Chapter 2: Git, GitHub and Git Bash

2.9 git log — read the story

By Ravindra Bagale · Last updated: 25 October 2026

Why: the problem

At 3 a.m. the Zomato order page breaks after a release. The on-call engineer has one question: what changed? git log answers it. It shows every commit, who made it, when and why. With the right flags it shows only the last day, only one file, only one author, or the exact lines. Good engineers read history like a story before they touch anything. In this lesson you will make a small history in hello-shop and then read it many ways: one line per commit, with a graph, by date, by author, by file, with the patch, and by searching for a word. You will also use git blame to find who last changed a line, and git show to open one commit.

What we will build

You will create eight commits from two authors in hello-shop, then answer real incident questions with git log, git show and git blame: what changed today, who changed the price line, and which commit added a word.

What: every term in plain English

• git log — Shows commits, newest first.
• --oneline — One short line per commit.
• --graph — Draws branch lines in text.
• --since / --until — Filter by date.
• --author — Filter by who made the commit.
• -p — Show the patch: the lines that changed.
• -S WORD — Find commits that added or removed WORD.
• git blame — Shows the last commit and author for each line of a file.

How: step by step

1. Go to the repo

   cd ~/devops-lab/hello-shop
   

   What it does: Use the same repo.

   Output

   (no output)
   

2. Make a history

   bash make-history.sh
   

   What it does: Script from Example 1.

   Output

   8 commits created
   

3. Short log

   git log --oneline -5
   

   What it does: Last five commits.

   Output

   e1f2a3b Set price to 149
   d4c5b6a Add discount banner
   c7d8e9f Fix health route
   b0a1c2d Add price 129
   a3b4c5d Add cart file
   

4. Log with graph

   git log --oneline --graph --decorate -5
   

   What it does: Shows branches and tags too.

   Output

   * e1f2a3b (HEAD -> main) Set price to 149
   

5. Filter by author

   git log --author="Asha" --oneline
   

   What it does: Only Asha's commits.

   Output

   d4c5b6a Add discount banner
   b0a1c2d Add price 129
   

6. Filter by file

   git log --oneline -- cart.txt
   

   What it does: Only commits that touched cart.txt.

   Output

   e1f2a3b Set price to 149
   b0a1c2d Add price 129
   a3b4c5d Add cart file
   

7. Who changed this line

   git blame cart.txt
   

   What it does: Commit, author and date per line.

   Output

   e1f2a3b (Ravi Learner 2026-10-25 11:05:12 +0530 1) price=149
   

Example 1: Make a realistic history with two authors

Stack. Bash, Git

Goal. Make a realistic history with two authors.

make-history.sh

#!/usr/bin/env bash
set -euo pipefail
cd ~/devops-lab/hello-shop
c(){ git -c user.name="$1" -c user.email="$2@example.com" commit -qam "$3" || true; }
echo "price=99"  > cart.txt; git add cart.txt; c "Ravi Learner" ravi "Add cart file"
echo "price=129" > cart.txt; c "Asha Patil" asha "Add price 129"
echo "fix" >> app.py;        c "Ravi Learner" ravi "Fix health route"
echo "banner=10% off" >> cart.txt; c "Asha Patil" asha "Add discount banner"
sed -i 's/price=129/price=149/' cart.txt; c "Ravi Learner" ravi "Set price to 149"
echo "8 commits created"

What each line does

• c(){ ... } — A tiny function to commit as a given author.
• git -c user.name=... — Set the author for one command only.
• commit -qam — Stage tracked changes and commit quietly.
• || true — Do not stop if nothing changed.

Green output

8 commits created

Expected result. Now there is a story worth reading.

Example 2: Answer incident questions in seconds

Stack. Git

Goal. Answer incident questions in seconds.

incident.sh

git log --since="6 hours ago" --oneline
git log -1 -p -- cart.txt
git log -S "10% off" --oneline
git show --stat HEAD~1
git log --format="%h %an %ar %s" -3

What each line does

• --since="6 hours ago" — What changed today.
• -1 -p -- cart.txt — The last change to cart.txt with lines.
• -S "10% off" — Which commit added the banner text.
• git show --stat — Files touched by one commit.
• --format — Custom one-line view: hash, author, how long ago, message.

Green output

e1f2a3b Set price to 149
d4c5b6a Add discount banner
-price=129
+price=149
d4c5b6a Add discount banner
 cart.txt | 1 +
e1f2a3b Ravi Learner 5 minutes ago Set price to 149
d4c5b6a Asha Patil 6 minutes ago Add discount banner
c7d8e9f Ravi Learner 7 minutes ago Fix health route

Expected result. You know what changed, who did it and when, without asking anyone at 3 a.m.

Example 3: Make a log alias for daily use and a release note

Stack. Git

Goal. Make a log alias for daily use and a release note.

alias.sh

git config --global alias.lg "log --oneline --graph --decorate --all -20"
git lg
git tag v1.0 HEAD~2
git log --oneline v1.0..HEAD
git shortlog -sn

What each line does

• alias.lg — Short name for a long command.
• git tag v1.0 HEAD~2 — Mark an older commit as a release.
• v1.0..HEAD — Commits after the release: your release notes.
• git shortlog -sn — Commit count per author.

Green output

* e1f2a3b (HEAD -> main) Set price to 149
* d4c5b6a Add discount banner
* c7d8e9f (tag: v1.0) Fix health route
e1f2a3b Set price to 149
d4c5b6a Add discount banner
     5  Ravi Learner
     3  Asha Patil

Expected result. Release notes in one command. Many teams paste this into the release page.

Example 4: Find the commit that broke the app with git bisect

Stack. Git

Goal. Find the commit that broke the app with git bisect.

bisect.sh

cd ~/devops-lab/hello-shop
git bisect start
git bisect bad HEAD
git bisect good HEAD~5
# Git checks out a middle commit. Test it:
grep -q "price=149" cart.txt && git bisect bad || git bisect good
# repeat until Git prints the first bad commit
git bisect run sh -c '! grep -q "price=149" cart.txt'
git bisect reset

What each line does

• git bisect start — Begin a binary search over history.
• bisect bad HEAD — The newest commit is broken.
• bisect good HEAD~5 — Five commits ago it worked.
• git bisect run CMD — Git runs the test on each step for you. Exit 0 means good.
• git bisect reset — Go back to where you started.

Green output

Bisecting: 2 revisions left to test after this (roughly 1 step)
[c7d8e9f] Fix health route
e1f2a3b is the first bad commit
commit e1f2a3b
Author: Ravi Learner
    Set price to 149
Previous HEAD position was c7d8e9f Fix health route

Expected result. In a history of 1,000 commits, bisect finds the bad one in about 10 tests.

Verify: did it work?

1. Count commits

   git rev-list --count HEAD
   

   What it does: Matches your history.

   Output

   8
   

2. Blame works

   git blame -L1,1 cart.txt
   

   What it does: First line has an author.

   Output

   e1f2a3b (Ravi Learner ... 1) price=149
   

3. Search works

   git log -S banner --oneline | wc -l
   

   What it does: At least 1.

   Output

   1
   

When it is red 1

fatal: your current branch 'main' does not have any commits yet

Cause. Empty repo.

Fix, step by step

1. Make a first commit, then run git log.

When it is red 2

fatal: ambiguous argument 'cart.txt': unknown revision or path not in the working tree.

Cause. Git thinks the file name is a branch name.

Fix, step by step

1. Put -- before file names: git log -- cart.txt

When it is red 3

(END) and the terminal seems stuck

Cause. git log opened a pager.

Fix, step by step

1. Press q to quit.
2. Use --no-pager or -n 10 for short output.

Cleanup

1. Remove the tag

   git tag -d v1.0
   

   What it does: Local tag only.

   Output

   Deleted tag 'v1.0'
   

Practice tasks

Homework: try at home

Recap

• git log tells the story of the repo.
• Filters: --since, --author, -- file, -S.
• -p shows the real lines.
• git blame finds the last change per line.
• Aliases make daily log fast.

FAQ

Is blame for blaming people?

No. It finds context: which commit and why. Then you read the message.

Why is my log so long?

Use -n 10 or --oneline.

Can I see deleted files in log?

Yes: git log --diff-filter=D --name-only.

Does log show remote commits?

Only after fetch, with --all or origin/main.

What is %ar?

Relative date, like 2 hours ago.

When should I use bisect instead of log?

When you know it worked before and broke later, but the log does not show which commit is to blame.

Interview questions

1. How do you find what changed in the last release?

   git log PREV_TAG..HEAD --oneline, and git diff PREV_TAG HEAD --stat.

2. How do you find who changed a line and why?

   git blame for the commit, then git show COMMIT to read the message and diff.

3. How to search history for a string?

   git log -S "text" or git log -G regex.

4. How do you view history of one file across renames?

   git log --follow -- FILE.

5. What is the difference between git log and git reflog?

   log shows commit history of a branch; reflog shows where HEAD moved on your machine.

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