How to read a Git diff and use GitHub blame

Alex Tutor

Last time we found the price change through the file list. Today we do it properly, the way you'll handle any "who changed this and why?" question.

Open config/prices.json. In the top right corner of the file there's a History button. It lists every commit that ever changed this one file.

You

Only eleven commits, grouped by date. And the newest is at the top, like you warned.

Alex Tutor

Right: GitHub lists history newest first, while our diagrams run oldest first. Engineers see the same list in the terminal with git log.

The whole repository has hundreds of commits, but a file's history is short and focused. That's usually the fastest way in.

Alex Tutor

Question 1

You need every change made to config/prices.json this year. What do you open?

Choose one answerThe 412 Commits counter on the repository page, then read every commitThe file, then its History buttonThe READMEThe Actions tab

Alex Tutor

Here's why the file view is so handy. These are the last six commits on main; only the two highlighted ones touched config/prices.json.

Alex Tutor

Last six commits on `main`The file's History would list only the two highlighted commits, newest first.

5b8f3a2Update cat food photos
77c1e90Add express delivery price
9d04e61Fix typo in footer
a41c09eRaise delivery price to 59 UAH
e7c3b19Add summer promo banner
f02a6d4Update FAQ answersmain

You

So the branch history is everything the team did, and the file history is just the slice that matters for my question.

Alex Tutor

Exactly: the same commits, filtered differently. The branch history also has filters by author and by date, for when you know who or when but not where.

Alex Tutor

Click a commit in that list and you get its diff: what changed compared with the previous version. Here's the one from a41c09e.

Alex Tutor

Diff of `config/prices.json` in commit a41c09e

   "currency": "UAH",
-  "delivery": 49,
+  "delivery": 59,
   "freeDeliveryFrom": 800

A line starting with - (red on GitHub) was removed, a line with + (green) was added. Lines without a sign are unchanged context.

You

So the line wasn't edited, it was removed and added again?

Alex Tutor

That's how diffs show an edit: the old version in red, the new one in green, and GitHub highlights the exact characters that differ. The grey lines around it are context, so you know where you are in the file.

At the top of each file GitHub also shows counters like +1 −1: lines added and removed. And a Split view puts the old version on the left and the new one on the right, if that reads easier.

Alex Tutor

Question 2

Match each part of a diff with its meaning.

Tap an answer, then tap the row it belongs to. You can also drag.

A red line starting with -
A green line starting with +
A grey line with no sign
+12 −3 above a file

Answers left to place

Alex Tutor

Prices are one-liners. Copy changes look richer. Here's a real one from the delivery FAQ that marketing asked for last month.

You

Show me. I want to see if I can read it without help.

Alex Tutor

Go for it. Read the minus lines first, then the plus lines, then count what's new.

You

Minus lines first, then plus lines, then count. Got it.

Alex Tutor

One tip: the file name above the diff matters. The same sentence can live in two places, and a diff always belongs to one file.

Alex Tutor

Question 3

Here is the diff of pages/faq/delivery.md:

 ## Delivery
-Orders arrive in 2–3 days.
+Orders arrive in 1–2 days in Kyiv and 2–4 days elsewhere.
 Free delivery from 1000 UAH.
+Express delivery: 99 UAH.

Which statements are true? Select all that apply.

Choose all that applyThe delivery-time sentence was rewrittenThe free-delivery line was changedThe ## Delivery heading was deletedA new line about express delivery was addedThe file's counters would read +2 −1

You

History works when I know the file. But what if I'm staring at one specific line and want its story?

Alex Tutor

Then switch the file view from Code to Blame. Next to every line GitHub shows the last commit that changed it: author, message and date, plus a colour strip from older to newer.

You

"Blame" sounds a bit aggressive for a team tool.

Dmytro

It's an old name, and nobody uses it to point fingers. I run blame on my own code all the time, mostly to find out what past me was thinking.

Alex Tutor

Think of it as "where did this line come from?". In the terminal it's git blame; on GitHub it's one button.

Alex Tutor

Question 4

In the Blame view, what does GitHub show next to each line?

Choose one answerThe person who first created the fileWho approved the line in code reviewHow many customers have seen the lineThe last commit that changed that line: author, message and date

Alex Tutor

Now the full chain for our price. Blame on "delivery": 59 shows commit a41c09e. Its message ends with (#128), the pull request. And the PR says "Closes #120", the issue finance opened.

Alex Tutor

From one line to the reason behind it

a41c09e Raise delivery price to 59 UAH
What changed, who and when (found with Blame)
Raise delivery price after courier rate change#128
The discussion: why, who reviewed it, when it was merged
Courier rates up from 1 June#120
Where the request came from: finance

You

Line, commit, pull request, issue. Four clicks, and I have the whole story with the finance reasoning attached.

Alex Tutor

That's the move. The commit tells you what and when; the pull request and the issue tell you why and who agreed.

Dmytro

That's also why I end commit messages with (#128) and write Closes #120 in pull requests. Without those links the trail stops at the commit, and you're back to asking around.

Alex Tutor

Question 5

Put the steps in order to find out why a line in config/prices.json looks the way it does.

Tap an item, then tap where it should go. You can also drag or use the arrows.

Dmytro

One warning. Blame shows the last commit that touched a line, and sometimes that commit only moved or reformatted it.

Say I re-indent the whole prices file. Blame then shows me on every line, with "Reformat prices.json", even though I didn't change a single price.

You

So how do I get past that?

Alex Tutor

Next to each commit in the blame view there's a small icon, View blame prior to this change. It shows the same file as it was just before that commit. Or open the file's History and look one step further back.

Alex Tutor

Question 6

Blame on the line "freeDeliveryFrom": 1000 shows the commit "Reformat prices.json", which only changed indentation. How do you find the commit that actually set the value to 1000?

Choose one answerUse View blame prior to this change, or step back through the file's HistoryAssume the reformat author set the valueSearch the README for "1000"It can't be found: blame only keeps the latest commit

Alex Tutor

Time to close the support ticket. Their question was: why does delivery cost 59 UAH now instead of 49?

You

And I can answer with facts, not guesses: the commit, the pull request and the issue.

Alex Tutor

Write it so a support agent can copy it straight to a customer. Short, friendly, no Git words.

Dmytro

And if you mention the date, take it from the PR, not from memory. Mine is terrible.

Alex Tutor

Question 7Short answer · AI-checked task

From the history you found: commit a41c09e "Raise delivery price to 59 UAH" (merged on 3 June), PR #128 "Raise delivery price after courier rate change", issue #120 "Courier rates up from 1 June" opened by finance. Write a 2–3 sentence answer support can send to a customer asking why delivery now costs 59 UAH instead of 49.

Write your answer and get a score with feedback from our AI reviewer.

Log in to get AI feedback

Quick recap of your toolkit: History for every change to a file, the diff for exactly what changed, Blame for where a line came from, and the linked pull request and issue for why.

Case closed, and module 1 done: you can read a repository, a commit and its history. Next module: branches, where parallel work lives.

You

And where Dmytro broke the checkout button, I suspect.

Dmytro

Allegedly. And it was fixed within ten minutes, as the history will happily show you.

Alex Tutor

That’s the lesson. You answered every task — nicely done.

Back to the course