What You’ll Learn

• How to interpret git diff output
• How to customize git diff context

What You’ll Need

• A text file editor that shows line numbers (like Vim or Visual Studio Code)
• Git

• In order to illustrate Git’s diff function, we will create a text file, track it with Git, make some changes, and then use git diff to make a comparison.
• For the text file, we will use “The Zen of Python” by Tim Peters. If you have Python installed, you can generate this text using the import this command, or you can just copy it from this page.

>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!"

• Next, copy the text into a text editor and save it in a directory of your choice. Name the file “zen.”

• Use a terminal to navigate to the directory where you saved the file.
• If the directory is not already under revision control, you will need to use the git init command to create a local repository for that directory location. Once this is done, a hidden “.git” directory will be created. This is where Git stores all the revision control information.
• Now we can add the file to the Git staging area using git add zen. This tells Git that we want it to track changes made to this file.
• You can check to see that the file was added using git status.

### ~ % cd ~/gitTest
### gitTest % ls -la
total 8
drwxr-xr-x   3 owner  staff    96 May 18 14:36 .
drwxr-xr-x@ 77 owner  staff  2464 May 18 14:36 ..
-rw-r--r--   1 owner  staff   857 May 18 14:31 zen
### gitTest % git init
Initialized empty Git repository in ~/gitTest/.git/

### gitTest % ls -la
total 8
drwxr-xr-x   4 owner  staff   128 May 18 14:37 .
drwxr-xr-x@ 77 owner  staff  2464 May 18 14:36 ..
drwxr-xr-x  10 owner  staff   320 May 18 14:38 .git
-rw-r--r--   1 owner  staff   857 May 18 14:31 zen

### gitTest % git add zen
### gitTest % git status
On branch master

No commits yet

Changes to be committed:
  (use "git rm --cached <file>..." to unstage)

	new file:   zen

• Now we will modify the zen file so that we will have some changes to examine. I added several changes to the file, as shown in the image.

Original

Modified

• Now that you have saved the changes to the zen file, examine the changes with git diff zen.

git diff shows the differences in two sections. The first has this header:

@@ -1,5 +1,5 @@

The “-1,5” means that git diff is showing us lines from the original file (-) starting at line 1, and it is showing us a total of five lines from the older version.

The “+1,5” means that git diff is showing us lines from the updated file (+) starting at line 1, and it is showing us a total of five lines from the updated version.

In this case, I simply replaced blank line 2 with “Sam Was Here!” The red minus sign (-) and the green plus sign (+) in the output are both referencing changed line 2. In the original, indicated by the minus sign (-), line 2 is blank. In the current version, indicated by the plus sign (+), we see the update to line 2: “Sam Was Here!” Another way to think about this is that the blank line 2 was removed (-) and that “Sam Was Here!” was added (+) as the new line 2.

Next, git diff shows us another section of the file indicated by:

@@ -12,10 +12,9 @@

The “-12,10” means that git diff is showing us lines from the original file (-) starting at line 12, and it is showing us a total of 10 lines from the older version.

The “+12,9” means that git diff is showing us lines from the updated file (+) starting at line 12, and it is showing us a total of nine lines from the updated version. Note that the section marker “@@ -12,10 +12,9 @@” is appended to line 11 to give additional context.

Here, I removed the original line 15 and also modified the last line.

By default, git diff provides three lines of context before and after the changes it is identifying. This can be changed using the -U# flag.

• Running git diff -U1 zen gives the following output with only one line of context around the changes:

• Running git diff -U5 zen gives the following output with five lines of context around the changes:

Conclusion

You should now have a better understanding of the git diff output and hopefully be able to use it more effectively!

Learn More

• Run git diff --help
• git diff Documentation
• Introduction to Version Control—Git and GitHub