Protect characters being misinterpreted as markup (_, <, and >).

[hgbook-git] / tour.mdwn

## Chapter 2
A tour of git: the basics

### 2.0 Copyright

This document is a modified version of a document originally titled
"Distributed revision control with Mercurial" and originally authored
by Bryan O’Sullivan. The original document was obtained from
<http://hgbook.red-bean.com/>.

Copyright © 2006, 2007 Bryan O’Sullivan.

This material may be distributed only subject to the terms and
conditions set forth in version 1.0 of the Open Publication
License. Please refer to Appendix D for the license text.

As this is a modified version, the name of Bryan O'Sullivan is used
only to properly credit him with the original text. The appearance of
his name here explicitly does not assert or imply his endorsement of
this modified document.

Portions Copyright © 2007 Carl Worth.

Changes made by Carl include the following:

  * 2007-09-27:
    * Convert from HTML to markdown source syntax
    * Eliminate all content except Chapter 2 and Appendix D
    * Eliminate line numbers from examples
    * Modified to describe git instead of mercurial

The source of this modified version can be obtained via git:

        git clone git://cworth.org/git/hgbook-git

or

        git clone http://cworth.org/git/hgbook-git

and can be [browsed online](http://git.cworth.org/git/hgbook-git)

### 2.1 Installing git on your system

Prebuilt binary packages of git are available for many popular
operating systems. These make it easy to start using git on your
computer immediately.

#### 2.1.1 Linux

Because each Linux distribution has its own packaging tools, policies,
and rate of development, it’s difficult to give a comprehensive set of
instructions on how to install git binaries. The version of
git that you will end up with can vary depending on how active
the person is who maintains the package for your distribution.

To keep things simple, I will focus on installing git from the
command line under the most popular Linux distributions. Most of these
distributions provide graphical package managers that will let you
install git with a single click. The package name to look for is
often git, but is sometimes git-core, (due to an unfortunate name
with git, meaning GNU Interactive Tools).

  * Debian

        apt-get install git-core

  * Fedora Core

        yum install git

  * Gentoo

        emerge dev-util/git

  * OpenSUSE

        yum install git

  * Ubuntu

        apt-get install git-core

#### 2.1.2 Mac OS X

A git-core package is available through
[macports](http://macports.org). Once macports is enabled, the command
to install git is:

        port install git-core

#### 2.1.3 Windows

Git has long been available as part of cygwin, and works reasonably
well in that environment. Some people find cygwin a particularly
inelegant approach to running git and would prefer a "native"
solution. To this end, the [msysgit
project](http://code.google.com/p/msysgit/) is rapidly putting
together a solution including various packages with full
installers. These include GitMe, a package to install the entire
development environment necessary to work on improving the msysgit
port of git, and WinGit, a package for installing just git itself
without the development environment, (still in Alpha as of September
2007).

### 2.2 Getting started

To begin, we’ll use the “git version” command to find out whether git
is actually installed properly. Versions 1.5 and newer of git are much
more friendly to new users than versions 1.4 and older. If you aren't
yet running version 1.5 or newer, it's highly recommended that you
upgrade.

        $ git version
        git version 1.5.3.2

#### 2.2.1 Built-in help

Git provides a built-in help system. This is invaluable for those
times when you find yourself stuck trying to remember how to run a
command. If you are completely stuck, simply run “git help”; it will
print a brief list of commonly-used commands, along with a description
of what each does. If you ask for help on a specific command (such as
"git help init"), it prints more detailed information. [XXX: Does "git
help \<foo\>" work universally as a built-in or does it expect man to be
present and just call out to "man git-\<foo\>"?]

        [XXX: The original hgbook includes the complete output of "hg
        help init" at this point. I'm not including the corresponding
        "git help init" output as it would be excessively long. The
        description alone is quite reasonable, (other than a
        not-too-helpful aside about the obsolete git-init-db command),
        but it only comes after a full screen's worth of options
        details. Might it make sense to have a more summarized help
        output for "git help <foo>" than all of the documentation
        available for git-<foo>? And perhaps alos provide a "git -v
        help" similar to "hg -v help" for more?]

### 2.3 Working with a repository

In git, everything happens inside a repository. The repository
for a project contains all of the files that “belong to” that project,
along with a historical record of the project’s files.

There’s nothing particularly magical about a repository; it is simply
a directory tree in your filesystem that git treats as
special. You can rename or delete a repository any time you like,
using either the command line or your file browser.

#### 2.3.1 Creating a copy of a remote repository

The "git clone" command is used to create a local copy of a remote
repository. This is generally the first git operation you will use
when beginning to work with an existing project.

We've assembled a simple repository that will be used in the examples
throughout this chapter. Go ahead and clone this repository now so
that you will be able to follow along:

        $ git clone git://cworth.org/git/hello
        Initialized empty Git repository in /tmp/hello/.git/
        remote: Generating pack...
        remote: Done counting 15 objects.
        remote: Deltifying 15 objects...
        remote:  100% (15/15) done
        remote: Total 15 (delta 2), reused 15 (delta remote: 2)
        Indexing 15 objects...
         100% (15/15) done
        Resolving 2 deltas...
         100% (2/2) done

If for some reason you are prevented from talking on the git: port,
then there is also the capability to clone a repository (less
efficiently) over http:

        $ git clone http://cworth.org/git/hello
        Initialized empty Git repository in /tmp/hello/.git/
        Getting alternates list for http://cworth.org/git/hello
        Getting pack list for http://cworth.org/git/hello
        Getting index for pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
        Getting pack 04ecb061314ecbd60fa0610ecf55a1cbf85ea294
         which contains a1a0e8b392b17caf50325498df54802fe3c03710
        walk a1a0e8b392b17caf50325498df54802fe3c03710
        walk 72d4f10e4a27dbb09ace1503c20dbac1912ee451
        walk 13ed136b983a9c439eddeea8a1c2076cffbb685f
        walk 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
        walk db7117a9dd9a6e57e8632ea5848e1101eee0fbde

If our clone succeeded, we should now have a local directory called
hello. This directory will contain some files.

        $ ls -l
        total 4
        drwxr-xr-x 3 cworth cworth 4096 2007-09-27 16:40 hello
        $ ls hello
        hello.c  Makefile

These files have the same contents and history in our repository as
they do in the repository we cloned.

Every git repository is complete, self-contained, and
independent. It contains its own private copy of a project’s files and
history. A cloned repository remembers the location of the repository
it was cloned from, but it does not communicate with that repository,
or any other, unless you tell it to.

What this means for now is that we’re free to experiment with our
repository, safe in the knowledge that it’s a private “sandbox” that
won’t affect anyone else.

#### 2.3.2 Creating copy of a local repository

As mentioned above, a repository can be copied through normal
file-copying commands. But it's useful to use "git clone" even when
just making a local copy of a repository. Using "git clone" will be
much faster and will use much less space than a normal copy. In fact,
local clones are impressively fast. Go ahead and make a local clone
now as follows:

        $ git clone hello hello-clone
        Initialized empty Git repository in /tmp/hello-clone/.git/
        0 blocks

        [XXX Git says "empty" here, (presumably from the git-init
        part), but shouldn't the command also report the succesful
        clone which makes it non-empty? And what the heck does "0
        blocks" mean? This is not friendly output.]

In fact, let's make a couple more clones at the same time. This isn't
just to drive home the speed and storage benefits of local clones, but
we'll use each of these cloned repositories in Section 2.8 when
discussing how to move commits between repositories:

           $ git clone hello hello-pull
           $ git clone hello hello-fetch
           $ git clone hello hello-remote

#### 2.3.2 What’s in a repository?

When we take a more detailed look inside a repository, we can see that
it contains a directory named .git. This is where git keeps all
of its metadata for the repository.

        $ cd hello
        $ ls -a
        .  ..  .git  hello.c  Makefile

The contents of the .git directory and its subdirectories are private
to git. Every other file and directory in the repository is
yours to do with as you please.

To introduce a little terminology, the .git directory is the “real”
repository, and all of the files and directories that coexist with it
are said to live in the working directory. An easy way to remember the
distinction is that the repository contains the history of your
project, while the working directory contains a snapshot of your
project at a particular point in history.

### 2.4 A tour through history

One of the first things we might want to do with a new, unfamiliar
repository is understand its history. The “git log” command gives us a
view of history.

        $ git log
        commit a1a0e8b392b17caf50325498df54802fe3c03710
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 15:43:07 2005 -0700
        
            Trim comments.
        
        commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:58 2005 -0700
        
            Get make to generate the final binary from a .o file.
        
        commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:43 2005 -0700
        
            Introduce a typo into hello.c.
        
        commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
        Author: Bryan O'Sullivan <mpm@selenic.com>
        Date:   Fri Aug 26 01:21:28 2005 -0700
        
            Create a makefile
        
        commit db7117a9dd9a6e57e8632ea5848e1101eee0fbde
        Author: Bryan O'Sullivan <mpm@selenic.com>
        Date:   Fri Aug 26 01:20:50 2005 -0700
        
            Create a standard "hello, world" program

By default, this command prints a brief paragraph of output for each
change to the project that was recorded. In git terminology, we
call each of these recorded events a commit.

The fields in a record of output from “git log” are as follows.

  * commit This field consists of a string of 40 hexadecimal characters.
    This is a unique identifier for referring to particular commits.
  * Author The identity of the person who authored the commit. This
    field consist of two sub-fields for the user's name and email
    address, (or at least an email-like idenitifer). Note that git
    stores a separate "Committer" field for the person who commited
    the change, (since often an author will email a change to a
    maintainer that commits it). The "git log" command doesn't display
    the Committer, but other git tools do.
  * Date The date and time on which the commit was authored, (again
    stored separately from the date the change was committed).
    timezone in which it was created. (The date and time are displayed
    in the timezone of the person who created the commit.)
  * commit message The text message that the creator of the commit
    entered to describe the commit, (generally a one-line summary
    followed by more supporting text).

The default output printed by “git log” is purely a summary; it is
missing a lot of detail.

#### 2.4.1 Commits, revisions, and talking to other people

As English is a notoriously sloppy language, and computer science has
a hallowed history of terminological confusion (why use one term when
four will do?), revision control has a variety of words and phrases
that mean the same thing. If you are talking about git history
with other people, you will find that what we have called a “commit”
is often called a "revision". In other systems, a similar notion
is referred to as a "changeset". You might even see abbreviations of
these terms such as "rev", "change", or even "cset".

While it may not matter much what word you use to refer to the concept
of “a commit”, it's important to know how to name “a specific
commit”. We have already seen one means of referring to a particular
commit, the 40-character hexadecimal string shown by "git log". These
commit identifiers are powerful because they are permanent, unique
identifiers that always identify the same commit in any copy of a
repository. If two users are examining a working directory associated
with the same commit identifier, then those two users have precisely
the same contents in all files, and exactly the same history leading
to that commit.

So there are places where it is often important to archive the
complete commit identifier, (perhaps in bug-tracking systems to
indicate a specific commit that fixes a bug, for example). But often,
in more casual settings, it's more convenient to use abbreviated
commit identifiers. Git accept any unique prefix of a commit
identifier, (and for reasonably-sized project the first 8 or 10
characters are almost always unique).

And unlike the permanent commit identifiers, git also provides
transient means of identifying commits. In fact, in day-to-day use of
git, you will probably use these names more than commit
identifiers. One example is branch names, (such as the default
"master" branch in any git repository), or any project-specific branch
names such as "stable", "experimental", or "crazy-insane-changes". Git
also provides a special name "HEAD" which always refers to the current
branch.

#### 2.4.2 Naming related commits

Git offers simple ways to name revisions that are related to
particular revisions in the history. One syntax is the ~ suffix which
refers to the parent of a commit, or if followed by a number, to the
Nth parent. For example, since "HEAD" refers to the most recent commit
in the current branch, "HEAD~", refers to the previous commit, and
"HEAD~2" refers to two commits back in the history.

Another useful syntax is .. which can be used to specify a range of
commits. So "origin..master" specifies everything that has been
committed to master since it diverged from origin.

#### 2.4.3 Viewing specific revisions

You can use "git log" to explore the range syntax just introduced. For
example, to see a list of the most recent 3 revisions you can use
"HEAD~3..", (the destination of the range is implicitly HEAD in this
case):

        $ git log HEAD~3..
        commit a1a0e8b392b17caf50325498df54802fe3c03710
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 15:43:07 2005 -0700
        
            Trim comments.
        
        commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:58 2005 -0700
        
            Get make to generate the final binary from a .o file.
        
        commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:43 2005 -0700
        
            Introduce a typo into hello.c.

#### 2.4.4 Other log filters

Besides filtering by commit identifiers, git allows you to easily
filter the log output according to which files (or directories) are
modified by listing them after "--" which is necessary to distinguish
commit names from file names:

        $ git log -- Makefile
        commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:58 2005 -0700
        
            Get make to generate the final binary from a .o file.
        
        commit 0a633bf58b45fcf1a8299d3c82cd1fd26d3f48f2
        Author: Bryan O'Sullivan <mpm@selenic.com>
        Date:   Fri Aug 26 01:21:28 2005 -0700
        
            Create a makefile

And "git log" can also filter based on the dates at which commits were
created:

        $ git log --since="2 weeks ago" --until="yesterday"

        [XXX: By default, "git log" displays author dates as "Date"
        but then uses commit dates when given a --since option. That
        seems like broken defaults to me. Why the inconsistency?]

Another useful option is -n or --max-count which, unsurprisingly,
limits the maximum number of commits to be displayed.

#### 2.4.5 More detailed information

While the default information printed by “git log” is useful if you
already know what you’re looking for, you may need to see more details
of the change, such as the "diffstat" information with --stat:

        $ git log --stat --max-count=3
        commit a1a0e8b392b17caf50325498df54802fe3c03710
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 15:43:07 2005 -0700
        
            Trim comments.
        
         hello.c |    8 ++------
         1 files changed, 2 insertions(+), 6 deletions(-)
        
        commit 72d4f10e4a27dbb09ace1503c20dbac1912ee451
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:58 2005 -0700
        
            Get make to generate the final binary from a .o file.
        
         Makefile |    2 ++
         1 files changed, 2 insertions(+), 0 deletions(-)
        
        commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:43 2005 -0700
        
            Introduce a typo into hello.c.
        
         hello.c |    2 +-
         1 files changed, 1 insertions(+), 1 deletions(-)

Or perhaps you'd like to see the actual patch content of each change,
which you can get with -p. That commit with the word typo in its name
looks suspicous, so let's tak a closer look. Remember that we can name
it as master~3, HEAD~3, or any prefix of its commit identifier, (such
as 13ed136b):

        $ git log -p -n 1 13ed136b
        commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:43 2005 -0700
        
            Introduce a typo into hello.c.
        
        diff --git a/hello.c b/hello.c
        index ed55ec0..80b260c 100644
        --- a/hello.c
        +++ b/hello.c
        @@ -11,6 +11,6 @@
         
         int main(int argc, char **argv)
         {
        -       printf("hello, world!\n");
        +       printf("hello, world!\");
                return 0;
         }

Of course, wanting to see all this information for a single commit is
such a common operation that it's given its own name in git, "git
show". So "git show 13ed136b" is a much easier way to get exactly the
same output:

        $ git show 13ed136b
        commit 13ed136b983a9c439eddeea8a1c2076cffbb685f
        Author: Bryan O'Sullivan <bos@serpentine.com>
        Date:   Tue Sep 6 13:15:43 2005 -0700
        
            Introduce a typo into hello.c.
        
        diff --git a/hello.c b/hello.c
        index ed55ec0..80b260c 100644
        --- a/hello.c
        +++ b/hello.c
        @@ -11,6 +11,6 @@
         
         int main(int argc, char **argv)
         {
        -       printf("hello, world!\n");
        +       printf("hello, world!\");
                return 0;
         }

### 2.5 All about command options

Let’s take a brief break from exploring git commands to discuss
a pattern in the way that they work; you may find this useful to keep
in mind as we continue our tour.

Git has a consistent and straightforward approach to dealing
with the options that you can pass to commands. It follows the
conventions for options that are common to modern Linux and Unix
systems.

  * Most options have long names. For example, as we’ve already seen,
    the “git log" command accepts a --max-count=\<number\> option.
  * Some options have short, single-character names. Often these are
    aliases for long commands, (such as "-n \<number\>" instead of
    --max-count=\<number\>), but sometimes the option exists in
    short-form with no long-form equivalent, (such as -p). [XXX: It
    wouldn't hurt to fix this by adding --patch, etc. right?]
  * Long options start with two dashes (e.g. --max-count), while short
    options start with one (e.g. -n).

  * Option naming and usage is consistent across commands. For
    example, every command that lets you specify a commit identifier
    or range will accept the same expressions, (HEAD~3,
    origin..master, 72d4f10e, etc), while any command that can be
    limited by paths will accept the same expressions ("-- doc/
    some-file.c"), etc.

Many commands that print output of some kind can be made more quiet by
passing the -q or --quiet options.

### 2.6 Making and reviewing changes

Now that we have a grasp of viewing history in git, let’s take a look
at making some changes and examining them. You should be working
within the "hello" directory that we originally cloned.

In the repository, we have a file hello.c that contains the classic
“hello, world” program. But this program currently has a syntax error
and won't compile, (there's a missing 'n' between the '\' and the '"'
on the line containing printf). Let's fix that now. You can use a text
editor to correct the file, or you can copy-and-paste the sed command
below:

        $ sed -i '/printf/s/\\"/\\n"/' hello.c

After you've made that change, the “git status” command will tell you
what git knows about the files in the repository.

        $ ls 
        hello.c  Makefile
        $ git status
        # On branch master
        # Changed but not updated:
        #   (use "git add <file>..." to update what will be committed)
        #
        #       modified:   hello.c
        #
        no changes added to commit (use "git add" and/or "git commit -a")

First "git status" tells us that the current branch is "master". This
means that the master branch is what will be updated when we create a
new commit.

Note: In git a branch is a very simple notion---it's simply a name
that points to a particular commit, (literally nothing more than a
pointer---look at the contents of .git/refs/heads/master if you're
curious). The fact that a branch is so light is what makes the
creation of new branches an instantaneous operation in git. Together
with the ease of merging, git makes branches a joy to work with. But
we'll delay talk of branching and merging to a future chapter.

Next “git status” prints a line with "modified" for each modified
file---in this case just hello.c. The “git status” command will not
print any output for files that have not been modified.

Notice that we didn’t need to inform git that we were going to modify
the file before we started, or that we had modified the file after we
were done; it was able to figure this out itself.

It’s a little bit helpful to know that we’ve modified hello.c, but we
might prefer to know exactly what changes we’ve made to it. To do
this, we use the “git diff” command.

        $ git diff
        diff --git a/hello.c b/hello.c
        index 9a3ff79..ea364d3 100644
        --- a/hello.c
        +++ b/hello.c
        @@ -7,6 +7,6 @@
         
         int main(int argc, char **argv)
         {
        -       printf("hello, world!\");
        +       printf("hello, world!\n");
                return 0;
         }

### 2.7 Recording changes in a new commit

We can modify files, build and test our changes, and use “git status”
and “git diff” to review our changes, until we’re satisfied with what
we’ve done and arrive at a natural stopping point where we want to
record our work in a new commit.

If you'd like, go ahead and build the program now with "make" and run
it to ensure it works. Everything should look good so we're just about
ready to make a commit. Unsurprisingly, we'll be using "git commit" to
do this.

#### 2.7.1 Introducing yourself to git

Before you run "git commit" though, you should introduce yourself to
git.  Git records your name and email address with each change that
you commit, (as both author and committer unless you tell it
otherwise), so that you and others will later be able to tell who made
each change.

Git tries to automatically figure out a sensible name and address to
attribute to both author and committer if you haven't explicitly told
it a name and address. And it tries a lot, (detailed below). If you're
not interested in these details, you might want to skip to the next
section which explains how to avoid all this guesswork and tell git
what your name and email address are.

Here is a list of all the guessing that git will attempt. It will
attempt each of the following methods, in order, (stopping for each of
the author and committer name and email as soon as a value is found):

  1. If you specify a --author option to the “git commit” command on
     the command line, followed by a "Real Name \<email@example.com\>"
     string, then this name and addresss will be used for the author
     fields. The committer fields will still be determined as
     below. This option is very helpful for when applying a commit
     originally authored by someone other than yourself.
  2. If any of the `GIT_AUTHOR_NAME`, `GIT_AUTHOR_EMAIL`,
     `GIT_COMMITTER`_NAME, or `GIT_COMMITER_EMAIL` environment variables
     are set, then those values will be used for the corresponding
     fields.
  3. If you have a file in your home directory called .gitconfig, with
     name or email settings in the [user] section, then these values
     will be used to set any remaining author and committer
     fields. For more details on the contents of this file, refer to
     section 2.7.1 below.
  4. If you have a file in the local repository called .git/config,
     again with name or email settings in the [user] section, then
     these values will be used to set any remaining author and
     committer fields.
  5. If you have set the `EMAIL` environment variable, this will be used
     to set author and committer email addresses if still unset.
  6. git will query your system to find out your real name from
     available GECOS field and your username, hostname, and domain to
     construct an email address, (or at least an identifier resembling
     an email address).

If all of these mechanisms fail, "git commit" will fail, printing an
error message instructing you how to use "git config" to tell git your
name and email address.

You should think of the `GIT_AUTHOR`/`COMMITER_NAME`/`EMAIL` environment
variables and the --author option to the “git commit” command as ways
to override git’s default selection. For normal use, the simplest and
most robust way to set your information is by creating a .gitconfig
file, (either manually or with the "git config" command); see below
for details.

##### Creating a git configuration file

To set your name and email address, just use the following commands:

        git config --global user.name "Your Name"
        git config --global user.email "you@example.com"

The --global option means that this command will set global
information, (affecting all repositories on this machine), in the
.gitconfig file in your home directory. Alternately, you could omit
the --global which would make the change take effect only in the local
repository. This is convenient if you want to have different email
addresses associated with different projects, for example.

Of course, git's configuration file is a simple-to-edit plain-text
file, so instead of using the above commands, you can also just edit
the files directly. Use your favorite editor to create a file called
.gitconfig in your home directory, (or if you ran the above commands
then it will be there already). The initial contents of your
.gitconfig should look like this.

        # This is a git configuration file. 
        [user]
                name = Your Name
                email = you@example.com

Similarly, you can make a repository-specific configuration by editing
.git/config in the local repository. It will already have some
sections present, (created by the "git clone"), just add a [user]
section as above.

The “[user]” line begins a section of the config file, so you can read
the “name = ...” line as meaning “set the value of the name item in
the user section”. This is the same notion expressed with the
"user.name" syntax on the git-config command line.  A section
continues until a new section begins, or the end of the file. Git
ignores empty lines and treats any text from “#” to the end of a line
as a comment.

##### Choosing your name and email

You can use any text you like as the value of the name and email
configuration items, since this information is for reading by other
people, not for interpreting by git. It is conventional to use your
actual name as well as a valid email address. But some poepl, (notably
Linus Torvalds, the original author of git), actually like the default
username@hostname convention that git falls back on without any
additional information about an email address. There's no requirement
that the email address actually be valid, and perhaps it's useful to
be reminded which machine was used to create particular commits. So
choose the name and email you wish, or follow a particular project's
conventions.

#### 2.7.2 Writing a commit message

When we commit a change, git drops us into a text editor to
enter a message that will describe the modifications we’ve made in
this commit. This is called the commit message. It will be a record
for readers of what we did and why, and it will be printed by “git log”
after we’ve finished committing.

        $ git commit -a

Note: The -a on the command-line instructs git to commit the new
content of *all* tracked files that have been modified. This is a
convenience over explicitly listing filenames to be committed on the
"git commit" command line. It is useful to use "git commit \<files\>"
when there is a need to commit only some subset of the files that have
been modified.

If new files need to be committed for the first time, just use "git
add \<file\>" before "git commit -a". If a file needs to be removed,
just remove it as normal before committing and "git commit -a" will
notice that---it does not need to be explicitly told about the
removal.

The editor that the “git commit” command drops us into will contain an
empty line, followed by a number of lines starting with “#”. These
lines contain the same information as seen in "git status" before:

        empty line
        # Please enter the commit message for your changes.
        # (Comment lines starting with '#' will not be included)
        # On branch master
        # Changes to be committed:
        #   (use "git reset HEAD <file>..." to unstage)
        #
        #       modified:   hello.c
        #
        # Untracked files:
        #   (use "git add <file>..." to include in what will be committed)
        #
        #       hello
        #       hello.o

Notice that two untracked files (hello and hello.o) have now appeared
from the build process. Git is reminding us of these in case we
intended to commit them as well, (in which case we would need to "git
add" them). We don't actually want to commit these files so we will
ignore them for now, (and we could tell git to ignore them as well by
listing them in a .gitignore file).

Git will ignore all lines in the commit message that start with “#”;
it uses them only to give us information on the commit. Modifying or
deleting these lines has no effect.

#### 2.7.3 Writing a good commit message

A good commit message will generally have a single line that
summarizes the commit, a blank line, and then one or more pargraphs
with supporting detail. Since many tools only print the first line of
a commit message by default, it’s important that the first line stands
alone.

One example of a first-line-only viewer is "git log
--pretty=short". Other examples include graphical history viewers such
as gitk and gitview, and web-based viewers such as gitweb and cgit.

Here’s a real example of a commit message that doesn’t follow
this guideline, and hence has a summary that is not readable.

        $ git log --pretty=short
        commit 3ef5535144da88a854f7930503845cd44506c2e2
        Author: Censored Person <censored.person@example.org>
        
            include buildmeister/commondefs.   Add an exports and install

As far as the remainder of the contents of the commit message are
concerned, there are no hard-and-fast rules. git itself doesn’t
interpret or care about the contents of the commit message, though
your project may have policies that dictate a certain kind of
formatting.

My personal preference is for short, but informative, commit messages
that tell me something that I can’t figure out with a quick glance at
the output of "git log --stat" or “git log -p", (so repeating the list
of all modified files is not useful, for example).

#### 2.7.4 Aborting a commit

If you decide that you don’t want to commit while in the middle of
editing a commit message, simply exit from your editor without saving
the file that it’s editing. This will cause nothing to happen to
either the repository or the working directory.

#### 2.7.5 Admiring our new handiwork

Once we’ve finished the commit, we can use the “git show” command to
display the commit we just created. As discussed previously, this
command produces output that is identical to “git log -p”, but for
only a single revision, (and the most recent revision by default):

        $ git show
        commit fd21e5d6c5eedee70137229ebf348c25181812ab
        Author: Carl Worth <cworth@cworth.org>
        Date:   Fri Sep 28 12:50:16 2007 -0700
        
            Fixed the typo so the program actuall complies now.
        
        diff --git a/hello.c b/hello.c
        index 9a3ff79..ea364d3 100644
        --- a/hello.c
        +++ b/hello.c
        @@ -7,6 +7,6 @@
         
         int main(int argc, char **argv)
         {
        -       printf("hello, world!\");
        +       printf("hello, world!\n");
                return 0;
         }

Note that you will not see the same commit identifier for your commit,
even if the change you made is identical to mine. The commit
identifier incorporates not only the contents of the files, but commit
message, the author and committer names and emails, and the author and
commit dates. (OK, so now you probably know enough to be able to guess
the right command to produce a commit with exactly the commit
identifier shown above. Can you do it?)

#### 2.7.6 Fixing up a broken commit (before anyone else sees it)

So now that we've cloned a local repository, made a change to the
code, setup our name and email address, and made a careful commit,
we're just about ready to share our change with the world. But wait,
that commit message has some really embarrassing misspellings in
it. Wouldn't it be nice to touch those up before I post this commit
with a never-to-be-changed again commit identifier?

This is the exact situation for which "git commit --amend" was

So I can just run that now and fix the broken commit message:

        $ git commit --amend

Here's the final result:

        $ git show
        commit 3c54ac672ec1130b36837f1b708054a7a1d402de
        Author: Carl Worth <cworth@cworth.org>
        Date:   Fri Sep 28 12:50:16 2007 -0700
        
            Fixed the typo so the program actually compiles now.
        
        diff --git a/hello.c b/hello.c
        index 9a3ff79..ea364d3 100644
        --- a/hello.c
        +++ b/hello.c
        @@ -7,6 +7,6 @@
         
         int main(int argc, char **argv)
         {
        -       printf("hello, world!\");
        +       printf("hello, world!\n");
                return 0;
         }

Note that we didn't use "commit -a" this time. This means that "git
commit --amend" will amend only the commit message and not any of the
actual files being tracked, (even if some of them had been modified
between the commits).

It's also possible to use "git commit -a --amend" to similarly fix up
mistakes noticed in code. That will replace the most recent commit
with a different commit based on any new changes to files.

I do feel a little hesitant to mention "git commit -a --amend". It's a
handy command for fixing up something like a misspelling in a comment
in the code. But if there is anything more significant than that, then
it would generally be better to create an additional commit rather
than amending an existing commit. This is important for several
reasons:

  * The amend operation will destroy a state that was previously saved
    in a commit. If it's just the commit message being changed, then
    that's no big deal. But if the contents are being amended, then a
    mistake could eliminate something valuable.

  * All commits should be logically independent and as small as
    possible. Abusing "git commit -a --amend" can cause a small commit
    to grow and acquire unrelated changes.

It's worth emphasizing the value of minimal, independent commits. The
smaller the changes are the more useful the history will be when
actually using the history, not just viewing it. This is particularly
important when doing "git bisect"---that's a powerful tool for
isolating a single commit that introduces a bug. And it's much more
helpful when the commit it isolates is as small as possible.

One advantage of using git over some other systems is that the commit
speed is blazingly fast. The tool doesn't punish you at all for
committing as often as you get our project into a state that is worth
saving. "Commit early, commit often" is a well-supported mode of
operation with git.

### 2.8 Sharing changes

We mentioned earlier that repositories in git are
self-contained. This means that the commit we just created exists
only in our hello repository. Let’s look at a few ways that we can
propagate this change into other repositories.

#### 2.8.1 Pulling changes from the original repository

Recall that in Section 2.3.2 we made several local clones of the hello
repository before we made any commits. This allows us to simulate what
happens when upstream changes have been committed after you originally
cloned.

The simplest, (and quite common), scenario is that you inherently
trust any changes in the original repository and you want to pull
these directly into your clone. This might be the case if you are
using git simply to track the progress of a project without making any
changes.

In this case, the operation is as simple as just calling "git pull":

        $ cd ../hello-pull
        $ git pull
        remote: Generating pack...
        Unpacking 3 objects...
         100% (3/3) done
        remote: Done counting 5 objects.
        Result has 3 objects.
        Deltifying 3 objects...
         100% remote: (3/3) done
        Total 3 (delta 1), reused 0 (delta 0)
        * refs/remotes/origin/master: fast forward to branch 'master' of /tmp/hello
          old..new: a1a0e8b..3c54ac6
        Updating a1a0e8b..3c54ac6
        Fast forward
         hello.c |    2 +-
         1 files changed, 1 insertions(+), 1 deletions(-)

XXX: Git is fairly noisy here, but a user has little need to care
about the several stages of operation involved here. As a tutorial
writer I'd like to say "ignore all that progress stuff, and look at
where the stat information starts" but it's hard for a new user to
even be able to understand that. I think it would be ideal if all of
the progress-tracking spew were reduced to a single line. Something
like "Computing (100%) Transferring (100%)" or whatever.

After (lots!) of progresss indication, git gives a report of which
files were modified, (which is very useful for getting a quick feel
for what happened). If you would like more details on what changes
came in, git provides a range that is perfect for examining. Let's
take a look (again, the commit identifiers will be different for you
--- just copy-and-paste the range that git prints):

        $ git log a1a0e8b..3c54ac6
        commit 3c54ac672ec1130b36837f1b708054a7a1d402de
        Author: Carl Worth <cworth@cworth.org>
        Date:   Fri Sep 28 12:50:16 2007 -0700
        
            Fixed the typo so the program actually compiles now.

As expected, we received just the one commit.

So that's all that's needed in the common case. Just run "git pull"
everytime you want to pull in new changes that have landed in the
upstream repository.

Note: Mercurial users who are reading this might wonder if there's a
need for the equivalent of "hg update" after doing a "git pull". And
the answer is no. Unlike mercurial, "git pull", (and "git merge") will
automatically update the working-directory files as necessary.

#### 2.8.2 Using fetch and merge separately to pull

Sometimes you may not know if you want to pull in the changes from the
remote repository or not. It's useful to be able to examine them
before accepting them into our branch. The "git pull" command shown in
the previous section is conceptually the combination of two command,
"git fetch" and "git merge". We can use these commands separately to
examine the change before accepting it.

So let's do that within the hello-fetch clone we made earlier. First
we will do the fetch:

        $ cd ../hello-fetch
        $ git fetch
        remote: Generating pack...
        Unpacking 3 objects...
        remote: Done counting 5 objects.
        Result has 3 objects.
        Deltifying 3 objects...
         100% remote: (3/3) done
        Total 3 (delta 1), reused 0 (delta 0)
         100% (3/3) done
        * refs/remotes/origin/master: fast forward to branch 'master' of /tmp/hello/
          old..new: a1a0e8b..3c54ac6

You may notice that the output here looks very much like the first
portion of the output from "git pull". This is no coincidence.  The
new changes have been "fetched" into the current repository and are
stored into "origin/master" and have not been into the current
"master" branch. Remember that "master" is our current branch. So now,
"origin/master" is the state of the master branch that exists in the
"origin" repository, (the one we cloned from).

The most convenient way to examine the fetched changes is with the
"master..origin" range notation:

        $ git log master..origin
        commit 3c54ac672ec1130b36837f1b708054a7a1d402de
        Author: Carl Worth <cworth@cworth.org>
        Date:   Fri Sep 28 12:50:16 2007 -0700
        
            Fixed the typo so the program actually compiles now.

Another helpful way of visualizing what happened with "git fetch" here
is to run "gitk --all", which gives a graphical representation of all
branches. Here is what it would look like:

[[img gitk-fetch.png]]

Notice that origin/master points to a single commit that was committed
on top of the state pointed to by the "master" branch.

Let's assume we are happy with the changes and we want to include them
into our master branch. To do this we simply run "git merge origin":

        $ git merge origin
        Updating a1a0e8b..3c54ac6
        Fast forward
         hello.c |    2 +-
         1 files changed, 1 insertions(+), 1 deletions(-)

Again, you'll see that this precisely matches the final portion of the
output from "git pull". Using "git fetch" and "git merge" let us
achieve exactly what "git pull" did, but we were able to stop in the
middle to examine the situation, (and we could have decided to reject
the changes and not merge them---leaving our master branch unchanged).

##### On merges and "fast forward"

You'll notice that we've been seeing the phrase "fast forward" several
times. This is a special-case operation performed by "git merge" where
a branch can be advanced along a linear sequence. This happens
whenever you pull changes that build directly on top of the same
commit you have as your most recent commit. In other words, there was
never any divergence or simultaneous commits created in parallel in
multiple repositories. If there had been parallel commits, then "git
merge" would actually introduce a new merge commit to tie the two
commits together.

When a non-fast-forward merge occurs, there is always the possibility
that a conflict occurs. In this case, "git merge" will leave conflict
markers in the files and instruct you to resolve the conflicts. When
you are finished, you would issue a "git commit -a" to create the
merge commit.

#### 2.8.3 Using "git remote" to pull changes other repositories

We've already described how "git pull" will pull in changes from the
repository which was the origin of the clone operation. Git also
provides excellent support for pulling changes from any other
repository as well, (distributed, rather than centralized
development).

If you have a situation where you want to pull a single time from some
repository, then you can simply give the path or URL of the repository
on the "git pull" command line. However, it's often the case that if
you want to pull changes from a repository once, you'll want to pull
changes from that same repository again in the future. This is where
the "git remote" notion is extremely useful---it allows you to
associate simple names, (and behaviors), with remote repository URLs

We've already seen one instance of "git remote" which is the creation
of the "origin" remote which happens automatically during "git
clone". Let's now create another. Let's assume you are going to be
working in the hello-remote repository and you'd like to pull changes
from the hello-pull repository, where your friend "fred" has been
making changes. Here's how to setup the new remote:

       $ cd ../hello-remote
       $ git remote add fred ../hello-pull

So that's a "git remote add" command line followed by an arbitrary
name you'd like for the new remote (fred) and the URL of the remote
(../hello-pull). Obviously, the URL could be a git:// URL or any other
git-supported URL in addition to a local path.

The "git remote" command is really just a helper for adding some
entries to the .git/config file. You might find it more convenient to
edit that file directly once you get comfortable with things.

At this point the name "fred" will work much like the name "origin"
has worked in previous examples. For example, we can fetch the changes
fred has made with "git fetch fred":

        $ git fetch fred
        remote: Generating pack...
        Unpacking 3 objects...
        remote: Done counting 5 objects.
        Result has 3 objects.
        Deltifying 3 objects...
         100% remote: (3/3) done
        Total 3 (delta 1), reused 0 (delta 0)
         100% (3/3) done
        * refs/remotes/fred/master: storing branch 'master' of ../hello-pull
          commit: 3c54ac6

Notice that this command-line only differs from the "git fetch" we did
previously by explicitly naming which remote should be fetched. We
could have explicitly said "git fetch origin" earlier.

We can also list all known remote-tracking branches with "git branch
-r":

        $ git branch -r
          fred/master
          origin/HEAD
          origin/master

These remote-tracking branches make it very easy to collaborate with
people as they are working on experimental features not yet ready for
upstream inclusion. For example, if fred's latest code is still
trashing filesystems then he might not want to push it out the the
project's primary repository. But he may still want my help with
it. So he can push it to a branch in his own repository for which I've
got a remote. Then on my next "git fetch fred" I might notice a new
branch called fred/trashes-filesystems and I can examine his code with
a command such as "git log ..fred/trashed-filesystems".

So lots of side collaboration can go on easily, and people working
only with the primary repository never even have to see this dangerous
code. It's distributed development at its finest.

#### 2.8.4 Checking out previous revisions

It's often useful to examine the working-tree state of some specific
revision other than the tip of some branch. For example, maybe you
would like to build a particular tagged version, or maybe you'd like
to test the behavior of the code before a particular change was
introduced. To do this, use "git checkout" and pass it the name of any
revision, (with a branch name, a tag name, or any other commit
identifier). For example, to examine our project before the original
typo was introduced:

        $ git checkout 0a633bf5
        Note: moving to "0a633bf5" which isn't a local branch
        If you want to create a new branch from this checkout, you may do so
        (now or later) by using -b with the checkout command again. Example:
          git checkout -b <new_branch_name>
        HEAD is now at 0a633bf... Create a makefile

The note that git gives us is to indicate that we are checking out a
non-branch revision. This is perfectly fine if we are just exploring
history, but if we actually wanted to use this revision as the basis
for new commits, we would first have to create a new branch name as it
describes.

If we were to use "git checkout" with a branch name, then that would
change the current branch, (meaning that any new commits would advance
that branch pointer).

For now, let's return back to the tip of the master branch by just
checking it out again:

        $ git checkout master
        Previous HEAD position was 0a633bf... Create a makefile
        Switched to branch "master"

#### 2.8.5 Pushing changes to another repository

As an unsurprising parallel to "git pull", git also provides "git
push" for pushing changes to another repository. Now, generally the
purpose of pushing to a repository is to have some "collaboration
point" where potentially multiple people might be pushing or
pulling. Because there might be multiple people pushing into the
repository at any point, it wouldn't make sense to have a
working-directory associated with this repository.

For this, git has the notion of a "bare" repository, which is simply a
repository with no working directory. Let's create a new bare
repository and push some changes into it:

        $ cd ..
        $ mkdir hello-bare
        $ cd hello-bare
        $ git --bare init --shared

The --shared option sets up the necessary group file permissions so
that other users in my group will be able to push into this repository
as well.

Now lets return to our hello repository and push some changes to this
new repository. Since this is our very first push into this repository
we need to tell git which branches to push. The easiest way to do this
is to use --all to indicate all branches:

        $ cd ../hello
        $ git push ../hello-bare --all
        updating 'refs/heads/master'
          from 0000000000000000000000000000000000000000
          to   3c54ac672ec1130b36837f1b708054a7a1d402de
        Generating pack...
        Done counting 18 objects.
        Deltifying 18 objects...
         100% (18/18) done
        Writing 18 objects...
         100% (18/18) done
        Total 18 (delta 3), reused 15 (delta 2)
        Unpacking 18 objects...
         100% (18/18) done
        refs/heads/master: 0000000000000000000000000000000000000000 -> 3c54ac672ec1130b36837f1b708054a7a1d402de

For subsequent pushes we don't need to specify --all as "git push" by
default pushes all branches that exist in both the local and remote
repositories. Also, as with pull, instead of explicitly specifying a
URL, you may also specify a remote to push to. And by default, after
cloning a repository, "git push" with no other arguments will attempt
to push back to the same origin repository. As this is often exactly
what is wanted, you may find that "git push" alone is often exactly
what you need.

What happens if we try to pull or push changes and the receiving
repository already has those changes? Nothing too exciting.

        $ git push ../hello-bare
        Everything up-to-date

## Appendix D
Open Publication License

Version 1.0, 8 June 1999 

### D.1 Requirements on both unmodified and modified versions

The Open Publication works may be reproduced and distributed in whole
or in part, in any medium physical or electronic, provided that the
terms of this license are adhered to, and that this license or an
incorporation of it by reference (with any options elected by the
author(s) and/or publisher) is displayed in the reproduction.

Proper form for an incorporation by reference is as follows: 

Copyright (c) year by author’s name or designee. This material may be
distributed only subject to the terms and conditions set forth in the
Open Publication License, vx.y or later (the latest version is
presently available at
[http://www.opencontent.org/openpub/][http://www.opencontent.org/openpub/]).

The reference must be immediately followed with any options elected by
the author(s) and/or publisher of the document (see section D.6).

Commercial redistribution of Open Publication-licensed material is
permitted.

Any publication in standard (paper) book form shall require the
citation of the original publisher and author. The publisher and
author’s names shall appear on all outer surfaces of the book. On all
outer surfaces of the book the original publisher’s name shall be as
large as the title of the work and cited as possessive with respect to
the title.

### D.2 Copyright

The copyright to each Open Publication is owned by its author(s) or
designee.

### D.3 Scope of license

The following license terms apply to all Open Publication works,
unless otherwise explicitly stated in the document.

Mere aggregation of Open Publication works or a portion of an Open
Publication work with other works or programs on the same media shall
not cause this license to apply to those other works. The aggregate
work shall contain a notice specifying the inclusion of the Open
Publication material and appropriate copyright notice.

Severability. If any part of this license is found to be unenforceable
in any jurisdiction, the remaining portions of the license remain in
force.

No warranty. Open Publication works are licensed and provided “as is”
without warranty of any kind, express or implied, including, but not
limited to, the implied warranties of merchantability and fitness for
a particular purpose or a warranty of non-infringement.

### D.4 Requirements on modified works

All modified versions of documents covered by this license, including
translations, anthologies, compilations and partial documents, must
meet the following requirements:

  1. The modified version must be labeled as such. 
  2. The person making the modifications must be identified and the
     modifications dated.
  3. Acknowledgement of the original author and publisher if
     applicable must be retained according to normal academic citation
     practices.
  4. The location of the original unmodified document must be identified. 
  5. The original author’s (or authors’) name(s) may not be used to
     assert or imply endorsement of the resulting document without the
     original author’s (or authors’) permission.

### D.5 Good-practice recommendations

In addition to the requirements of this license, it is requested from
and strongly recommended of redistributors that:

  1. If you are distributing Open Publication works on hardcopy or
     CD-ROM, you provide email notification to the authors of your
     intent to redistribute at least thirty days before your
     manuscript or media freeze, to give the authors time to provide
     updated documents. This notification should describe
     modifications, if any, made to the document.
  2. All substantive modifications (including deletions) be either
     clearly marked up in the document or else described in an
     attachment to the document.
  3. Finally, while it is not mandatory under this license, it is
     considered good form to offer a free copy of any hardcopy and
     CD-ROM expression of an Open Publication-licensed work to its
     author(s).

### D.6 License options

The author(s) and/or publisher of an Open Publication-licensed
document may elect certain options by appending language to the
reference to or copy of the license. These options are considered part
of the license instance and must be included with the license (or its
incorporation by reference) in derived works.

  1. To prohibit distribution of substantively modified versions
     without the explicit permission of the author(s). “Substantive
     modification” is defined as a change to the semantic content of
     the document, and excludes mere changes in format or
     typographical corrections.

     To accomplish this, add the phrase “Distribution of substantively
     modified versions of this document is prohibited without the
     explicit permission of the copyright holder.” to the license
     reference or copy.

  2. To prohibit any publication of this work or derivative works in
     whole or in part in standard (paper) book form for commercial
     purposes is prohibited unless prior permission is obtained from
     the copyright holder.

     To accomplish this, add the phrase “Distribution of the work or
     derivative of the work in any standard (paper) book form is
     prohibited unless prior permission is obtained from the copyright
     holder.” to the license reference or copy.