Unit 3: Seeing Changes
What exactly did I do?
Unit 3 of 8 in Git and version control for kids. Its 4 lessons are What Changed?, The Diff That Vanished, Inside an Old Commit and Diff Detective — below is everything each one explains, and a question or two from it to try.
Every terminal session on this page was replayed before it shipped, on a git whose every message is tested against the real one.
This unit opens with a fortnight’s trial of everything — no card needed — or with a family plan, bought in the iPhone app. The first two units of every track are free for ever. Try it in the app.
🔬 What Changed?
Line by line
git status says *which files* changed. git diff says *what changed inside them*.
A line starting with - was there before. A line starting with + is there now. A line with a space is untouched, shown so you can see where you are.
Terminal
$ git diff diff --git a/poem.txt b/poem.txt index 1c27a27..0d47f1f 100644 --- a/poem.txt +++ b/poem.txt @@ -1,2 +1,2 @@ Roses are red -Violets are blue +Violets are purple
Adding, not replacing
When you only add a line, the diff has a + and no - at all. The @@ line is git telling you which line numbers it is talking about.
Terminal
$ git diff diff --git a/list.txt b/list.txt index 4245cc7..01cc058 100644 --- a/list.txt +++ b/list.txt @@ -1,2 +1,3 @@ milk eggs +bread
Try it yourself
In a diff, what does a line starting with - mean?
- It is a mistake
- That line used to be there and is not any more
- That line is new
- That line is a comment
What does this diff say happened?
Terminal
git diff- Nothing changed
- A line was added at the end
- A whole file was deleted
- A line was changed from one thing to another
Answer them in the app
🫥 The Diff That Vanished
Where did my diff go?
Run git add, then git diff, and git says nothing. It has not lost your work.
Plain git diff means "what is on my desk that is *not* in the box yet". Once it is in the box, there is nothing left to report.
Terminal
$ git diff diff --git a/hello.txt b/hello.txt index f893417..d7353d5 100644 --- a/hello.txt +++ b/hello.txt @@ -1 +1 @@ -hello +hello world $ git add hello.txt $ git diff
Asking about the box instead
git diff --staged shows what is in the staging area — in other words, exactly what your next commit will contain.
It is the last thing worth doing before you commit.
Terminal
$ git diff --staged diff --git a/hello.txt b/hello.txt index f893417..d7353d5 100644 --- a/hello.txt +++ b/hello.txt @@ -1 +1 @@ -hello +hello world
Try it yourself
You staged a change. Which command still shows it?
- git diff
- git diff --staged
- git status --short
- git log
Answer it in the app
🗝️ Inside an Old Commit
git show
git show opens up a commit: who made it, its message, and the diff of what it changed.
On its own it shows the newest commit. HEAD~1 means "one before the newest", HEAD~2 two before, and so on.
Terminal
$ git show
commit f75a37f0604a7a9b9d5c6fbe46745701153620b4 (HEAD -> main)
Author: You <you@example.com>
Date: Mon Jan 6 09:02:00 2025 +0000
Add eggs
diff --git a/list.txt b/list.txt
index a04b167..4245cc7 100644
--- a/list.txt
+++ b/list.txt
@@ -1 +1,2 @@
milk
+eggs
Comparing any two moments
git diff also takes two names. git diff HEAD~2 HEAD shows everything that changed across the last two commits at once — handy when you cannot remember what you did yesterday.
Terminal
$ git diff HEAD~2 HEAD diff --git a/f.txt b/f.txt index f08dc37..f844632 100644 --- a/f.txt +++ b/f.txt @@ -1 +1,3 @@ one +two +three
Try it yourself
What does HEAD~2 mean?
- The second commit ever made
- Two commits before where you are now
- The second file in the folder
- Two branches away
Answer it in the app
🏆 Diff Detective
Try it yourself
You have changed a file but not staged it. Which shows the change?
- git diff
- git diff --staged
- git show
- git log
A diff shows only + lines and no - lines. What happened?
- Lines were only added
- Lines were only deleted
- The file was renamed
- Nothing happened
Answer them in the app