1 [[meta title="Chapter 2: A tour of git: the basics"]]
5 This document is a modified version of a document originally titled
6 "Distributed revision control with Mercurial" and originally authored
7 by Bryan O’Sullivan. The original document was obtained from
8 <http://hgbook.red-bean.com/>.
10 Copyright © 2006, 2007 Bryan O’Sullivan.
12 This material may be distributed only subject to the terms and
13 conditions set forth in version 1.0 of the Open Publication
14 License. Please refer to Appendix D for the license text.
16 As this is a modified version, the name of Bryan O'Sullivan is used
17 only to properly credit him with the original text. The appearance of
18 his name here explicitly does not assert or imply his endorsement of
19 this modified document.
21 Portions Copyright © 2007 Carl Worth.
23 Changes made by Carl include the following:
26 * Convert from HTML to markdown source syntax
27 * Eliminate all content except Chapter 2 and Appendix D
28 * Eliminate line numbers from examples
29 * Modified to describe git instead of mercurial
31 The source of this modified version can be obtained via git:
33 git clone git://cworth.org/git/hgbook-git
35 git clone http://cworth.org/git/hgbook-git
37 and can be browsed online:
39 http://git.cworth.org/git/hgbook-git
41 ### 2.1 Installing git on your system
43 Prebuilt binary packages of git are available for many popular
44 operating systems. These make it easy to start using git on your
49 Because each Linux distribution has its own packaging tools, policies,
50 and rate of development, it’s difficult to give a comprehensive set of
51 instructions on how to install git binaries. The version of
52 git that you will end up with can vary depending on how active
53 the person is who maintains the package for your distribution.
55 To keep things simple, I will focus on installing git from the
56 command line under the most popular Linux distributions. Most of these
57 distributions provide graphical package managers that will let you
58 install git with a single click. The package name to look for is
59 often git, but is sometimes git-core, (due to an unfortunate name
60 with git, meaning GNU Interactive Tools).
64 apt-get install git-core
84 A git-core package is available through
85 [macports](http://macports.org). Once macports is enabled, the command
92 Git has long been available as part of cygwin, and works reasonably
93 well in that environment. Some people find cygwin a particularly
94 inelegant approach to running git and would prefer a "native"
95 solution. To this end, the [msysgit
96 project](http://code.google.com/p/msysgit/) is rapidly putting
97 together a solution including various packages with full
98 installers. These include GitMe, a package to install the entire
99 development environment necessary to work on improving the msysgit
100 port of git, and WinGit, a package for installing just git itself
101 without the development environment, (still in Alpha as of September
104 ### 2.2 Getting started
106 To begin, we’ll use the “git version” command to find out whether git
107 is actually installed properly. Versions 1.5 and newer of git are much
108 more friendly to new users than versions 1.4 and older. If you aren't
109 yet running version 1.5 or newer, it's highly recommended that you
115 #### 2.2.1 Built-in help
117 Git provides a built-in help system. This is invaluable for those
118 times when you find yourself stuck trying to remember how to run a
119 command. If you are completely stuck, simply run “git help”; it will
120 print a brief list of commonly-used commands, along with a description
121 of what each does. If you ask for help on a specific command (such as
122 "git help init"), it prints more detailed information. [XXX: Does "git
123 help <foo>" work universally as a built-in or does it expect man to be
124 present and just call out to "man git-<foo>"?]
126 [XXX: The original hgbook includes the complete output of "hg
127 help init" at this point. I'm not including the corresponding
128 "git help init" output as it would be excessively long. The
129 description alone is quite reasonable, (other than a
130 not-too-helpful aside about the obsolete git-init-db command),
131 but it only comes after a full screen's worth of options
132 details. Might it make sense to have a more summarized help
133 output for "git help <foo>" than all of the documentation
134 available for git-<foo>? And perhaps alos provide a "git -v
135 help" similar to "hg -v help" for more?]
137 ### 2.3 Working with a repository
139 In git, everything happens inside a repository. The repository
140 for a project contains all of the files that “belong to” that project,
141 along with a historical record of the project’s files.
143 There’s nothing particularly magical about a repository; it is simply
144 a directory tree in your filesystem that git treats as
145 special. You can rename or delete a repository any time you like,
146 using either the command line or your file browser.
148 #### 2.3.1 Making a local copy of a repository
150 Copying a repository is just a little bit special. While you could use
151 a normal file copying command to make a copy of a repository, it’s
152 best to use a built-in command that git provides. This command
153 is called “git clone”, because it creates an identical copy of an
156 $ git clone git://cworth.org/git/hello
157 Initialized empty Git repository in /tmp/hello/.git/
158 remote: Generating pack...
159 remote: Done counting 15 objects.
160 remote: Deltifying 15 objects...
161 remote: 100% (15/15) done
162 remote: Total 15 (delta 2), reused 15 (delta remote: 2)
163 Indexing 15 objects...
165 Resolving 2 deltas...
168 If for some reason you are prevented from talking on the git: port,
169 then there is also the capability to clone a repository (less
170 efficiently) over http:
172 $ git clone http://cworth.org/git/hello
173 Initialized empty Git repository in /tmp/hello/.git/
174 Getting alternates list for http://cworth.org/git/hello
175 Getting pack list for http://cworth.org/git/hello
176 Getting index for pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
177 Getting pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
178 which contains a1a0e8b392b17caf50325498df54802fe3c03710
179 walk a1a0e8b392b17caf50325498df54802fe3c03710
180 walk 72d4f10e4a27dbb09ace1503c20dbac1912ee451
181 walk 13ed136b983a9c439eddeea8a1c2076cffbb685f
182 walk 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
183 walk db7117a9dd9a6e57e8632ea5848e1101eee0fbde
185 If our clone succeeded, we should now have a local directory called
186 hello. This directory will contain some files.
190 drwxr-xr-x 3 cworth cworth 4096 2007-09-27 16:40 hello
194 These files have the same contents and history in our repository as
195 they do in the repository we cloned.
197 Every git repository is complete, self-contained, and
198 independent. It contains its own private copy of a project’s files and
199 history. A cloned repository remembers the location of the repository
200 it was cloned from, but it does not communicate with that repository,
201 or any other, unless you tell it to.
203 What this means for now is that we’re free to experiment with our
204 repository, safe in the knowledge that it’s a private “sandbox” that
205 won’t affect anyone else.
207 #### 2.3.2 What’s in a repository?
209 When we take a more detailed look inside a repository, we can see that
210 it contains a directory named .git. This is where git keeps all
211 of its metadata for the repository.
215 . .. .git hello.c Makefile
217 The contents of the .git directory and its subdirectories are private
218 to git. Every other file and directory in the repository is
219 yours to do with as you please.
221 To introduce a little terminology, the .git directory is the “real”
222 repository, and all of the files and directories that coexist with it
223 are said to live in the working directory. An easy way to remember the
224 distinction is that the repository contains the history of your
225 project, while the working directory contains a snapshot of your
226 project at a particular point in history.
228 ### 2.4 A tour through history
230 One of the first things we might want to do with a new, unfamiliar
231 repository is understand its history. The “git log” command gives us a
235 commit a1a0e8b392b17caf50325498df54802fe3c03710
236 Author: Bryan O'Sullivan <bos@serpentine.com>
237 Date: Tue Sep 6 15:43:07 2005 -0700
241 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
242 Author: Bryan O'Sullivan <bos@serpentine.com>
243 Date: Tue Sep 6 13:15:58 2005 -0700
245 Get make to generate the final binary from a .o file.
247 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
248 Author: Bryan O'Sullivan <bos@serpentine.com>
249 Date: Tue Sep 6 13:15:43 2005 -0700
251 Introduce a typo into hello.c.
253 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
254 Author: Bryan O'Sullivan <mpm@selenic.com>
255 Date: Fri Aug 26 01:21:28 2005 -0700
259 commit db7117a9dd9a6e57e8632ea5848e1101eee0fbde
260 Author: Bryan O'Sullivan <mpm@selenic.com>
261 Date: Fri Aug 26 01:20:50 2005 -0700
263 Create a standard "hello, world" program
265 By default, this command prints a brief paragraph of output for each
266 change to the project that was recorded. In git terminology, we
267 call each of these recorded events a commit.
269 The fields in a record of output from “git log” are as follows.
271 * commit This field consists of a string of 40 hexadecimal characters.
272 This is a unique identifier for referring to particular commits.
273 * Author The identity of the person who authored the commit. This
274 field consist of two sub-fields for the user's name and email
275 address, (or at least an email-like idenitifer). Note that git
276 stores a separate "Committer" field for the person who commited
277 the change, (since often an author will email a change to a
278 maintainer that commits it). The "git log" command doesn't display
279 the Committer, but other git tools do.
280 * Date The date and time on which the commit was authored, (again
281 stored separately from the date the change was committed).
282 timezone in which it was created. (The date and time are displayed
283 in the timezone of the person who created the commit.)
284 * commit message The text message that the creator of the commit
285 entered to describe the commit, (generally a one-line summary
286 followed by more supporting text).
288 The default output printed by “git log” is purely a summary; it is
289 missing a lot of detail.
291 #### 2.4.1 Commits, revisions, and talking to other people
293 As English is a notoriously sloppy language, and computer science has
294 a hallowed history of terminological confusion (why use one term when
295 four will do?), revision control has a variety of words and phrases
296 that mean the same thing. If you are talking about git history
297 with other people, you will find that what we have called a “commit”
298 is often called a "revision". In other systems, a similar notion
299 is referred to as a "changeset". You might even see abbreviations of
300 these terms such as "rev", "change", or even "cset".
302 While it may not matter much what word you use to refer to the concept
303 of “a commit”, it's important to know how to name “a specific
304 commit”. We have already seen one means of referring to a particular
305 commit, the 40-character hexadecimal string shown by "git log". These
306 commit identifiers are powerful because they are permanent, unique
307 identifiers that always identify the same commit in any copy of a
308 repository. If two users are examining a working directory associated
309 with the same commit identifier, then those two users have precisely
310 the same contents in all files, and exactly the same history leading
313 So there are places where it is often important to archive the
314 complete commit identifier, (perhaps in bug-tracking systems to
315 indicate a specific commit that fixes a bug, for example). But often,
316 in more casual settings, it's more convenient to use abbreviated
317 commit identifiers. Git accept any unique prefix of a commit
318 identifier, (and for reasonably-sized project the first 8 or 10
319 characters are almost always unique).
321 And unlike the permanent commit identifiers, git also provides
322 transient means of identifying commits. In fact, in day-to-day use of
323 git, you will probably use these names more than commit
324 identifiers. One example is branch names, (such as the default
325 "master" branch in any git repository), or any project-specific branch
326 names such as "stable", "experimental", or "crazy-insane-changes". Git
327 also provides a special name "HEAD" which always refers to the current
330 #### 2.4.2 Naming related commits
332 Git offers simple ways to name revisions that are related to
333 particular revisions in the history. One syntax is the ~ suffix which
334 refers to the parent of a commit, or if followed by a number, to the
335 Nth parent. For example, since "HEAD" refers to the most recent commit
336 in the current branch, "HEAD~", refers to the previous commit, and
337 "HEAD~2" refers to two commits back in the history.
339 Another useful syntax is .. which can be used to specify a range of
340 commits. So "origin..master" specifies everything that has been
341 committed to master since it derived from origin.
343 #### 2.4.3 Viewing specific revisions
345 You can use "git log" to explore the range syntax just introduced. For
346 example, to see a list of the most recent 3 revisions you can use
347 "HEAD~3..", (the destination of the range is implicitly HEAD in this
351 commit a1a0e8b392b17caf50325498df54802fe3c03710
352 Author: Bryan O'Sullivan <bos@serpentine.com>
353 Date: Tue Sep 6 15:43:07 2005 -0700
357 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
358 Author: Bryan O'Sullivan <bos@serpentine.com>
359 Date: Tue Sep 6 13:15:58 2005 -0700
361 Get make to generate the final binary from a .o file.
363 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
364 Author: Bryan O'Sullivan <bos@serpentine.com>
365 Date: Tue Sep 6 13:15:43 2005 -0700
367 Introduce a typo into hello.c.
369 #### 2.4.4 Other log filters
371 Besides filtering by commit identifiers, git allows you to easily
372 filter the log output according to which files (or directories) are
373 modified by listing them after "--" wihch is necessary to distinguish
374 commit names from file names:
376 $ git log -- Makefile
377 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
378 Author: Bryan O'Sullivan <bos@serpentine.com>
379 Date: Tue Sep 6 13:15:58 2005 -0700
381 Get make to generate the final binary from a .o file.
383 commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
384 Author: Bryan O'Sullivan <mpm@selenic.com>
385 Date: Fri Aug 26 01:21:28 2005 -0700
389 And "git log" can also filter based on the dates at which commits were
392 $ git log --since="2 weeks ago" --until="yesterday"
394 Another useful option is -n or --max-count which, unsurprisingly,
395 limits the maximum number of commits to be displayed.
397 #### 2.4.5 More detailed information
399 While the default information printed by “git log” is useful if you
400 already know what you’re looking for, you may need to see more details
401 of the change, such as the "diffstat" information with --stat:
403 $ git log --stat --max-count=3
404 commit a1a0e8b392b17caf50325498df54802fe3c03710
405 Author: Bryan O'Sullivan <bos@serpentine.com>
406 Date: Tue Sep 6 15:43:07 2005 -0700
411 1 files changed, 2 insertions(+), 6 deletions(-)
413 commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
414 Author: Bryan O'Sullivan <bos@serpentine.com>
415 Date: Tue Sep 6 13:15:58 2005 -0700
417 Get make to generate the final binary from a .o file.
420 1 files changed, 2 insertions(+), 0 deletions(-)
422 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
423 Author: Bryan O'Sullivan <bos@serpentine.com>
424 Date: Tue Sep 6 13:15:43 2005 -0700
426 Introduce a typo into hello.c.
429 1 files changed, 1 insertions(+), 1 deletions(-)
431 Or perhaps you'd like to see the actual patch content of each change,
432 which you can get with -p. That commit with the word typo in its name
433 looks suspicous, so let's tak a closer look. Remember that we can name
434 it as master~3, HEAD~3, or any prefix of its commit identifier, (such
437 $ git log -p -n 1 13ed136b
438 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
439 Author: Bryan O'Sullivan <bos@serpentine.com>
440 Date: Tue Sep 6 13:15:43 2005 -0700
442 Introduce a typo into hello.c.
444 diff --git a/hello.c b/hello.c
445 index ed55ec0..80b260c 100644
450 int main(int argc, char **argv)
452 - printf("hello, world!\n");
453 + printf("hello, world!\");
457 Of course, wanting to see all this information for a single commit is
458 such a common operation that it's given its own name in git, "git
459 show". So "git show 13ed136b" is a much easier way to get exactly the
463 commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
464 Author: Bryan O'Sullivan <bos@serpentine.com>
465 Date: Tue Sep 6 13:15:43 2005 -0700
467 Introduce a typo into hello.c.
469 diff --git a/hello.c b/hello.c
470 index ed55ec0..80b260c 100644
475 int main(int argc, char **argv)
477 - printf("hello, world!\n");
478 + printf("hello, world!\");
482 ### 2.5 All about command options
484 Let’s take a brief break from exploring git commands to discuss
485 a pattern in the way that they work; you may find this useful to keep
486 in mind as we continue our tour.
488 Git has a consistent and straightforward approach to dealing
489 with the options that you can pass to commands. It follows the
490 conventions for options that are common to modern Linux and Unix
493 * Most options have long names. For example, as we’ve already seen,
494 the “git log" command accepts a --max-count=<number> option.
495 * Some options have short, single-character names. Often these are
496 aliases for long commands, (such as "-n <number>" instead of
497 --max-count=<number>), but sometimes the option exists in
498 short-form with no long-form equivalent, (such as -p). [XXX: It
499 wouldn't hurt to fix this by adding --patch, etc. right?]
500 * Long options start with two dashes (e.g. --max-count), while short
501 options start with one (e.g. -n).
503 * Option naming and usage is consistent across commands. For
504 example, every command that lets you specify a commit identifier
505 or range will accept the same expressions, (HEAD~3,
506 origin..master, 72d4f10e, etc), while any command that can be
507 limited by paths will accept the same expressions ("-- doc/
510 Many commands that print output of some kind can be made more quiet by
511 passing the -q or --quiet options.
513 ### 2.6 Making and reviewing changes
515 Now that we have a grasp of viewing history in git, let’s take a
516 look at making some changes and examining them.
518 The first thing we’ll do is isolate our experiment in a repository of
519 its own. We use the “git clone” command, but we don’t need to clone a
520 copy of the remote repository. Since we already have a copy of it
521 locally, we can just clone that instead. This is much faster than
522 cloning over the network, and cloning a local repository uses less
523 disk space in most cases, too.
526 $ git clone hello my-hello
527 Initialized empty Git repository in /tmp/my-hello/.git/
530 [XXX We say "empty" here, (presumably from the git-init part),
531 but shouldn't the command also report the succesful clone
532 which makes it non-empty? And what the heck does "0 blocks"
535 As an aside, it’s often good practice to keep a “pristine” copy of a
536 remote repository around, which you can then make temporary clones of
537 to create sandboxes for each task you want to work on. This lets you
538 work on multiple tasks in parallel, each isolated from the others
539 until it’s complete and you’re ready to integrate it back. Because
540 local clones are so cheap, there’s almost no overhead to cloning and
541 destroying repositories whenever you want.
543 Alternatively, you can achieve much the same effect by creating
544 multiple branches in a single repository, (but we won't go into detail
545 on how to do that in this chapter). Some people greatly appreciate
546 having multiple branches in a single repository rather than having
547 many repositories cluttering up their filesystem. Other people prefer
548 the ability to have working-tree changes, and intermediate build
549 files, etc. each isolated in a separate repository per branch. Both
550 modes are very well-supported by git, so it's really a matter of which
551 you find most appropriate at any time given your tastes and project
554 In our my-hello repository, we have a file hello.c that contains the
555 classic “hello, world” program. Let’s use the ancient and venerable
556 sed command to edit this file so that it prints a second line of
557 output. (I’m only using sed to do this because it’s easy to write a
558 scripted example this way. Since you’re not under the same constraint,
559 you probably won’t want to use sed; simply use your preferred text
560 editor to do the same thing.)
562 $ sed -i '/printf/a\\tprintf("hello again!\\n");' hello.c
564 The “git status” command will tell us what git knows about the files
571 # Changed but not updated:
572 # (use "git add <file>..." to update what will be committed)
576 no changes added to commit (use "git add" and/or "git commit -a")
578 We see that “git status” command prints a line with "modified" for
579 hello.c. The “git status” command will not print any output for files
580 that have not been modified.
582 Notice that we didn’t need to inform git that we were going to modify
583 the file before we started, or that we had modified the file after we
584 were done; it was able to figure this out itself.
586 It’s a little bit helpful to know that we’ve modified hello.c, but we
587 might prefer to know exactly what changes we’ve made to it. To do
588 this, we use the “git diff” command.
591 diff --git a/hello.c b/hello.c
592 index 9a3ff79..6d28887 100644
596 int main(int argc, char **argv)
598 printf("hello, world!\");
599 + printf("hello again!\n");
603 ### 2.7 Recording changes in a new commit
605 We can modify files, build and test our changes, and use “git status”
606 and “git diff” to review our changes, until we’re satisfied with what
607 we’ve done and arrive at a natural stopping point where we want to
608 record our work in a new commit.
610 The “git commit” command lets us create a new changeset; we’ll usually
611 refer to this as “making a commit” or “committing”.
613 #### 2.7.1 Setting up a username
615 When you try to run “git commit” for the first time, it might not do
616 exactly what you want. Git records your name and address with each
617 change that you commit, (as both author and committer unless you tell
618 it otherwise), so that you and others will later be able to tell who
619 made each change. Git tries to automatically figure out a sensible
620 name and address to attribute to both author and committer. It will
621 attempt each of the following methods, in order, (stopping for each field as soon as a value is found):
623 1. If you specify a --author option to the “git commit” command on
624 the command line, followed by a "Real Name <email@example.com>"
625 string, then this name and addresss will be used for the author
626 fields. The committer fields will still be determined as
627 below. This option is very helpful for when applying a commit
628 originally authored by someone other than yourself.
629 2. If any of the GIT_AUTHOR_NAME, GIT_AUTHOR_EMAIL,
630 GIT_COMMITTER_NAME, or GIT_COMMITER_EMAIL environment variables
631 are set, then those values will be used for the corresponding
633 3. If you have a file in your home directory called .gitconfig, with
634 name or email settings in the [user] section, then these values
635 will be used to set any remaining author and committer
636 fields. For more details on the contents of this file, refer to
638 4. If you have a file in the local repository called .git/config,
639 again with name or email settings in the [user] section, then
640 these values will be used to set any remaining author and
642 5. If you have set the EMAIL environment variable, this will be used
643 to set author and committer email addresses if still unset.
644 6. git will query your system to find out your real name from
645 available GECOS field and your username, hostname, and domain to
646 construct an email address, (or at least an identifier resembling
649 If all of these mechanisms fail, "git commit" will fail, printing an
650 error message instructing you how to use "git config" to tell git your
651 name and email address.
653 You should think of the GIT_AUTHOR/COMMITER_NAME/EMAIL environment
654 variables and the --author option to the “git commit” command as ways
655 to override git’s default selection. For normal use, the simplest and
656 most robust way to set your information is by creating a .gitconfig
657 file, (either manually or with the "git config" command); see below
660 ##### Creating a git configuration file
662 To set your name and email address, just use the following commands:
664 git config --global user.name "Your Name"
665 git config --global user.email "you@example.com"
667 The --global option means that this command will set global
668 information, (affecting all repositories on this machine), in the
669 .gitconfig file in your home directory. Alternately, you could omit
670 the --global which would make the change take effect only in the local
671 repository. This is convenient if you want to have different email
672 addresses associated with different projects, for example.
674 Of course, git's configuration file is a simple-to-edit plain-text
675 file, so instead of using the above commands, you can also just edit
676 the files directly. Use your favorite editor to create a file called
677 .gitconfig in your home directory, (or if you ran the above commands
678 then it will be there already). The initial contents of your
679 .gitconfig should look like this.
681 # This is a git configuration file.
684 email = you@example.com
686 Similarly, you can make a repository-specific configuration by editing
687 .git/config in the local repository. It will already have some
688 sections present, (created by the "git clone"), just add a [user]
691 The “[user]” line begins a section of the config file, so you can read
692 the “name = ...” line as meaning “set the value of the name item in
693 the user section”. This is the same notion expressed with the
694 "user.name" syntax on the git-config command line. A section
695 continues until a new section begins, or the end of the file. Git
696 ignores empty lines and treats any text from “#” to the end of a line
699 ##### Choosing a user name
701 You can use any text you like as the value of the name and email
702 configuration items, since this information is for reading by other
703 people, not for interpreting by git. It is conventional to use a valid
704 email address, but some, (notably Linus Torvalds, the original author
705 of git), actually like the default user@hostname convention that git
706 falls back on without any additional information. There's no
707 requirement that the email address actually be valid, and perhaps it's
708 useful to be reminded which machine was used to create particular
711 #### 2.7.2 Writing a commit message
713 When we commit a change, git drops us into a text editor to
714 enter a message that will describe the modifications we’ve made in
715 this commit. This is called the commit message. It will be a record
716 for readers of what we did and why, and it will be printed by “git log”
717 after we’ve finished committing.
721 Note: The -a on the command-line instructs git to commit all changes
722 to tracked files. Without this, "git commit" will only commit changes
723 that have been previously staged for committing with "git add
724 file". The most common usage is to commit with "git commit -a" and
725 only use "git add file; git commit" when there is a need to commit
726 only some subset of changes that have been made.
728 The editor that the “git commit” command drops us into will contain an
729 empty line, followed by a number of lines starting with “#”.
732 # Please enter the commit message for your changes.
733 # (Comment lines starting with '#' will not be included)
735 # Changes to be committed:
736 # (use "git reset HEAD <file>..." to unstage)
741 git ignores the lines that start with “#”; it uses them only
742 to tell us which files it’s recording changes to. Modifying or
743 deleting these lines has no effect.
745 #### 2.7.3 Writing a good commit message
747 A good commit message will generally have a single line that
748 summarizes the commit, a blank line, and then one or more pargraphs
749 with supporting detail. Since many tools only print the first line of
750 a commit message by default, it’s important that the first line stands
753 One example of a first-line-only viewer is "git log
754 --pretty=short". Other examples include graphical history viewers such
755 as gitk and gitview, and web-based viewers such as gitweb and cgit.
757 Here’s a real example of a commit message that doesn’t follow
758 this guideline, and hence has a summary that is not readable.
760 $ git log --pretty=short
761 commit 3ef5535144da88a854f7930503845cd44506c2e2
762 Author: Censored Person <censored.person@example.org>
764 include buildmeister/commondefs. Add an exports and install
766 As far as the remainder of the contents of the commit message are
767 concerned, there are no hard-and-fast rules. git itself doesn’t
768 interpret or care about the contents of the commit message, though
769 your project may have policies that dictate a certain kind of
772 My personal preference is for short, but informative, commit messages
773 that tell me something that I can’t figure out with a quick glance at
774 the output of “git log -p".
776 #### 2.7.4 Aborting a commit
778 If you decide that you don’t want to commit while in the middle of
779 editing a commit message, simply exit from your editor without saving
780 the file that it’s editing. This will cause nothing to happen to
781 either the repository or the working directory.
783 #### 2.7.5 Admiring our new handiwork
785 Once we’ve finished the commit, we can use the “git show” command to
786 display the commit we just created. As discussed previously, this
787 command produces output that is identical to “git log -p”, but for
788 only a single revision, (and the most recent revision by default):
791 commit 018cfb742be6176443ffddac454e593e802ddf3e
792 Author: Carl Worth <cworth@cworth.org>
793 Date: Thu Sep 27 23:55:00 2007 -0700
795 Added an extra line of output.
797 If I would have been clever I would have fixed that old typo
800 diff --git a/hello.c b/hello.c
801 index 9a3ff79..6d28887 100644
805 int main(int argc, char **argv)
807 printf("hello, world!\");
808 + printf("hello again!\n");
812 Note that you will not see the same commit identifier for your commit,
813 even if the change you made is identical to mine. The commit
814 identifier incorporates not only the contents of the files, but commit
815 message, the author and committer names and emails, and the author and
816 commit dates. (OK, so now you probably know enough to be able to guess
817 the right command to produce a commit with exactly the commit
818 identifier shown above. Can you do it?)
820 #### 2.7.6 Fixing up a broken commit (before anyone else sees it)
822 So now that we've cloned a local repository, made a change to the
823 code, setup our name and email address, and made a commit with a
824 careful message, we're just about ready to share our change with the
825 world. But wait, we forgot to try to compile it didn't we?
828 cc -c -o hello.o hello.c
829 hello.c:10:9: warning: missing terminating " character
830 hello.c:10:9: warning: missing terminating " character
831 hello.c: In function ‘main’:
832 hello.c:10: error: missing terminating " character
833 hello.c:11: error: expected ‘)’ before ‘;’ token
834 hello.c:13: warning: passing argument 1 of ‘printf’ makes pointer from integer without a cast
835 hello.c:13: error: expected ‘;’ before ‘}’ token
836 make: *** [hello.o] Error 1
838 Oh look. The code's broken and doesn't compile. We don't want to share
839 code in this state. For situations where you notice one tiny detail
840 that got left out of the last commit, (a silly syntax error, a
841 misspelling in a comment or commit messsage), git provides a very
842 handy tool for just changing the last commit.
844 So fix that typo, (a missing 'n' between the '\' and the '"'), with
845 your editor or with something like this:
847 sed -i 's/\\"/\\n"/' hello.c
849 And then you can just amend the previous commit rather than creating a
850 new one with the --amend option to "git commit":
852 $ git commit -a --amend
854 Note that we use -a to include the code change here. And that helps
855 point out a situation where "git commit" is useful without the -a
856 option, "git commit --amend" is a useful command for amend just the
857 last commit message, without committing any new code changes, even if
858 some files have been modified in the working tree.
860 And here's the final result:
863 commit 839b58d021c618bd0e1d336d4d5878a0082672e6
864 Author: Carl Worth <cworth@cworth.org>
865 Date: Thu Sep 27 23:55:00 2007 -0700
867 Added an extra line of output and fixed the typo bug.
869 diff --git a/hello.c b/hello.c
870 index 9a3ff79..ca750e0 100644
875 int main(int argc, char **argv)
877 - printf("hello, world!\");
878 + printf("hello, world!\n");
879 + printf("hello again!\n");
883 I can't help but point out that this really was a poor example for
884 --amend. The end result is a single commit that does two independent
885 things, (fixes one bug and adds one new feature). It's much better to
886 create a code history where each commit makes an independent change,
887 (and as small as possible). This is important for several reasons:
889 * Small changes are easier to review
891 * Independent changes are easier to split up if only part of the
892 series gets accepted "upstream" for one reason or another.
894 * The smaller the changes are the more useful the history will be
895 when actually using the history, not just viewing it. This is
896 particularly important when doing "git bisect"---that's a powerful
897 tool for isolating the single commit that introduces a bug. And
898 it's much more powerful if the commit it isolates is as small as
901 So it's a good thing this document is available under a license that
902 allows for distribution of modified versions. Someone should clean up
903 the --amend example to not teach bad habits like I did above. [Note:
904 All this bad-habit stuff was introduced by me, and was not present in
905 Bryan's original chapter. -Carl]
907 ### 2.8 Sharing changes
909 We mentioned earlier that repositories in git are
910 self-contained. This means that the commit we just created exists
911 only in our my-hello repository. Let’s look at a few ways that we can
912 propagate this change into other repositories.
914 #### 2.8.1 Pulling changes from another repository
916 To get started, let’s clone our original hello repository, which does
917 not contain the change we just committed. We’ll call our temporary
918 repository hello-pull.
921 $ git clone hello hello-pull
922 Initialized empty Git repository in /tmp/hello-pull/.git/
925 We could use the “git pull” command to apply changes from my-hello to
926 our master branch in hello-pull. However, blindly pulling unknown
927 changes into a repository is a somewhat scary prospect. The "git pull"
928 command is coneptually the combination of two commands, "git fetch"
929 and "git merge"; we can run those separately to examine the changes
930 before applying them locally. First we do the fetch:
933 $ git fetch ../my-hello
934 remote: Generating pack...
935 Unpacking 3 objects...
937 remote: Done counting 5 objects.
938 Result has 3 objects.
939 Deltifying 3 objects...
940 100% remote: (3/3) done
941 Total 3 (delta 1), reused 0 (delta 0)
943 The fetched commits (or commit in this case) are available as the name
944 FETCH_HEAD. [XXX: Shouldn't git-fetch print that name out to the user
945 if the user didn't provide a specific branch name to fetch into.] And
946 the difference between what we had before and what exists on
947 FETCH_HEAD can easily be examined with the ..FETCH_HEAD range
950 $ git log ..FETCH_HEAD
951 commit 839b58d021c618bd0e1d336d4d5878a0082672e6
952 Author: Carl Worth <cworth@cworth.org>
953 Date: Thu Sep 27 23:55:00 2007 -0700
955 Added an extra line of output and fixed the typo bug.
957 Since these commits actually exist in the local repository now, we
958 don't need to fetch or pull them from the remote repository again---we
959 can now use "git merge" to apply the previously fetched commits. (A
960 mercurial user might notice here that git does not have the race
961 condition between "hg incoming" and "hg pull" that mercurial has since
962 the commits are fetched only once.)
964 $ git merge FETCH_HEAD
965 Updating a1a0e8b..839b58d
968 1 files changed, 2 insertions(+), 1 deletions(-)
970 Notice that "git merge" reports that our branch pointer has been
971 updated from a1a0e8b to 839b58d. Also, this is a "fast forward"
972 meaning that the new commits are a linear sequence on top of the
973 commit we already hand. In other words, there wasn't any divergence
974 between these two repositories so no actual "merge" commit was
977 This separation of fetch and merge is useful when you need to
978 carefully review some changes before applying them. But often you're
979 in a situation where you know you trust the remote repository and you
980 simply want to pull those changes as conveniently as possible, (no
981 extra commands, no typing a magic name like FETCH_HEAD). This is the
982 case when the tracking upstream development of a project with git. And
983 in that case, the above steps are as simple as just executing "git
984 pull". So let's repeat all that the simpler way:
987 $ git clone hello hello-tracking
988 Initialized empty Git repository in /tmp/hello-tracking/.git/
991 $ git pull ../my-hello
992 remote: Generating pack...
993 remote: Done counting 5 objects.
994 Result has 3 objects.
995 Deltifying 3 objects...
996 Unpacking 3 objects...
997 remote: 100% (3/3) done
998 Total 3 (delta 1), reused 0 (delta 0)
1000 Updating a1a0e8b..839b58d
1003 1 files changed, 2 insertions(+), 1 deletions(-)
1005 It should be plain to see that the "git pull" command really did the
1006 combined sequence of "git fetch" and "git merge". Also, if you want to
1007 pull from the same repository you cloned from originally, (which is
1008 the common case for the upstream-tracking scenario), then "git pull"
1009 with no explicit repository is suffcient, and it will default to
1010 pulling from the same repository as the original clone.
1012 [XXX: The structure of the preceding section follows that of the
1013 original hgbook. But an alternate structure that arranged to pull from
1014 the originally cloned repository (as would be common) would allow for
1015 more straightforward use of git's features. For example, instead of
1016 the silly FETCH_HEAD stuff it would allow for "git fetch" and "git log
1017 master..origin" to be a very nice replacement for "hg
1018 incoming". Similarly, below, "git log origin..master" would make a
1019 nice replacement for "hg outgoing" which is something I didn't offer
1020 at all. One could also use git's remotes with the myriad repositories
1021 as used here, but it would require doing things like "git remote add
1022 <some-name> ../hello-pull" and that seems like a bit much to introduce
1023 for a turorial of this level. If nothing else, if the above section
1024 seems a little intimidating, understand that it's because things are
1025 not presented in the most natural "git way", (and I'm a little too
1026 tired to fix it tonight).]
1028 #### 2.8.2 Checking out previous revisions
1030 If any users of mercurial are reading this, they might wonder if
1031 there's a need for the equivalent of "hg update" after doing a "git
1032 pull". And the answer is no. Unlike mercurial, "git pull" and "git
1033 merge" will automatically update the workind-directory files as
1036 But there's another function provided by "hg update" which is to
1037 update the working-directory files to a particular revision. In git,
1038 this functionality is provided by the "git checkout" command. To
1039 checkout a particular branch, tag, or an arbitrary revions, simply
1040 give the appropriate name to the "git checkout" command. For example,
1041 to examine the files as they existed before the original typo
1042 introduction, we could do:
1044 $ git checkout 0a633bf5
1045 Note: moving to "0a633bf5" which isn't a local branch
1046 If you want to create a new branch from this checkout, you may do so
1047 (now or later) by using -b with the checkout command again. Example:
1048 git checkout -b <new_branch_name>
1049 HEAD is now at 0a633bf... Create a makefile
1051 The note that git gives us is to indicate that we are checking out a
1052 non-branch revision. This is perfectly fine if we are just exploring
1053 history, but if we actually wanted to use this revision as the basis
1054 for new commits, we would first have to create a new branch name as it
1057 For now, let's return back to the tip of the master branch by just
1058 checking it out again:
1060 $ git checkout master
1061 Previous HEAD position was 0a633bf... Create a makefile
1062 Switched to branch "master"
1064 #### 2.8.3 Pushing changes to another repository
1066 Git lets us push changes to another repository, from the repository
1067 we’re currently visiting. As with previous examples, above, we’ll
1068 first create a temporary repository to push our changes into. But
1069 instead of using "git clone", this time we'll use "git init" to make a
1070 repository from an empty directory. We do this to create a "bare"
1071 repository which is simply a repository that has no working-directory
1072 files associated with it. In general, you should only push to bare
1079 Initialized empty Git repository in /tmp/hello-push/
1081 And then we'll go back to our my-hello repository to perform the
1082 push. Since this is our very first push into this repository we need
1083 to tell git which branches to push. The easiest way to do this is to
1084 use --all to indicate all branches:
1087 $ git push ../hello-push --all
1088 updating 'refs/heads/master'
1089 from 0000000000000000000000000000000000000000
1090 to 839b58d021c618bd0e1d336d4d5878a0082672e6
1092 Done counting 18 objects.
1093 Deltifying 18 objects...
1095 Writing 18 objects...
1097 Total 18 (delta 3), reused 0 (delta 0)
1098 Unpacking 18 objects...
1100 refs/heads/master: 0000000000000000000000000000000000000000 -> 839b58d021c618bd0e1d336d4d5878a0082672e6
1102 For subsequent pushes we don't need to specify --all as "git push"
1103 will push all branches that exist in both the local and remote
1106 What happens if we try to pull or push changes and the receiving
1107 repository already has those changes? Nothing too exciting.
1109 $ git push ../hello-push
1110 Everything up-to-date
1112 #### 2.8.4 Sharing changes over a network
1114 The commands we have covered in the previous few sections are not
1115 limited to working with local repositories. Each works in exactly the
1116 same fashion over a network connection; simply pass in a URL or an ssh
1117 host:/path/name specification instead of a local path.
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