Git Diff Comparison and Change Analysis
Comparing differences is an important tool for understanding code changes. By comparing the differences between different versions, you can clearly see what was changed, why it was changed, and the scope of the changes.
1. Basics of Comparing Differences
(1) The Three Areas of Git
Git has three important areas, and understanding them is key to mastering diffs:
graph TB
A[Workspace<br/>Working Directory] -->|git add| B[Buffer<br/>Staging Area]
B -->|git commit| C[Repository<br/>Repository]
A -.->|git diff| B
B -.->|git diff --staged| C
A -.->|git diff HEAD| C
style A fill:#fff3cd
style B fill:#d4edda
style C fill:#c3e6cb
- Working Directory: The directory where files are actually edited
- Staging Area: Changes ready to be committed (also known as the staging area)
- Repository: The history of all commits
(2) Scenarios for Comparing Differences
- Pre-submission check: Confirm the content you want to submit
- Code Review: Review others' changes
- Version Comparison: Compare the differences between different versions
- Branch Comparison: Compare the differences between different branches
- Conflict Analysis: Understanding the Causes of Conflict
(3) Overview of the diff Command
git diff Used to display differences. Basic usage:
BASH
git diff [options] [<commit>] [--] [<path>...]
2. Comparing the Three Regions
(1) Working Directory vs. Temporary Directory
Compare the differences between the working directory and the staging area—that is, unstaged changes.
▶ Example: Viewing Unstaged Changes
BASH
# Edit File
echo "new line" >> README.md
# View Workspace Changes（Relative to the temporary storage area）
git diff
# Output：
# diff --git a/README.md b/README.md
# index abc1234..def5678 100644
# --- a/README.md
# +++ b/README.md
# @@ -1,3 +1,4 @@
# # My Project
#
# ## Features
# +new line
# View a Specific File
git diff README.md
# View Multiple Files
git diff file1.js file2.js
(2) Staging Area vs. Repository
Compare the differences between the staging area and the repository—that is, changes that have been staged but not yet committed.
▶ Example: View staged changes
BASH
# Add or modify to the staging area
git add README.md
# View changes in the staging area（Compared to the latest submission）
git diff --staged
# Or use the old syntax
git diff --cached
# Output：
# diff --git a/README.md b/README.md
# index abc1234..def5678 100644
# --- a/README.md
# +++ b/README.md
# @@ -1,3 +1,4 @@
# # My Project
#
# ## Features
# +new line
(3) Working Directory vs. Repository
Compare the differences between the working directory and the repository—that is, all uncommitted changes.
▶ Example: View all unsaved changes
BASH
# View the workspace relative toHEADthe differences
git diff HEAD
# View the differences between the workspace and a specific commit
git diff a1b2c3d
# View the differences in the workspace compared to the last commit
git diff HEAD^
# The output includes all changes, both staged and unstaged.
3. Interpreting the Difference Output
(1) diff output format
▶ Example: Understanding diff Output
BASH
git diff
# Output Format Description：
# diff --git a/file.js b/file.js <- GitDifferential Head
# index abc1234..def5678 100644 <- File Mode
# --- a/file.js <- Original Document（-Indicates deletion）
# +++ b/file.js <- New Document（+Indicates addition）
# @@ -1,3 +1,4 @@ <- Change Location (hunk header)
# # My Project <- Context Line（Starts with a space）
#
# ## Features <- Context Line
# +new line <- Add a row（+Introduction）
# -old line <- Delete Row（-Introduction）
(2) Hunk Header Format
hunk header format: @@ -a,b +c,d @@
-a,b: The original file starts on line a and has a total of b lines+c,d: The new file starts on line c and has a total of d lines
TEXT
@@ -10,5 +10,7 @@ function login() {
This means that the 5 lines starting on line 10 of the original file have become 7 lines starting on line 10 of the new file.
(3) Statistical Information
▶ Example: Viewing a Summary of Statistics
BASH
# Display Statistics
git diff --stat
# Output：
# README.md | 1 +
# file1.js | 5 ++---
# file2.js | 3 +++
# 3 files changed, 5 insertions(+), 3 deletions(-)
# Show brief statistics
git diff --shortstat
# Output：
# 3 files changed, 5 insertions(+), 3 deletions(-)
# Display Numerical Statistics
git diff --numstat
# Output：
# 1 0 README.md
# 2 3 file1.js
# 3 0 file2.js
4. Comparing Different Versions
(1) Compare the two commits
▶ Example: Submission Comparison
BASH
# Compare the two commits
git diff a1b2c3d d4e5f6g
# Compare a commit to the current workspace
git diff a1b2c3d
# Compare a commit to the staging area
git diff --staged a1b2c3d
# Compare Adjacent Commits
git diff HEAD^ HEAD
# View the changes introduced by a specific commit
git show a1b2c3d
# Equivalent to
git diff a1b2c3d^ a1b2c3d
(2) Compare Branches
▶ Example: Branch Comparison
BASH
# Compare Two Branches
git diff main feature
# ViewfeatureBranch relative tomainChanges to
git diff main...feature
# ViewmainBranch relative tofeatureChanges to
git diff feature...main
# Compare a specific file across branches
git diff main feature -- file.js
# View changes made after the branch was forked
git diff $(git merge-base main feature) feature
(3) Comparison Tags
▶ Example: Version Comparison
BASH
# Compare the Two Versions
git diff v1.0.0 v2.0.0
# View the changes in a specific version
git diff v1.0.0 HEAD
# View the release notes
git diff v1.0.0 v2.0.0 --stat
# Output：
# src/auth.js | 25 +++++++++++++++++++++++++
# src/user.js | 10 +++++-----
# package.json | 2 +-
# 3 files changed, 32 insertions(+), 5 deletions(-)
5. Detailed Explanation of the diff Option
(1) Common Options
| Option | Description | Purpose |
|---|---|---|
--staged |
Staging Area vs. Repository | View Staged Changes |
--stat |
View Statistics | View and Edit Statistics |
--shortstat |
Brief Statistics | View Total Changes |
--name-only |
Show filenames only | View list of modified files |
--name-status |
Display Status | View File Modification Status |
-w |
Ignore whitespace | Ignore differences between spaces and tabs |
▶ Example: Using the diff option
BASH
# Show only the names of modified files
git diff --name-only
# Output：
# README.md
# file1.js
# file2.js
# Display File Modification Status
git diff --name-status
# Output：
# M README.md <- Edit
# A file1.js <- New
# D file2.js <- Delete
# R old.js new.js <- Rename
# Ignore differences in whitespace characters
git diff -w
# Ignore all spaces
git diff -w --ignore-blank-lines
# Show differences at the word level
git diff --word-diff
# Highlight in color
git diff --color-words
(2) Context Control
▶ Example: Controlling the Number of Lines in the Context
BASH
# Reduce the number of context lines
git diff -U1
# The output displays only1Line Context：
# @@ -10,3 +10,4 @@ function login() {
# const user = authenticate();
# + if (!user) return;
# return user;
# Do not display context
git diff -U0
# Increase the number of context lines
git diff -U10
(3) Path Restrictions
▶ Example: Limiting the Comparison Range
BASH
# Compare only specific files
git diff -- file.js
# Compare only specific directories
git diff -- src/
# Exclude certain files
git diff -- . ':!*.test.js'
# Compare only a specific type of file
git diff -- '*.js'
6. Real-World Applications
(1) Pre-submission Check
▶ Example: Thoroughly check before submitting
BASH
# 1. View Uncommitted Changes
git diff
# 2. View Staged Changes
git diff --staged
# 3. View Statistics on All Changes
git diff HEAD --stat
# 4. Submit after verifying that everything is correct
git commit -m "feat: Add a New Feature"
(2) Code Review
▶ Example: Reviewing Someone Else's Code
BASH
# View the changes in a specific commit
git show a1b2c3d
# View statistics for a specific commit
git show --stat a1b2c3d
# View the list of files modified in a specific commit
git show --name-only a1b2c3d
# Compare the differences between the two branches
git diff main...feature
# ViewPull Requestthe differences
git diff origin/main...origin/feature
(3) Conflict Analysis
▶ Example: Analyzing Merge Conflicts
BASH
# View the content to be merged
git diff main...feature
# View common ancestors up tofeatureChanges to
git diff $(git merge-base main feature) feature
# View files that may be in conflict
git diff --name-only main...feature
# View the specific details of the conflict
git diff main feature -- file.js
(4) Confirmation of Version Rollback
▶ Example: Confirming the rollback content
BASH
# View content that would be lost if you undo the action
git diff HEAD HEAD~1
# View rollback statistics
git diff HEAD HEAD~1 --stat
# View the revision history for a specific file
git diff HEAD HEAD~1 -- file.js
# Execute the rollback after confirmation
git reset --hard HEAD~1
❓ FAQ
Q What is the difference between git diff, git diff --staged, and git diff HEAD?
A
git diff Compare the working directory and the staging area (unstaged changes); git diff --staged Compare the staging area and the repository (staged changes); git diff HEAD Compare the working directory and the repository (all unsubmitted changes).Q How can I view only the names of modified files without seeing their actual contents?
A Use
git diff --name-only to display only the filenames, or use git diff --stat to display the filenames and modification statistics.Q What do the "+" and "-" symbols in the diff output mean?
A Lines beginning with "+" indicate new content, lines beginning with "-" indicate deleted content, and lines beginning with a space are the original text (unchanged).
Q How do I compare the differences between two branches?
A Use
git diff main feature to compare two branches, and use git diff main...feature to view the changes in the feature branch relative to the main branch (starting from the fork point).Q How do I ignore differences in spacing?
A Use
git diff -w to ignore differences between spaces and tabs, and use git diff --ignore-space-at-eol to ignore differences in trailing spaces.📖 Summary
- Git has three areas: the working directory, the staging area, and the repository; the
diffcommand can compare any two of these areas. - Working Directory vs. Staging Area:
git diffView unstaged changes - Staging Area vs. Repository:
git diff --stagedView Staged Changes - Working Directory vs. Repository:
git diff HEADView all uncommitted changes - Compare Versions: Compare differences between commits, branches, and tags
- Extensive diff options: --stat for statistics, --name-only for a list of files, -w to ignore whitespace
📝 Exercises
-
Basic Exercise: Create a Git repository. After modifying a file, use
git diffto view unstaged changes. After staging the changes, usegit diff --stagedto view staged changes. -
Advanced Exercise: Create two branches, make changes to the same file on different branches, use
git diffto compare the differences between the two branches, and analyze the conflicts that may arise. -
Challenge: Use the various options of
git diff(such as --stat, --name-status, -w, etc.) to analyze the changes made to a real-world project and generate a complete change report.
