Module 3 Lesson 3: Viewing history with git log

Once you have a series of commits, you need a way to see them. The git log command is your primary tool for exploring the history of your repository.

In this lesson, we master the various ways to view and filter your logs so you can find exactly what you're looking for.

1. The Basic Log

The simplest command shows all commits in reverse chronological order:

git log

This output includes:

commit <SHA-1>: The unique ID.
Author: Who made the change.
Date: When it was made.
The Message: Your explanation of the change.

2. Condensing the Log

On large projects, the default log is too long. Use these flags to make it readable:

git log --oneline: Shows each commit on a single line (ID and summary only).
git log -n <number>: Shows only the last <number> of commits.
git log --stat: Shows which files were changed and how many lines were added/deleted in each commit.

# Example of a clean, readable log
git log --oneline -n 5

3. Visualizing the Tree

Git is a "graph" of commits. You can visualize how your history branches and joins (which we'll study in Module 4) with this power-user command:

git log --oneline --graph --all

Many developers create an alias for this (Module 10) because it's so useful.

graph TD
    C3["c8a1b2e - Add footer (HEAD -> main)"]
    C2["7f3d9a1 - Fix header typo"]
    C1["1a2b3c4 - Initial commit"]
    
    C3 --> C2
    C2 --> C1

4. Filtering the History

You can search for specific commits using filters:

By Author: git log --author="Sudeep"
By Date: git log --since="2 weeks ago" or git log --until="2025-01-01"
By Keyword: git log --grep="bug" (searches commit messages).
By File: git log -- index.html (only shows commits that touched index.html).

Lesson Exercise

Goal: Navigate a "Dense" Log.

Go to any repository you cloned in Module 2 (e.g., the Linux kernel or a smaller project).
Run git log --oneline -n 10.
Try to find commits only by a specific author.
Run git log --grep="fix" and see how many bug fixes appear in the recent history.
Use git log --stat on a single commit to see exactly which files were modified.

Observation: You'll find that the "SHA-1" IDs are the keys. If you want to see exactly what happened in commit abc1234, you can use git show abc1234.

Summary

In this lesson, we established:

git log is the standard tool for viewing history.
--oneline and -n make long logs manageable.
--graph helps visualize the project's structure.
You can filter the log by author, date, keyword, and file path.

Next Lesson: History is great for looking back, but what if you made a mistake? We’ll learn the art of Undoing changes.