2. Git Basics
6. Git Tools
7.2 Customizing Git - 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
.gitattribute 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.
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
To tell Git to treat all
pbxproj files as binary data, add the following line to your
*.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. In the 1.6 series of Git, you can also use a macro that is provided that means
Diffing Binary Files
In the 1.6 series of Git, you can use the Git 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.
Because this is a pretty cool and not widely known feature, I’ll go over a few examples. First, you’ll use this 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
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
strings program to convert Word documents into readable text files, which it will then diff properly:
$ git config diff.word.textconv strings
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
strings 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 @@ -8,7 +8,8 @@ re going to cover Version Control Systems (VCS) and Git basics re going to cover how to get it and set it up for the first time if you don t already have it on your system. In Chapter Two we will go over basic Git usage - how to use Git for the 80% -s going on, modify stuff and contribute changes. If the book spontaneously +s going on, modify stuff and contribute changes. If the book spontaneously +Let's see if this works.
Git successfully and succinctly tells me that I added the string "Let’s see if this works", which is correct. It’s not perfect — it adds a bunch of random stuff at the end — but it certainly works. If you can find or write a Word-to-plain-text converter that works well enough, that solution will likely be incredibly effective. 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.
Another interesting problem you can solve this way involves diffing image files. One way to do this is to run JPEG 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 : 2009:04:21 07:02:45-07:00 +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.
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 text.txt $ git checkout -- text.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 committed ("clean", see Figure 7-2) and just before they’re checked out ("smudge", see Figure 7-3). These filters can be set to do all sorts of fun things.
Figure 7-2. The “smudge” filter is run on checkout.
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:
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 = STDIN.read 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.
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
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:
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:
$ cat LAST_COMMIT Last commit date: $Format:Tue Apr 21 08:38:48 2009 -0700$
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:
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.