Git Diff
The git diff command shows the exact differences between two versions of a file — what lines were added, what lines were removed, and what lines remained unchanged.
Think of it as a "compare documents" feature. Just as a word processor can show what changed between two versions of a document (tracked changes), git diff shows the same for code files.
Reading the diff Output
Before learning the commands, it is important to understand how to read the diff output:
diff --git a/index.html b/index.html
index 3a5b6c7..8d9e0f1 100644
--- a/index.html
+++ b/index.html
@@ -1,4 +1,5 @@
<html>
<body>
- <h1>Hello</h1>
+ <h1>Hello World</h1>
+ <p>Welcome to my site</p>
</body>
</html>
| Symbol | Meaning |
|---|---|
--- | The original version of the file (before change) |
+++ | The new version of the file (after change) |
- (red) | Line that was removed |
+ (green) | Line that was added |
| No symbol | Line that did not change (context) |
@@ -1,4 +1,5 @@ | Shows which line numbers changed in each version |
Common git diff Use Cases
1. Compare Working Directory to Last Commit
git diffShows changes in the working directory that are NOT yet staged. In other words, what has been modified since the last commit, but not yet added with git add.
2. Compare Staged Changes to Last Commit
git diff --staged(Also written as git diff --cached)
Shows what is currently in the staging area compared to the last commit. This is very useful before committing — to review exactly what will be saved.
3. Compare Working Directory + Staged to Last Commit
git diff HEADShows all changes (both staged and unstaged) compared to the last commit.
Practical Example
# Create a file and make the first commit
echo "<h1>Hello</h1>" > index.html
git add index.html
git commit -m "Add index.html"
# Modify the file
echo "<p>Welcome</p>" >> index.html
# See what changed (not yet staged)
git diff
# Output shows: + <p>Welcome</p> (new line added)
# Stage the file
git add index.html
# Now git diff shows nothing (changes are staged, not unstaged)
git diff
# Output: (empty - nothing unstaged)
# But this shows what's staged vs last commit
git diff --staged
# Output shows: + <p>Welcome</p>
Comparing Two Specific Commits
git diff a3f92b1 b7c3d45Shows all differences between commit a3f92b1 and commit b7c3d45. Commit hashes come from git log --oneline.
Comparing Two Branches
git diff main feature-loginShows all differences between the main branch and the feature-login branch. This is very useful before merging a branch.
Comparing a Specific File Only
git diff index.htmlShows changes only in index.html, ignoring all other files.
Comparing a Specific File Between Two Branches
git diff main feature-login -- style.cssShows how style.css differs between the main and feature-login branches.
Show Only the Names of Changed Files
git diff --name-onlyOutput:
index.html
style.css
No details — just the list of which files changed.
Show a Summary of Changes
git diff --statOutput:
index.html | 3 ++-
style.css | 10 +++++++---
2 files changed, 10 insertions(+), 3 deletions(-)
Summary of Common git diff Commands
| Command | What it Compares |
|---|---|
git diff | Unstaged changes vs last commit |
git diff --staged | Staged changes vs last commit |
git diff HEAD | All changes (staged + unstaged) vs last commit |
git diff branch1 branch2 | Two branches |
git diff commit1 commit2 | Two specific commits |
git diff --name-only | Only the file names that changed |
git diff --stat | A summary of how much changed in each file |
Summary
git diff shows the exact line-by-line differences between versions of files. Lines prefixed with + were added, and lines prefixed with - were removed. Use git diff to see unstaged changes, git diff --staged to review what will be committed, and git diff branch1 branch2 to compare branches.