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Customizing Git

So far, I’ve covered the basics of how Git works and how to use it, and I’ve introduced a number of tools that Git provides to help you use it easily and efficiently. In this chapter, I’ll go through some operations that you can use to make Git operate in a more customized fashion by introducing several important configuration settings and the hooks system. With these tools, it’s easy to get Git to work exactly the way you, your company, or your group needs it to.

Git Configuration

As you briefly saw in Chapter 1, you can specify Git configuration settings with the git config command. One of the first things you did was set up your name and e-mail address:

$ git config --global "John Doe"
$ git config --global

Now you’ll learn a few of the more interesting options that you can set in this manner to customize your Git usage.

You saw some simple Git configuration details in the first chapter, but I’ll go over them again quickly here. Git uses a series of configuration files to determine non-default behavior that you may want. The first place Git looks for these values is in an /etc/gitconfig file, which contains values for every user on the system and all of their repositories. If you pass the option --system to git config, it reads and writes from this file specifically.

The next place Git looks is the ~/.gitconfig file, which is specific to each user. You can make Git read and write to this file by passing the --global option.

Finally, Git looks for configuration values in the config file in the Git directory (.git/config) of whatever repository you’re currently using. These values are specific to that single repository. Each level overwrites values in the previous level, so values in .git/config trump those in /etc/gitconfig, for instance. You can also set these values by manually editing the file and inserting the correct syntax, but it’s generally easier to run the git config command.

Basic Client Configuration

The configuration options recognized by Git fall into two categories: client side and server side. The majority of the options are client side—configuring your personal working preferences. Although tons of options are available, I’ll only cover the few that either are commonly used or can significantly affect your workflow. Many options are useful only in edge cases that I won’t go over here. If you want to see a list of all the options your version of Git recognizes, you can run

$ git config --help

The manual page for git config lists all the available options in quite a bit of detail.


By default, Git uses whatever you’ve set as your default text editor or else falls back to the Vi editor to create and edit your commit and tag messages. To change that default to something else, you can use the core.editor setting:

$ git config --global core.editor emacs

Now, no matter what is set as your default shell editor variable, Git will fire up Emacs to edit messages.


If you set this to the path of a file on your system, Git will use that file as the default message when you commit. For instance, suppose you create a template file at $HOME/.gitmessage.txt that looks like this:

subject line

what happened

[ticket: X]

To tell Git to use it as the default message that appears in your editor when you run git commit, set the commit.template configuration value:

$ git config --global commit.template $HOME/.gitmessage.txt
$ git commit

Then, your editor will open to something like this for your placeholder commit message when you commit:

subject line

what happened

[ticket: X]
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
# modified:   lib/test.rb
".git/COMMIT_EDITMSG" 14L, 297C

If you have a commit-message policy in place, then putting a template for that policy on your system and configuring Git to use it by default can help increase the chance of that policy being followed regularly.


The core.pager setting determines what pager is used when Git pages output such as log and diff. You can set it to more or to your favorite pager (by default, it’s less), or you can turn it off by setting it to a blank string:

$ git config --global core.pager ''

If you run that, Git will page the entire output of all commands, no matter how long it is.


If you’re making signed annotated tags (as discussed in Chapter 2), setting your GPG signing key as a configuration setting makes things easier. Set your key ID like so:

$ git config --global user.signingkey <gpg-key-id>

Now, you can sign tags without having to specify your key every time with the git tag command:

$ git tag -s <tag-name>


You can put patterns in your project’s .gitignore file to have Git not see them as untracked files or try to stage them when you run git add on them, as discussed in Chapter 2. However, if you want another file outside of your project to hold those values or have extra values, you can tell Git where that file is with the core.excludesfile setting. Simply set it to the path of a file that has content similar to what a .gitignore file would have.


This option is available only in Git 1.6.1 and later. If you mistype a command in Git, it shows you something like this:

$ git com
git: 'com' is not a git-command. See 'git --help'.

Did you mean this?

If you set help.autocorrect to 1, Git will automatically run the command if it has only one match under this scenario.

Colors in Git

Git can color its output to your terminal, which can help you visually parse the output quickly and easily. A number of options can help you set the coloring to your preference.


Git automatically colors most of its output if you ask it to. You can get very specific about what you want colored and how; but to turn on all the default terminal coloring, set color.ui to true:

$ git config --global color.ui true

When that value is set, Git colors its output if the output goes to a terminal. Other possible settings are false, which never colors the output, and always, which sets colors all the time, even if you’re redirecting Git commands to a file or piping them to another command.

You’ll rarely want color.ui = always. In most scenarios, if you want color codes in your redirected output, you can instead pass a --color flag to the Git command to force it to use color codes. The color.ui = true setting is almost always what you’ll want to use.


If you want to be more specific about which commands are colored and how, Git provides verb-specific coloring settings. Each of these can be set to true, false, or always:


In addition, each of these has subsettings you can use to set specific colors for parts of the output, if you want to override each color. For example, to set the meta information in your diff output to blue foreground, black background, and bold text, you can run

$ git config --global color.diff.meta "blue black bold"

You can set the color to any of the following values: normal, black, red, green, yellow, blue, magenta, cyan, or white, or, if your terminal supports more than 16 colors, an arbitrary numeric color value (between 0 and 255 on a 256-color terminal). If you want an attribute like bold in the previous example, you can choose from bold, dim, ul, blink, and reverse.

See the git config manpage for all the subsettings you can configure, if you want to do that.

External Merge and Diff Tools

Although Git has an internal implementation of diff, which is what you’ve been using, you can set up an external tool instead. You can also set up a graphical merge conflict-resolution tool instead of having to resolve conflicts manually. I’ll demonstrate setting up the Perforce Visual Merge Tool (P4Merge) to do your diffs and merge resolutions, because it’s a nice graphical tool and it’s free.

If you want to try this out, P4Merge works on all major platforms, so you should be able to do so. I’ll use path names in the examples that work on Mac and Linux systems; for Windows, you’ll have to change /usr/local/bin to an executable path in your environment.

You can download P4Merge here:

To begin, you’ll set up external wrapper scripts to run your commands. I’ll use the Mac path for the executable; in other systems, it will be where your p4merge binary is installed. Set up a merge wrapper script named extMerge that calls your binary with all the arguments provided:

$ cat /usr/local/bin/extMerge
/Applications/ $*

The diff wrapper checks to make sure seven arguments are provided and passes two of them to your merge script. By default, Git passes the following arguments to the diff program:

path old-file old-hex old-mode new-file new-hex new-mode

Because you only want the old-file and new-file arguments, you use the wrapper script to pass the ones you need.

$ cat /usr/local/bin/extDiff
[ $# -eq 7 ] && /usr/local/bin/extMerge "$2" "$5"

You also need to make sure these tools are executable:

$ sudo chmod +x /usr/local/bin/extMerge
$ sudo chmod +x /usr/local/bin/extDiff

Now you can set up your config file to use your custom merge resolution and diff tools. This takes a number of custom settings: merge.tool to tell Git what strategy to use, mergetool.*.cmd to specify how to run the command, mergetool.trustExitCode to tell Git if the exit code of that program indicates a successful merge resolution or not, and diff.external to tell Git what command to run for diffs. So, you can either run four config commands

$ git config --global merge.tool extMerge
$ git config --global mergetool.extMerge.cmd \
    'extMerge "$BASE" "$LOCAL" "$REMOTE" "$MERGED"'
$ git config --global mergetool.trustExitCode false
$ git config --global diff.external extDiff

or you can edit your ~/.gitconfig file to add these lines:

  tool = extMerge
[mergetool "extMerge"]
  cmd = extMerge \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"
  trustExitCode = false
  external = extDiff

After all this is set, if you run diff commands such as this:

$ git diff 32d1776b1^ 32d1776b1

Instead of getting the diff output on the command line, Git fires up P4Merge, which looks something like Figure 7-1.

Insert 18333fig0701.png Figure 7-1. P4Merge.

If you try to merge two branches and subsequently have merge conflicts, you can run the command git mergetool; it starts P4Merge to let you resolve the conflicts through that GUI tool.

The nice thing about this wrapper setup is that you can change your diff and merge tools easily. For example, to change your extDiff and extMerge tools to run the KDiff3 tool instead, all you have to do is edit your extMerge file:

$ cat /usr/local/bin/extMerge
/Applications/ $*

Now, Git will use the KDiff3 tool for diff viewing and merge conflict resolution.

Git comes preset to use a number of other merge-resolution tools without your having to set up the cmd configuration. You can set your merge tool to kdiff3, opendiff, tkdiff, meld, xxdiff, emerge, vimdiff, or gvimdiff. If you’re not interested in using KDiff3 for diff but rather want to use it just for merge resolution, and the kdiff3 command is in your path, then you can run

$ git config --global merge.tool kdiff3

If you run this instead of setting up the extMerge and extDiff files, Git will use KDiff3 for merge resolution and the normal Git diff tool for diffs.

Formatting and Whitespace

Formatting and whitespace issues are some of the more frustrating and subtle problems that many developers encounter when collaborating, especially cross-platform. It’s very easy for patches or other collaborated work to introduce subtle whitespace changes because editors silently introduce them or Windows programmers add carriage returns at the end of lines they touch in cross-platform projects. Git has a few configuration options to help with these issues.


If you’re programming on Windows or using another system but working with people who are programming on Windows, you’ll probably run into line-ending issues at some point. This is because Windows uses both a carriage-return character and a linefeed character for newlines in its files, whereas Mac and Linux systems use only the linefeed character. This is a subtle but incredibly annoying fact of cross-platform work.

Git can handle this by auto-converting CRLF line endings into LF when you commit, and vice versa when it checks out code onto your filesystem. You can turn on this functionality with the core.autocrlf setting. If you’re on a Windows machine, set it to true — this converts LF endings into CRLF when you check out code:

$ git config --global core.autocrlf true

If you’re on a Linux or Mac system that uses LF line endings, then you don’t want Git to automatically convert them when you check out files; however, if a file with CRLF endings accidentally gets introduced, then you may want Git to fix it. You can tell Git to convert CRLF to LF on commit but not the other way around by setting core.autocrlf to input:

$ git config --global core.autocrlf input

This setup should leave you with CRLF endings in Windows checkouts but LF endings on Mac and Linux systems and in the repository.

If you’re a Windows programmer doing a Windows-only project, then you can turn off this functionality, recording the carriage returns in the repository by setting the config value to false:

$ git config --global core.autocrlf false


Git comes preset to detect and fix some whitespace issues. It can look for four primary whitespace issues — two are enabled by default and can be turned off, and two aren’t enabled by default but can be activated.

The two that are turned on by default are trailing-space, which looks for spaces at the end of a line, and space-before-tab, which looks for spaces before tabs at the beginning of a line.

The two that are disabled by default but can be turned on are indent-with-non-tab, which looks for lines that begin with eight or more spaces instead of tabs, and cr-at-eol, which tells Git that carriage returns at the end of lines are OK.

You can tell Git which of these you want enabled by setting core.whitespace to the values you want on or off, separated by commas. You can disable settings by either leaving them out of the setting string or prepending a - in front of the value. For example, if you want all but cr-at-eol to be set, you can do this:

$ git config --global core.whitespace \

Git will detect these issues when you run a git diff command and try to color them so you can possibly fix them before you commit. It will also use these values to help you when you apply patches with git apply. When you’re applying patches, you can ask Git to warn you if it’s applying patches with the specified whitespace issues:

$ git apply --whitespace=warn <patch>

Or you can have Git try to automatically fix the issue before applying the patch:

$ git apply --whitespace=fix <patch>

These options apply to the git rebase command as well. If you’ve committed whitespace issues but haven’t yet pushed upstream, you can run a rebase with the --whitespace=fix option to have Git automatically fix whitespace issues as it’s rewriting the patches.

Server Configuration

Not nearly as many configuration options are available for the server side of Git, but there are a few interesting ones you may want to take note of.


By default, Git doesn’t check for consistency all the objects it receives during a push. Although Git can check to make sure each object still matches its SHA-1 checksum and points to valid objects, it doesn’t do that by default on every push. This is a relatively expensive operation and may add a lot of time to each push, depending on the size of the repository or the push. If you want Git to check object consistency on every push, you can force it to do so by setting receive.fsckObjects to true:

$ git config --system receive.fsckObjects true

Now, Git will check the integrity of your repository before each push is accepted to make sure faulty clients aren’t introducing corrupt data.


If you rebase commits that you’ve already pushed and then try to push again, or otherwise try to push a commit to a remote branch that doesn’t contain the commit that the remote branch currently points to, you’ll be denied. This is generally good policy; but in the case of the rebase, you may determine that you know what you’re doing and can force-update the remote branch with a -f flag to your push command.

To disable the ability to force-update remote branches to non-fast-forward references, set receive.denyNonFastForwards:

$ git config --system receive.denyNonFastForwards true

The other way you can do this is via server-side receive hooks, which I’ll cover in a bit. That approach lets you do more complex things like deny non-fast-forwards to a certain subset of users.


One of the workarounds to the denyNonFastForwards policy is for the user to delete the branch and then push it back up with the new reference. In newer versions of Git (beginning with version 1.6.1), you can set receive.denyDeletes to true:

$ git config --system receive.denyDeletes true

This denies branch and tag deletion over a push across the board — no user can do it. To remove remote branches, you must remove the ref files from the server manually. There are also more interesting ways to do this on a per-user basis via ACLs, as you’ll learn at the end of this chapter.

Git Attributes

Some of these settings can also be specified for a path, so that Git applies those settings only for a subdirectory or subset of files. These path-specific settings are called Git attributes and are set either in a .gitattributes file in one of your directories (normally the root of your project) or in the .git/info/attributes file if you don’t want the attributes file committed with your project.

Using attributes, you can do things like specify separate merge strategies for individual files or directories in your project, tell Git how to diff non-text files, or have Git filter content before you check it into or out of Git. In this section, you’ll learn about some of the attributes you can set on your paths in your Git project and see a few examples of using this feature in practice.

Binary Files

One cool trick for which you can use Git attributes is telling Git which files are binary (in cases it otherwise may not be able to figure out) and giving Git special instructions about how to handle those files. For instance, some text files may be machine generated and not diffable, whereas some binary files can be diffed — you’ll see how to tell Git which is which.

Identifying Binary Files

Some files look like text files but for all intents and purposes are to be treated as binary data. For instance, Xcode projects on the Mac contain a file that ends in .pbxproj, which is basically a JSON (plain text javascript data format) dataset written out to disk by the IDE that records your build settings and so on. Although it’s technically a text file, because it’s all ASCII, you don’t want to treat it as such because it’s really a lightweight database — you can’t merge the contents if two people changed it, and diffs generally aren’t helpful. The file is meant to be consumed by a machine. In essence, you want to treat it like a binary file.

To tell Git to treat all pbxproj files as binary data, add the following line to your .gitattributes file:

*.pbxproj -crlf -diff

Now, Git won’t try to convert or fix CRLF issues; nor will it try to compute or print a diff for changes in this file when you run git show or git diff on your project. You can also use a built-in macro binary that means -crlf -diff:

*.pbxproj binary

Diffing Binary Files

In Git, you can use the attributes functionality to effectively diff binary files. You do this by telling Git how to convert your binary data to a text format that can be compared via the normal diff. But the question is how do you convert binary data to a text? The best solution is to find some tool that does conversion for your binary format to a text representation. Unfortunately, very few binary formats can be represented as human readable text (imagine trying to convert audio data to a text). If this is the case and you failed to get a text presentation of your file's contents, it's often relatively easy to get a human readable description of that content, or metadata. Metadata won't give you a full representation of your file's content, but in any case it's better than nothing.

We'll make use of the both described approaches to get usable diffs for some widely used binary formats.

Side note: There are different kinds of binary formats with a text content, which are hard to find usable converter for. In such a case you could try to extract a text from your file with the strings program. Some of these files may use an UTF-16 encoding or other "codepages" and strings won’t find anything useful in there. Your mileage may vary. However, strings is available on most Mac and Linux systems, so it may be a good first try to do this with many binary formats.

MS Word files

First, you’ll use the described technique to solve one of the most annoying problems known to humanity: version-controlling Word documents. Everyone knows that Word is the most horrific editor around; but, oddly, everyone uses it. If you want to version-control Word documents, you can stick them in a Git repository and commit every once in a while; but what good does that do? If you run git diff normally, you only see something like this:

$ git diff
diff --git a/chapter1.doc b/chapter1.doc
index 88839c4..4afcb7c 100644
Binary files a/chapter1.doc and b/chapter1.doc differ

You can’t directly compare two versions unless you check them out and scan them manually, right? It turns out you can do this fairly well using Git attributes. Put the following line in your .gitattributes file:

*.doc diff=word

This tells Git that any file that matches this pattern (.doc) should use the "word" filter when you try to view a diff that contains changes. What is the "word" filter? You have to set it up. Here you’ll configure Git to use the catdoc program, which was written specifically for extracting text from a binary MS Word documents (you can get it from, to convert Word documents into readable text files, which it will then diff properly:

$ git config diff.word.textconv catdoc

This command adds a section to your .git/config that looks like this:

[diff "word"]
	textconv = catdoc

Now Git knows that if it tries to do a diff between two snapshots, and any of the files end in .doc, it should run those files through the "word" filter, which is defined as the catdoc program. This effectively makes nice text-based versions of your Word files before attempting to diff them.

Here’s an example. I put Chapter 1 of this book into Git, added some text to a paragraph, and saved the document. Then, I ran git diff to see what changed:

$ git diff
diff --git a/chapter1.doc b/chapter1.doc
index c1c8a0a..b93c9e4 100644
--- a/chapter1.doc
+++ b/chapter1.doc
@@ -128,7 +128,7 @@ and data size)
 Since its birth in 2005, Git has evolved and matured to be easy to use
 and yet retain these initial qualities. It’s incredibly fast, it’s
 very efficient with large projects, and it has an incredible branching
-system for non-linear development.
+system for non-linear development (See Chapter 3).

Git successfully and succinctly tells me that I added the string "(See Chapter 3)", which is correct. Works perfectly!

OpenDocument Text files

The same approach that we used for MS Word files (*.doc) can be used for OpenDocument Text files (*.odt) created by

Add the following line to your .gitattributes file:

*.odt diff=odt

Now set up the odt diff filter in .git/config:

[diff "odt"]
	binary = true
	textconv = /usr/local/bin/odt-to-txt

OpenDocument files are actually zip’ped directories containing multiple files (the content in an XML format, stylesheets, images, etc.). We’ll need to write a script to extract the content and return it as plain text. Create a file /usr/local/bin/odt-to-txt (you are free to put it into a different directory) with the following content:

#! /usr/bin/env perl
# Simplistic OpenDocument Text (.odt) to plain text converter.
# Author: Philipp Kempgen

if (! defined($ARGV[0])) {
	print STDERR "No filename given!\n";
	print STDERR "Usage: $0 filename\n";
	exit 1;

my $content = '';
open my $fh, '-|', 'unzip', '-qq', '-p', $ARGV[0], 'content.xml' or die $!;
	local $/ = undef;  # slurp mode
	$content = <$fh>;
close $fh;
$_ = $content;
s/<text:span\b[^>]*>//g;           # remove spans
s/<text:h\b[^>]*>/\n\n*****  /g;   # headers
s/<text:list-item\b[^>]*>\s*<text:p\b[^>]*>/\n    --  /g;  # list items
s/<text:list\b[^>]*>/\n\n/g;       # lists
s/<text:p\b[^>]*>/\n  /g;          # paragraphs
s/<[^>]+>//g;                      # remove all XML tags
s/\n{2,}/\n\n/g;                   # remove multiple blank lines
s/\A\n+//;                         # remove leading blank lines
print "\n", $_, "\n\n";

And make it executable

chmod +x /usr/local/bin/odt-to-txt

Now git diff will be able to tell you what changed in .odt files.

Image files

Another interesting problem you can solve this way involves diffing image files. One way to do this is to run PNG files through a filter that extracts their EXIF information — metadata that is recorded with most image formats. If you download and install the exiftool program, you can use it to convert your images into text about the metadata, so at least the diff will show you a textual representation of any changes that happened:

$ echo '*.png diff=exif' >> .gitattributes
$ git config diff.exif.textconv exiftool

If you replace an image in your project and run git diff, you see something like this:

diff --git a/image.png b/image.png
index 88839c4..4afcb7c 100644
--- a/image.png
+++ b/image.png
@@ -1,12 +1,12 @@
 ExifTool Version Number         : 7.74
-File Size                       : 70 kB
-File Modification Date/Time     : 2009:04:17 10:12:35-07:00
+File Size                       : 94 kB
+File Modification Date/Time     : 2009:04:21 07:02:43-07:00
 File Type                       : PNG
 MIME Type                       : image/png
-Image Width                     : 1058
-Image Height                    : 889
+Image Width                     : 1056
+Image Height                    : 827
 Bit Depth                       : 8
 Color Type                      : RGB with Alpha

You can easily see that the file size and image dimensions have both changed.

Keyword Expansion

SVN- or CVS-style keyword expansion is often requested by developers used to those systems. The main problem with this in Git is that you can’t modify a file with information about the commit after you’ve committed, because Git checksums the file first. However, you can inject text into a file when it’s checked out and remove it again before it’s added to a commit. Git attributes offers you two ways to do this.

First, you can inject the SHA-1 checksum of a blob into an $Id$ field in the file automatically. If you set this attribute on a file or set of files, then the next time you check out that branch, Git will replace that field with the SHA-1 of the blob. It’s important to notice that it isn’t the SHA of the commit, but of the blob itself:

$ echo '*.txt ident' >> .gitattributes
$ echo '$Id$' > test.txt

The next time you check out this file, Git injects the SHA of the blob:

$ rm test.txt
$ git checkout -- test.txt
$ cat test.txt
$Id: 42812b7653c7b88933f8a9d6cad0ca16714b9bb3 $

However, that result is of limited use. If you’ve used keyword substitution in CVS or Subversion, you can include a datestamp — the SHA isn’t all that helpful, because it’s fairly random and you can’t tell if one SHA is older or newer than another.

It turns out that you can write your own filters for doing substitutions in files on commit/checkout. These are the "clean" and "smudge" filters. In the .gitattributes file, you can set a filter for particular paths and then set up scripts that will process files just before they’re checked out ("smudge", see Figure 7-2) and just before they’re committed ("clean", see Figure 7-3). These filters can be set to do all sorts of fun things.

Insert 18333fig0702.png Figure 7-2. The “smudge” filter is run on checkout.

Insert 18333fig0703.png Figure 7-3. The “clean” filter is run when files are staged.

The original commit message for this functionality gives a simple example of running all your C source code through the indent program before committing. You can set it up by setting the filter attribute in your .gitattributes file to filter *.c files with the "indent" filter:

*.c     filter=indent

Then, tell Git what the "indent" filter does on smudge and clean:

$ git config --global filter.indent.clean indent
$ git config --global filter.indent.smudge cat

In this case, when you commit files that match *.c, Git will run them through the indent program before it commits them and then run them through the cat program before it checks them back out onto disk. The cat program is basically a no-op: it spits out the same data that it gets in. This combination effectively filters all C source code files through indent before committing.

Another interesting example gets $Date$ keyword expansion, RCS style. To do this properly, you need a small script that takes a filename, figures out the last commit date for this project, and inserts the date into the file. Here is a small Ruby script that does that:

#! /usr/bin/env ruby
data =
last_date = `git log --pretty=format:"%ad" -1`
puts data.gsub('$Date$', '$Date: ' + last_date.to_s + '$')

All the script does is get the latest commit date from the git log command, stick that into any $Date$ strings it sees in stdin, and print the results — it should be simple to do in whatever language you’re most comfortable in. You can name this file expand_date and put it in your path. Now, you need to set up a filter in Git (call it dater) and tell it to use your expand_date filter to smudge the files on checkout. You’ll use a Perl expression to clean that up on commit:

$ git config filter.dater.smudge expand_date
$ git config filter.dater.clean 'perl -pe "s/\\\$Date[^\\\$]*\\\$/\\\$Date\\\$/"'

This Perl snippet strips out anything it sees in a $Date$ string, to get back to where you started. Now that your filter is ready, you can test it by setting up a file with your $Date$ keyword and then setting up a Git attribute for that file that engages the new filter:

$ echo '# $Date$' > date_test.txt
$ echo 'date*.txt filter=dater' >> .gitattributes

If you commit those changes and check out the file again, you see the keyword properly substituted:

$ git add date_test.txt .gitattributes
$ git commit -m "Testing date expansion in Git"
$ rm date_test.txt
$ git checkout date_test.txt
$ cat date_test.txt
# $Date: Tue Apr 21 07:26:52 2009 -0700$

You can see how powerful this technique can be for customized applications. You have to be careful, though, because the .gitattributes file is committed and passed around with the project but the driver (in this case, dater) isn’t; so, it won’t work everywhere. When you design these filters, they should be able to fail gracefully and have the project still work properly.

Exporting Your Repository

Git attribute data also allows you to do some interesting things when exporting an archive of your project.


You can tell Git not to export certain files or directories when generating an archive. If there is a subdirectory or file that you don’t want to include in your archive file but that you do want checked into your project, you can determine those files via the export-ignore attribute.

For example, say you have some test files in a test/ subdirectory, and it doesn’t make sense to include them in the tarball export of your project. You can add the following line to your Git attributes file:

test/ export-ignore

Now, when you run git archive to create a tarball of your project, that directory won’t be included in the archive.


Another thing you can do for your archives is some simple keyword substitution. Git lets you put the string $Format:$ in any file with any of the --pretty=format formatting shortcodes, many of which you saw in Chapter 2. For instance, if you want to include a file named LAST_COMMIT in your project, and the last commit date was automatically injected into it when git archive ran, you can set up the file like this:

$ echo 'Last commit date: $Format:%cd$' > LAST_COMMIT
$ echo "LAST_COMMIT export-subst" >> .gitattributes
$ git add LAST_COMMIT .gitattributes
$ git commit -am 'adding LAST_COMMIT file for archives'

When you run git archive, the contents of that file when people open the archive file will look like this:

Last commit date: $Format:Tue Apr 21 08:38:48 2009 -0700$

Merge Strategies

You can also use Git attributes to tell Git to use different merge strategies for specific files in your project. One very useful option is to tell Git to not try to merge specific files when they have conflicts, but rather to use your side of the merge over someone else’s.

This is helpful if a branch in your project has diverged or is specialized, but you want to be able to merge changes back in from it, and you want to ignore certain files. Say you have a database settings file called database.xml that is different in two branches, and you want to merge in your other branch without messing up the database file. You can set up an attribute like this:

database.xml merge=ours

And then define a dummy ours merge strategy with:

git config --global merge.ours.driver true

If you merge in the other branch, instead of having merge conflicts with the database.xml file, you see something like this:

$ git merge topic
Auto-merging database.xml
Merge made by recursive.

In this case, database.xml stays at whatever version you originally had.

Git Hooks

Like many other Version Control Systems, Git has a way to fire off custom scripts when certain important actions occur. There are two groups of these hooks: client side and server side. The client-side hooks are for client operations such as committing and merging. The server-side hooks are for Git server operations such as receiving pushed commits. You can use these hooks for all sorts of reasons, and you’ll learn about a few of them here.

Installing a Hook

The hooks are all stored in the hooks subdirectory of the Git directory. In most projects, that’s .git/hooks. By default, Git populates this directory with a bunch of example scripts, many of which are useful by themselves; but they also document the input values of each script. All the examples are written as shell scripts, with some Perl thrown in, but any properly named executable scripts will work fine — you can write them in Ruby or Python or what have you. These example hook files end with .sample; you’ll need to rename them.

To enable a hook script, put a file in the hooks subdirectory of your Git directory that is named appropriately and is executable. From that point forward, it should be called. I’ll cover most of the major hook filenames here.

Client-Side Hooks

There are a lot of client-side hooks. This section splits them into committing-workflow hooks, e-mail-workflow scripts, and the rest of the client-side scripts.

Committing-Workflow Hooks

The first four hooks have to do with the committing process. The pre-commit hook is run first, before you even type in a commit message. It’s used to inspect the snapshot that’s about to be committed, to see if you’ve forgotten something, to make sure tests run, or to examine whatever you need to inspect in the code. Exiting non-zero from this hook aborts the commit, although you can bypass it with git commit --no-verify. You can do things like check for code style (run lint or something equivalent), check for trailing whitespace (the default hook does exactly that), or check for appropriate documentation on new methods.

The prepare-commit-msg hook is run before the commit message editor is fired up but after the default message is created. It lets you edit the default message before the commit author sees it. This hook takes a few options: the path to the file that holds the commit message so far, the type of commit, and the commit SHA-1 if this is an amended commit. This hook generally isn’t useful for normal commits; rather, it’s good for commits where the default message is auto-generated, such as templated commit messages, merge commits, squashed commits, and amended commits. You may use it in conjunction with a commit template to programmatically insert information.

The commit-msg hook takes one parameter, which again is the path to a temporary file that contains the current commit message. If this script exits non-zero, Git aborts the commit process, so you can use it to validate your project state or commit message before allowing a commit to go through. In the last section of this chapter, I’ll demonstrate using this hook to check that your commit message is conformant to a required pattern.

After the entire commit process is completed, the post-commit hook runs. It doesn’t take any parameters, but you can easily get the last commit by running git log -1 HEAD. Generally, this script is used for notification or something similar.

The committing-workflow client-side scripts can be used in just about any workflow. They’re often used to enforce certain policies, although it’s important to note that these scripts aren’t transferred during a clone. You can enforce policy on the server side to reject pushes of commits that don’t conform to some policy, but it’s entirely up to the developer to use these scripts on the client side. So, these are scripts to help developers, and they must be set up and maintained by them, although they can be overridden or modified by them at any time.

E-mail Workflow Hooks

You can set up three client-side hooks for an e-mail-based workflow. They’re all invoked by the git am command, so if you aren’t using that command in your workflow, you can safely skip to the next section. If you’re taking patches over e-mail prepared by git format-patch, then some of these may be helpful to you.

The first hook that is run is applypatch-msg. It takes a single argument: the name of the temporary file that contains the proposed commit message. Git aborts the patch if this script exits non-zero. You can use this to make sure a commit message is properly formatted or to normalize the message by having the script edit it in place.

The next hook to run when applying patches via git am is pre-applypatch. It takes no arguments and is run after the patch is applied, so you can use it to inspect the snapshot before making the commit. You can run tests or otherwise inspect the working tree with this script. If something is missing or the tests don’t pass, exiting non-zero also aborts the git am script without committing the patch.

The last hook to run during a git am operation is post-applypatch. You can use it to notify a group or the author of the patch you pulled in that you’ve done so. You can’t stop the patching process with this script.

Other Client Hooks

The pre-rebase hook runs before you rebase anything and can halt the process by exiting non-zero. You can use this hook to disallow rebasing any commits that have already been pushed. The example pre-rebase hook that Git installs does this, although it assumes that next is the name of the branch you publish. You’ll likely need to change that to whatever your stable, published branch is.

After you run a successful git checkout, the post-checkout hook runs; you can use it to set up your working directory properly for your project environment. This may mean moving in large binary files that you don’t want source controlled, auto-generating documentation, or something along those lines.

Finally, the post-merge hook runs after a successful merge command. You can use it to restore data in the working tree that Git can’t track, such as permissions data. This hook can likewise validate the presence of files external to Git control that you may want copied in when the working tree changes.

Server-Side Hooks

In addition to the client-side hooks, you can use a couple of important server-side hooks as a system administrator to enforce nearly any kind of policy for your project. These scripts run before and after pushes to the server. The pre hooks can exit non-zero at any time to reject the push as well as print an error message back to the client; you can set up a push policy that’s as complex as you wish.

pre-receive and post-receive

The first script to run when handling a push from a client is pre-receive. It takes a list of references that are being pushed from stdin; if it exits non-zero, none of them are accepted. You can use this hook to do things like make sure none of the updated references are non-fast-forwards; or to check that the user doing the pushing has create, delete, or push access or access to push updates to all the files they’re modifying with the push.

The post-receive hook runs after the entire process is completed and can be used to update other services or notify users. It takes the same stdin data as the pre-receive hook. Examples include e-mailing a list, notifying a continuous integration server, or updating a ticket-tracking system — you can even parse the commit messages to see if any tickets need to be opened, modified, or closed. This script can’t stop the push process, but the client doesn’t disconnect until it has completed; so, be careful when you try to do anything that may take a long time.


The update script is very similar to the pre-receive script, except that it’s run once for each branch the pusher is trying to update. If the pusher is trying to push to multiple branches, pre-receive runs only once, whereas update runs once per branch they’re pushing to. Instead of reading from stdin, this script takes three arguments: the name of the reference (branch), the SHA-1 that reference pointed to before the push, and the SHA-1 the user is trying to push. If the update script exits non-zero, only that reference is rejected; other references can still be updated.

An Example Git-Enforced Policy

In this section, you’ll use what you’ve learned to establish a Git workflow that checks for a custom commit message format, enforces fast-forward-only pushes, and allows only certain users to modify certain subdirectories in a project. You’ll build client scripts that help the developer know if their push will be rejected and server scripts that actually enforce the policies.

I used Ruby to write these, both because it’s my preferred scripting language and because I feel it’s the most pseudocode-looking of the scripting languages; thus you should be able to roughly follow the code even if you don’t use Ruby. However, any language will work fine. All the sample hook scripts distributed with Git are in either Perl or Bash scripting, so you can also see plenty of examples of hooks in those languages by looking at the samples.

Server-Side Hook

All the server-side work will go into the update file in your hooks directory. The update file runs once per branch being pushed and takes the reference being pushed to, the old revision where that branch was, and the new revision being pushed. You also have access to the user doing the pushing if the push is being run over SSH. If you’ve allowed everyone to connect with a single user (like "git") via public-key authentication, you may have to give that user a shell wrapper that determines which user is connecting based on the public key, and set an environment variable specifying that user. Here I assume the connecting user is in the $USER environment variable, so your update script begins by gathering all the information you need:

#!/usr/bin/env ruby

refname = ARGV[0]
oldrev  = ARGV[1]
newrev  = ARGV[2]
user    = ENV['USER']

puts "Enforcing Policies... \n(#{refname}) (#{oldrev[0,6]}) (#{newrev[0,6]})"

Enforcing a Specific Commit-Message Format

Your first challenge is to enforce that each commit message must adhere to a particular format. Just to have a target, assume that each message has to include a string that looks like "ref: 1234" because you want each commit to link to a work item in your ticketing system. You must look at each commit being pushed up, see if that string is in the commit message, and, if the string is absent from any of the commits, exit non-zero so the push is rejected.

You can get a list of the SHA-1 values of all the commits that are being pushed by taking the $newrev and $oldrev values and passing them to a Git plumbing command called git rev-list. This is basically the git log command, but by default it prints out only the SHA-1 values and no other information. So, to get a list of all the commit SHAs introduced between one commit SHA and another, you can run something like this:

$ git rev-list 538c33..d14fc7

You can take that output, loop through each of those commit SHAs, grab the message for it, and test that message against a regular expression that looks for a pattern.

You have to figure out how to get the commit message from each of these commits to test. To get the raw commit data, you can use another plumbing command called git cat-file. I’ll go over all these plumbing commands in detail in Chapter 9; but for now, here’s what that command gives you:

$ git cat-file commit ca82a6
tree cfda3bf379e4f8dba8717dee55aab78aef7f4daf
parent 085bb3bcb608e1e8451d4b2432f8ecbe6306e7e7
author Scott Chacon <> 1205815931 -0700
committer Scott Chacon <> 1240030591 -0700

changed the version number

A simple way to get the commit message from a commit when you have the SHA-1 value is to go to the first blank line and take everything after that. You can do so with the sed command on Unix systems:

$ git cat-file commit ca82a6 | sed '1,/^$/d'
changed the version number

You can use that incantation to grab the commit message from each commit that is trying to be pushed and exit if you see anything that doesn’t match. To exit the script and reject the push, exit non-zero. The whole method looks like this:

$regex = /\[ref: (\d+)\]/

# enforced custom commit message format
def check_message_format
  missed_revs = `git rev-list #{$oldrev}..#{$newrev}`.split("\n")
  missed_revs.each do |rev|
    message = `git cat-file commit #{rev} | sed '1,/^$/d'`
    if !$regex.match(message)
      puts "[POLICY] Your message is not formatted correctly"
      exit 1

Putting that in your update script will reject updates that contain commits that have messages that don’t adhere to your rule.

Enforcing a User-Based ACL System

Suppose you want to add a mechanism that uses an access control list (ACL) that specifies which users are allowed to push changes to which parts of your projects. Some people have full access, and others only have access to push changes to certain subdirectories or specific files. To enforce this, you’ll write those rules to a file named acl that lives in your bare Git repository on the server. You’ll have the update hook look at those rules, see what files are being introduced for all the commits being pushed, and determine whether the user doing the push has access to update all those files.

The first thing you’ll do is write your ACL. Here you’ll use a format very much like the CVS ACL mechanism: it uses a series of lines, where the first field is avail or unavail, the next field is a comma-delimited list of the users to which the rule applies, and the last field is the path to which the rule applies (blank meaning open access). All of these fields are delimited by a pipe (|) character.

In this case, you have a couple of administrators, some documentation writers with access to the doc directory, and one developer who only has access to the lib and tests directories, and your ACL file looks like this:


You begin by reading this data into a structure that you can use. In this case, to keep the example simple, you’ll only enforce the avail directives. Here is a method that gives you an associative array where the key is the user name and the value is an array of paths to which the user has write access:

def get_acl_access_data(acl_file)
  # read in ACL data
  acl_file ="\n").reject { |line| line == '' }
  access = {}
  acl_file.each do |line|
    avail, users, path = line.split('|')
    next unless avail == 'avail'
    users.split(',').each do |user|
      access[user] ||= []
      access[user] << path

On the ACL file you looked at earlier, this get_acl_access_data method returns a data structure that looks like this:

 "schacon"=>["lib", "tests"],

Now that you have the permissions sorted out, you need to determine what paths the commits being pushed have modified, so you can make sure the user who’s pushing has access to all of them.

You can pretty easily see what files have been modified in a single commit with the --name-only option to the git log command (mentioned briefly in Chapter 2):

$ git log -1 --name-only --pretty=format:'' 9f585d


If you use the ACL structure returned from the get_acl_access_data method and check it against the listed files in each of the commits, you can determine whether the user has access to push all of their commits:

# only allows certain users to modify certain subdirectories in a project
def check_directory_perms
  access = get_acl_access_data('acl')

  # see if anyone is trying to push something they can't
  new_commits = `git rev-list #{$oldrev}..#{$newrev}`.split("\n")
  new_commits.each do |rev|
    files_modified = `git log -1 --name-only --pretty=format:'' #{rev}`.split("\n")
    files_modified.each do |path|
      next if path.size == 0
      has_file_access = false
      access[$user].each do |access_path|
        if !access_path || # user has access to everything
          (path.index(access_path) == 0) # access to this path
          has_file_access = true
      if !has_file_access
        puts "[POLICY] You do not have access to push to #{path}"
        exit 1


Most of that should be easy to follow. You get a list of new commits being pushed to your server with git rev-list. Then, for each of those, you find which files are modified and make sure the user who’s pushing has access to all the paths being modified. One Rubyism that may not be clear is path.index(access_path) == 0, which is true if path begins with access_path — this ensures that access_path is not just in one of the allowed paths, but an allowed path begins with each accessed path.

Now your users can’t push any commits with badly formed messages or with modified files outside of their designated paths.

Enforcing Fast-Forward-Only Pushes

The only thing left is to enforce fast-forward-only pushes. To do so, you can simply set the receive.denyDeletes and receive.denyNonFastForwards settings. But enforcing this with a hook will also work, and you can modify it to do so only for certain users or whatever else you come up with later.

The logic for checking this is to see if any commits are reachable from the older revision that aren’t reachable from the newer one. If there are none, then it was a fast-forward push; otherwise, you deny it:

# enforces fast-forward only pushes
def check_fast_forward
  missed_refs = `git rev-list #{$newrev}..#{$oldrev}`
  missed_ref_count = missed_refs.split("\n").size
  if missed_ref_count > 0
    puts "[POLICY] Cannot push a non fast-forward reference"
    exit 1


Everything is set up. If you run chmod u+x .git/hooks/update, which is the file into which you should have put all this code, and then try to push a non-fast-forward reference, you’ll get something like this:

$ git push -f origin master
Counting objects: 5, done.
Compressing objects: 100% (3/3), done.
Writing objects: 100% (3/3), 323 bytes, done.
Total 3 (delta 1), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
Enforcing Policies...
(refs/heads/master) (8338c5) (c5b616)
[POLICY] Cannot push a non fast-forward reference
error: hooks/update exited with error code 1
error: hook declined to update refs/heads/master
To git@gitserver:project.git
 ! [remote rejected] master -> master (hook declined)
error: failed to push some refs to 'git@gitserver:project.git'

There are a couple of interesting things here. First, you see this where the hook starts running.

Enforcing Policies...
(refs/heads/master) (8338c5) (c5b616)

Notice that you printed that out to stdout at the very beginning of your update script. It’s important to note that anything your script prints to stdout will be transferred to the client.

The next thing you’ll notice is the error message.

[POLICY] Cannot push a non fast-forward reference
error: hooks/update exited with error code 1
error: hook declined to update refs/heads/master

The first line was printed out by you, the other two were Git telling you that the update script exited non-zero and that is what is declining your push. Lastly, you have this:

To git@gitserver:project.git
 ! [remote rejected] master -> master (hook declined)
error: failed to push some refs to 'git@gitserver:project.git'

You’ll see a remote rejected message for each reference that your hook declined, and it tells you that it was declined specifically because of a hook failure.

Furthermore, if the ref marker isn’t there in any of your commits, you’ll see the error message you’re printing out for that.

[POLICY] Your message is not formatted correctly

Or if someone tries to edit a file they don’t have access to and push a commit containing it, they will see something similar. For instance, if a documentation author tries to push a commit modifying something in the lib directory, they see

[POLICY] You do not have access to push to lib/test.rb

That’s all. From now on, as long as that update script is there and executable, your repository will never be rewound and will never have a commit message without your pattern in it, and your users will be sandboxed.

Client-Side Hooks

The downside to this approach is the whining that will inevitably result when your users’ commit pushes are rejected. Having their carefully crafted work rejected at the last minute can be extremely frustrating and confusing; and furthermore, they will have to edit their history to correct it, which isn’t always for the faint of heart.

The answer to this dilemma is to provide some client-side hooks that users can use to notify them when they’re doing something that the server is likely to reject. That way, they can correct any problems before committing and before those issues become more difficult to fix. Because hooks aren’t transferred with a clone of a project, you must distribute these scripts some other way and then have your users copy them to their .git/hooks directory and make them executable. You can distribute these hooks within the project or in a separate project, but there is no way to set them up automatically.

To begin, you should check your commit message just before each commit is recorded, so you know the server won’t reject your changes due to badly formatted commit messages. To do this, you can add the commit-msg hook. If you have it read the message from the file passed as the first argument and compare that to the pattern, you can force Git to abort the commit if there is no match:

#!/usr/bin/env ruby
message_file = ARGV[0]
message =

$regex = /\[ref: (\d+)\]/

if !$regex.match(message)
  puts "[POLICY] Your message is not formatted correctly"
  exit 1

If that script is in place (in .git/hooks/commit-msg) and executable, and you commit with a message that isn’t properly formatted, you see this:

$ git commit -am 'test'
[POLICY] Your message is not formatted correctly

No commit was completed in that instance. However, if your message contains the proper pattern, Git allows you to commit:

$ git commit -am 'test [ref: 132]'
[master e05c914] test [ref: 132]
 1 files changed, 1 insertions(+), 0 deletions(-)

Next, you want to make sure you aren’t modifying files that are outside your ACL scope. If your project’s .git directory contains a copy of the ACL file you used previously, then the following pre-commit script will enforce those constraints for you:

#!/usr/bin/env ruby

$user    = ENV['USER']

# [ insert acl_access_data method from above ]

# only allows certain users to modify certain subdirectories in a project
def check_directory_perms
  access = get_acl_access_data('.git/acl')

  files_modified = `git diff-index --cached --name-only HEAD`.split("\n")
  files_modified.each do |path|
    next if path.size == 0
    has_file_access = false
    access[$user].each do |access_path|
    if !access_path || (path.index(access_path) == 0)
      has_file_access = true
    if !has_file_access
      puts "[POLICY] You do not have access to push to #{path}"
      exit 1


This is roughly the same script as the server-side part, but with two important differences. First, the ACL file is in a different place, because this script runs from your working directory, not from your Git directory. You have to change the path to the ACL file from this

access = get_acl_access_data('acl')

to this:

access = get_acl_access_data('.git/acl')

The other important difference is the way you get a listing of the files that have been changed. Because the server-side method looks at the log of commits, and, at this point, the commit hasn’t been recorded yet, you must get your file listing from the staging area instead. Instead of

files_modified = `git log -1 --name-only --pretty=format:'' #{ref}`

you have to use

files_modified = `git diff-index --cached --name-only HEAD`

But those are the only two differences — otherwise, the script works the same way. One caveat is that it expects you to be running locally as the same user you push as to the remote machine. If that is different, you must set the $user variable manually.

The last thing you have to do is check that you’re not trying to push non-fast-forwarded references, but that is a bit less common. To get a reference that isn’t a fast-forward, you either have to rebase past a commit you’ve already pushed up or try pushing a different local branch up to the same remote branch.

Because the server will tell you that you can’t push a non-fast-forward anyway, and the hook prevents forced pushes, the only accidental thing you can try to catch is rebasing commits that have already been pushed.

Here is an example pre-rebase script that checks for that. It gets a list of all the commits you’re about to rewrite and checks whether they exist in any of your remote references. If it sees one that is reachable from one of your remote references, it aborts the rebase:

#!/usr/bin/env ruby

base_branch = ARGV[0]
if ARGV[1]
  topic_branch = ARGV[1]
  topic_branch = "HEAD"

target_shas = `git rev-list #{base_branch}..#{topic_branch}`.split("\n")
remote_refs = `git branch -r`.split("\n").map { |r| r.strip }

target_shas.each do |sha|
  remote_refs.each do |remote_ref|
    shas_pushed = `git rev-list ^#{sha}^@ refs/remotes/#{remote_ref}`
    if shas_pushed.split("\n").include?(sha)
      puts "[POLICY] Commit #{sha} has already been pushed to #{remote_ref}"
      exit 1

This script uses a syntax that wasn’t covered in the Revision Selection section of Chapter 6. You get a list of commits that have already been pushed up by running this:

git rev-list ^#{sha}^@ refs/remotes/#{remote_ref}

The SHA^@ syntax resolves to all the parents of that commit. You’re looking for any commit that is reachable from the last commit on the remote and that isn’t reachable from any parent of any of the SHAs you’re trying to push up — meaning it’s a fast-forward.

The main drawback to this approach is that it can be very slow and is often unnecessary — if you don’t try to force the push with -f, the server will warn you and not accept the push. However, it’s an interesting exercise and can in theory help you avoid a rebase that you might later have to go back and fix.


You’ve covered most of the major ways that you can customize your Git client and server to best fit your workflow and projects. You’ve learned about all sorts of configuration settings, file-based attributes, and event hooks, and you’ve built an example policy-enforcing server. You should now be able to make Git fit nearly any workflow you can dream up.