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1. What if you wanted to save the temporary changes after all Easy git checkout b dirty and commit before switching back to the master branch Whenever you want to return to the dirty changes simply type git checkout dirty We touched upon this command in an earlier chapter when discussing loading old states At last we can tell the whole story the files change to the requested state but we must leave the master branch Any commits made from now on take your files down a different road which can be named later In other words after checking out an old state Git automatically puts you in a new unnamed branch which can be named and saved with git checkout b 15 Chapter 5 Branch Wizardry 5 3 Quick Fixes You re in the middle of something when you are told to drop everything and fix a newly discovered bug git commit a git checkout b fixes SHA1_HASH Then once you ve fixed the bug git commit a m Bug fixed git push to the central repository git checkout master and resume work on your original task 5 4 Uninterrupted Workflow Some projects require your code to be reviewed before you may submit it To make life easier for those reviewing your code if you have a big change to make you might break it into two or more parts and get each part separately reviewed What if the second part cannot be written until the first part is approved and checked in In many version contro2. Other times you want to hop to an old state briefly In this case type git checkout SHA1_HASH This takes you back in time while preserving newer commits However like time travel in a science fiction movie if you now edit and commit you will be in an alternate reality because your actions are different to what they were the first time around This alternate reality is called a branch and we ll have more to say about this later For now just remember that git checkout master will take you back to the present Uncommitted changes travel in time with you when you run checkout To take the computer game analogy again git reset hard load an old save and delete all saved games newer than the one just loaded e git checkout load an old game but if you play on the game state will deviate from the newer saves you made the first time around Any saved games you make now will end up in a separate branch representing the alternate reality you have entered We deal with this later You can choose only to restore particular files and subdirectories by appending them after the command Don t like cutting and pasting hashes Then use Chapter 3 Basic Tricks git checkout 10 minutes ago Other time specifications work too For example you can ask for the 5th last saved state git checkout 5 3 2 1 Reverting In a court of law events can be stricken from the record Likewise you can pi
3. How does Git know you renamed a file even though you never mentioned the fact explicitly Sure you may have run git mv but that is exactly the same as a git rm followed by a git add Git heuristically ferrets out renames and copies between successive versions In fact it can detect chunks of code being moved or copied around between files Though it cannot cover all cases it does a decent job and this feature is always improving If it fails to work for you try options enabling more expensive copy detection and consider upgrading 7 4 Indexing For every tracked file Git records information such as its size creation time and last modification time in a file known as the index To determine whether a file has changed Git compares its current stats with that held the index If they match then Git can skip reading the file again 28 Chapter 7 Secrets Revealed Since stat calls are vastly cheaper than reading file contents if you only edit a few files Git can update its state in almost no time 7 5 Bare Repositories You may have been wondering what format those online Git repositories use They re plain Git repositories just like your git directory except they ve got names like proj git and they have no working directory associated with them Most Git commands expect the Git index to live in git and will fail on these bare repositories Fix this by setting the GIT_DIR environment variable to the path of
4. depth option This is much faster but the resulting clone has reduced functionality 8 6 Volatile Projects Git was written to be fast with respect to the size of the changes Humans make small edits from version to version A one liner bugfix here a new feature there emended comments and so forth But if your files are radically different in successive revisions then on each commit your history necessarily grows by the size of your whole project There is nothing any version control system can do about this but standard Git users will suffer more since normally histories are cloned The reasons why the changes are so great should be examined Perhaps file formats should be changed Minor edits should only cause minor changes to at most a few files Or perhaps a database or backup archival solution is what is actually being sought not a version control system For example version control may be ill suited for managing photos periodically taken from a webcam If the files really must be constantly morphing and they really must be versioned a possibility is to use Git in a centralized fashion One can create shallow clones which checks out little or no history of the project Of course many Git tools will be unavailable and fixes must be submitted as patches This is probably fine as it s unclear why anyone would want the history of wildly unstable files 31 Chapter 8 Git Shortcomings Another example is a project de
5. including this guide http repo or cz w gitmagic git http gitorious org is another Git hosting site aimed at open source projects http github com hosts open source projects for free including this guide http github com blynn gitmagic tree master and private projects for a fee Chapter 2 Introduction Pll use an analogy to introduce version control See the Wikipedia entry on revision control http en wikipedia org wiki Revision_control for a saner explanation 2 1 Work is Play I ve played computer games almost all my life In contrast I only started using version control systems as an adult I suspect I m not alone and comparing the two may make these concepts easier to explain and understand Think of editing your code or document or whatever as playing a game Once you ve made a lot of progress you d like to save To do so you click on the Save button in your trusty editor But this will overwrite the old version It s like those old school games which only had one save slot sure you could save but you could never go back to an older state Which was a shame because your previous save might have been right at a exceptionally fun part of the game that you d like to revisit one day Or worse still your current save is in an unwinnable state and you have to start again 2 2 Version Control When editing you can Save As a different file or copy the file somewhere first b
6. 1l soon suffer Developers constantly struggle with virtual red tape and bureaucracy The secret is the git directory in your working directory Git keeps the history of your project here The initial stops it showing up in 1s listings Except when you re pushing and pulling changes all version control operations operate within this directory You have total control over the fate of your files because Git doesn t care what you do to them Git can easily recreate a saved state from git at any time 7 2 Integrity Most people associate cryptography with keeping information secret but another equally important goal is keeping information safe Proper use of cryptographic hash functions can prevent accidental or malicious data corruption A SHAT hash can be thought of as a unique 160 bit ID number for every string of bytes you ll encounter in your life Actually more than that every string of bytes that any human will ever use over many lifetimes The hash of the whole contents of a file can be viewed as a unique ID number for that file An important observation is that a SHA1 hash is itself a string of bytes so we can hash strings of bytes containing other hashes Roughly speaking all files handled by Git are referred to by their unique ID not by their filename All data resides in files in the git objects subdirectory where you won t find any normal filenames The 27 Chapter 7 Secrets Revealed contents of fi
7. about to attempt something drastic I like to save the current state so I can go back and try again should things go awry Take a snapshot of all files in the current directory with git init git add git commit m My first backup The above sequence of commands should be memorized or placed in a script as they will be reused frequently Then if something goes wrong run git reset hard to go back to where you were To save the state again git commit a m Another backup 3 1 1 Add Delete Rename The above will only keep track of the files that were present when you first ran git add If you add new files or subdirectories you ll have to tell Git git add NEWFILES Similarly if you want Git to forget about certain files maybe because you ve deleted them git rm OLDFILES Renaming a file is the same as removing the old name and adding the new name There s also the shortcut git mv which has the same syntax as the mv command For example git mv OLDFILE NEWFILE Chapter 3 Basic Tricks 3 2 Advanced Undo Redo Sometimes you just want to go back and forget about every change past a certain point because they re all wrong Then git log shows you a list of recent commits and their SHA 1 hashes Next type git reset hard SHA1_HASH to restore the state to a given commit and erase all newer commits from the record permanently
8. script is ready for release execute git commit a m Next release and your users can upgrade their version by changing to the directory containing your script and typing S git pull Your users will never end up with a version of your script you don t want them to see Obviously this trick works for anything not just scripts 3 6 What Have Done Find out what changes you ve made since the last commit with git diff Or since yesterday git diff yesterday Chapter 3 Basic Tricks Or between a particular version and 2 versions ago git diff SHAl_HASH 2 Chapter 4 Cloning Around In older version control systems checkout is the standard operation to get files You checkout a bunch of files in the requested saved state In Git and other distributed version control systems cloning is the standard operation To get files you create a clone of the entire repository In other words you practically create a mirror of the central server Anything the main repository can do you can do 4 1 Sync Computers This is the reason I first used Git I can tolerate making tarballs or using rsync for backups and basic syncing But sometimes I edit on my laptop other times on my desktop and the two may not have talked to each other in between Initialize a Git repository and commit your files as above on one machine Then on the other git clone other computer path to files to create a
9. second copy of the files and Git repository From now on git commit a git pull other computer path to files will pull in the state of the files on the other computer into the one you re working on If you ve recently made conflicting edits in the same file Git will let you know and you should commit again after resolving them 4 2 Classic Source Control Initialize a Git repository for your files git init git add git commit m Initial commit On the central server initialize an empty Git repository with some name and start the Git daemon if necessary GIT_DIR proj git git init git daemon detach it might already be running 10 Chapter 4 Cloning Around Some public hosts such as repo or cz http repo or cz will have a different method for setting up the initially empty Git repository such as filling in a form on a webpage Push your project to the central server with git push git central server path to proj git HEAD We re ready To check out source a developer types git clone git central server path to proj git After making changes the code is checked in to the main server by git commit a git push If the main server has been updated the latest version needs to be checked out before the push To sync to the latest version git commit a S git pull 4 3 Forking a Project Sick of the way a project is being run Think you could do a better j
10. tthe oh sana 11 AA Ultimate Backups si ereite capt odas piadosa aora acia 11 4 5 Eight Speed Multitask coincida callo cono te dee ch svebe conch si E E E stander islas 12 4 6 Guerilla Version Control corre iicviscevesvaviavicn see ap ano dan oben diana E E ER 12 IA sei sccessensbnessendestesecsecesesossaniccess 14 5 1 The Boss Key urticaria 2 2 Dirty Work sii nade ila lint dhe dint iaa SQUAD esti ees Meet 5 4 Uninterrupted Workflow 5 5 Reorganizing a Medley SSSy 1 c c scetssseesscepessetasteneseveun oee ere r E e EE EER e rE ar EE TeS 17 3 0 Managine Branches a 17 5 7 Work How YOu Want ds 18 5 7 1 Personal Experience eu ish ek iene ends see ad Gee aes dorsales 18 6 Git Grandmastery O 20 OTe Source Releases sey wie vases idea hice Wada vas oo tte laa ii sados 20 6 2 Changelog Generation SHH ssccssus oE EE Ea Eker EE T rE r ES EEEE ESSEER E p EEEE EE Ts e 20 6 35 GILOVer SSH HFIP cui tii 20 6 4 Commit What Changed 000 eee ee E o EE EEEE ENEE SE EEIE EEr ES EEEE ea 21 62d ES tand Corrected ua i E REA E NE iia 21 0 6 And Then Some SEE A A EAA AEE eastern tees 21 6 7 Local Changes Last 0d tas tip Aioria 22 6 8 My Commit Is Too Big aer eE E E EEEE non nooo n conca enn cnn cnn non A nro nana ninia 22 6 95 Don t Lose Your HEAD SS 00 ii 22 iii 6 10 HEADLINE le 23 6 11 Makine History SS eiii ideas 24 6 12 Building On Git A a 25 6 13 Dating Stints bi
11. Git Magic Ben Lynn Git Magic by Ben Lynn Table of Contents 1 Preface S puuri arn AN EE ANA AEAN A 1 T t Thanks A TR 1 j AA Bi in 1 2 Introduction s ccecssssrssssresssressssessesesessensesessesensssesessesesseeesensesessesensesssessceeseesesessesessenesesseseseeseseesess 2 ZW oras Play its 2 2 2 Version Control anios 2 230 Distributed Control srta nta sevsescecescvsees iia le Esad ines cares ir tees 2 2 3 1 A Silly Superstition tyes Alektra Ae he ae aa a ee 3 24 Merge Conflicts SSH ss tanda di 4 3 Basic Tricks cscsssssrssssresssresssrsssssssssensesensesensesessscessscesesessesessesensesssessseesessesessssessesesessesessesessesess 5 SAL PAVING State iii 5 3 1 1 Add Delete Rename cacon ancora AEE tesencnactcsvestecetees 5 3 2 Advanced Undo REGG ie eeraa e a a lib iii 6 3 2 1 Reverting s 2 aici vel aA en pelea iene ed Rie eed aes Sate 7 3 3 Downloading Files SSS esha n awk esas Sows dei E iincii eewvesty 7 3 4 The Bleeding Edge ase cx ccc sccsecs seis eues eror eases EEEo EE EErEE EENE EENE EEEE VEST KEETE odas 7 3 5 Instant P blishins 0 ii 7 3 06 What Have T Done din ise dada les aby 8 4 Cloning Around sscsscsssssssssscsscsssssssssscsssssesssescessssessnssnssssessssnessesssssssssossesessessnssessessessessesseees 10 4 1 Sync Computers hito iia 10 4 2 Classic Source Control SSF ave coronar Ra side 10 4 3 ForkKing a Project 2 65 pak
12. ces and perhaps soon you will have a script or two of your own that calls Git One easy trick is to use built in git aliases shorten your most frequently used commands git config global alias co checkout git config global get regexp alias display current aliases alias co checkout git co foo same as git checkout foo Another is to print the current branch in the prompt or window title Invoking git symbolic ref HEAD shows the current branch name In practice you most likely want to remove the refs heads and ignore errors git symbolic ref HEAD 2 gt dev null cut b 12 See the Git homepage http git or cz for more examples 6 13 Daring Stunts Recent versions of Git make it difficult for the user to accidentally destroy data This is perhaps the most compelling reason to upgrade 25 Chapter 6 Git Grandmastery Nonetheless there are times you truly want to destroy data We show how to override the safeguards for common commands Only use them if you know what you are doing Checkout If you have uncommitted changes a plain checkout fails To destroy your changes and checkout a given commit anyway use the force flag git checkout f COMMIT On the other hand if you specify particular paths for checkout then there are no safety checks The supplied paths are quietly overwritten Take care if you use checkout in this manner Reset Reset also fails in the presence of uncommitt
13. ces each consisting of related files Use git submodule if you still want to keep everything in a single repository 8 3 Who s Editing What Some version control systems force you to explicitly tag a file before editing While this is especially annoying when this involves talking to a central server it does have two benefits 1 Diffs are quick because only the tagged files need be examined 2 When a central server stores the tags one can discover who else is working on the file With appropriate scripting you can achieve the same with Git This requires cooperation from the programmer who should execute particular scripts when editing a file 30 Chapter 8 Git Shortcomings 8 4 File History Since Git records project wide changes reconstructing the history of a single file requires more work than in version control systems that track individual files The penalty is typically slight and well worth having as other operations are incredibly efficient For example git checkout is faster than cp a and project wide deltas compress better than collections of file based deltas 8 5 Initial Clone Creating a clone is more expensive than checking out code in other version control systems when there is a lengthy history The initial cost is worth paying in the long run as most future operations will then be fast and offline However in some situations it may be preferable to create a shallow clone with the
14. ck for any number of parts 5 5 Reorganizing a Medley Perhaps you like to work on all aspects of a project in the same branch You want to keep works in progress to yourself and want others to see your commits only when they have been neatly organized Start a couple of branches git checkout b sanitized git checkout b medley Next work on anything fix bugs add features add temporary code and so forth committing often along the way Then git checkout sanitized git cherry pick SHA1_HASH applies a given commit to the sanitized branch With appropriate cherry picks you can construct a branch that contains only permanent code and has related commits grouped together 5 6 Managing Branches Type git branch 17 Chapter 5 Branch Wizardry to list all the branches There is always a branch named master and you start here by default Some advocate leaving the master branch untouched and creating new branches for your own edits See git help branch for other branch operations 5 7 Work How You Want Applications such as Mozilla Firefox http www mozilla com allow you to open multiple tabs and multiple windows Switching tabs gives you different content in the same window Git branching is like tabs for your working directory Continuing this analogy Git cloning is like opening a new window Being able to do both easily makes for a better user experience On a higher level seve
15. ck specific commits to undo git commit a git revert SHA1_HASH will undo just the commit with the given hash Running git log reveals the revert is recorded as a new commit 3 3 Downloading Files Get a copy of a project managed with Git by typing git clone git server path to files For example to get all the files I used to create this site git clone git git or cz gitmagic git We ll have much to say about the clone command soon 3 4 The Bleeding Edge If you ve already downloaded a copy of a project using git clone you can upgrade to the latest version with git pull Chapter 3 Basic Tricks 3 5 Instant Publishing Let s say you ve written a script you d like to share with others You could just tell them to download from your computer but if they do so while you re improving the script or making experimental changes they could wind up in trouble Of course this is why release cycles exist Developers work on code and when they feel it s suitable for others they release the code To do this with Git in the directory where your script resides git init git add git commit m First release Then tell your users to run git clone your computer path to script to download your script This assumes they have ssh access If not run git daemon and tell your users to instead run git clone git your computer path to script From now on every time your
16. d 25 7 Secrets Revealed omononnononoonononnonesnoneconeconen canon conan ocnononnonocononconon canon cononosnonoconocconooonosoononcononosconesoss 27 TL Jovis bility iaa 27 TD OO 27 13 Intelligence Sass iii 28 RA eaea aa anea eee ee E E cacncbevteuscels cob venees dens curves sees coda EO acento 28 TDs Bare REPOSitOres yes ses sseus e E cases tes EE E L E E E E E E E E a 29 7 02 GIES ON SINS id 29 8 Git Shortcomings sess ccnsseescesseesocadcccessasessecsseasesesoseussodensasoasecescnessenssbeoseaseedovaacdsessensduaveasscasodadesassesud e 30 8 1 Microsoft Windows SHS a r a 30 8 2 Unrelated Piles Ss ati its 30 8 3 Who s Editing Whit 000 Ea EEE E E EE A OEC a 30 8 4 Fil History F ii r E E E E EE E E TETEE hee EE A Ee 30 8 amp 3 Initials Clone as ia 31 8 6 Volatile Projects Ei araa araisa naear hess a aa ETa Eo EaR T Ers RESSA ESE EEO iria 31 97 Global Counter rn aei eeo tensed eE EE E A E OKEERE EEKEREN Eaa 32 8 8 Empty Subdirectories sirere es r pc neto Ipiales Eri 32 8 97 InitiakCommit n a E KEE E E E E genes E A G 32 Chapter 1 Preface Git http git or cz is a version control Swiss army knife A reliable versatile multipurpose revision control tool whose extraordinary flexibility makes it tricky to learn let alone master I m recording what I ve figured out so far in these pages because I initially had difficulty understanding the Git user manual http www kernel org pub softwa
17. e you can commit and pull changes from the other clone git pull the other clone 4 6 Guerilla Version Control Are you working on a project that uses some other version control system and you sorely miss Git Then initialize a Git repository in your working directory git init S git add git commit m Initial commit then clone it at light speed git clone some new directory Now go to the new directory and work here instead using Git to your heart s content Once in a while you ll want to sync with everyone else in which case go to the original directory sync using the other version control system and type git add git commit m Sync with everyone else Then go to the new directory and run 12 Chapter 4 Cloning Around git commit a m Description of my changes git pull The procedure for giving your changes to everyone else depends on the other version control system The new directory contains the files with your changes Run whatever commands of the other version control system are needed to upload them to the central repository Tip If you want to interact with a Subversion repository check out the git svn command which can automate all this and more for you 13 Chapter 5 Branch Wizardry Instant branching and merging are the most lethal of Git s killer features Problem External factors inevitably necessitate context switching A severe bug manifests i
18. ed changes To force it through run git reset hard COMMIT Branch Deleting branches fails if this causes changes to be lost To force a deletion type git branch D BRANCH instead of d Similarly attempting to overwrite a branch via a move fails if data loss would ensue To force a branch move type git branch M SOURCE TARGET instead of m Unlike checkout and reset the destruction is deferred The changes are still stored in the git subdirectory and can be retrieved by recovering the appropriate hash from git logs see HEAD hunting above The data is only deleted the next time garbage is collected 26 Chapter 7 Secrets Revealed We take a peek under the hood and explain how Git performs its miracles I will skimp over details For in depth descriptions refer to the user manual http www kernel org pub software scm git docs user manual html 7 1 Invisibility How can Git be so unobtrusive Aside from occasional commits and merges you can work as if you were unaware that version control exists That is until you need it and that s when you re glad Git was watching over you the whole time Other version control systems don t let you forget about them Permissions of files may be read only unless you explicitly tell the server which files you intend to edit The central server might be keeping track of who s checked out which code and when When the network goes down you
19. eedruns are real life examples players specializing in different levels of the same game collaborate to produce amazing results How would you set up a system so they can get at each other s saves easily And upload new ones In the old days every project used centralized version control A server somewhere held all the saved games Nobody else did Every player kept at most a few saved games on their machine When a player wanted to make progress they d download the latest save from the main server play a while save and upload back to the server for everyone else to use What if a player wanted to get an older saved game for some reason Maybe the current saved game is in an unwinnable state because somebody forgot to pick up an object back in level three and they want to find the latest saved game where the game can still be completed Or maybe they want to compare two older saved games to see how much work a particular player did There could be many reasons to want to see an older revision but the outcome is the same They have to ask the central server for that old saved game The more saved games they want the more they need to communicate The new generation of version control systems of which Git is a member are known as distributed systems and can be thought of as a generalization of centralized systems When players download from the main server they get every saved game not just the latest one It s as if they re mirro
20. efore saving if you want to savour old versions Maybe compress them too to save space This is a primitive and labour intensive form of version control Computer games improved on this long ago many of them providing multiple automatically timestamped save slots Let s make the problem slightly tougher Say you have a bunch of files that go together such as source code for a project or files for a website Now if you want to keep an old version you have to archive a whole directory Keeping many versions around by hand is inconvenient and quickly becomes expensive With some computer games a saved game really does consist of a directory full of files These games hide this detail from the player and present a convenient interface to manage different versions of this directory Version control systems are no different They all have nice interfaces to manage a directory of stuff You can save the state of the directory every so often and you can load any one of the saved states later on Unlike most computer games they re usually smart about conserving space Typically only a few files change between version to version and not by much Storing the differences instead of entire new copies saves room Chapter 2 Introduction 2 3 Distributed Control Now imagine a very difficult computer game So difficult to finish that many experienced gamers all over the world decide to team up and share their saved games to try to beat it Sp
21. elog by typing git log gt ChangeLog 6 3 Git Over SSH HTTP Suppose you have ssh access to your web server but it does not have Git installed Then download compile and install Git in your account Create a repository in your web directory GIT_DIR proj git git init and in the proj git directory run git bare update server info chmod a x hooks post update From your computer push via ssh git push web server path to proj git master and others get your project via git clone http web server proj git 20 Chapter 6 Git Grandmastery 6 4 Commit What Changed Telling Git when you ve added deleted and renamed files gets tedious Instead you can type git add git add u Git will look at the files in the current directory and work out the details by itself Instead of the second add command run git commit a if you also intend to commit at this time You can perform the above in a single pass with git ls files d m o z xargs 0 git update index add remov The z and 0 options prevent ill side effects from filenames containing strange characters Note this command adds ignored files You may want to use the x or xX option 6 5 Stand Corrected Did you just commit but wish you had typed a different message Realized you forgot to add a file Then git commit amend can help you out Since this changes the history only do this if you have y
22. es are someone has already written a script to export the whole history to Git Otherwise take a look at git fast import This command takes text input in a specific format and creates Git history from scratch Typically a script is cobbled together and run once to feed this command migrating the project in a single shot As an example paste the following listing into temporary file such as tmp history commit refs heads master committer Alice lt alice example com gt Thu 01 Jan 1970 00 00 00 0000 data lt lt EOT Initial commit EO 100644 inline hello c data lt lt EOT include lt stdio h gt int main printf Hello world n return 0 commit refs heads master committer Bob lt bob example com gt Tue 14 Mar 2000 01 59 26 0800 data lt lt EOT Replace printf with write EO 100644 inline hello c data lt lt EOT include lt unistd h gt int main write 1 Hello world n 14 return 0 24 Chapter 6 Git Grandmastery Then create a Git repository from this temporary file by typing mkdir project cd project git init git fast import lt tmp history You can checkout the latest version of the project with git checkout master 6 12 Building On Git In true UNIX fashion Git s design allows it to be easily used as a low level component of other programs There are GUI interfaces web interfaces alternative command line interfa
23. es in any type of revision control system Suppose Alice and Bob have both independently downloaded the latest saved game They both play the game a bit more and save What if they both want to submit their new saves Let s say Alice uploads first Then when Bob tries to upload his save the system realizes that his state does not chronologically follow Alice s state Let us revert to editing a text file Our computer game analogy becomes too thinly stretched since most games never worry about this So suppose Alice has inserted a line at the beginning of a file and Bob appends one at the end They both upload their changes Most systems will automatically deduce a reasonable course of action accept and merge their changes so both Alice s and Bob s edits are applied Now suppose both Alice and Bob have made distinct edits to the same line Then it is impossible to resolve the conflict without human intervention The second person to upload is informed of a merge conflict and they must either choose one edit over another or revise the line entirely More complex situations can arise Version control systems handle the simpler cases themselves and leave the difficult cases for humans Usually their behaviour is configurable Chapter 3 Basic Tricks Rather than diving into a sea of Git commands use these elementary examples to get your feet wet Despite their simplicity each of them are useful 3 1 Saving State When I m
24. et to push your changes otherwise your tree will diverge from other trees Of course if you control all the other trees too then there is no problem since you can overwrite them 6 6 And Then Some Let s suppose the previous problem is ten times worse After a lengthy session you ve made a bunch of commits But you re not quite happy with the way they re organized and some of those commit messages could use rewording This is quite likely if you ve been saving early and saving often Then type git rebase i HEAD 10 and the last 10 commits will appear in your favourite EDITOR A sample excerpt pick 5c6eb73 Added repo or cz link 21 Chapter 6 Git Grandmastery pick a31la64 Reordered analogies in Work How You Want pick 100834f Added push target to Makefile Then e Remove commits by deleting lines Reorder commits by reordering lines e Replace pick with edit to mark a commit for amending e Replace pick with squash to merge a commit with the previous one Next run git commit amend if you marked a commit for editing Otherwise run git rebase continue Again only do this if no one else has a clone of your tree 6 7 Local Changes Last You re working on an active project You make some local commits over time and then you sync with the official tree with a merge This cycle repeats itself a few times before you re ready to push to the central tree But now the histor
25. it taking many features for granted I did not know what centralized systems were like and assumed they were similar Later I was forced to use one Often I have a flaky internet connection This matters little with Git but makes development unbearable with a centralized system 18 Chapter 5 Branch Wizardry Additionally I found myself conditioned to avoid certain commands because of the latencies involved which ultimately prevented me from following the work flow I wanted When I had to run a slow command the interruption to my train of thought dealt a disproportionate amount of damage While waiting for server communcation to complete I d do something else to pass the time such as check email or write documentation By the time I returned to the original task the command had already finished long ago I would then spend a long time trying to remember what I was doing 19 Chapter 6 Git Grandmastery This pretentiously named page is my dumping ground for uncategorized Git tricks 6 1 Source Releases For my projects Git tracks exactly the files I d like to archive and release to users To create a tarball of the source code I run git archiv format tar prefix proj 1 2 3 HEAD 6 2 Changelog Generation It s good practice to keep a changelog http en wikipedia org wiki Changelog and some projects even require it If you ve been committing frequently which you should generate a Chang
26. l systems you d have to send the first part to the reviewers and then wait until it has been approved before starting on the second part Actually that s not quite true but in these systems editing Part II before Part I has been submitted involves a lot of suffering and hardship In Git branching and merging are painless a technical term for fast and local So after you ve committed the first part and sent it for review git checkout b part2 Next code the second part of the big change without waiting for the first part to be accepted When the first part is approved and submitted git checkout master git merge part2 git branch d part2 don t need this branch anymore and the second part of the change is ready to review But wait What if it wasn t that simple Say you made a mistake in the first part which you have to correct before submitting No problem First switch back to the master branch with 16 Chapter 5 Branch Wizardry git checkout master Fix the issue with the first part of the change and hope it gets approved If not we simply repeat this step Youll probably want to merge the fixed version of Part I into Part II as well git checkout part2 git merge master Now it s the same as before Once the first part has been approved and submitted git checkout master git merge part2 git branch d part2 and again the second part is ready to be reviewed It s easy to extend this tri
27. les are strings of bytes we call blobs and they are divorced from their filenames The filenames are recorded somewhere though They live in tree objects which are lists of filenames along with the IDs of their contents Since the tree itself is a string of bytes it too has a unique ID which is how it is stored in the git objects subdirectory Trees can appear on the lists of other trees hence a directory tree and all the files within may be represented by trees and blobs Lastly a commit contains a message a few tree IDs and information on how they are related to each other A commit is also a string of bytes hence it too has a unique ID You can see for yourself take any hash you see in the git objects directory and type git cat file p SHA1_HASH Now suppose somebody tries to rewrite history and attempts to change the contents of a file in an ancient version Then the ID of the file will change since it s now a different string of bytes This changes the ID of any tree object referencing this file which in turn changes the ID of all commit objects involving this tree The corruption in the bad repository is exposed when everyone realizes all the commits since the mutilated file have the wrong IDs I ve ignored details such as file permissions and signatures But in short so long as the 20 bytes representing the last commit are safe it s impossible to tamper with a Git repository 7 3 Intelligence
28. n the released version suddenly without warning and must be fixed as soon as possible at all costs The deadline for a certain feature is imminent The guy who wrote a certain function is about to leave so you should drop what you are doing and get him to help you understand it Interrupting your train of thought can be detrimental to your productivity and the slower and less convenient it is to switch contexts the greater the loss With centralized version control we must download a fresh working copy from the central server Distributed systems fare better as we can clone the desired version locally But cloning still entails copying the whole working directory as well as the entire history up to the given point Even though Git reduces the cost of this with file sharing and hard links the project files themselves must be recreated in their entirety in the new working directory Solution Git has a better tool for these situations that is much faster and more space efficient than cloning git branch With this magic word the files in your directory suddenly shapeshift from one version to another This transformation can do more than merely go back or forward in history Your files can morph from the last release to the experimental version to the current development version to your friend s version and so on 5 1 The Boss Key Ever play one of those games where at the push of a button the boss key the screen would instantly di
29. ob Then on your server git clone git main server path to files Next tell everyone about your fork of the project at your server At any later time you can merge in the changes from the original project with git pull 4 4 Ultimate Backups Want numerous tamper proof geographically diverse redundant archives If your project has many developers don t do anything Every clone of your code is effectively a backup Not just of the current state of the project but of your project s entire history Thanks to cryptographic hashing if anyone s clone becomes corrupted it will be spotted as soon as they try to communicate with others 11 Chapter 4 Cloning Around If your project is not so popular find as many servers as you can to host clones The truly paranoid should always write down the latest 20 byte SHA1 hash of the HEAD somewhere safe It has to be safe not private For example publishing it in a newspaper would work well because it s hard for an attacker to alter every copy of a newspaper 4 5 Light Speed Multitask Say you want to work on several features in parallel Then after committing your project git clone some new directory Git exploits hard links and file sharing as much as safely possible to create this clone so it will be ready in a flash and you can now work on two independent features simultaneously For example you can edit one clone while the other is compiling At any tim
30. on t worry for commands like these Git saves the original HEAD as a tag called ORIG_HEAD and you can return safe and sound with S git reset ORIG_HEAD 6 10 HEAD hunting Perhaps ORIG_HEAD isn t enough Perhaps you ve just realized you made a monumental mistake last month and you need to go back to an ancient commit in a long forgotten branch It s hard to lose Git commits permanently even after deleting branches As long as you never run git ge prune your commits are preserved forever and can be restored at any time The trouble is finding the appropriate hash You could look at all the hash values in git objects and use trial and error to find the one you want But there s a much easier way Git records every hash of a commit it computes in git logs The subdirectory refs contains the history of all activity on all branches while the file HEAD shows every hash value it has ever taken The latter can be used to find hashes of commits on branches that have been accidentally lopped off The reflog command provides a friendly interface to these log files Try git reflog 23 Chapter 6 Git Grandmastery and see its manpage for more information Eventually you may want to run git ge prune to recover space Be aware that doing so prevents you from recovering lost HEADs 6 11 Making History Want to migrate a project to Git If it s managed with one of the more well known systems then chanc
31. pending on firmware which takes the form of a huge binary file The history of the firmware is uninteresting to users and updates compress poorly so firmware revisions would unnecessarily blow up the size of the repository In this case the source code should be stored in a Git repository and the binary file should be kept separately To make life easier one could distribute a script that uses Git to clone the code and rsync or a Git shallow clone for the firmware 8 7 Global Counter Some centralized version control systems maintain a positive integer that increases when a new commit is accepted Git refers to changes by their hash which is better in many circumstances But some people like having this integer around Luckily it s easy to write scripts so that with every update the central Git repository increments an integer perhaps in a tag and associates it with the hash of the latest commit Every clone could maintain such a counter but this would probably be useless since only the central repository and its counter matters to everyone 8 8 Empty Subdirectories Empty subdirectories cannot be tracked Create dummy files to work around this problem The current implementation of Git rather than its design is to blame for this drawback With luck once Git gains more traction more users will clamour for this feature and it will be implemented 8 9 Initial Commit A stereotypical computer scientist counts f
32. ral Linux window managers allow you to have multiple desktops which means you can instantly view a different state of the desktop This is similar to branching in Git whereas Git cloning would be like attaching another monitor where you can open more windows Yet another example is the screen http www gnu org software screen utility This gem allows you to create destroy and switch between multiple terminal sessions in the same terminal Instead of opening new terminals clone you can use the same one if you run screen branch In fact you can do a lot more with screen but that s a topic for another text Cloning branching and merging are fast and local in Git encouraging you to use the combination that best suits you Git allows you to work exactly how you want 5 7 1 Personal Experience The equivalent of branching and cloning in centralized version control systems when it exists requires network communication This precludes working offline and means these systems need more expensive network infrastructure especially as the number of users grows Most importantly these operations will be slower to some degree usually to the point where users won t bother using them unless absolutely necessary And when they absolutely have to run slow commands productivity suffers because of an interrupted work flow I experienced these phenomena first hand Git was the first version control system I used and I grew accustomed to
33. re scm git docs user manual html As Arthur C Clarke observed any sufficiently advanced technology is indistinguishable from magic This is a great way to approach Git newbies can ignore its inner workings and view Git as a gizmo that can amaze friends and infuriate enemies with its wondrous abilities Rather than go into details we provide rough instructions for particular effects After repeated use gradually you will understand how each trick works and how to tailor the recipes for your needs Other Editions Single webpage book html barebones HTML with no CSS PDF file book pdf printer friendly 1 1 Thanks Kudos to Dustin Sallings Alberto Bertogli James Cameron Douglas Livingstone Michael Budde Richard Albury Tarmigan and Derek Mahar for suggestions and improvements If I ve left you out please tell me because I often forget to update this section 1 2 Links I once listed some references but it s too time consuming to maintain Besides anyone can simply use a search engine http www google com to find Git tutorials http www google com search q git tutorial guides http www google com search q git guide and comparisons http www google com search q git comparison with Subversion http www google com search q git subversion Mercurial http www google com search q git mercurial and other version control systems Free Git hosting http repo or cz hosts free projects
34. ring the central server This initial cloning operation can be expensive especially if there s a long history but it pays off in the long run One immediate benefit is that when an old save is desired for any reason communication with the central server is unnecessary 2 3 1 A Silly Superstition A popular misconception is that distributed systems are ill suited for projects requiring an official central repository Nothing could be further from the truth Photographing someone does not cause their soul to be stolen Similarly cloning the master repository does not diminish its importance A good first approximation is that anything a centralized version control system can do a well designed distributed system can do better Network resources are simply costlier than local resources While we shall later see there are drawbacks to a distributed approach one is less likely to make erroneous comparisons with this rule of thumb Chapter 2 Introduction A small project may only need a fraction of the features offered by such a system But would you use Roman numerals when calculating with small numbers Moreover your project may grow beyond your original expectations Using Git from the outset is like carrying a Swiss army knife even though you mostly use it to open bottles On the day you desperately need a screwdriver you ll be glad you have more than a plain bottle opener 2 4 Merge Conflicts An interesting problem aris
35. rom 0 rather than 1 Unfortunately with respect to commits git does not adhere to this convention Many commands are unfriendly before the initial commit Additionally some corner cases must be handled specially such as rebasing a branch with a different initial commit Git would benefit from defining the zero commit as soon as a repository is constructed HEAD would be set to the string consisting of 20 zero bytes This special commit represents an empty tree with no parent at some time predating all Git repositories 32 Chapter 8 Git Shortcomings Then running git log for example would inform the user that no commits have been made yet instead of exiting with a fatal error Similarly for other tools Every initial commit is implicitly a descendant of this zero commit For example rebasing an unrelated branch would cause the whole branch to be grafted on to the target Currently all but the initial commit is applied resulting in a merge conflict One workaround is to use git checkout followed by git commit C on the initial commit then rebase the rest There are worse cases unfortunately If several branches with different initial commits are merged together then rebasing the result requires substantial manual intervention 33
36. splay a spreadsheet or something So if the boss walked in the office while you were playing the game you could quickly hide this fact In some directory echo I m smarter than my boss gt myfile txt git init git add git commit m Initial commit min wn We have created a Git repository that tracks one text file containing a certain message Now type git checkout b boss nothing seems to change after this echo My boss is smarter than me gt myfile txt 14 Chapter 5 Branch Wizardry git commit a m Another commit It looks like we ve just overwritten our file and committed it But it s an illusion Type git checkout master switch to original version of the file and hey presto The text file is restored And if the boss decides to snoop around this directory type git checkout boss switch to version suitable for boss eyes You can switch between the two versions of the file as much as you like and commit to each independently 5 2 Dirty Work Say you re working on some feature and for some reason you need to go back to an old version and temporarily put in a few prints statements to see how something works Then git commit a git checkout SHA1_HASH Now you can add ugly temporary code all over the place You can even commit these changes When you re done git checkout master to return to your original work Observe that any uncommitted changes are carried over
37. the bare repository or running Git within the directory itself with the bare option 7 6 Git s Origins This Linux Kernel Mailing List post http lkml org Ikm1 2005 4 6 121 describes the chain of events that led to Git The entire thread is a fascinating archaeological site for Git historians 29 Chapter 8 Git Shortcomings There are some Git issues I ve swept under the carpet Some can be handled easily with scripts and hooks some require reorganizing or redefining the project and for the few remaining annoyances one will just have to wait Or better yet pitch in and help I ve been playing around with some version control system ideas and wrote an experimental system based on Git blynn gg which addresses some of these problems 8 1 Microsoft Windows Git on Microsoft Windows can be cumbersome e Cygwin http cygwin com a Linux like environment for Windows contains a Windows port of Git http cygwin com packages git Git on MSys http code google com p msysgit is an alternative requiring minimal runtime support though a few of the commands need some work 8 2 Unrelated Files If your project is very large and contains many unrelated files that are constantly being changed Git may be disadvantaged more than other systems because single files are not tracked Git tracks changes to the whole project which is usually beneficial A solution is to break up your project into pie
38. y in your local Git clone is a messy jumble of your changes and the official changes You d prefer to see all your changes in one contiguous section and after all the official changes This is a job for git rebase as described above In many cases you can use the onto flag and avoid interaction Also see the manpage for other amazing uses of this command which really deserves a chapter of its own You can split commits You can even rearrange branches of a tree 6 8 My Commit Is Too Big Have you neglected to commit for too long Been coding furiously and forgotten about source control until now Made a series of unrelated changes because that s your style No worries use git add i or git commit i to interactively choose which edits should belong to the next commit 22 Chapter 6 Git Grandmastery 6 9 Don t Lose Your HEAD The HEAD tag is like a cursor that normally points at the latest commit advancing with each new commit Some Git commands let you move it For example git reset HEAD 3 will move the HEAD three commits backwards in time Thus all Git commands now act as if you hadn t made those last three commits while your files remain in the present See the git reset man page for some applications But how can you go back to the future The past commits do not know anything of the future If you have the SHA1 of the original HEAD then git reset SHA1 But suppose you never took it down D
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