Using git diff between branches

This guide will focus on how to use git diff to compare different branches, a command that is especially useful for reviewing changes before merging branches or understanding differences between a local branch and the main branch.

Basic usage of git diff for branch comparison

Comparing two branches

To see the differences between two branches, you can use the following command:

git diff branch1 branch2

This command will show the diff output of what is different between the end of branch1 and the end of branch2. It lists all the changes made in branch2 relative to branch1.

diff --git a/file1.txt b/file1.txt
index 9d1ed08..4b3792e 100644
--- a/file1.txt
+++ b/file1.txt
@@ -1,4 +1,4 @@
-Hello, this is branch1.
+Hello, this is branch2.
 This file contains sample text.
 Here we demonstrate a simple diff.
-This line is the same in both branches.
+This line has been changed in branch2.

• diff --git a/file1.txt b/file1.txt: indicates that the differences are being shown for the file named file1.txt.
• index 9d1ed08..4b3792e 100644: shows the SHA-1 hashes of the file versions before and after the change along with the file's mode (which remains unchanged here, 100644 indicates a regular file).
• Lines beginning with - and +:
  • Lines that start with a - show content that exists in branch1 but was changed or removed in branch2.
  • Lines that start with a + show what the content has been changed to in branch2, or what has been added.

In this example:

• The first line of the file changed from "Hello, this is branch1." in branch1 to "Hello, this is branch2." in branch2.
• Another line "This line is the same in both branches." in branch1 was changed to "This line has been changed in branch2." in branch2.

Advanced usage

Comparing specific files between branches

If you only want to compare a specific file between two branches, you can specify the file name at the end of the command:

git diff branch1 branch2 -- path/to/file

Example:

To compare a file named example.txt between develop and main branches:

git diff develop main -- example.txt

This will only show you the differences in example.txt and will ignore all other files.

Additional git diff options and features

List only filenames

If you are only interested in seeing which files have changed, not the actual line-by-line differences, you can use:

git diff branch1 branch2 --name-only

This command will list the filenames that are different between two branches, without displaying the actual content differences.

Output differences as a patch

For generating a patch file from the differences between two branches, use:

git diff branch1 branch2 > changes.patch

This command will create a file called changes.patch which can be applied to another branch using git apply. For more information on applying a patch, see this guide on Git patches.

Viewing diffs with commit graphs

Using git log with diff options can help visualize the commit history alongside the differences:

git log --graph --oneline --decorate --all --patch

This displays an ASCII graph of the commit history, showing which commits are included in each branch and what changes those commits introduced.

Common issues and solutions

Branch not up-to-date

If you find that git diff does not show the expected changes, ensure that your branches are up to date with their remote counterparts:

git fetch origin
git diff origin/branch1 origin/branch2

This compares the remote versions of the branches, which is useful if there have been updates to the branch that you do not yet have locally.

For further reading on examining the differences between branches, see the official Git documentation.