2 A tour of git: the basics
6 This document is a modified version of a document originally titled
7 "Distributed revision control with Mercurial" and originally authored
8 by Bryan O’Sullivan. The original document was obtained from
9 <http://hgbook.red-bean.com/>.
11 Copyright © 2006, 2007 Bryan O’Sullivan.
13 This material may be distributed only subject to the terms and
14 conditions set forth in version 1.0 of the Open Publication
15 License. Please refer to Appendix D for the license text.
17 As this is a modified version, the name of Bryan O'Sullivan is used
18 only to properly credit him with the original text. The appearance of
19 his name here explicitly does not assert or imply his endorsement of
20 this modified document.
22 Portions Copyright © 2007 Carl Worth.
24 Changes made by Carl include the following:
27 * Convert from HTML to markdown source syntax
28 * Eliminate all content except Chapter 2 and Appendix D
29 * Eliminate line numbers from examples
30 * Modified to describe git instead of mercurial
32 The source of this modified version can be obtained via git:
34 git clone git://cworth.org/git/hgbook-git
38 git clone http://cworth.org/git/hgbook-git
40 and can be [browsed online](http://git.cworth.org/git/hgbook-git)
42 ### 2.1 Installing git on your system
44 Prebuilt binary packages of git are available for many popular
45 operating systems. These make it easy to start using git on your
50 Because each Linux distribution has its own packaging tools, policies,
51 and rate of development, it’s difficult to give a comprehensive set of
52 instructions on how to install git binaries. The version of
53 git that you will end up with can vary depending on how active
54 the person is who maintains the package for your distribution.
56 To keep things simple, I will focus on installing git from the
57 command line under the most popular Linux distributions. Most of these
58 distributions provide graphical package managers that will let you
59 install git with a single click. The package name to look for is
60 often git, but is sometimes git-core, (due to an unfortunate name
61 with git, meaning GNU Interactive Tools).
65 apt-get install git-core
81 apt-get install git-core
85 A git-core package is available through
86 [macports](http://macports.org). Once macports is enabled, the command
93 Git has long been available as part of cygwin, and works reasonably
94 well in that environment. Some people find cygwin a particularly
95 inelegant approach to running git and would prefer a "native"
96 solution. To this end, the [msysgit
97 project](http://code.google.com/p/msysgit/) is rapidly putting
98 together a solution including various packages with full
99 installers. These include GitMe, a package to install the entire
100 development environment necessary to work on improving the msysgit
101 port of git, and WinGit, a package for installing just git itself
102 without the development environment, (still in Alpha as of September
105 ### 2.2 Getting started
107 To begin, we’ll use the “git version” command to find out whether git
108 is actually installed properly. Versions 1.5 and newer of git are much
109 more friendly to new users than versions 1.4 and older. If you aren't
110 yet running version 1.5 or newer, it's highly recommended that you
116 #### 2.2.1 Built-in help
118 Git provides a built-in help system. This is invaluable for those
119 times when you find yourself stuck trying to remember how to run a
120 command. If you are completely stuck, simply run “git help”; it will
121 print a brief list of commonly-used commands, along with a description
122 of what each does. If you ask for help on a specific command (such as
123 "git help init"), it prints more detailed information. [XXX: Does `git
124 help <foo>` work universally as a built-in or does it expect man to be
125 present and just call out to `man git-<foo>`?]
127 [XXX: The original hgbook includes the complete output of "hg
128 help init" at this point. I'm not including the corresponding
129 "git help init" output as it would be excessively long. The
130 description alone is quite reasonable, (other than a
131 not-too-helpful aside about the obsolete git-init-db command),
132 but it only comes after a full screen's worth of options
133 details. Might it make sense to have a more summarized help
134 output for "git help <foo>" than all of the documentation
135 available for git-<foo>? And perhaps alos provide a "git -v
136 help" similar to "hg -v help" for more?]
138 ### 2.3 Working with a repository
140 In git, everything happens inside a repository. The repository
141 for a project contains all of the files that “belong to” that project,
142 along with a historical record of the project’s files.
144 There’s nothing particularly magical about a repository; it is simply
145 a directory tree in your filesystem that git treats as
146 special. You can rename or delete a repository any time you like,
147 using either the command line or your file browser.
149 #### 2.3.1 Creating a copy of a remote repository
151 The "git clone" command is used to create a local copy of a remote
152 repository. This is generally the first git operation you will use
153 when beginning to work with an existing project.
155 We've assembled a simple repository that will be used in the examples
156 throughout this chapter. Go ahead and clone this repository now so
157 that you will be able to follow along:
159 $ git clone git://cworth.org/git/hello
160 Initialized empty Git repository in /tmp/hello/.git/
161 remote: Generating pack...
162 remote: Done counting 15 objects.
163 remote: Deltifying 15 objects...
164 remote: 100% (15/15) done
165 remote: Total 15 (delta 2), reused 15 (delta remote: 2)
166 Indexing 15 objects...
168 Resolving 2 deltas...
171 If for some reason you are prevented from talking on the git: port,
172 then there is also the capability to clone a repository (less
173 efficiently) over http:
175 $ git clone http://cworth.org/git/hello
176 Initialized empty Git repository in /tmp/hello/.git/
177 Getting alternates list for http://cworth.org/git/hello
178 Getting pack list for http://cworth.org/git/hello
179 Getting index for pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
180 Getting pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
181 which contains a1a0e8b392b17caf50325498df54802fe3c03710
182 walk a1a0e8b392b17caf50325498df54802fe3c03710
183 walk 72d4f10e4a27dbb09ace1503c20dbac1912ee451
184 walk 13ed136b983a9c439eddeea8a1c2076cffbb685f
185 walk 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
186 walk db7117a9dd9a6e57e8632ea5848e1101eee0fbde
188 If our clone succeeded, we should now have a local directory called
189 hello. This directory will contain some files.
193 drwxr-xr-x 3 cworth cworth 4096 2007-09-27 16:40 hello
197 These files have the same contents and history in our repository as
198 they do in the repository we cloned.
200 Every git repository is complete, self-contained, and
201 independent. It contains its own private copy of a project’s files and
202 history. A cloned repository remembers the location of the repository
203 it was cloned from, but it does not communicate with that repository,
204 or any other, unless you tell it to.
206 What this means for now is that we’re free to experiment with our
207 repository, safe in the knowledge that it’s a private “sandbox” that
208 won’t affect anyone else.
210 #### 2.3.2 Creating copy of a local repository
212 As mentioned above, a repository can be copied through normal
213 file-copying commands. But it's useful to use "git clone" even when
214 just making a local copy of a repository. Using "git clone" will be
215 much faster and will use much less space than a normal copy. In fact,
216 local clones are impressively fast. Go ahead and make a local clone
219 $ git clone hello hello-clone
220 Initialized empty Git repository in /tmp/hello-clone/.git/
223 [XXX Git says "empty" here, (presumably from the git-init
224 part), but shouldn't the command also report the succesful
225 clone which makes it non-empty? And what the heck does "0
226 blocks" mean? This is not friendly output.]
228 In fact, let's make a couple more clones at the same time. This isn't
229 just to drive home the speed and storage benefits of local clones, but
230 we'll use each of these cloned repositories in Section 2.8 when
231 discussing how to move commits between repositories:
233 $ git clone hello hello-pull
234 $ git clone hello hello-fetch
235 $ git clone hello hello-remote
237 #### 2.3.2 What’s in a repository?
239 When we take a more detailed look inside a repository, we can see that
240 it contains a directory named .git. This is where git keeps all
241 of its metadata for the repository.
245 . .. .git hello.c Makefile
247 The contents of the .git directory and its subdirectories are private
248 to git. Every other file and directory in the repository is
249 yours to do with as you please.
251 To introduce a little terminology, the .git directory is the “real”
252 repository, and all of the files and directories that coexist with it
253 are said to live in the working directory. An easy way to remember the
254 distinction is that the repository contains the history of your
255 project, while the working directory contains a snapshot of your
256 project at a particular point in history.
258 ### 2.4 A tour through history
260 One of the first things we might want to do with a new, unfamiliar
261 repository is understand its history. The “git log” command gives us a
265 commit a1a0e8b392b17caf50325498df54802fe3c03710
266 Author: Bryan O'Sullivan <bos@serpentine.com>
267 Date: Tue Sep 6 15:43:07 2005 -0700
271 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
272 Author: Bryan O'Sullivan <bos@serpentine.com>
273 Date: Tue Sep 6 13:15:58 2005 -0700
275 Get make to generate the final binary from a .o file.
277 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
278 Author: Bryan O'Sullivan <bos@serpentine.com>
279 Date: Tue Sep 6 13:15:43 2005 -0700
281 Introduce a typo into hello.c.
283 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
284 Author: Bryan O'Sullivan <mpm@selenic.com>
285 Date: Fri Aug 26 01:21:28 2005 -0700
289 commit db7117a9dd9a6e57e8632ea5848e1101eee0fbde
290 Author: Bryan O'Sullivan <mpm@selenic.com>
291 Date: Fri Aug 26 01:20:50 2005 -0700
293 Create a standard "hello, world" program
295 This command prints a record of output for each change to the project
296 that was recorded. In git terminology, we call each of these recorded
299 The default fields in a record of output from “git log” are as follows.
301 * commit This field consists of a string of 40 hexadecimal characters.
302 This is a unique identifier for referring to particular commits.
303 * Author The identity of the person who authored the commit. This
304 field consist of two sub-fields for the user's name and email
305 address, (or at least an email-like idenitifer). Note that git
306 also stores a separate "Committer" field for the person who
307 commited the change, (since often an author will email a change to
308 a maintainer that commits it). See below for how to instruct "git
309 log" to display it as well.
310 * Date The date and time on which the commit was authored, (again
311 stored separately from the date the change was committed).
312 timezone in which it was created. (The date and time are displayed
313 in the timezone of the person who created the commit.)
314 * commit message The text message that the creator of the commit
315 entered to describe the commit, (generally a one-line summary
316 followed by more supporting text).
318 The output of the "git log" command can be made more or less verbose
319 by means of the --pretty option. For example, with "git log
320 --pretty=short" the commit identifier will be omitted and only the
321 first line of each commit message will be show. And with "git log
322 --pretty=fuller", (the name 'fuller' is in contrast to the default
323 --pretty=full), the committer name and dates will be printed in
324 addition to the author name and dates.
326 #### 2.4.1 Commits, revisions, and talking to other people
328 As English is a notoriously sloppy language, and computer science has
329 a hallowed history of terminological confusion (why use one term when
330 four will do?), revision control has a variety of words and phrases
331 that mean the same thing. If you are talking about git history
332 with other people, you will find that what we have called a “commit”
333 is often called a "revision". In other systems, a similar notion
334 is referred to as a "changeset". You might even see abbreviations of
335 these terms such as "rev", "change", or even "cset".
337 While it may not matter much what word you use to refer to the concept
338 of “a commit”, it's important to know how to name “a specific
339 commit”. We have already seen one means of referring to a particular
340 commit, the 40-character hexadecimal string shown by "git log". These
341 commit identifiers are powerful because they are permanent, unique
342 identifiers that always identify the same commit in any copy of a
343 repository. If two users are examining a working directory associated
344 with the same commit identifier, then those two users have precisely
345 the same contents in all files, and exactly the same history leading
348 So there are places where it is often important to archive the
349 complete commit identifier, (perhaps in bug-tracking systems to
350 indicate a specific commit that fixes a bug, for example). But often,
351 in more casual settings, it's more convenient to use abbreviated
352 commit identifiers. Git accept any unique prefix of a commit
353 identifier, (and for reasonably-sized project the first 8 or 10
354 characters are almost always unique).
356 And unlike the permanent commit identifiers, git also provides
357 transient means of identifying commits. In fact, in day-to-day use of
358 git, you will probably use these names more than commit
359 identifiers. One example is branch names, (such as the default
360 "master" branch in any git repository), or any project-specific branch
361 names such as "stable", "experimental", or "crazy-insane-changes". Git
362 also provides a special name "HEAD" which always refers to the current
365 #### 2.4.2 Naming related commits
367 Git offers simple ways to name revisions that are related to
368 particular revisions in the history. One syntax is the ~ suffix which
369 refers to the parent of a commit, or if followed by a number, to the
370 Nth parent. For example, since "HEAD" refers to the most recent commit
371 in the current branch, "HEAD~", refers to the previous commit, and
372 "HEAD~2" refers to two commits back in the history.
374 Another useful syntax is .. which can be used to specify a range of
375 commits. So "origin..master" specifies everything that has been
376 committed to master since it diverged from origin.
378 #### 2.4.3 Viewing specific revisions
380 You can use "git log" to explore the range syntax just introduced. For
381 example, to see a list of the most recent 3 revisions you can use
382 "HEAD~3..", (the destination of the range is implicitly HEAD in this
386 commit a1a0e8b392b17caf50325498df54802fe3c03710
387 Author: Bryan O'Sullivan <bos@serpentine.com>
388 Date: Tue Sep 6 15:43:07 2005 -0700
392 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
393 Author: Bryan O'Sullivan <bos@serpentine.com>
394 Date: Tue Sep 6 13:15:58 2005 -0700
396 Get make to generate the final binary from a .o file.
398 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
399 Author: Bryan O'Sullivan <bos@serpentine.com>
400 Date: Tue Sep 6 13:15:43 2005 -0700
402 Introduce a typo into hello.c.
404 #### 2.4.4 Other log filters
406 Besides filtering by commit identifiers, git allows you to easily
407 filter the log output according to which files (or directories) are
408 modified by listing them after "--" which is necessary to distinguish
409 commit names from file names:
411 $ git log -- Makefile
412 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
413 Author: Bryan O'Sullivan <bos@serpentine.com>
414 Date: Tue Sep 6 13:15:58 2005 -0700
416 Get make to generate the final binary from a .o file.
418 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
419 Author: Bryan O'Sullivan <mpm@selenic.com>
420 Date: Fri Aug 26 01:21:28 2005 -0700
424 And "git log" can also filter based on the dates at which commits were
427 $ git log --since="2 weeks ago" --until="yesterday"
429 [XXX: By default, "git log" displays author dates as "Date"
430 but then uses commit dates when given a --since option. That
431 seems like broken defaults to me. Why the inconsistency?]
433 Another useful option is -n or --max-count which, unsurprisingly,
434 limits the maximum number of commits to be displayed.
436 #### 2.4.5 More detailed information
438 While the default information printed by “git log” is useful if you
439 already know what you’re looking for, you may need to see more details
440 of the change, such as the "diffstat" information with --stat:
442 $ git log --stat --max-count=3
443 commit a1a0e8b392b17caf50325498df54802fe3c03710
444 Author: Bryan O'Sullivan <bos@serpentine.com>
445 Date: Tue Sep 6 15:43:07 2005 -0700
450 1 files changed, 2 insertions(+), 6 deletions(-)
452 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
453 Author: Bryan O'Sullivan <bos@serpentine.com>
454 Date: Tue Sep 6 13:15:58 2005 -0700
456 Get make to generate the final binary from a .o file.
459 1 files changed, 2 insertions(+), 0 deletions(-)
461 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
462 Author: Bryan O'Sullivan <bos@serpentine.com>
463 Date: Tue Sep 6 13:15:43 2005 -0700
465 Introduce a typo into hello.c.
468 1 files changed, 1 insertions(+), 1 deletions(-)
470 Or perhaps you'd like to see the actual patch content of each change,
471 which you can get with -p. That commit with the word typo in its name
472 looks suspicous, so let's tak a closer look. Remember that we can name
473 it as master~3, HEAD~3, or any prefix of its commit identifier, (such
476 $ git log -p -n 1 13ed136b
477 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
478 Author: Bryan O'Sullivan <bos@serpentine.com>
479 Date: Tue Sep 6 13:15:43 2005 -0700
481 Introduce a typo into hello.c.
483 diff --git a/hello.c b/hello.c
484 index ed55ec0..80b260c 100644
489 int main(int argc, char **argv)
491 - printf("hello, world!\n");
492 + printf("hello, world!\");
496 Of course, wanting to see all this information for a single commit is
497 such a common operation that it's given its own name in git, "git
498 show". So "git show 13ed136b" is a much easier way to get exactly the
502 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
503 Author: Bryan O'Sullivan <bos@serpentine.com>
504 Date: Tue Sep 6 13:15:43 2005 -0700
506 Introduce a typo into hello.c.
508 diff --git a/hello.c b/hello.c
509 index ed55ec0..80b260c 100644
514 int main(int argc, char **argv)
516 - printf("hello, world!\n");
517 + printf("hello, world!\");
521 ### 2.5 All about command options
523 Let’s take a brief break from exploring git commands to discuss
524 a pattern in the way that they work; you may find this useful to keep
525 in mind as we continue our tour.
527 Git has a consistent and straightforward approach to dealing
528 with the options that you can pass to commands. It follows the
529 conventions for options that are common to modern Linux and Unix
532 * Most options have long names. For example, as we’ve already seen,
533 the “git log" command accepts a `--max-count=<number>` option.
534 * Some options have short, single-character names. Often these are
535 aliases for long commands, (such as `-n <number>` instead of
536 `--max-count=<number>`), but sometimes the option exists in
537 short-form with no long-form equivalent, (such as `-p`). [XXX: It
538 wouldn't hurt to fix this by adding `--patch`, etc. right?]
539 * Long options start with two dashes (e.g. `--max-count`), while short
540 options start with one (e.g. `-n`).
542 * Option naming and usage is consistent across commands. For
543 example, every command that lets you specify a commit identifier
544 or range will accept the same expressions, (HEAD~3,
545 origin..master, 72d4f10e, etc), while any command that can be
546 limited by paths will accept the same expressions ("-- doc/
549 Many commands that print output of some kind can be made more quiet by
550 passing the -q or --quiet options.
552 ### 2.6 Making and reviewing changes
554 Now that we have a grasp of viewing history in git, let’s take a look
555 at making some changes and examining them. You should be working
556 within the "hello" directory that we originally cloned.
558 In the repository, we have a file hello.c that contains the classic
559 “hello, world” program. But this program currently has a syntax error
560 and won't compile, (there's a missing 'n' between the '\' and the '"'
561 on the line containing printf). Let's fix that now. You can use a text
562 editor to correct the file, or you can copy-and-paste the sed command
565 $ sed -i '/printf/s/\\"/\\n"/' hello.c
567 After you've made that change, the “git status” command will tell you
568 what git knows about the files in the repository.
574 # Changed but not updated:
575 # (use "git add <file>..." to update what will be committed)
579 no changes added to commit (use "git add" and/or "git commit -a")
581 First "git status" tells us that the current branch is "master". This
582 means that the master branch is what will be updated when we create a
585 Note: In git a branch is a very simple notion---it's simply a name
586 that points to a particular commit, (literally nothing more than a
587 pointer---look at the contents of .git/refs/heads/master if you're
588 curious). The fact that a branch is so light is what makes the
589 creation of new branches an instantaneous operation in git. Together
590 with the ease of merging, git makes branches a joy to work with. But
591 we'll delay talk of branching and merging to a future chapter.
593 Next “git status” prints a line with "modified" for each modified
594 file---in this case just hello.c. The “git status” command will not
595 print any output for files that have not been modified.
597 Notice that we didn’t need to inform git that we were going to modify
598 the file before we started, or that we had modified the file after we
599 were done; it was able to figure this out itself.
601 It’s a little bit helpful to know that we’ve modified hello.c, but we
602 might prefer to know exactly what changes we’ve made to it. To do
603 this, we use the “git diff” command.
606 diff --git a/hello.c b/hello.c
607 index 9a3ff79..ea364d3 100644
612 int main(int argc, char **argv)
614 - printf("hello, world!\");
615 + printf("hello, world!\n");
619 ### 2.7 Recording changes in a new commit
621 We can modify files, build and test our changes, and use “git status”
622 and “git diff” to review our changes, until we’re satisfied with what
623 we’ve done and arrive at a natural stopping point where we want to
624 record our work in a new commit.
626 If you'd like, go ahead and build the program now with "make" and run
627 it to ensure it works. Everything should look good so we're just about
628 ready to make a commit. Unsurprisingly, we'll be using "git commit" to
631 #### 2.7.1 Introducing yourself to git
633 Before you run "git commit" though, you should introduce yourself to
634 git. Git records your name and email address with each change that
635 you commit, (as both author and committer unless you tell it
636 otherwise), so that you and others will later be able to tell who made
639 Git tries to automatically figure out a sensible name and address to
640 attribute to both author and committer if you haven't explicitly told
641 it a name and address. And it tries a lot, (detailed below). If you're
642 not interested in these details, you might want to skip to the next
643 section which explains how to avoid all this guesswork and tell git
644 what your name and email address are.
646 Here is a list of all the guessing that git will attempt. It will
647 attempt each of the following methods, in order, (stopping for each of
648 the author and committer name and email as soon as a value is found):
650 1. If you specify a --author option to the “git commit” command on
651 the command line, followed by a `"Real Name <email@example.com>"`
652 string, then this name and addresss will be used for the author
653 fields. The committer fields will still be determined as
654 below. This option is very helpful for when applying a commit
655 originally authored by someone other than yourself.
656 2. If any of the `GIT_AUTHOR_NAME`, `GIT_AUTHOR_EMAIL`,
657 `GIT_COMMITTER`_NAME, or `GIT_COMMITER_EMAIL` environment variables
658 are set, then those values will be used for the corresponding
660 3. If you have a file in your home directory called .gitconfig, with
661 name or email settings in the [user] section, then these values
662 will be used to set any remaining author and committer
663 fields. For more details on the contents of this file, refer to
665 4. If you have a file in the local repository called .git/config,
666 again with name or email settings in the [user] section, then
667 these values will be used to set any remaining author and
669 5. If you have set the `EMAIL` environment variable, this will be used
670 to set author and committer email addresses if still unset.
671 6. git will query your system to find out your real name from
672 available GECOS field and your username, hostname, and domain to
673 construct an email address, (or at least an identifier resembling
676 If all of these mechanisms fail, "git commit" will fail, printing an
677 error message instructing you how to use "git config" to tell git your
678 name and email address.
680 You should think of the `GIT_AUTHOR`/`COMMITER_NAME`/`EMAIL` environment
681 variables and the --author option to the “git commit” command as ways
682 to override git’s default selection. For normal use, the simplest and
683 most robust way to set your information is by creating a .gitconfig
684 file, (either manually or with the "git config" command); see below
687 ##### Creating a git configuration file
689 To set your name and email address, just use the following commands:
691 git config --global user.name "Your Name"
692 git config --global user.email "you@example.com"
694 The --global option means that this command will set global
695 information, (affecting all repositories on this machine), in the
696 .gitconfig file in your home directory. Alternately, you could omit
697 the --global which would make the change take effect only in the local
698 repository. This is convenient if you want to have different email
699 addresses associated with different projects, for example.
701 Of course, git's configuration file is a simple-to-edit plain-text
702 file, so instead of using the above commands, you can also just edit
703 the files directly. Use your favorite editor to create a file called
704 .gitconfig in your home directory, (or if you ran the above commands
705 then it will be there already). The initial contents of your
706 .gitconfig should look like this.
708 # This is a git configuration file.
711 email = you@example.com
713 Similarly, you can make a repository-specific configuration by editing
714 .git/config in the local repository. It will already have some
715 sections present, (created by the "git clone"), just add a [user]
718 The “[user]” line begins a section of the config file, so you can read
719 the “name = ...” line as meaning “set the value of the name item in
720 the user section”. This is the same notion expressed with the
721 "user.name" syntax on the git-config command line. A section
722 continues until a new section begins, or the end of the file. Git
723 ignores empty lines and treats any text from “#” to the end of a line
726 ##### Choosing your name and email
728 You can use any text you like as the value of the name and email
729 configuration items, since this information is for reading by other
730 people, not for interpreting by git. It is conventional to use your
731 actual name as well as a valid email address. But some people, (notably
732 Linus Torvalds, the original author of git), actually like the default
733 username@hostname convention that git falls back on without any
734 additional information about an email address. There's no requirement
735 that the email address actually be valid, and perhaps it's useful to
736 be reminded which machine was used to create particular commits. So
737 choose the name and email you wish, or follow a particular project's
740 #### 2.7.2 Writing a commit message
742 When we commit a change, git drops us into a text editor to
743 enter a message that will describe the modifications we’ve made in
744 this commit. This is called the commit message. It will be a record
745 for readers of what we did and why, and it will be printed by “git log”
746 after we’ve finished committing.
750 Note: The -a on the command-line instructs git to commit the new
751 content of *all* tracked files that have been modified. This is a
752 convenience over explicitly listing filenames to be committed on the
753 `git commit` command line. It is useful to use `git commit <files>`
754 when there is a need to commit only some subset of the files that have
757 If new files need to be committed for the first time, just use `git
758 add <file>` before `git commit -a`. If a file needs to be removed,
759 just remove it as normal before committing and `git commit -a` will
760 notice that---it does not need to be explicitly told about the
763 The editor that the `git commit` command drops us into will contain an
764 empty line, followed by a number of lines starting with “#”. These
765 lines contain the same information as seen in `git status` before:
768 # Please enter the commit message for your changes.
769 # (Comment lines starting with '#' will not be included)
771 # Changes to be committed:
772 # (use "git reset HEAD <file>..." to unstage)
777 # (use "git add <file>..." to include in what will be committed)
782 Notice that two untracked files (hello and hello.o) have now appeared
783 from the build process. Git is reminding us of these in case we
784 intended to commit them as well, (in which case we would need to "git
785 add" them). We don't actually want to commit these files so we will
786 ignore them for now, (and we could tell git to ignore them as well by
787 listing them in a .gitignore file).
789 Git will ignore all lines in the commit message that start with “#”;
790 it uses them only to give us information on the commit. Modifying or
791 deleting these lines has no effect.
793 #### 2.7.3 Writing a good commit message
795 A good commit message will generally have a single line that
796 summarizes the commit, a blank line, and then one or more pargraphs
797 with supporting detail. Since many tools only print the first line of
798 a commit message by default, it’s important that the first line stands
801 One example of a first-line-only viewer is "git log
802 --pretty=short". Other examples include graphical history viewers such
803 as gitk and gitview, and web-based viewers such as gitweb and cgit.
805 Here’s a real example of a commit message that doesn’t follow
806 this guideline, and hence has a summary that is not readable.
808 $ git log --pretty=short
809 commit 3ef5535144da88a854f7930503845cd44506c2e2
810 Author: Censored Person <censored.person@example.org>
812 include buildmeister/commondefs. Add an exports and install
814 As far as the remainder of the contents of the commit message are
815 concerned, there are no hard-and-fast rules. git itself doesn’t
816 interpret or care about the contents of the commit message, though
817 your project may have policies that dictate a certain kind of
820 My personal preference is for short, but informative, commit messages
821 that tell me something that I can’t figure out with a quick glance at
822 the output of "git log --stat" or “git log -p", (so repeating the list
823 of all modified files is not useful, for example).
825 #### 2.7.4 Aborting a commit
827 If you decide that you don’t want to commit while in the middle of
828 editing a commit message, simply exit from your editor without saving
829 the file that it’s editing. This will cause nothing to happen to
830 either the repository or the working directory.
832 #### 2.7.5 Admiring our new handiwork
834 Once we’ve finished the commit, we can use the “git show” command to
835 display the commit we just created. As discussed previously, this
836 command produces output that is identical to “git log -p”, but for
837 only a single revision, (and the most recent revision by default):
840 commit fd21e5d6c5eedee70137229ebf348c25181812ab
841 Author: Carl Worth <cworth@cworth.org>
842 Date: Fri Sep 28 12:50:16 2007 -0700
844 Fixed the typo so the program actuall complies now.
846 diff --git a/hello.c b/hello.c
847 index 9a3ff79..ea364d3 100644
852 int main(int argc, char **argv)
854 - printf("hello, world!\");
855 + printf("hello, world!\n");
859 Note that you will not see the same commit identifier for your commit,
860 even if the change you made is identical to mine. The commit
861 identifier incorporates not only the contents of the files, but commit
862 message, the author and committer names and emails, and the author and
863 commit dates. (OK, so now you probably know enough to be able to guess
864 the right command to produce a commit with exactly the commit
865 identifier shown above. Can you do it?)
867 #### 2.7.6 Fixing up a broken commit (before anyone else sees it)
869 So now that we've cloned a local repository, made a change to the
870 code, setup our name and email address, and made a careful commit,
871 we're just about ready to share our change with the world. But wait,
872 that commit message has some really embarrassing misspellings in
873 it. Wouldn't it be nice to touch those up before I post this commit
874 with a never-to-be-changed again commit identifier?
876 This is the exact situation for which "git commit --amend" was
877 invented. So I can just run that now and fix the broken commit
882 Here's the final result:
885 commit 3c54ac672ec1130b36837f1b708054a7a1d402de
886 Author: Carl Worth <cworth@cworth.org>
887 Date: Fri Sep 28 12:50:16 2007 -0700
889 Fixed the typo so the program actually compiles now.
891 diff --git a/hello.c b/hello.c
892 index 9a3ff79..ea364d3 100644
897 int main(int argc, char **argv)
899 - printf("hello, world!\");
900 + printf("hello, world!\n");
904 Note that we didn't use "commit -a" this time. This means that "git
905 commit --amend" will amend only the commit message and not any of the
906 actual files being tracked, (even if some of them had been modified
907 between the commits).
909 It's also possible to use "git commit -a --amend" to similarly fix up
910 mistakes noticed in code. That will replace the most recent commit
911 with a different commit based on any new changes to files.
913 I do feel a little hesitant to mention "git commit -a --amend". It's a
914 handy command for fixing up something like a misspelling in a comment
915 in the code. But if there is anything more significant than that, then
916 it would generally be better to create an additional commit rather
917 than amending an existing commit. This is important for several
920 * The amend operation will destroy a state that was previously saved
921 in a commit. If it's just the commit message being changed, then
922 that's no big deal. But if the contents are being amended, then a
923 mistake could eliminate something valuable.
925 * All commits should be logically independent and as small as
926 possible. Abusing "git commit -a --amend" can cause a small commit
927 to grow and acquire unrelated changes.
929 It's worth emphasizing the value of minimal, independent commits. The
930 smaller the changes are the more useful the history will be when
931 actually using the history, not just viewing it. This is particularly
932 important when doing "git bisect"---that's a powerful tool for
933 isolating a single commit that introduces a bug. And it's much more
934 helpful when the commit it isolates is as small as possible.
936 One advantage of using git over some other systems is that the commit
937 speed is blazingly fast. The tool doesn't punish you at all for
938 committing as often as you get our project into a state that is worth
939 saving. "Commit early, commit often" is a well-supported mode of
942 ### 2.8 Sharing changes
944 We mentioned earlier that repositories in git are
945 self-contained. This means that the commit we just created exists
946 only in our hello repository. Let’s look at a few ways that we can
947 propagate this change into other repositories.
949 #### 2.8.1 Pulling changes from the original repository
951 Recall that in Section 2.3.2 we made several local clones of the hello
952 repository before we made any commits. This allows us to simulate what
953 happens when upstream changes have been committed after you originally
956 The simplest, (and quite common), scenario is that you inherently
957 trust any changes in the original repository and you want to pull
958 these directly into your clone. This might be the case if you are
959 using git simply to track the progress of a project without making any
962 In this case, the operation is as simple as just calling "git pull":
966 remote: Generating pack...
967 Unpacking 3 objects...
969 remote: Done counting 5 objects.
970 Result has 3 objects.
971 Deltifying 3 objects...
972 100% remote: (3/3) done
973 Total 3 (delta 1), reused 0 (delta 0)
974 * refs/remotes/origin/master: fast forward to branch 'master' of /tmp/hello
975 old..new: a1a0e8b..3c54ac6
976 Updating a1a0e8b..3c54ac6
979 1 files changed, 1 insertions(+), 1 deletions(-)
981 XXX: Git is fairly noisy here, but a user has little need to care
982 about the several stages of operation involved here. As a tutorial
983 writer I'd like to say "ignore all that progress stuff, and look at
984 where the stat information starts" but it's hard for a new user to
985 even be able to understand that. I think it would be ideal if all of
986 the progress-tracking spew were reduced to a single line. Something
987 like "Computing (100%) Transferring (100%)" or whatever.
989 After (lots!) of progresss indication, git gives a report of which
990 files were modified, (which is very useful for getting a quick feel
991 for what happened). If you would like more details on what changes
992 came in, git provides a range that is perfect for examining. Let's
993 take a look (again, the commit identifiers will be different for you
994 --- just copy-and-paste the range that git prints):
996 $ git log a1a0e8b..3c54ac6
997 commit 3c54ac672ec1130b36837f1b708054a7a1d402de
998 Author: Carl Worth <cworth@cworth.org>
999 Date: Fri Sep 28 12:50:16 2007 -0700
1001 Fixed the typo so the program actually compiles now.
1003 As expected, we received just the one commit.
1005 So that's all that's needed in the common case. Just run "git pull"
1006 everytime you want to pull in new changes that have landed in the
1007 upstream repository.
1009 Note: Mercurial users who are reading this might wonder if there's a
1010 need for the equivalent of "hg update" after doing a "git pull". And
1011 the answer is no. Unlike mercurial, "git pull", (and "git merge") will
1012 automatically update the working-directory files as necessary.
1014 #### 2.8.2 Using fetch and merge separately to pull
1016 Sometimes you may not know if you want to pull in the changes from the
1017 remote repository or not. It's useful to be able to examine them
1018 before accepting them into our branch. The "git pull" command shown in
1019 the previous section is conceptually the combination of two command,
1020 "git fetch" and "git merge". We can use these commands separately to
1021 examine the change before accepting it.
1023 So let's do that within the hello-fetch clone we made earlier. First
1024 we will do the fetch:
1028 remote: Generating pack...
1029 Unpacking 3 objects...
1030 remote: Done counting 5 objects.
1031 Result has 3 objects.
1032 Deltifying 3 objects...
1033 100% remote: (3/3) done
1034 Total 3 (delta 1), reused 0 (delta 0)
1036 * refs/remotes/origin/master: fast forward to branch 'master' of /tmp/hello/
1037 old..new: a1a0e8b..3c54ac6
1039 You may notice that the output here looks very much like the first
1040 portion of the output from "git pull". This is no coincidence. The
1041 new changes have been "fetched" into the current repository and are
1042 stored into "origin/master" and have not been into the current
1043 "master" branch. Remember that "master" is our current branch. So now,
1044 "origin/master" is the state of the master branch that exists in the
1045 "origin" repository, (the one we cloned from).
1047 The most convenient way to examine the fetched changes is with the
1048 "master..origin" range notation:
1050 $ git log master..origin
1051 commit 3c54ac672ec1130b36837f1b708054a7a1d402de
1052 Author: Carl Worth <cworth@cworth.org>
1053 Date: Fri Sep 28 12:50:16 2007 -0700
1055 Fixed the typo so the program actually compiles now.
1057 Another helpful way of visualizing what happened with "git fetch" here
1058 is to run "gitk --all", which gives a graphical representation of all
1059 branches. Here is what it would look like:
1061 [[img gitk-fetch.png]]
1063 Notice that origin/master points to a single commit that was committed
1064 on top of the state pointed to by the "master" branch.
1066 Let's assume we are happy with the changes and we want to include them
1067 into our master branch. To do this we simply run "git merge origin":
1070 Updating a1a0e8b..3c54ac6
1073 1 files changed, 1 insertions(+), 1 deletions(-)
1075 Again, you'll see that this precisely matches the final portion of the
1076 output from "git pull". Using "git fetch" and "git merge" let us
1077 achieve exactly what "git pull" did, but we were able to stop in the
1078 middle to examine the situation, (and we could have decided to reject
1079 the changes and not merge them---leaving our master branch unchanged).
1081 ##### On merges and "fast forward"
1083 You'll notice that we've been seeing the phrase "fast forward" several
1084 times. This is a special-case operation performed by "git merge" where
1085 a branch can be advanced along a linear sequence. This happens
1086 whenever you pull changes that build directly on top of the same
1087 commit you have as your most recent commit. In other words, there was
1088 never any divergence or simultaneous commits created in parallel in
1089 multiple repositories. If there had been parallel commits, then "git
1090 merge" would actually introduce a new merge commit to tie the two
1093 When a non-fast-forward merge occurs, there is always the possibility
1094 that a conflict occurs. In this case, "git merge" will leave conflict
1095 markers in the files and instruct you to resolve the conflicts. When
1096 you are finished, you would issue a "git commit -a" to create the
1099 #### 2.8.3 Using "git remote" to pull changes from other repositories
1101 We've already described how "git pull" will pull in changes from the
1102 repository which was the origin of the clone operation. Git also
1103 provides excellent support for pulling changes from any other
1104 repository as well, (distributed, rather than centralized
1107 If you have a situation where you want to pull a single time from some
1108 repository, then you can simply give the path or URL of the repository
1109 on the "git pull" command line. However, it's often the case that if
1110 you want to pull changes from a repository once, you'll want to pull
1111 changes from that same repository again in the future. This is where
1112 the "git remote" notion is extremely useful---it allows you to
1113 associate simple names, (and behaviors), with remote repository URLs
1115 We've already seen one instance of "git remote" which is the creation
1116 of the "origin" remote which happens automatically during "git
1117 clone". Let's now create another. Let's assume you are going to be
1118 working in the hello-remote repository and you'd like to pull changes
1119 from the hello-pull repository, where your friend "fred" has been
1120 making changes. Here's how to setup the new remote:
1122 $ cd ../hello-remote
1123 $ git remote add fred ../hello-pull
1125 So that's a "git remote add" command line followed by an arbitrary
1126 name you'd like for the new remote (fred) and the URL of the remote
1127 (../hello-pull). Obviously, the URL could be a git:// URL or any other
1128 git-supported URL in addition to a local path.
1130 The "git remote" command is really just a helper for adding some
1131 entries to the .git/config file. You might find it more convenient to
1132 edit that file directly once you get comfortable with things.
1134 At this point the name "fred" will work much like the name "origin"
1135 has worked in previous examples. For example, we can fetch the changes
1136 fred has made with "git fetch fred":
1139 remote: Generating pack...
1140 Unpacking 3 objects...
1141 remote: Done counting 5 objects.
1142 Result has 3 objects.
1143 Deltifying 3 objects...
1144 100% remote: (3/3) done
1145 Total 3 (delta 1), reused 0 (delta 0)
1147 * refs/remotes/fred/master: storing branch 'master' of ../hello-pull
1150 Notice that this command-line only differs from the "git fetch" we did
1151 previously by explicitly naming which remote should be fetched. We
1152 could have explicitly said "git fetch origin" earlier.
1154 We can also list all known remote-tracking branches with "git branch
1162 These remote-tracking branches make it very easy to collaborate with
1163 people as they are working on experimental features not yet ready for
1164 upstream inclusion. For example, if fred's latest code is still
1165 trashing filesystems then he might not want to push it out the the
1166 project's primary repository. But he may still want my help with
1167 it. So he can push it to a branch in his own repository for which I've
1168 got a remote. Then on my next "git fetch fred" I might notice a new
1169 branch called fred/trashes-filesystems and I can examine his code with
1170 a command such as "git log ..fred/trashed-filesystems".
1172 So lots of side collaboration can go on easily, and people working
1173 only with the primary repository never even have to see this dangerous
1174 code. It's distributed development at its finest.
1176 #### 2.8.4 Checking out previous revisions
1178 It's often useful to examine the working-tree state of some specific
1179 revision other than the tip of some branch. For example, maybe you
1180 would like to build a particular tagged version, or maybe you'd like
1181 to test the behavior of the code before a particular change was
1182 introduced. To do this, use "git checkout" and pass it the name of any
1183 revision, (with a branch name, a tag name, or any other commit
1184 identifier). For example, to examine our project before the original
1185 typo was introduced:
1187 $ git checkout 0a633bf5
1188 Note: moving to "0a633bf5" which isn't a local branch
1189 If you want to create a new branch from this checkout, you may do so
1190 (now or later) by using -b with the checkout command again. Example:
1191 git checkout -b <new_branch_name>
1192 HEAD is now at 0a633bf... Create a makefile
1194 The note that git gives us is to indicate that we are checking out a
1195 non-branch revision. This is perfectly fine if we are just exploring
1196 history, but if we actually wanted to use this revision as the basis
1197 for new commits, we would first have to create a new branch name as it
1200 If we were to use "git checkout" with a branch name, then that would
1201 change the current branch, (meaning that any new commits would advance
1202 that branch pointer).
1204 For now, let's return back to the tip of the master branch by just
1205 checking it out again:
1207 $ git checkout master
1208 Previous HEAD position was 0a633bf... Create a makefile
1209 Switched to branch "master"
1211 #### 2.8.5 Pushing changes to another repository
1213 As an unsurprising parallel to "git pull", git also provides "git
1214 push" for pushing changes to another repository. Now, generally the
1215 purpose of pushing to a repository is to have some "collaboration
1216 point" where potentially multiple people might be pushing or
1217 pulling. Because there might be multiple people pushing into the
1218 repository at any point, it wouldn't make sense to have a
1219 working-directory associated with this repository.
1221 For this, git has the notion of a "bare" repository, which is simply a
1222 repository with no working directory. Let's create a new bare
1223 repository and push some changes into it:
1228 $ git --bare init --shared
1230 The --shared option sets up the necessary group file permissions so
1231 that other users in my group will be able to push into this repository
1234 Now lets return to our hello repository and push some changes to this
1235 new repository. Since this is our very first push into this repository
1236 we need to tell git which branches to push. The easiest way to do this
1237 is to use --all to indicate all branches:
1240 $ git push ../hello-bare --all
1241 updating 'refs/heads/master'
1242 from 0000000000000000000000000000000000000000
1243 to 3c54ac672ec1130b36837f1b708054a7a1d402de
1245 Done counting 18 objects.
1246 Deltifying 18 objects...
1248 Writing 18 objects...
1250 Total 18 (delta 3), reused 15 (delta 2)
1251 Unpacking 18 objects...
1253 refs/heads/master: 0000000000000000000000000000000000000000 -> 3c54ac672ec1130b36837f1b708054a7a1d402de
1255 For subsequent pushes we don't need to specify --all as "git push" by
1256 default pushes all branches that exist in both the local and remote
1257 repositories. Also, as with pull, instead of explicitly specifying a
1258 URL, you may also specify a remote to push to. And by default, after
1259 cloning a repository, "git push" with no other arguments will attempt
1260 to push back to the same origin repository. As this is often exactly
1261 what is wanted, you may find that "git push" alone is often exactly
1264 What happens if we try to pull or push changes and the receiving
1265 repository already has those changes? Nothing too exciting.
1267 $ git push ../hello-bare
1268 Everything up-to-date
1271 Open Publication License
1273 Version 1.0, 8 June 1999
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