c9342dac8d38849f24ef44f27 type commit tag v1 5 0 tagger Junio C Hamano lt junkio cox net gt 1171411200 0000 GIT 1 5 0 BEGIN PGP SIGNATURE Version GnuPG v1 4 6 GNU Linux iD8DBOBF 01GqwMbZpPMRm50RAURiAJIONBLA7s2kq jkKlqlqqC57SbnmzQcdG4ui nLE L9aUXdWeTFPron96DLA 2E 0 2 END PGP SIGNATURE See the git tag 1 command to learn how to create and verify tag objects Note that git tag 1 can also be used to create lightweight tags which are not tag objects at all but just simple references whose names begin with refs tags 7 1 6 How Git stores objects efficiently pack files Newly created objects are initially created in a file named after the object s SHA 1 hash stored in git objects 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 l
45. at the above configuration can be performed by directly editing the file git config instead of using git remote 1 After configuring the remote the following three commands will do the same thing git fetch git example com proj git refs heads x refs remotes example x git fetch example refs heads x refs remotes example x git fetch example See git config 1 for more details on the configuration options mentioned above and git fetch 1 for more details on the refspec syntax 41 70 Chapter 7 Git concepts Git is built on a small number of simple but powerful ideas While it is possible to get things done without understanding them you will find Git much more intuitive if you do We start with the most important the object database and the index 7 1 The Object Database We already saw in Section 1 3 that all commits are stored under a 40 digit object name In fact all the information needed to represent the history of a project is stored in objects with such names In each case the name is calculated by taking the SHA 1 hash of the contents of the object The SHA 1 hash is a cryptographic hash function What that means to us is that it is impossible to find two different objects with the same name This has a number of advantages among others e Git can quickly determine whether two objects are identical or not just by comparing names e Since object names are computed the same way in every reposit
46. at this is how you intend to manage the branch It s also possible for a push to fail in this way when other people have the right to push to the same repository In that case the correct solution is to retry the push after first updating your work either by a pull or by a fetch followed by a rebase see the next section and gitcvs migration 7 for more 4 4 6 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 gitcvs migration 7 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 recom mended simply because the mode of collaboration that Git supports by exchanging patches and pulling from public reposito ries has so many advantages over the central shared repository e 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 e 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 pro
47. ating 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 8 70 1 9 Fetching branches from other repositories You can also track branches from repositories other than the one you cloned from using git remote 1 git remote add linux nfs git linux nfs org pub nfs 2 6 g1t git fetch linux nfs x 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 inthis case linux nfs git branch r linux nfs master origin master If you run git fetch lt remote gt later the remote tracking branches for the named lt remote gt 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 secti
48. ating the remote head ref If the remote head is not an ancestor to the local head the push fails reachable All of the ancestors of a given commit are said to be reachable from that commit More generally one object is reachable from another if we can reach the one from the other by a chain that follows tags to whatever they tag commits to their parents or trees and trees to the trees or blobs that they contain rebase To reapply a series of changes from a branch to a different base and reset the head of that branch to the result ref A 40 byte hex representation of a SHA1 or a name that denotes a particular object They may be stored in a file under S GIT_DIR refs directory or in the SGIT_DIR packed refs file reflog A reflog shows the local history of a ref In other words it can tell you what the 3rd last revision in this repository was and what was the current state in this repository yesterday 9 14pm See git reflog 1 for details refspec A refspec is used by fetch and push to describe the mapping between remote ref and local ref They are combined with a colon in the format lt src gt lt dst gt preceded by an optional plus sign For example git fetch SURL refs heads maste means grab the master branch head from the URL and store it as my origin branch head And git push SURL refs heads master refs heads to upstream means publish my master branch head as to upstream branch at URL See also git push 1 remote
49. ce of your own pointing to them 40 70 6 3 Forcing git fetch to do non fast forward updates Tf 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 6 4 Configuring remote tracking 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 git config 1 git config l core repositoryformatversion 0 core filemode tru core logallrefupdates true remote origin url git git kernel org pub scm git git git remote origin fetch refs heads x refs remotes origin x 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 git remote add example git example com proj git adds the following to git config remote example url git example com proj git fetch refs heads x refs remotes example x Also note th
50. ce 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 Chapter 7 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 1 3 1 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 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 descendant of X or that there is a chain of parents leading from commit Y to commit X 1 3 2 Understanding history History diagrams W
51. cent commit on a branch is referred to as the tip of that branch The tip of the branch is referenced by a branch head which moves forward as additional development is done on the branch A single Git repository can track an arbitrary number of branches but your working tree is associated with just one of them the current or checked out branch and HEAD points to that branch cache Obsolete for index chain A list of objects where each object in the list contains a reference to its successor for example the successor of a commit could be one of its parents changeset BitKeeper cvsps speak for commit Since Git does not store changes but states it really does not make sense to use the term changesets with Git checkout The action of updating all or part of the working tree with a tree object or blob from the object database and updating the index and HEAD if the whole working tree has been pointed at a new branch cherry picking In SCM jargon cherry pick means to choose a subset of changes out of a series of changes typically commits and record them as a new series of changes on top of a different codebase In Git this is performed by the git cherry pick command to extract the change introduced by an existing commit and to record it based on the tip of the current branch as a new commit clean A working tree is clean if it corresponds to the revision referenced by the current head Also see dirty 61 70
52. ch may leave you with no way to find the history it used to point to so use this command carefully 1 3 Understanding History Commits Every change in the history of a project is represented by a commit The git show 1 command shows the most recent commit on the current branch git show commit 17cf78166le6d38 737 15 53ab552 1e95960d7 Author Linus Torvalds lt torvalds ppc970 osdl org none gt Date Tue Apr 19 14 11 06 2005 0700 Remove duplicate getenv DB_ENVIRONMENT call Noted by Tony Luck diff git a init db c b init db c index 65898fa b002dc6 100644 a init db c b init db c 7 7 7 7 QQ int main int argc char argv char shal_dir getenv DB_ENVIRONMENT xpath char shal_dir path int len i if mkdir git 0755 lt 0 5 70 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 SHA 1 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 Sin
53. cmd_log both reside in built in 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 showorgit cat file For the sake of clarity let s stay with git cat file because it 59 70 e is plumbing and e 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 fil t s p lt type gt lt shal gt if get_shal argv 2 shal 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__shal It tries to i
54. contents of the index git ls files stag 100644 63c918c667fa005ff12ad89437f2fdc80926e21c 0 gitignore 100644 5529b198e8d14decbe4ad99db3f7fb632de0439d 0 mailmap 100644 6 87c4664981e4397625791c8ea3bbb5 2279a3 0 COPYING 100644 a37b2152bd26be2c2289e1f57a292534a5la93c7 0 Documentation gitignore 100644 fhefe9a45b00a54b58d94d06eca48b03d40a50e0 0 Documentation Makefile 100644 251llaef8d89ab52be5ecb6a5e46236b4b6bcd07ea O xdiff xtypes h 100644 2ade97b2574a9f77e7ae14002a4e07a6a38e46d07 0 xdiff xutils c 100644 d5de8292e05e7c36c4b68857c1cf9855e3d2f 70a 0 xdiff xutils h Note that in older documentation you may see the index called the current directory cache or just the cache It has three important properties 1 The index contains all the information necessary to generate a single uniquely determined tree object For example running git commit 1 generates this tree object from the index stores it in the object database and uses it as the tree object associated with the new commit 2 The index enables fast comparisons between the tree object it defines and the working tree It does this by storing some additional data for each entry such as the last modified time This data is not displayed above and is not stored in the created tree object but it can be used to determine quickly which files in the working directory differ from what was stored in the index and thus save Git from having to read all of the data from such
55. ct type may be a blob representing the contents of a file or another tree representing the contents of a subdirectory Since trees and blobs like all other objects are named by the SHA 1 hash of their contents two trees have the same SHA 1 name if and only if their contents including recursively the contents of all subdirectories are identical This allows Git to quickly determine the differences between two related tree objects since it can ignore any entries with identical object names Note in the presence of submodules trees may also have commits as entries See Chapter 8 for documentation Note that the files all have mode 644 or 755 Git actually only pays attention to the executable bit 7 1 3 Blob Object You can use git show 1 to examine the contents of a blob take for example the blob in the entry for COPYING from the tree above git show 6ff87c4664 Note that the only valid version of the GPL as far as this project is concerned is _this_ particular version of the license ie v2 not v2 2 or v3 x or whatever unless explicitly otherwise stated A blob object is nothing but a binary blob of data It doesn t refer to anything else or have attributes of any kind 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 dir
56. current HEAD git branch d new delete branch new Instead of basing a 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 67 70 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 git remote example origin git remote show example x remot xampl URL Tracked remote branches master next xampl git fetch example git branch r A 3 Exploring history git example com project git list remote repositories get details git example com project git update branches from example list all remote branches Search for regr
57. d dircache You are waaaaay behind See index directory The list you get with ls dirty A working tree is said to be dirty if it contains modifications which have not been committed to the current branch ent Favorite synonym to tree ish by some total geeks See http en wikipedia org wiki Ent_ Middle earth for an in depth explanation Avoid this term not to confuse people evil merge An evil merge is a merge that introduces changes that do not appear in any parent fast forward A fast forward is a special type of merge where you have a revision and you are merging another branch s changes that happen to be a descendant of what you have In such these cases you do not make a new merge commit but instead just update to his revision This will happen frequently on a remote tracking branch of a remote repository fetch Fetching a branch means to get the branch s head ref from a remote repository to find out which objects are missing from the local object database and to get them too See also git fetch 1 file system Linus Torvalds originally designed Git to be a user space file system i e the infrastructure to hold files and directories That ensured the efficiency and speed of Git Git archive Synonym for repository for arch people gitfile A plain file git at the root of a working tree that points at the directory that is the real repository 62 70 grafts Grafts enables two otherwise diff
58. d all changes are calculated by comparing the contents of the tree referred to by this commit with the trees associated with its parents In particular Git does not attempt to record file renames explicitly though it can identify cases where the existence of the same file data at changing paths suggests a rename See for example the M option to git diff 1 A commit is usually created by git commit 1 which creates a commit whose parent is normally the current HEAD and whose tree is taken from the content currently stored in the index 7 1 2 Tree Object The ever versatile git show 1 command can also be used to examine tree objects but git ls tree 1 will give you more details git ls tree fb3a8bdd0ce 100644 blob 63c918c667fa005ff12ad89437f2fdc80926e21c gitignore 100644 blob 5529b198e8d14dechbe4ad99db3f7fb632de0439d mailmap 100644 blob 6f 87c4664981e4397625791c8ea3bbb5f2279a3 COPYING 040000 tree 2fb783e477100ce076f6bf57e4a6f026013dc745 Documentation 100755 blob 3c0032cec592a765692234f1cba47dfdcc3a9200 GIT VERSION GEN 100644 blob 289b046a443c0647624607d471289b2c7dcd470b INSTALL 100644 blob 4eb463797adc693dc168b926b6932ff53f17d0b1 Makefile 100644 blob 548142c327a6790ff8821d67c2eeleff7ab656b52 README 43 70 As you can see a tree object contains a list of entries each with a mode object type SHA 1 name and name sorted by name It represents the contents of a single directory tree The obje
59. d top mnemonic makes the pattern match from the root of the working tree even when you are running the command from inside a subdirectory Currently only the slash is recognized as the magic signature but it is envisioned that we will support more types of magic in later versions of Git A pathspec with only a colon means there is no pathspec This form should not be combined with other pathspec parent A commit object contains a possibly empty list of the logical predecessor s in the line of development i e its parents pickaxe The term pickaxe refers to an option to the diffcore routines that help select changes that add or delete a given text string With the pickaxe al1 option it can be used to view the full changeset that introduced or removed say a particular line of text See git diff 1 64 70 plumbing Cute name for core Git porcelain Cute name for programs and program suites depending on core Git presenting a high level access to core Git Porcelains expose more of a SCM interface than the plumbing pull Pulling a branch means to fetch it and merge it See also git pull 1 push Pushing a branch means to get the branch s head ref from a remote repository find out if it is a direct ancestor to the branch s local head ref and in that case putting all objects which are reachable from the local head ref and which are missing from the remote repository into the remote object database and upd
60. display the given version of the file 3 10 4 Temporarily setting aside work in progress While you are in the middle of working on something complicated you find an unrelated but obvious and trivial bug You would like to fix it before continuing You can use git stash 1 to save the current state of your work and after fixing the bug or optionally after doing so on a different branch and then coming back unstash the work in progress changes git stash save work in progress for foo feature This command will save your changes away to the stash and reset your working tree and the index to match the tip of your current branch Then you can make your fix as usual edit and test git commit a m blorpl typofix After that you can go back to what you were working on with git stash pop git stash pop 3 11 Ensuring good performance On large repositories Git depends on compression to keep the history information from taking up too much space on disk or in memory Some Git commands may automatically run git gc 1 so you don t have to worry about running it manually However compressing a large repository may take a while so you may want to call gc explicitly to avoid automatic compression kicking in when it is not convenient 23 70 3 12 Ensuring reliability 3 12 1 Checking the repository for corruption The git fsck 1 command runs a number of self consistency checks on the repository and reports on any problems
61. ds to limit the scope of operations to some subset of the tree or worktree See the documentation of each command for whether paths are relative to the current directory or toplevel The pathspec syntax is as follows e any path matches itself e the pathspec up to the last slash represents a directory prefix The scope of that pathspec is limited to that subtree e the rest of the pathspec is a pattern for the remainder of the pathname Paths relative to the directory prefix will be matched against that pattern using fnmatch 3 in particular and can match directory separators For example Documentation jpg will match all jpg files in the Documentation subtree including Documentation chap ter_1 figure_1 jpg A pathspec that begins with a colon has special meaning In the short form the leading colon is followed by zero or more magic signature letters which optionally is terminated by another colon and the remainder is the pattern to match against the path The optional colon that terminates the magic signature can be omitted if the pattern begins with a character that cannot be a magic signature and is not a colon In the long form the leading colon is followed by a open parenthesis a comma separated list of zero or more magic words and a close parentheses and the remainder is the pattern to match against the path The magic signature consists of an ASCII symbol that is not alphanumeric top The magic wor
62. e between some first skipped commits and a later bad commit There are also ways to automate the bisecting process if you have a test script that can tell a good from a bad commit See git bisect 1 for more information about this and other git bisect features 2 2 Naming commits We have seen several ways of naming commits already e 40 hexdigit object name e 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 references HEAD refers to the head of the current branch There are many more see the SPECIFYING REVISIONS section of the gitrevisions 7 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 Ur Recall that merge commits may have more than one parent by default you can also choose and follow the first parent listed in the commit but git show HI git show HI AD 1 show the first parent of HEAD AD 2 show the second parent of HEAD B B 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 t
63. e 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 lt Branch A O O o lt master o o o lt Branch B If we need to talk about a particular commit the character o may be replaced with another letter or number 1 3 3 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 1 4 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 lt branch gt create a new branch named lt branch gt referencing the same point in history as the current branch 6 70 git branch lt branch gt lt start point gt create a new branch named lt branch gt referencing lt start point gt which may be specified any way you like in cluding using a branch name or a tag name git branch d lt branch gt delete the branc
64. eating 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 lt commit1 gt lt commit2 gt 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 lt commitname gt 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 tr m u lt origtree gt lt yourtree
65. ectory tree and renaming a file does not change the object that file is associated with Note that any tree or blob object can be examined using git show 1 with the lt revision gt lt path gt syntax This can sometimes be useful for browsing the contents of a tree that is not currently checked out 7 1 4 Trust If you receive the SHA 1 name of a blob from one source and its contents from another possibly untrusted source you can still trust that those contents are correct as long as the SHA 1 name agrees This is because the SHA 1 is designed so that it is infeasible to find different contents that produce the same hash Similarly you need only trust the SHA 1 name of a top level tree object to trust the contents of the entire directory that it refers to and if you receive the SHA 1 name of a commit from a trusted source then you can easily verify the entire history of commits reachable through parents of that commit and all of those contents of the trees referred to by those commits 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 SHA 1 h
66. elp during a merge All of the changes that Git was able to merge automatically are already added to the index file so 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 lt lt lt lt lt lt lt HEAD file txt Hello world Goodbye gt gt gt gt gt gt gt 77976da35a11db4580b80ae27e8d65caf5208086 file txt 20 70 Recall that the commit which will be committed 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 git show 3 file txt the version from MERGE_HEAD When you ask git diff 1 to show the conflicts it runs a three way diff between the conflicted merge results in the work tree with stages 2 and 3 to show only hunks whose contents come from both sides mixed in other words when a hunk s merge results come only from stage 2 that part is not conflicting and is not shown Same for stage 3 The diff above shows the differences be
67. en HEAD and the index what would be committed if you ran commit now ly difference between the index file and your e S git diff working directory changes that would not be included if you ran commit now difference between HEAD and working tree what would be committed if you ran commit a now a brief per file summary of the above git diff HEAD i i i i i F F ij git status You can also use git gui 1 to create commits view changes in the index and the working tree files and individually select diff hunks for inclusion in the index by right clicking on the diff hunk and choosing Stage Hunk For Commit 18 70 3 4 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 The text up to the first blank line in a commit message is treated as the commit title and that title is used throughout Git For example git format patch 1 turns a commit into email and it uses the title on the Subject line and the rest of the commit in the body 3 5 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 g
68. ere one is a descendant of the other so the above output shows that eO5dbOfd actually is an ancestor of v1 5 0 rcl Alternatively note that git log v1 5 0 rcl e05db0fd will produce empty output if and only if v1 5 0 rcl includes eOSdbOfd because it outputs only commits that are not reachable from v1 5 0 rc1 As yet another alternative the 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 rcl v1 5 0 rc2 e05db0fd Fix warnings in shal_file c use C99 printf format if available v1 5 0 rc0 GIT v1 5 0 preview t vi 5 0 rel GIT v1 5 0 rel vl 5 0 re2 GIT vl 5 0 re2 then search for a line that looks like e05db0fd Fix warnings in shal_file c use C99 printf format if available Which shows that e05dbOfd is reachable from itself from v1 5 0 rcl and from v1 5 0 rc2 but not from v1 5 0 rc0 14 70 2 7 4 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 git show ref 1 git show ref heads bf62196b5e363d73353a9dcf094c59595f 3153b7 refs heads core tutorial db768d45504c1bb46f63ee9 d6e1772bd047e05bf9 refs heads maint a07157ac624b252
69. erent lines of development to be joined together by recording fake ancestry information for commits This way you can make Git pretend the set of parents a commit has is different from what was recorded when the commit was created Configured via the git info grafts file hash In Git s context synonym to object name head A named reference to the commit at the tip of a branch Heads are stored in a filein SGIT_DIR refs heads directory except when using packed refs See git pack refs 1 HEAD The current branch In more detail Your working tree is normally derived from the state of the tree referred to by HEAD HEAD is a reference to one of the heads in your repository except when using a detached HEAD in which case it directly references an arbitrary commit head ref A synonym for head hook During the normal execution of several Git commands call outs are made to optional scripts that allow a developer to add functionality or checking Typically the hooks allow for a command to be pre verified and potentially aborted and allow for a post notification after the operation is done The hook scripts are found in the GIT_DIR hooks directory and are enabled by simply removing the sample suffix from the filename In earlier versions of Git you had to make them executable index A collection of files with stat information whose contents are stored as objects The index is a stored version of your working tree Truth be told it can
70. essions git bisect start git bisect bad git bisect good v2 6 13 rc2 Bisecting 675 revisions left gitk visualize and browse history git log list all commits git log src _modifying src git log v2 6 15 v2 6 16 sin v2 6 216 Mot ine 12 615 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 S Git diff V2 6 15 772 616 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 current version is bad last known good revision to test after this test here then 68 70 git bisect good if this revision is good or git bisect bad if this revision is bad repeat until done A 4 Making changes Make sure Git knows who to blame cat gt gt gitconfig lt lt 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 fi
71. formation How do you tell which version of the file has been lost The easiest way to do this is with git log raw all full history somedirectory myfile Because you re asking for raw output you ll now get something like commit abc Author Date 100644 100644 4b9458b newsha M somedirectory myfile commit xyz Author Date 100644 100644 oldsha 4b9458b M somedirectory myfile 47 70 This tells you that the immediately following version of the file was newsha and that the immediately preceding version was oldsha You also know the commit messages that went with the change from oldsha to 4b9458b and with the change from 4b9458b to newsha If you ve been committing small enough changes you may now have a good shot at reconstructing the contents of the in between state 4b9458b If you can do that you can now recreate the missing object with git hash object w lt recreated file gt and your repository is good again Btw you could have ignored the fsck and started with doing a git log raw all and just looked for the sha of the missing object 4b9458b in that whole thing It s up to you Git does have a lot of information it is just missing one particular blob version 7 2 The index The index is a binary file generally kept in git index containing a sorted list of path names each with permissions and the SHA 1 of a blob object git Is files 1 can show you the
72. g git checkout b my todo copy origin todo You can also check out origin todo directly to examine it or write a one off patch See detached head Note that the name origin is just the name that Git uses by default to refer to the repository that you cloned from 1 7 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 e 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 Newly created refs are actually stored in the git refs directory under the path given by their name However for efficiency reasons they may also be packed together in a single file see git pack refs 1 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 gitrevisions 7 1 8 Upd
73. g 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 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 12 70 2 5 Generating diffs You can generate diffs between any two versions using git diff 1 git diff master test That will produce the diff between the tips of the two branches If you d prefer to find the diff from their common ancestor to test you can use three dots instead of two git diff master test Sometimes what you want instead is a set of patches for this you can use git format patch 1 git format
74. gt lt targettree gt 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 9 5 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 l1s files unmerged command An example git read tree m Sorig HEAD target git ls files unmerged 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1 hello c 100644 06fabta24256dc7e560efa5687fa84b51f0263c3a 2 hello c 100644 cc44c73eb783565da5831b4d820c962954019b69 3 hello c Each line of the git 1s files unmerged output begins with the blob mode bits blob SHA 1 stage number and the filename The stage number is Git s way to say which tree it came from stage 1 corresponds to the orig tree stage 2 to the HEAD tree and stage 3 to the target tree 56 70 Earlier we said that trivial merges are done inside git read tree m For example if the file did not change from Sorig to HEAD nor target or if the file changed from Sorig to HEAD and Sorig to target the same way obviously the final outcome
75. h lt branch gt 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 lt branch gt 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 lt branch gt make the current branch lt branch gt updating the working directory to reflect the version referenced by lt branch gt git checkout b lt new gt lt start point gt create a new branch lt new gt referencing lt start point gt 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 1 5 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 lt new_branch_name gt HEAD is now at 427abfa Linux v2 6 17 The HEAD then
76. he associated blob and or tree objects A tree is equivalent to a directory tree ish A ref pointing to either a commit object a tree object or a tag object pointing to a tag or commit or tree object unmerged index An index which contains unmerged index entries unreachable object An object which is not reachable from a branch tag or any other reference upstream branch The default branch that is merged into the branch in question or the branch in question is rebased onto It is configured via branch lt name gt remote and branch lt name gt merge If the upstream branch of A is origin B sometimes we say A is tracking origin B working tree The tree of actual checked out files The working tree normally contains the contents of the HEAD commit s tree plus any local changes that you have made but not yet committed 66 70 Appendix A Git Quick Reference This is a quick summary of the major commands the previous chapters explain how these work in more detail A 1 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 A 2 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
77. he 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 11 70 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 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 e05dab0fd4f31dde7005f075a84f96b360d05984b 2 3 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 git tag 1 man page for details 2 4 Browsing revisions The 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 S git log v2 5 commits since not reachable from v2 5 git lo
78. ic branches for a Linux subsystem maintainer 16 16 16 16 18 18 18 19 19 21 21 21 21 22 22 22 22 23 23 23 23 24 Rewriting history and maintaining patch series S1 Crap the perleti palco senies so na A OR a ee A A Ree a da 5 2 Keeping a patch seres up to date using git rebase c o cos eee maea a ee Ree Oe eee ee a 3 3 REWITINE smele commit e ociosa ee we be eee a hee eee we Sel Reordering or selecting tram a patch series oo cerneret RR wR RN RD OR ee oe eRe eee ES 5 9 Using interactive rebases c scio i d o eoe a Ree eee A A Be we 0 IAS o E e a A A A e ad 5 7 Problems with rewriting history occ a a A da he 5 8 Why bisecting merge commits can be harder than bisecting linear history o Advanced branch management 6 1 Fetching individual pranches o o os pe esaa eda a YR ee AES 8S 02 gitieihandiast ATorwWaniS co ci A a a E OR A A a E eG 6 3 Forcing git fetch todo non fast forward updates lt ociosa ar Ee et ek 6 4 Configuring remote tracking branches creuse eke ee ee Git concepts TL Tebo Database aos ph Ce A OR eR A SE Se eee oS Tal Commit ODECE cc oko Be eke be ee eo bh ee he ee hw Phe 1 EA Se BEEPS Sa ee eee ehh heh wea eed sd amp 9450 BIOWODIEGL ocio bebe eet eb be eh ee eds ea dae ehh bw ee eee ee PAS WMS sa rr dr ee Be a A oe A ee ee Se eg TS REOS o one SAGE Se be Pa Saha Ge eRe BAGS Page a SG wal Rod owe 7 1 6 How Git stores objects efficient
79. ich probably only Git developers need to understand 10 1 Object storage format All objects have a statically determined type which identifies the format of the object i e how it is used and how it can refer wow to other objects There are currently four different object types blob tree commit and tag 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 SHA 1 hash that is used to name the object is the hash of the original data plus this header so shal sum file does not match the object name for file Historical note in the dawn of the age of Git the hash was the SHA 1 of the compressed object As aresult 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 lt ascii type without space gt lt space gt lt ascii decimal size gt lt byte 0 gt lt binary object data gt 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 a
80. ing 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 x c You see Git is actually the best tool to find out about the source of Git itself 60 70 Chapter 11 Git Glossary alternate object database Via the alternates mechanism a repository can inherit part of its object database from another object database which is called alternate bare repository A bare repository is normally an appropriately named directory with a git suffix that does not have a locally checked out copy of any of the files under revision control That is all of the Git administrative and control files that would normally be present in the hidden git sub directory are directly present in the repository git directory instead and no other files are present and checked out Usually publishers of public repositories make bare repositories available blob object Untyped object e g the contents of a file branch A branch is an active line of development The most re
81. is what is in HEAD What the above example shows is that file he 1lo c was changed from orig to HEAD and Sorig 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 gt hello c 1 git cat file blob 06fa6a2 gt hello c 2 git cat file blob cc44c73 gt 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 the 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 runs git cat file three times for this There is a 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 merg s resolve is implemented with 57 70 Chapter 10 Hacking Git This chapter covers internal details of the Git implementation wh
82. it add on them But it quickly becomes annoying to have these untracked files lying around e g they make git add practically useless and they keep showing up in the output of git status You can tell Git to ignore certain files by creating a file called git ignore in the top level of your working directory with contents such as Lines starting with are considered comments Ignore any file named foo txt foo txt Ignore generated html files html except foo html which is maintained by hand foo html Ignore objects and archives x oa See gitignore 5 for a detailed explanation of the syntax You can also place gitignore files in other directories in your working tree and they will apply to those directories and their subdirectories The git ignore files can be added to your repository like any other files Gust run git add gitignore and git commit as usual which is convenient when the exclude patterns such as patterns matching build output files would also make sense for other users who clone your repository If you wish the exclude patterns to affect only certain repositories instead of every repository for a given project you may instead put them in a file in your repository named git info exclude or in any file specified by the core excludesfil configuration variable Some Git commands can also take exclude patterns directly on the command line See gitignore 5 for the details 3 6 How to merge
83. ject either by mutual agreement or because a maintainer becomes unresponsive or difficult to work with e The lack of a central group of committers means there is less need for formal decisions about who is in and who is out 4 4 7 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 30 70 4 5 Examples 4 5 1 Maintaining topic branches for a Linux subsystem maintainer This describes how Tony Luck uses Git in his role as maintainer of the A64 architecture for the Linux kernel He uses two public branches e 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 e 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 tracking branch n
84. k up the submodule commits in the repository and manually check them out earlier versions won t recognize the submodules at all To see how submodule support works create for example four example repositories that can be used later as a submodule mkdir git cd git S for iinabcd do mkdir Si cd i git init echo module i gt Si txt git add i txt git commit m Initial commit submodule i cd done Now create the superproject and add all the submodules mkdir super 49 70 cd super git init S for i in a bod do git submodule add git i i done Note Do not use local URLs here if you plan to publish your superproject See what files git submodule created ls a git gitmodules a b c d The git submodule add lt repo gt lt path gt command does a couple of things e It clones the submodule from lt repo gt to the given lt path gt under the current directory and by default checks out the master branch e It adds the submodule s clone path to the gitmodules 5 file and adds this file to the index ready to be committed e It adds the submodule s current commit ID to the index ready to be committed Commit the superproject git commit m Add submodules a b c and d Now clone the superproject cd git clone super cloned cd cloned The submodule directories are there but they re empty ls a a git submodule status d266b9873ad50488163
85. l this even further 32 70 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 S1 in test release git checkout 1 amp amp git pull origin iT origin before git rev parse refs remotes origin master git fetch origin after S git rev parse refs remotes origin master if Sbefore Safter then git log before after git shortlog fi iT echo usage 0 origin test release 1 gt amp 2 exit 1 iT esac merge script Merge a branch into either the test or release branch pname 0 usage echo usage Spname branch test release 1 gt amp 2 exit 1 git show ref q verify refs heads 1 echo Can t see branch lt 1 gt 1 gt amp 2 usage case 2 in test release if git log 2 1 wc c eq 0 J then echo 1 already merged into 2 1 gt amp 2 exit 1 EL git checkout 2 amp amp git pull 1 iT x usage mv esac status script report on status of my ia64 Git tree 33 70 gb tput setab 2 rb tput setab 1 restore tput setab 9 if then fi for branch in do done git rev list test release we c gt 0 echo rb Warning commits in release that are git log test release git show ref heads sed s x
86. le 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 A 5 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 A 6 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 69 70 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 A 7 Repository maintenance Check for corruption git fsck Recompress remove unused cruft git gc 70 70 Appendix B Notes
87. leanly 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 gt your public repo i you pull they pull they push V their public repo lt their repo We explain how to do this in the following sections 4 4 1 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 4 4 2 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
88. ly pack files o o os 5 Se 45 sra eH EMRE EHS ES Silat Dangling objets oe we a ee Bee da Gea as TAS RKecovernng irom repository conmupyon sos p e a fei RR a Ee eR ES e El Theme bono s te SS RSE mi ES PRS ER RES Re eed we ad ee SS la Submodules 8 amp 1 Pitfalls with sabmodules ono caca ER RS eee ee ee ee eed we ed Low level Git operations 9 1 Object access and mamipuwlation 2 654242 e BPS baw ee Seer bee BEEP Eee Bee bie 92 The Workthow e me s o eee hee eee ba eee eb eee ee eb bw eA eee eo 92 Orea MEN ee ee ok a A Oe ee A ek See Le eS 9 22 IAC gt DDJECTOALIDASE ca ak Ra GR Ee BAGS Bagh a eG wal bod e e io OBS database MEE 20009 ee eS e Rea Se EME SSeS HoH SESS oe 9 24 index gt working directory ee ses tere a eae Ge ae ee ga Tyne Tall togeier nce EI e a ee ee ee oe Ee Os ES A eee eS 93 Ezamining thedata so RS A ae SS RRS ER Oe A eh a GeO OA Meremanmlipletiees 2 eco SS kee She Shit eR SEE SESS ERS REESE Ben ESE e 95 Merging multiple trees continvied s lt r c a ew ee eee 34 34 34 35 36 36 37 37 37 39 39 39 40 40 41 41 42 42 43 43 43 44 45 45 47 48 50 vi 10 Hacking Git 10 1 Object storage format o oe e ER A RR ewe eR a e 10 2 A birda eve view of Gits source code o o eac serta AA a AA 11 Git Glossary A Git Quick Reference Al Creanga new Toposit ry oe co a A ee baa Pez INIUAIAS Branches e i ct auan e AR Ye e A eee ee amp E ee Hs DERIO EA IE A Gee Bey a
89. ly 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 53 70 update index 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 The previously introduced git add 1 is just a wrapper for git update index 1 9 2 2 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
90. may just be to send them as patches in email First use 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 git format patch can include an initial cover letter You can insert commentary on individual patches after the three dash line which format pat ch places after the commit message but before the patch itself If you use git notes to track your cover letter material git format patch notes will include the commit s notes in a similar manner 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 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 4 3 Importing patches to a project Git also provides a tool called 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 pat ches 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 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
91. mp AA AA NS A A E AA Makine changes oo A A ee A E Bo MOME ata E RR A A BO we SA e A e amp e a de a AGO SDA your chinte aaa a a AO A AA A e A7 Repository niamtengnce e s bw eee A A ew ee RR eG eS B Notes and todo list for this manual 57 57 57 60 66 66 66 67 68 68 68 69 70 1 70 Part I Git User s Manual for version 1 5 3 or newer 2 70 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 Chapter 1 and Chapter 2 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 Chapter 3 and Chapter 4 Further chapters cover more specialized topics Comprehensive reference documentation is available through the man pages or git help 1 command For example for the command git clone lt repo gt you can either use man git clone or git help clone With the latter you can use the manual viewer of your choice see git help 1 for more information See also Appendix A for a brief overview of Git commands without any explanation Finally see Appendix B for ways that you can help make this manual more complete 3 70 Chapter 1 Repositories and Branches 1 1 How to get a Gi
92. n one step git pull origin master In fact if you have master checked out then this branch has been configured by git clone to get changes from the HEAD branch of the origin repository So often you can accomplish the above with just a simple git pull This command will fetch changes from the remote branches to your remote tracking branches origin x and merge the default branch into the current branch More generally a branch that is created from a remote tracking branch will pull by default from that branch See the descrip tions of the branch lt name gt remote and branch lt name gt merge options in git config 1 and the discussion of the track option in git checkout 1 to learn how to control these defaults 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 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 26 70 4 2 Submitting patches to a project If you just have a few changes the simplest way to submit them
93. name gt remote and remote lt name gt push options in git config 1 for details 29 70 4 4 5 What to do when a push fails If a push would not result in a fast forward of the remote branch then it will fail with an error like error remote refs heads master is not an ancestor of local refs heads master Maybe you are not up to date and need to pull first error failed to push to ssh yourserver com you proj git This can happen for example if you e use git reset hard to remove already published commits or e use git commit amend to replace already published commits as in Section 3 10 2 or e use git rebase to rebase any already published commits as in Section 5 2 You may force git push to perform the update anyway by preceding the branch name with a plus sign git push ssh yourserver com you proj git master Note the addition of the sign Alternatively you can use the f flag to force the remote update as in git push f ssh yourserver com you proj git master Normally whenever a branch head in a public repository is modified it is modified to point to a descendant of the commit that it pointed to before By forcing a push in this situation you break that convention See Section 5 7 Nevertheless this is a common practice for people that need a simple way to publish a work in progress patch series and it is an acceptable compromise as long as you warn other developers th
94. nd verifies their internal consistency in addition to just verifying their superficial consistency through the hash 10 2 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 commit Also we do not call it cache any more but rather 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 58 70 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
95. nterpret 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 shal Two things are interesting here e get_shal 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 e the variable shal in the function signature of get_shal isunsigned char but is actually expected to be a pointer tounsigned 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 x it is the binary representation as opposed to the ASCII representation in hex characters which is passed as char x You will see both of these things throughout the code Now for the meat case 0 buf read_object_with_reference shal argv 1 amp 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_refer 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 l 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 correspond
96. on of git config 1 for details 9 70 Chapter 2 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 hierarchy 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 2 1 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 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 65934a92028b88e83e2b0f8b36618fe503349f8el 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 in no branch HEAD is now detached from any branch and points directly to a commit with commit id 65934 that is reachable from master but not from v2 6 18 Compile and test it and see whether it crashes Assume it does crash Then git bi
97. oose 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 technical pack format txt To put the loose objects into a pack just run git repack S 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 3e54ad29d5b2e05838c75df582c65257b8d08elc 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 git gc 1 command performs packing pruning and more for you so is normally the only high level command you need 45 70 7 1 7 Dangling objects The 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
98. ory the same content stored in two repositories will always be stored under the same name e Git can detect errors when it reads an object by checking that the object s name is still the SHA 1 hash of its contents See Section 10 1 for the details of the object formatting and SHA 1 calculation wow There are four different types of objects blob tree commit and tag e A blob object is used to store file data e A tree object 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 e A commit object ties such directory hierarchies together into a directed acyclic graph of revisions each commit contains the object name of exactly one tree designating the directory hierarchy at the time of the commit In addition a commit refers to parent commit objects that describe the history of how we arrived at that directory hierarchy e A tag object symbolically identifies and can be used to sign other objects It contains the object name and type of another object a symbolic name of course and optionally a signature The object types in some more detail 42 70 7 1 1 Commit Object The commit object links a physical state of a tree with a description of how we got there and why Use the pretty raw option to git show 1 or git log 1 to examine your favorite commit git show s pretty raw 2be7fcb476
99. oving from one representation to the other 9 2 5 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 54 70 git commit tr lt tree gt p lt parent gt p lt parent2 gt 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
100. patch master test will generate a file with a patch for each commit reachable from test but not from master 2 6 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 2 7 Examples 2 7 1 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 we 1 Alternatively you may often see this sort of thing done with the lower level command git rev list 1 which just lists the SHA 1 s of all the given commits git rev list origin mybranch we 1 2 7 2 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 e05db0fd4f31dde70
101. repository below Otherwise all you need to do is start 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 git daemon 1 man page for details See especially the examples section 4 4 3 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 28 70 mv proj git home you public_html proj git cd proj git git bare update server info mv hooks post update sample hooks post update UN 1N UY Ur For an explanation of the last two lines see git update server info 1 and githooks 5 Advertise the URL of proj git Anybody else should then be able to clone or pull from that URL for example with a command line like git clone http yourserver com you proj git See also setup git server over http for a slightly more sophisticated setup using WebDAV which also allows pushing over HTTP 4 4 4 Pushing changes to a public repository
102. 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 3 7 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 git commit 1 will fail git commit file txt needs merge Also git status 1 will list those files as unmerged and the files with conflicts will have conflict markers added like this lt lt lt lt lt lt lt HEAD file txt Hello world Goodbye gt gt gt gt gt gt gt 779716da35a11db4580b80ae27e8d65caf5208086 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 3 7 1 Getting conflict resolution h
103. s may prove helpful 16 70 Chapter 3 Developing with Git 3 1 Telling Git your name Before creating any commits you should introduce yourself to Git The easiest way to do so is to use git config 1 git config global user name Your Name Comes Here git config global user email you yourdomain example com Which will add the following to a file named git config in your home directory user name Your Name Comes Here email you yourdomain example com See the CONFIGURATION FILE section of git config 1 for details on the configuration file The file is plain text so you can also edit it with your favorite editor 3 2 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 UN 1N UN Y nn 3 3 How to make a commit Creating a new commit takes three steps 1 Making some changes to the working directory using your favorite editor 17 70 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
104. sect bad Bisecting 1769 revisions left to test after this 7eff82c8b1511017ae605f0c9 ac275a7e21b867 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 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 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 10 70 git bisect visualize which will run gitk and label the commit it chose with a marker that says bisect Choose a safe looking commit nearby note its commit id and check it out with git reset hard fb47ddb2db then test run bisect goodorbisect bad as appropriate and continue Instead of git bisect visualize and then git reset hard fb47ddb2db you might just want to tell Git that you want to skip the current commit git bisect skip In this case though Git may not eventually be able to tell the first bad on
105. story that includes merge commits However when the commit that it finds is a merge commit the user may need to work harder than usual to figure out why that commit introduced a problem Imagine this history Suppose that on the upper line of development the meaning of one of the functions that exists at Z is changed at commit X The commits from Z leading to A change both the function s implementation and all calling sites that exist at Z as well as new calling sites they add to be consistent There is no bug at A Suppose that in the meantime on the lower line of development somebody adds a new calling site for that function at commit Y The commits from Z leading to B all assume the old semantics of that function and the callers and the callee are consistent with each other There is no bug at B either 38 70 Suppose further that the two development lines merge cleanly at C so no conflict resolution is required Nevertheless the code at C is broken because the callers added on the lower line of development have not been converted to the new semantics introduced on the upper line of development So if all you know is that D is bad that Z is good and that git bisect 1 identifies C as the culprit how will you figure out that the problem is due to this change in semantics When the result of agit bisect is a non merge commit you should normally be able to discover the problem by examining just that commit Developers can
106. t 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 git clone 1 command to download a copy of an existing repository 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 1inux 2 6 in the examples above After you cd into this directory you will see that it contains a copy of the project files called the working tree together with a special top level directory named git which contains all the information about the history of the project 1 2 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 of the project s contents In Git each such version is called a commit Those snapshots aren t necessarily all arranged in a single line from oldest to newest instead work may simultaneously proceed along parallel lines of development called branches which may merge and diverge
107. tand its inner workings 9 1 Object access and manipulation The git cat file 1 command can show the contents of any object though the higher level git show 1 is usually more useful The git commit tree 1 command allows constructing commits with arbitrary parents and trees A tree can be created with git write tree 1 and its data can be accessed by git ls tree 1 Two trees can be compared with git diff tree 1 A tag is created with git mktag 1 and the signature can be verified by git verify tag 1 though it is normally simpler to use git tag 1 for both 9 2 The Workflow High level operations such as git commit 1 git checkout 1 and git reset 1 work by moving data between the working tree the index and the object database Git provides low level operations which perform each of these steps individually 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 between the index file and either the database or the working directory Thus there are four main combinations 9 2 1 working directory index The git update index 1 command updates the index with information from the working directory You generally update the index information by just specifying the filename you want to update like so git update index filenam but to avoid common mistakes with filename globbing etc the command will not normal
108. tween 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 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 j git diff 3 file txt diff against stage 3 git diff theirs file txt same as the above UN 1N GN N Ur The 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
109. ubversion 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 More on recovery from repository corruption See http marc theaimsgroup com l git amp m 117263864820799 amp w 2 http marc theai MN git amp m 117147855503798 amp w 2
110. ush mytree or push just one of the test and release branches using git push mytree test or 31 70 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 a recent stable tag of Linus s branch Picking a stable base for your branch will 1 help you by avoiding inclusion of unrelated and perhaps lightly tested changes 2 help future bug hunters that use git bisect to find problems git checkout b speed up spinlocks v2 6 35 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 S patch test commit patch test commit x 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 amp amp 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
111. veral modules in a single commit while moving things around or updating APIs and translations Git does not allow partial checkouts so duplicating this approach in Git would force developers to keep a local copy of modules they are not interested in touching Commits in an enormous checkout would be slower than you d expect as Git would have to scan every directory for changes If modules have a lot of local history clones would take forever On the plus side distributed revision control systems can much better integrate with external sources In a centralized model a single arbitrary snapshot of the external project is exported from its own revision control and then imported into the local revision control on a vendor branch All the history is hidden With distributed revision control you can clone the entire external history and much more easily follow development and re merge local changes Git s submodule support allows a repository to contain as a subdirectory a checkout of an external project Submodules maintain their own identity the submodule support just stores the submodule repository location and commit ID so other developers who clone the containing project superproject can easily clone all the submodules at the same revision Partial checkouts of the superproject are possible you can tell Git to clone none some or all of the submodules The git submodule 1 command is available since Git 1 5 3 Users with Git 1 5 2 can loo
112. verwhelmed 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 5 2 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 UUU Ur Y 35 70 You have performed no merges into mywork so it is just a simple linear sequence of patches on top of origin o 0 0 lt origin a b c lt mywork Some more interesting work has been done in the upstream project and origin has advanced O 0 O 0 0 o lt origin a b c lt 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 0 O 0 0 o lt origin x a b c m lt mywork
113. will also add a dirty to the work tree side when generating patch output or used with the submodule option git diff diff git a sub b sub a sub b sub ee 1 1 ee Subproject commit 3f356705649b5d566d97ff843cf1933592292a453 Subproject commit 3f356705649b5d566d97ff843cf19335922 2453 dirty git diff submodule Submodule sub 3f35670 3f35670 dirty You also should not rewind branches in a submodule beyond commits that were ever recorded in any superproject It s not safe to run git submodule update if you ve made and committed changes within a submodule without checking out a branch first They will be silently overwritten cat a txt module a echo line added from private2 gt gt a txt git commit a m line added inside private2 cd git submodule update Submodule path a checked out d266b9873ad50488163457f025db7cdd9683d88b cd a cat a txt module a Note The changes are still visible in the submodule s reflog If you have uncommitted changes in your submodule working tree git submodule update will not overwrite them In stead you get the usual warning about not being able switch from a dirty branch 52 70 Chapter 9 Low level Git operations Many of the higher level commands were originally implemented as shell scripts using a smaller core of low level Git commands These can still be useful when doing unusual things with Git or just as a way to unders
114. y can be later deepened with git fetch 1 symref Symbolic reference instead of containing the SHA 1 id itself it is of the format ref refs some thing and when referenced it recursively dereferences to this reference HEAD is a prime example of a symref Symbolic references are manipulated with the git symbolic ref 1 command tag A ref under refs tags namespace that points to an object of an arbitrary type typically a tag points to either a tag or a commit object In contrast to a head a tag is not updated by the commit command A Git tag has nothing to do with a Lisp tag which would be called an object type in Git s context A tag is most typically used to mark a particular point in the commit ancestry chain tag object An object containing a ref pointing to another object which can contain a message just like a commit object It can also contain a PGP signature in which case it is called a signed tag object topic branch A regular Git branch that is used by a developer to identify a conceptual line of development Since branches are very easy and inexpensive it is often desirable to have several small branches that each contain very well defined concepts or small incremental yet related changes tree Either a working tree or a tree object together with the dependent blob and tree objects i e a stored representation of a working tree tree object An object containing a list of file names and modes along with refs to t
115. you ll need to use gzip explicitly git archive format tar prefix project HEAD gzip gt latest tar gz 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 15 70 bin sh stable 1 last 2 new 3 echo git tag vSnew echo git archiv prefix linux Snew vSnew gzip 9 gt linux new tar gz echo git diff v stable v new gzip 9 gt patch Snew gz echo git log no merges vSnew vSlast gt ChangeLog new echo git shortlog no merges v new vSlast gt ShortLog echo git diff stat summary M vSlast v new gt diffstat new and then he just cut and pastes the output commands after verifying that they look OK 2 7 6 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 abbrev 40 pretty oneline grep B 1 git hash object filename Figuring out why this works is left as an exercise to the advanced student The git log 1 git diff tree 1 and git hash object 1 man page

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