Viewing History and Differences

Introduction

Understanding how to view commit history and differences between versions is crucial for debugging and understanding project evolution.

Viewing Commit History

Basic Log

git log

This shows all commits with:

• Commit hash
• Author and email
• Date
• Commit message

Compact View

git log --oneline

Shows only the first line of each commit message with shortened hash.

Showing Differences

View Uncommitted Changes

git diff

Shows differences between working directory and staging area.

View Staged Changes

git diff --staged

Or:

git diff --cached

Shows differences between staging area and last commit.

Compare Two Commits

git diff commit1..commit2

Using short hashes:

git diff abc1234..def5678

Compare Branches

git diff main..feature-branch

Shows all changes in feature-branch that are not in main.

Compare Specific Files

git diff commit1..commit2 -- filename.txt

Practical Examples

Example 1: See What Changed in Last Commit

git show

Or:

git show HEAD

Example 2: See Changes in Specific File

git log -p filename.txt

Shows history of changes to a specific file.

Example 3: Who Changed What

git log --author="John"

Shows commits by a specific author.

Example 4: Find When a Line Was Added

git log -S "function_name"

Searches for commits that added or removed that string.

Example 5: Show File at Specific Commit

git show HEAD~3:filename.txt

Shows the content of a file at a specific commit.

Pretty Log Formats

Short Summary

git log --pretty=format:"%h - %an, %ar : %s"

Oneline with Graph

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

JSON Output

git log --pretty=format:"{}" --json

Useful for scripting.

Searching History

By Message

git log --grep="fix"

Finds commits with "fix" in the message.

By Date

git log --since="2024-01-01"
git log --until="2024-12-31"

By File

git log -- filename.txt

Shows commits affecting that file.

By Changes

git log -S "search_term"

Finds commits adding or removing that term.

Blame

Who Changed Each Line

git blame filename.txt

Shows who last modified each line and when.

Blame Specific Lines

git blame -L 10,20 filename.txt

Shows blame for lines 10-20.

Bisect

Find Bad Commit

Binary search through history:

git bisect start
git bisect bad
git bisect good v1.0.0

GIT will checkout commits for you to test. Mark as good or bad until you find the problematic commit.

End Bisect

git bisect reset

Summary

Essential commands for viewing history:

• git log - View commit history
• git diff - See changes between versions
• git show - View specific commit
• git blame - See who changed each line
• git bisect - Find problematic commits

Next Lesson

Learn about remote repositories and how to collaborate with others.