Session 3 · Looking Back

⏱ about 60 minutes · 🧰 you need: Session 2 done (the my-website repo)

The big idea

A time machine is only useful if you can look around in the past. This session teaches the “reading” commands: see the history in different ways, see exactly what changed, and tell Git which files to ignore.

Everyday example: flipping through the photo album. git log is the list of captions, git show opens one page, and git diff is the “spot the difference” puzzle between two photos.

cd ~/git-lab/my-website

1. git log in useful shapes

git log --oneline

1a9b0f6 (HEAD -> main) Add about page
8c4d7e2 Add greeting to homepage
3f2a1b9 Add homepage

Command	What you get
git log --oneline	one line per commit
git log -3	only the last 3 commits
git log --stat	which files changed in each commit, and how many lines
git log -p	the full changes (a patch) in each commit
git log --author="Asha"	only commits by a person
git log --since="2 days ago"	commits from a time window
git log --grep="login"	commits whose message contains a word
git log -- index.html	only commits that touched this file
git log --oneline --graph --all	a text drawing of all branches (very handy from Session 5)

git log --stat -2

commit 1a9b0f6... (HEAD -> main)
Author: Asha Kumar <asha@example.com>
Date:   Fri Oct 16 10:42:11 2026 +0530

    Add about page

 about.html | 1 +
 1 file changed, 1 insertion(+)

commit 8c4d7e2...
    Add greeting to homepage

 index.html | 1 +
 1 file changed, 1 insertion(+)

Tip

A set-and-forget shortcut for the graph view: git config --global alias.lg "log --oneline --graph --all --decorate". After that, git lg does it.

2. git show: open one page of the album

git show 8c4d7e2

commit 8c4d7e2...
Author: Asha Kumar <asha@example.com>
Date:   Fri Oct 16 10:41:50 2026 +0530

    Add greeting to homepage

diff --git a/index.html b/index.html
index 4b2c1a0..9d3e7f1 100644
--- a/index.html
+++ b/index.html
@@ -1 +1,2 @@
 <h1>Welcome to my website</h1>
+<p>Hello from Chennai!</p>

You can use the short ID, or relatives of HEAD (HEAD is “the commit I am on now”):

Name	Meaning
HEAD	the latest commit on your current branch
HEAD~1 (or HEAD^)	one commit before that (the parent)
HEAD~3	three commits back

git show HEAD~1

3. How to read a diff

Every diff uses the same notation:

@@ -1 +1,2 @@
 <h1>Welcome to my website</h1>      ← no sign: unchanged line (context)
+<p>Hello from Chennai!</p>          ← + : line ADDED
-<p>Old text</p>                     ← - : line REMOVED

A changed line appears as one - (old) and one + (new). @@ -1 +1,2 @@ says “starting at line 1 of the old file, 1 line; starting at line 1 of the new file, 2 lines”. You can ignore the numbers at first and just look for + and -.

4. git diff: what did I change?

Make a change but do not commit:

echo "<footer>(c) 2026</footer>" >> index.html
git diff

diff --git a/index.html b/index.html
--- a/index.html
+++ b/index.html
@@ -1,2 +1,3 @@
 <h1>Welcome to my website</h1>
 <p>Hello from Chennai!</p>
+<footer>(c) 2026</footer>

Three versions of “the file” exist at the same time (working folder, staging, last commit), so Git has three diffs:

git diff compares working folder to staging, git diff --staged compares staging to the last commit, git diff HEAD compares working folder to the last commit.

Command	Compares	Use it to answer
git diff	working folder ↔ staging area	“What have I changed but not yet staged?”
git diff --staged	staging area ↔ last commit	“What will go into my next commit?”
git diff HEAD	working folder ↔ last commit	“Everything changed since my last commit?”
git diff 3f2a1b9 1a9b0f6	any two commits	“What changed between these versions?”
git diff --stat	add to any of the above	a summary: files and line counts only

git add index.html
git diff            # prints nothing: nothing is unstaged now
git diff --staged   # shows the footer line: it is staged

Important

Habit to build: before every commit run git status and git diff --staged. It takes five seconds and catches stray debug lines, passwords and wrong files.

Commit it:

git commit -m "Add footer to homepage"

5. Telling Git to ignore files: .gitignore

Some files should never be in your history: secrets (.env), downloaded libraries (node_modules/), compiled files, editor settings, logs. Create a file named .gitignore listing patterns:

mkdir logs
echo "debug output" > logs/app.log
echo "SECRET_KEY=abc123" > .env
git status

Untracked files:
	.env
	logs/

cat > .gitignore <<'EOF'
# secrets
.env

# logs
logs/
*.log

# macOS / editor noise
.DS_Store
.vscode/
EOF
git status

Untracked files:
	.gitignore

.env and logs/ vanished from the list. Commit the .gitignore itself so the whole team shares it:

git add .gitignore
git commit -m "Add .gitignore"

Pattern	Matches
*.log	every file ending in .log, anywhere
logs/	the folder logs and everything in it
/build	only a build in the top folder
!keep.log	exception: do not ignore this file
# text	a comment

Warning

.gitignore only affects files Git is not yet tracking. If you committed .env earlier, ignoring it later does not remove it. Stop tracking it (keeping the file on disk) with git rm --cached .env, commit, and change any secret that was exposed, because it is still in the history (see Session 12).

6. Other useful look-around commands

Command	Use
git status -s	short status: M modified, A added, ?? untracked
git ls-files	list every file Git tracks
git blame index.html	who last changed each line, and in which commit
git shortlog -sn	commits per author

git blame index.html

3f2a1b9 (Asha Kumar 2026-10-16 10:40:12 +0530 1) <h1>Welcome to my website</h1>
8c4d7e2 (Asha Kumar 2026-10-16 10:41:50 +0530 2) <p>Hello from Chennai!</p>
b7e9c01 (Asha Kumar 2026-10-16 11:05:33 +0530 3) <footer>(c) 2026</footer>

Common mistakes

Mistake	Fix
git diff shows nothing, yet status shows a change	The change is staged. Use git diff --staged.
git log is “stuck” on a screen	It opened a pager. Press q to quit, space for the next page.
.gitignore does not hide a file	The file is already tracked. Use git rm --cached <file>.
Named the file gitignore.txt or .gitignore.txt	It must be exactly .gitignore.

Hands-on exercises

Tryout 1 · Find. In my-website, list only the commits whose message contains the word about, one per line.

Answer

git log --oneline --grep="about" -i

(-i ignores upper/lower case.)

Tryout 2 · Compare. Show what changed between the first and the latest commit, as a summary of files and line counts.

Answer

git log --oneline            # note the first commit's ID, e.g. 3f2a1b9
git diff --stat 3f2a1b9 HEAD

Tryout 3 · Ignore. Create a file notes.tmp and a folder cache/ with a file inside. Make Git ignore both without ignoring .gitignore. Verify with git status.

Answer

echo "x" > notes.tmp
mkdir cache && echo "y" > cache/data
printf "*.tmp\ncache/\n" >> .gitignore
git status -s

Only M .gitignore appears. Commit it: git commit -am "Ignore tmp files and cache".

Recap

• ✅ git log (--oneline, --stat, -p, --graph, --author, --grep) reads history.

• ✅ git show <commit> opens one commit. HEAD~1 means “one before”.

• ✅ In a diff, + is added and - is removed.

• ✅ git diff (unstaged), git diff --staged (about to commit), git diff HEAD (everything).

• ✅ .gitignore keeps secrets and junk out. git blame tells who changed a line.

Next: Session 4 · Undoing Things.