git diff

git diff: show differences between commits, branches, files, or the working directory.

The git diff command is used to display the differences between different states of your Git repository, such as the differences between your working directory and the staging area, or between two commits. It helps you see what changes have been made, which can be useful for reviewing your work before committing or for understanding the history of your project.

By default, git diff shows the differences between your working directory and the staging area. This allows you to see what changes you've made that haven't been staged for commit yet. You can also use git diff to compare specific files, branches, or commits.

git diff is particularly useful when you want to:

• Review the changes you've made before staging or committing them.
• Compare different versions of your project to see how it has evolved over time.
• Identify the specific lines of code that were changed in a particular commit.
• Understand the differences between branches before merging them.

Examples

git diff

git diff --staged

git diff 1234abc 5678def

git diff main feature-branch

git diff main feature-branch -- path/to/file.js

Understanding the output

Let's consider we've modified a file named index.html by adding some new tags and modifying an existing one. Here is how the output of the git diff command might look:

diff --git a/index.html b/index.html
index 1c2s3z5..4f5g6h7 100644
--- a/index.html
+++ b/index.html
@@ -8,1 +8,3 @@
-<h1>Old header</h1>
+<h2>New header</h2>
+
+<p>New text.</p>

This output format is called a patch, and it's designed to show us what has changed in the file. Ironically, while the output is meant to be human-readable, the design choice was made in the 1970s, so it may not be as intuitive as modern interfaces. Although it may look a bit cryptic at first, there's a pattern to it. Here's how to read it:

• The first line indicates that the diff is comparing two versions of the index.html file.
• The second line shows the Git internal metadata. This is not important for understanding the changes in the file.
• The --- and +++ lines and the file paths show what corresponds to the - and + lines respectively.
• The @@ line shows the line numbers for the changes in the old and new versions of the file.
• Lines prefixed with - were removed in the new version.
• Lines prefixed with + were added in the new version.
• Unchanged lines are shown for context but have no prefix.

This output tells us that:

• The file index.html has changed, as indicated by the paths in the first few lines.
• The line <h1>Old header</h1> has been removed.
• It's replaced by a new line <h2>New header</h2>.
• A new block <p>New text.</p> has been added to the bottom, as shown by a line starting with +.

A patch file like this may include multiple changes to the file, each separated by a @@ line. It may also include several files.