Git Diff

Git has a great diff utility built in that can give you statistics or a patch file given any combination of tree objects, working directory and index.

Two common uses of this include seeing what you've worked on but not committed yet, and creating a patch file to send to someone over email (though there is a much preferred way to share changes which we will learn about in the "distributed workflow" section a bit later).

What has changed?

If you simply run git diff with no arguments, it will show you the differences between your current working directory and your index, that is, the last time you ran git add on your files.

For example, if I add my email to the README file and run it, I will see this:

$ git diff
diff --git a/README b/README
index 569b350..26c6ac8 100644
--- a/README
+++ b/README
@@ -5,4 +5,4 @@ This library calls git commands and returns the output.

 It is an example for the Git Peepcode book.

-Author : Scott Chacon
+Author : Scott Chacon (schacon@gmail.com)

You can also use git diff to show you some spiffy stats for a diff, rather than a patch file, if you want to see a wider overview of what changed, then drill down into specific files later. Here are some examples getting stats, the first for the differences between two commits and the second a summary between a commit and the current HEAD.

$ git diff --numstat a11bef06a3f65..cf25cc3bfb0
3       1       README
1       1       Rakefile
4       5       lib/simplegit.rb

$ git diff --stat 0576fac35..
 README           |    4 +++-
 Rakefile         |    2 +-
 lib/simplegit.rb |    4 ++++
 3 files changed, 8 insertions(+), 2 deletions(-)

If you want to see what the specific difference is in one of those files, you can just add a path limiter to the diff command.

$ git diff a11bef06a3f65..cf25cc3bfb0 -- Rakefile
diff --git a/Rakefile b/Rakefile
index a874b73..8f94139 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require 'rake/gempackagetask'
 spec = Gem::Specification.new do |s|
     s.platform  =   Gem::Platform::RUBY
     s.name      =   "simplegit"
-    s.version   =   "0.1.0"
+    s.version   =   "0.1.1"
     s.author    =   "Scott Chacon"
     s.email     =   "schacon@gmail.com"
     s.summary   =   "A simple gem for using Git in Ruby code."

You can use this command to detect changes between your index and any tree, or your working directory and any tree, your working directory and your index, etc.

Generating Patch Files

The default output of the git diff command is a valid patch file. If you pipe the output into a file and email it to someone, they can apply it with the 'patch' command. If you've done some work off of a project in an 'experiment' branch, you could create a patch file this way:

git diff master..experiment > experiment.patch

You can then email that file to anyone, who could apply it with the '-p1' argument:

$ patch -p1 < ~/experiment.patch
patching file lib/simplegit.rb

git diff