Git Magic
Contents
1. git bundle create newbundle HEAD lastbundle 6 5 Patches The Global Currency Patches are text representations of your changes that can be easily understood by computers and humans alike This gives them universal appeal You can email a patch to developers no matter what version control system they re using As long as your audience can read their email they can see your edits Similarly on your side all you require is an email account there s no need to setup an online Git repository Recall from the first chapter 26 Chapter 6 Git Grandmastery git diff COMMIT outputs a patch which can be pasted into an email for discussion In a Git repository type E git apply lt FILI to apply the patch In more formal settings when author names and perhaps signatures should be recorded generate the corresponding patches past a certain point by typing git format patch START_COMMIT The resulting files can be given to git send email or sent by hand You can also specify a range of commits git format patch START_COMMIT END_COMMIT On the receving end save an email to a file then type Gl git am lt FILI This applies the incoming patch and also creates a commit including information such as the author With a browser email client you may need to click a button to see the email in its raw original form before saving the patch to a file There are slight differences for mbox ba2. 38 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 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 pieces 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 mark a file in some way 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 marked files need be examined 2 One can discover who else is working on the file by asking the centr
3. Chapter 7 Secrets Revealed which pretty prints the given object 7 7 2 Trees But where are the filenames They must be stored somewhere at some stage Git gets around to the filenames during a commit git commit find git objects type f You should now see 3 objects This time I cannot tell you what the 2 new files are as it partly depends on the filename you picked We ll proceed assuming you chose rose If you didn t you can rewrite history to make it look like you did git filter branch tree filter mv YOUR_FILENAME rose find git objects type f Now you should see the file git objects 05 b217bb859794d08bb9e4 7 04cbda4b207 fbe9 because this is the SHA1 hash of its contents tree SP 32 NUL 100644 rose NUL 0Oxaa823728ea7d592acc69 b36875a482cdf3fd5c8d Check this file does indeed contain the above by typing echo 05b217bb859794d08bb9e4F7 04chda4b207fbe9 git cat file batch With zpipe it s easy to verify the hash zpipe d lt git objects 05 b217bb859794d08bb9e4fF7 04chda4b207fbe9 shalsum Hash verification is trickier via cat file because its output contains more than the raw uncompressed object file This file is a tree object a list of tuples consisting of a file type a filename and a hash In our example the file type is 100644 which means rose is a normal file and the hash is the blob object that contains the contents of rose Other possible file
4. hash of a commit and store it in the database after we already have stored all relevant trees blobs and parent commits The object database is immune to unexpected interruptions such as power outages We defeat even the most devious adversaries Suppose somebody attempts to stealthily modify the contents of a file in an ancient version of a project To keep the object database looking healthy they must also change the hash of the corresponding blob object since it s now a different string of bytes This means they ll have to change the hash of any tree object referencing the file and in turn change the hash of all commit objects involving such a tree in addition to the hashes of all the descendants of these commits This implies the hash of the official head differs to that of the bad repository By following the trail of mismatching hashes we can pinpoint the mutilated file as well as the commit where it was first corrupted In short so long as the 20 bytes representing the last commit are safe it s impossible to tamper with a Git repository What about Git s famous features Branching Merging Tags Mere details The current head is kept in the file git HEAD which contains a hash of a commit object The hash gets updated during a commit as well as many other commands Branches are almost the same they are files in git refs heads Tags too they live in git refs tags but they are updated by a different set of commands
5. maintenance http csdcf stanford edu status 1 Thanks Kudos to Dustin Sallings Alberto Bertogli James Cameron Douglas Livingstone Michael Budde Richard Albury Tarmigan Derek Mahar and Frode Aannevik for suggestions and improvements Thanks to Daniel Baumann for creating and maintaining the Debian package Thanks also to JunJie Meng and JiangWei for the Chinese translation If I ve left you out please tell me because I often forget to update this section My gratitute goes to many others for your support and praise I wish this were a real physical book so I could quote your generous words on the cover to promote it In seriousness I greatly appreciate each message Reading one always brightens my mood 2 License This guide is released under the GNU General Public License version 3 http www gnu org licenses gpl 3 0 html Naturally the source is kept in a Git repository and can be Preface obtained by typing git clone git repo or cz gitmagic git Creates gitmagic directory See below for other mirrors 3 Free Git hosting http repo or cz hosts free projects 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 vi Chapter 1 Introduction Pll
6. Lessons of History See git help filter branch which discusses this example and gives a faster method In general filter branch lets you alter large sections of history with a single command Afterwards the git refs original directory describes the state of affairs before the operation Check the filter branch command did what you wanted then delete this directory if you wish to run more filter branch commands Lastly replace clones of your project with your revised version if you want to interact with them later 5 5 Making History Want to migrate a project to Git If it s managed with one of the more well known systems then chances are someone has already written a script to export the whole history to Git Otherwise look up git fast import which reads text input in a specific format to create Git history from scratch Typically a script using this command is hastily cobbled together and run once 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 EOT commit refs heads master committer Bob lt bob example com gt Tue 14 Mar 2000 01 59 26 0800 data lt lt EOT Repl
7. but not all of our changes into the index and then permanently record this carefully rigged snapshot 6 8 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 back 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 help page for some applications But how can you go back to the future The past commits know nothing of the future If you have the SHA1 of the original HEAD then git reset SHA1 But suppose you never took it down Don 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 9 HEAD hunting Perhaps ORIG_HEAD isn t enough Perhaps you ve just realized you made a monumental mistake and you need to go back to an ancient commit in a long forgotten branch By default Git keeps a commit for at least two weeks even if you ordered Git to destroy the branch containing it 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 29 Chapter 6 Git Grandmastery Git records every hash of a commit it computes in git logs The subdirectory
8. creating new branches for your own edits The d and m options allow you to delete and move rename branches See git help branch 4 7 Temporary Branches After a while you may realize you are creating short lived branches frequently for similar reasons every other branch merely serves to save the current state so you can briefly hop back to an older state to fix a high priority bug or something It s analogous to changing the TV channel temporarily to see what else is on But instead of pushing a couple of buttons you have to create check out and delete temporary branches and commits Luckily Git has a shorcut that is as convenient as a TV remote control git stash This saves the current state in a temporary location a stash and restores the previous state Your working directory appears exactly as it was before you started editing and you can fix bugs pull in upstream changes and so on When you want to go back to the stashed state type git stash apply You may need to resolve some conflicts You can have multiple stashes and manipulate them in various ways See git help stash As you may have guessed Git maintains branches behind the scenes to perform this magic trick 4 8 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 worki
9. einen Covent 41 8 8 Empty SUDCIEECtOLIES camita ri tias 41 SO Initial Comida cgi ise 41 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 software 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 e Chinese translation http docs google com View id dfwthj68_675gz3bw8kj by JunJie Meng and JiangWei Single webpage book html barebones HTML with no CSS PDF file book pdf printer friendly e Debian gitmagic package http packages debian org search searchon names amp key words gitmagic get a fast and local copy of this site Ubuntu package Jaunty Jackalope http packages ubuntu com jaunty gitmagic also available Handy when this server is offline for
10. 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 25 Chapter 6 Git Grandmastery 6 4 Git Over Anything Want to synchronize repositories without servers or even a network connection Need to improvise during an emergency We ve seen git fast export and git fast import can convert repositories to a single file and back We could shuttle such files back and forth to transport git repositories over any medium but a more efficient tool is git bundle The sender creates a bundle git bundle create somefile HEAD then transports the bundle somefile to the other party somehow email thumb drive floppy disk an xxd printout and an OCR machine reading bits over the phone smoke signals etc The receiver retrieves commits from the bundle by typing git pull somefile The receiver can even do this from an empty repository Despite its size somefile contains the entire original git repository In larger projects eliminate waste by bundling only changes the other repository lacks git bundle create somefile HEAD COMMON_SHA1 If done frequently one could easily forget which commit was last sent The help page suggests using tags to solve this Namely after you send a bundle type git tag f lastbundle HEAD and create new refresher bundles with
11. types are executables symlinks or directories In the last case the hash points to a tree object If you ran filter branch you ll have old objects you no longer need Although they will be jettisoned automatically once the grace period expires we ll delete them now to make our toy example easier to follow rm r git refs original git reflog expir expire now all git prune 36 Chapter 7 Secrets Revealed For real projects you should typically avoid commands like this as you are destroying backups If you want a clean repository it is usually best to make a fresh clone Also take care when directly manipulating git what if a Git command is running at the same time or a sudden power outage occurs In general refs should be deleted with git update ref d though usually it s safe to remove refs original by hand 7 7 3 Commits We ve explained 2 of the 3 objects The third is a commit object Its contents depend on the commit message as well as the date and time it was created To match what we have here we ll have to tweak it a little git commit amend m Shakespear Change the commit message git filter branch env filter export GIT_AUTHOR_DATE Fri 13 Feb 2009 15 31 30 0800 GIT_AUTHOR_NAME Alice GIT_AUTHOR_EMATL alice example com GIT_COMMITTER_DATE Fri 13 Feb 2009 15 31 30 0800 GIT_COMMI ER_NAME Bob GIT_COMMITTER_EMAIL bobltexample com Rig timestamps and authors find g
12. 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 1 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 an 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 1 2 Version Control When editing you can Save As a different file or copy the file somewhere first before saving if you want to savour old versions You can 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
13. you chose It s because the SHA1 hash of blob SP 6 NUL Sweet LF is aa823728 ea7d592acc69b36875a482cdf3fd5c8d where SP is a space NUL is a zero byte and LF is a linefeed You can verify this by typing echo blob 6 001 sweet tr 001 000 shalsum This is written with the bash shell in mind other shells may be able to handle NUL on the command line obviating the need for the tr workaround Git is content addressable files are not stored according to their filename but rather by the hash of the data they contain in a file we call a blob object We can think of the hash as a unique ID for a file s contents so in a sense we are addressing files by their content The initial blob 6 is a just a header denoting the type of the object and the length of its contents in bytes to simplify internal bookkeeping This is how I knew what you would see The file s name is irrelevant only the data inside is used to construct the blob object You may be wondering what happens to identical files Try adding copies of your file with any filenames whatsoever The contents of git objects stay the same no matter how many copies you add Git only stores the data once By the way the files within git objects are compressed with zlib so you should not stare at them directly Filter them through zpipe d http www zlib net zpipe c or type git cat file p aa823728ea7d592acc69b36875a482cd 3fd5c8d 35
14. 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 from 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 a
15. Git Magic Ben Lynn Git Magic by Ben Lynn Table of Contents Preface socrii iiir eas AAAA AAAA AASA v A O NOS v PAATE NS EE E AAEE E race Shas wuts Petco A A E S v Ie Free Git hostina ereen a a E A E ew E E EAE esd au ee vi 1 INtroduction sssessssecssssssssersessesessesessesessssessssesessesesessesessesessessssassssesessesesscsesessesessessesessssesssseseseesesees 1 IR AAA nee Re aE i ee 1 1 2 Version Controlar iria ii de aia Earle caes iris 1 1 35 Distributed Control umi hehe Gee IA ee one een ee RT E ee 1 13 A Silly Superstition cn 2 14 Meroe Cond Eonia 3 2 Basic Ticks A E E oso cnrsoonss 4 DAMAS VIO Stalen nanan aii aes no a Se ace 4 2 1 1 Add Delete Rena it ies 4 2 2 Advanced Undo Redo eco TA rated i 4 222 REVES tie aiii ii 6 2 3 Downloading Files vs sisccesceiet cevapectes dessa sue encase ouenaubed orio uNa Eao bes neuee EESE EE KEE EE Ee ES a 6 2 Az The Bleeding Edge toos tica iia il 6 2 5 Instant Publishing siesena isos did led AE E d acid 6 2 6 What Have DO Ains E ds 7 USA O A RR 8 3 Cloning Around sesira orsin e Eia a OEE EEEE E NOAE EEES 9 SAL SYNC Compute iS en e aa r rees Eea uE E a E E EEE E E E EEE EE EVE KEE E 9 3 2 Classic Sour e Cool a a aaa ia aa iaiia 9 3 3 ForkKing a Project iii io id 10 3 4 Ultimate Backups nsii ar a o Ran pin ri ise 10 3 3 L1ghtSpeed Moltitask o cnica 11 3 6 Guerilla Versi n Control id is 11 4 Branch Wizard ry ccocoocononnononcnnononnonociononionocoo
16. ace printf with write EOT 100644 inline hello c data lt lt EOT include lt unistd h gt 21 Chapter 5 Lessons of History int main write 1 Hello world n 14 return 0 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 The git fast export command converts any git repository to the git fast import format whose output you can study for writing exporters and also to transport git repositories in a human readable format Indeed these commands can send repositories of text files over text only channels 5 6 Where Did It All Go Wrong You ve just discovered a broken feature in your program which you know for sure was working a few months ago Argh Where did this bug come from If only you had been testing the feature as you developed It s too late for that now However provided you ve been committing often Git can pinpoint the problem git bisect start git bisect bad SHA1_OF_BAD_VERSION git bisect good SHA1_OF_GOOD_VERSION Git checks out a state halfway in between Test the feature and if it s still broken git bisect bad If not replace bad with good Git again transports you to a state halfway between the known good and bad versions narrowing down the possibilities After a few iterations thi
17. al server who has marked it for editing 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 39 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 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 i
18. 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 2 Basic Tricks 2 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 SHA1 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 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 Also to stop Git complaining always commit or reset your changes before running checkout To take the computer game analogy again e 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 a
19. e 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 2 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 Indeed in my first months with Git I never ventured beyond the material in this chapter 2 1 Saving State About to attempt something drastic Before you do take a snapshot of all files in the current directory with git init git add git commit m My first backup Now if your new edits go awry run git reset hard to go back to where you were To save the state again git commit a m Another backup 2 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
20. eckout to fail 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 uncommitted 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 31 Chapter 6 Git Grandmastery 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 these two commands defer data destruction 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 By default they will be kept for at least two weeks Clean Some git commands refuse to proceed because they re worried about clobbering untracked files If you re certain that all untracked files and directories are expendable then delete them mercilessly with git clean f d Next time that pesky command will work 32 Chapter 7 Secrets Revealed We take a peek under the hood and explain how Git performs its miracles I will skimp ove
21. ect using git clone you can upgrade to the latest version with git pull Chapter 2 Basic Tricks 2 5 Instant Publishing Suppose 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 may work on a project frequently but they only make the code available when they feel it is presentable 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 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 2 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 2 Basic Tricks Or between a par
22. ems 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 1 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 1 4 Merge Conflicts For this topic our computer game analogy becomes too thinly stretched Instead let us again consider editing a document Suppose Alice inserts a line at the beginning of a file and Bob appends one at the end of his copy 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 ar
23. ermine 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 Since stat calls are considerably faster than file reads 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 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 34 Chapter 7 Secrets Revealed 7 7 The Object Database Here s how to write a Git like system from scratch in a few hours 7 7 1 Blobs First a magic trick Pick a filename any filename In an empty directory echo sweet gt YOUR_FILENAME git init git add find git objects type f mM UY wn You ll see git objects aa 823728ea7d592acc69b36875a482cdf3fd5c8d How do I know this despite not knowing the filename
24. ewspaper would work well because it s hard for an attacker to alter every copy of a newspaper 3 5 Light Speed Multitask Say you want to work on several features in parallel Then commit your project and run 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 time you can commit and pull changes from the other clone git pull the other clone 3 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 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 11 Chapter 3 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 co
25. get files you create a clone of the entire repository In other words you practically mirror the central server Anything the main repository can do you can do 3 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 on one machine Then on the other git clone other computer path to files to create a 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 3 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 Chapter 3 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 t
26. 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 1 Introduction 1 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 Speedruns 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 el
27. he 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 3 3 Forking a Project Sick of the way a project is being run Think you could do a better job 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 3 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 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 10 Chapter 3 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 n
28. ig global alias co checkout git config global get regexp alias display current aliases alias co checkout 30 Chapter 6 Git Grandmastery 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 The contrib subdirectory is a treasure trove of tools built on basic commands which in time may be promoted to official commands On Debian and Ubuntu this directory lives at usr share doc git core contrib One popular resident is workdir git new workdir Via clever symlinking this script creates a new working directory whose history is shared with the original respository git new workdir an existing repo new directory The new directory and files within can be thought of as a clone except since the history is shared the two trees automatically stay in sync There s no need to merge push or pull 6 11 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 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 Uncommitted changes cause ch
29. ion 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 4 1 The Boss Key Ever play one of those games where at the push of a button the boss key the screen would instantly display a spreadsheet or something So if the boss walked in the office while you were playing the game you could quickly hide it away 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 13 Chapter 4 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 be
30. it objects type f You should now see git objects 49 993fe130c4b3bf24857a15d7969c396b7bc187 which is the SHA 1 hash of its contents commit 158 NUL tree 05b217bb859794d08bb9e4 7 04chda4b207fbe9 LF author Alice lt alice example com gt 1234567890 0800 LF committer Bob lt bob example com gt 1234567890 0800 LF LF Shakespeare LF As before you can run zpipe or cat file to see for yourself This is the first commit so there are no parent commits but later commits will always have at least one parent 7 7 4 Indistinguishable From Magic There s little else to say We have just exposed the secret behind Git s powers It seems too simple it looks like you could mix together a few shell scripts and add a dash of C code to cook up the above in a matter of hours In fact this accurately describes the earliest versions of Git Nonetheless apart from ingenious packing tricks to save space and ingenious indexing tricks to save time we now know how Git deftly changes a filesystem into a database perfect for version control 37 Chapter 7 Secrets Revealed For example if any file within the object database is corrupted by a disk error then its hash will no longer match alerting us to the problem By hashing hashes of other objects we maintain integrity at all levels Commits are atomic that is a commit can never only partially record changes we can only compute the
31. ith 15 Chapter 4 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 You ll 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 trick for any number of parts 4 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 4 6 Managing Branches Type git branch 16 Chapter 4 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
32. me developers strongly feel history should be immutable warts and all Others feel trees should be made presentable before they are unleashed in public Git accommodates both viewpoints Like cloning branching and merging rewriting history is simply another power Git gives you It is up to you to use it wisely 5 1 Stand Corrected Did you just commit but wish you had typed a different message Then run git commit amend to change the last message Realized you forgot to add a file Run git add to add it and then run the above command Want to include a few more edits in that last commit Then make those edits and run git commit amend a 5 2 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 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 pick a311a64 Reordered analogies in Work How You Want pick 100834f Added push target to Makefile 19 Chapter 5 Lessons of History Then e Remove commits by deleting lines e 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 If you marked a commit fo
33. ng directory Continuing this analogy Git cloning is like opening a new window Being able to do both improves the user experience On a higher level several Linux window managers allow you to have multiple desktops you can instantly switch to a different desktop This is similar to branching in Git whereas Git cloning would be like attaching another monitor to gain another desktop Yet another example is the screen http www gnu org software screen utility This gem lets you 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 17 Chapter 4 Branch Wizardry Cloning branching and merging are fast and local in Git encouraging you to use the combination that best suits you Git lets you work exactly how you want 18 Chapter 5 Lessons of History A consequence of Git s distributed nature is that history can be edited easily But if you tamper with the past take care only rewrite that part of history which you alone possess Just as nations forever argue over who committed what atrocity if someone else has a clone whose version of history differs to yours you will have trouble reconciling when your trees interact Of course if you control all the other trees too then there is no problem since you can overwrite them So
34. nonconononnonoccononaononiononiononoononoononocnonocaonocoonocconosononcononosconosess 13 Ad THE BOSS Kevin io E 13 4 2 Dirty Wok dit 14 4 3 Quick Fires heb Alden eines Bete ne eae ee A ate 14 4 4 Uninterrupted Work Ho Wu 15 45 CIAO AN 16 4 0 Managing A ON 16 Ashe A asta sessuee seas ivece ses E EEEE E EE E E 17 438 Work How YOu Wat li abi 17 S Lessons O A vobsacsaa enssuseuasseascesovanstsuasns ucooasscassaasnsisueses ts 19 Dilo Stand Corrected soren onne rreren a aen title a on ea E 19 93254 And Then Some r e a E E A mea tl eked Aon ERE ea ORA 19 5 3 Local Changes Mastin iii 20 5 4 Rewrtins History aen eee ee eee E E ES ERTAIN 20 3 3 Makanig HIStory o eoero reee depts irte tibia 21 5 6 Where Did It All Go Wrong 0 eee cece ceseeseeeeceseeseecaecaeceacsaecesceeeeseseeecaessaesaecneceeeeseeeaseneeaes 22 3 7 Who Made It All Go Wrong iii iii lid 23 iii 5 8 Personal Experience 23 6 Git GrandMastery ccscsssrecssreessrsesersesererserersessrsessssesssssessesesersssessssessssessesessesssscsesssssessesessssesseses 25 Oy 1s Source E AAA RN 25 6 2 Changelog Generation iii it 25 6 3 Git Over SSH HTTP oo 25 GA Git Over A sees ieee iker EEEE rE r T EEEE T SEEE Ep REESE ERE Epe 26 6 5 Patches The Global Currency seisis cececeseeseeesceseeseeceecaecaceaecsceeeeeeeaecaessaesaecneceseeseeeaseneeags 26 6 6 Commit What Changed nsien ieres sass ascend cst seeebesscodsdeveas asaris Eiro rie SES ETa ESOS tess 27 6 7 My C
35. ntains the files with your changes Run whatever commands of the other version control system are needed to upload them to the central repository The git svn command automates the above for Subversion repositories and can also be used to export a Git project to a Subversion repository http google opensource blogspot com 2008 05 export git project to google code html 12 Chapter 4 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 in the released version without warning The deadline for a certain feature is moved closer A developer whose help you need for a key section of the project is about to leave In all cases you must abruptly drop what you are doing and focus on a completely different task Interrupting your train of thought can be detrimental to your productivity and the more cumbersome 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 Solut
36. ommit Is Too Big heis aeeoeiaeo e epee Ers aea Eoi EEEE cidade EEEE AE E E E EO SENES 28 6 7 1 Staged Changes picni eo i eer sere Ee E A EE ES EEr E T 28 6 8 Don t Eose Your HEAD Ad 29 6 9 m i SUDI ie DaI i a Fe EEE EE EE TTT 29 6 10 Burlding On linia raras E EE E ohare siete ls 30 6 11 Darme SNS niron nen e E E TA A abra 31 7 Secrets Revealed sccsssssssrssssrssssrsessrsesererserersessssessssessssssessesesessesessessssesessessssessssesesscseseseesessesenseses 33 TA IOVISIDI ni ASIA 33 V2 o O 33 TI Ate Sen Ce lid dt ie 34 TA Inde ius cantina 34 1 3 Bare Reposltorles scrisse eir eaae pea oree ee ere r E a EE EnS e SE ar Eeri 34 TO GIES OTIS A aa 34 1 1 The Object Database ireira errare eene aasre Soa sssvasbsccnhsseuscogeds aoee EE TOE oE SEER iesise sss oiT rsss 34 RABO ii ds Easa 35 A E CEO Ta IE 36 ESA A RN 37 7 7 4 Indistinguishable From MaglC cooconcnnonnocnonnnonnnnnnoncnnconnonnconnonncononnn cnn non non nc onc cnn cnnonnnrnnnnnos 37 8 Git Shortcomings csc secisesaicsecisesecesencs ed ccastedsccecssesussscsceseeesunie dio deselcedeaucgtesusdaesaseseeseadsnseedesedseboscaccasteceed 39 8 1 Micros Olt Wan dOws escocia dico don ecbe sean E A a EAEE OA ERAEN ERRE AE 39 8 2 Unrelated Fleitas Soteoceseateecs 39 8 3 Who s Editing What lina aed natin aide dined ala eaten 39 S Ac ON S EEES 39 SI Initial Cl Send cdo ee ta tapas baa Te even ada ee a a i eee 40 8 6 Volatile Projets conil ls 40 8 7 Global Comte VIA ceca
37. own changes or open a file for editing Centralized systems preclude working offline and need more expensive network infrastructure especially as the number of developers grows Most importantly all operations are slower to some degree usually to the point where users shun advanced commands unless absolutely necessary In extreme cases this is true of even the most basic commands When users must 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 I quickly grew accustomed to it taking many features for granted I simply assumed other systems were similar choosing a version control system ought to be no different from choosing a text editor or web browser I was shocked when later forced to use a centralized system My often flaky internet connection matters little with Git but makes development unbearable when it needs to be as reliable as local disk Additionally I found myself conditioned to avoid certain commands because of the latencies involved which ultimately prevented me from following my desired work flow 23 Chapter 5 Lessons of History 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 communication to complete I d do something else to pass the time such as check email or write documentation By the time I returned to
38. r 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 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 though
39. r editing then run git commit amend Otherwise run git rebase continue So commit early and commit often you can easily tidy up later with rebase 5 3 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 history 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 git help rebase for detailed examples of this amazing command You can split commits You can even rearrange branches of a tree 5 4 Rewriting History Occasionally you need the source control equivalent of airbrushing people out of official photos erasing them from history in a Stalinesque fashion For example suppose we intend to release a project but it involves a file that should be kept private for some reason Perhaps I left my credit card number in a text file and accidentally added it to the project Deleting the file is insufficient for the file can be accessed from older commits We must remove the file from all commits git filter branch tree filter rm top secret file HEAD 20 Chapter 5
40. 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 Instead of cutting and pasting hashes from the reflog try git checkout 10 minutes ago Or checkout the 5th last visited commit via git checkout 5 See the Specifying Revisions section of git help rev parse for more You may wish to configure a longer grace period for doomed commits For example git config gc pruneexpire 30 days means a deleted commit will only be permanently lost once 30 days have passed and git ge is run You may also wish to disable automatic invocations of git ge git conifg gc auto 0 in which case commits will only be deleted when you run git ge manually 6 10 Building On Git In true UNIX fashion Git s design allows it to be easily used as a low level component of other programs such as GUI and web interfaces alternative command line interfaces patch managements tools importing and conversion tools and so on In fact some Git commands are themselves scripts standing on the shoulders of giants With a little tinkering you can customize Git to suit your preferences One easy trick is to use built in Git aliases to shorten your most frequently used commands git conf
41. round 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 git checkout SHA1_HASH some file another fil Take care as this form of checkout can silently overwrite files To prevent accidents commit before running any checkout command especially when first learning Git In general whenever you feel unsure of the outcome of an operation Git command or not first run git commit a Chapter 2 Basic Tricks Don t like cutting and pasting hashes Then use git checkout My first b to jump to the commit that starts with a given message You can also ask for the 5th last saved state git checkout master 5 2 2 1 Reverting In a court of law events can be stricken from the record Likewise you can pick 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 2 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 2 4 The Bleeding Edge If you ve already downloaded a copy of a proj
42. s binary search will lead you to the commit that caused the trouble Once you ve finished your investigation return to your original state by typing git bisect reset Instead of testing every change by hand automate the search by running 22 Chapter 5 Lessons of History git bisect run COMMAND Git uses the return value of the given command typically a one off script to decide whether a change is good or bad the command should exit with code 0 when good 125 when the change should be skipped and anything else between and 127 if it is bad A negative return value aborts the bisect You can do much more the help page explains how to visualize bisects examine or replay the bisect log and eliminate known innocent changes for a speedier search 5 7 Who Made It All Go Wrong Like many other version control systems Git has a blame command git blame FILI El which annotates every line in the given file showing who last changed it and when Unlike many other version control systems this operation works offline reading only from local disk 5 8 Personal Experience In a centralized version control system history modification is a difficult operation and only available to administrators Cloning branching and merging are impossible without network communication So are basic operations such as browsing history or committing a change In some systems users require network connectivity just to view their
43. s 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 40 Chapter 8 Git Shortcomings Another example is a project depending 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
44. se 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 mirroring 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 unnecessaty 1 3 1 A Silly Superstition A popular misconception is that distributed syst
45. sed email clients but if you use one of these you re probably the sort of person who can figure them out easily without reading tutorials 6 6 Commit What Changed Telling Git when you ve added deleted and renamed files is troublesome for certain projects 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 27 Chapter 6 Git Grandmastery 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 7 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 Run git add p For each edit you made Git will show you the hunk of code that was changed and ask if it should be part of the next commit Answer with y or n You have other options such as postponing the decision type to learn more Once you re satisfied type git commit to commit precisely the changes you selected the staged changes Make sure you omit the a option otherwise Git
46. t 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 As a SHA 1 hash is itself a string of bytes we can hash strings of bytes containing other hashes This simple observation is surprisingly useful look up hash chains We ll later see how Git uses it to efficiently guarantee data integrity 33 Chapter 7 Secrets Revealed Briefly Git keeps your data in the git objects subdirectory where instead of normal filenames you ll find only IDs By using IDs as filenames as well as a few lockfiles and timestamping tricks Git transforms any humble filesystem into an efficient and robust database 7 3 Intelligence 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 det
47. t some time predating all Git repositories 41 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 42
48. the original task the command had finished long ago and I would waste more time trying to remember what I was doing Humans are bad at context switching There was also an interesting tragedy of the commons effect anticipating network congestion individuals would consume more bandwidth than necessary on various operations in an attempt to reduce future delays The combined efforts intensified congestion encouraging individuals to consume even more bandwidth next time to avoid even longer delays 24 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 Changelog by typing git log gt ChangeLog 6 3 Git Over SSH HTTP Suppose you have ssh access to a web server but Git is not installed Though less efficient than its native protocol Git can communicate over HTTP Download compile and install Git in your account and create a repository in your web directory GIT_DIR proj git git init In the proj git directory run git bare update
49. ticular version and 2 versions ago git diff SHA1_HASH master 2 In each case the output is a patch that can be applied with git apply Try also git whatchanged since 2 weeks ago Often Pll browse history with qgit http sourceforge net projects qgit instead due to its slick photogenic interface or tig http jonas nitro dk tig a text mode interface that works well over slow connections Alternatively install a web server run git instaweb and fire up any web browser 2 7 Exercise Let A B C D be four successive commits where B is the same as A except some files have been removed We want to add the files back at D and not at B How can we do this There are at least three solutions Assuming we are at D 1 The difference between A and B are the removed files We can create a patch representing this difference and apply it git diff BA git apply 2 Since we saved the files back at A we can retrieve them git checkout A FILES 3 We can view going from A to B as a change we want to undo git revert B Which choice is best Whichever you prefer most It is easy to get what you want with Git and often there are many ways to get it Chapter 3 Cloning Around In older version control systems checkout is the standard operation to get files You retrieve a bunch of files in the requested saved state In Git and other distributed version control systems cloning is the standard operation To
50. tween the two versions of the file as much as you like and commit to each independently 4 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 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 14 Chapter 4 Branch Wizardry 4 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 yo
51. u 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 4 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 control 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 submitting Part I involves 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 w
52. will commit all the edits What if you ve edited many files in many places Reviewing each change one by one becomes frustratingly mind numbing In this case use git add i whose interface is less straightforward but more flexible With a few keystrokes you can stage or unstage several files at a time or review and select changes in particular files only Alternatively run git commit interactive which automatically commits after you re done 6 7 1 Staged Changes So far I have deliberately avoided mentioning Git s famous index but we must now confront it to explain the above The index is a temporary staging area Git seldom shuttles data directly between your project and its history Rather Git first writes data to the index and then copies the data in the index to its final destination For example commit a is really a two step process The first step places a snapshot of the current state of every tracked file into the index The second step permanently records the snapshot now in the index 28 Chapter 6 Git Grandmastery Committing without the a option only performs the second step and only makes sense after running commands that somehow change the index such as git add Usually we can ignore the index and pretend we are reading straight from and writing straight to the history On this occasion we want finer control on what gets written to history and are forced to manipulate the index We place a snapshot of some
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