Explain git diff code

What this git diff sample shows

file headers

Each file starts with diff --git a/... b/..., where a/ is the old version and b/ the new one. The index 9853b5d..97c2803 100644 line gives short hashes of the file's contents before and after, and its mode: 100644 is an ordinary file that isn't executable (100755 would be). Then --- a/cart.js and +++ b/cart.js say which side the - and + lines come from.

hunks

@@ -1,11 +1,13 @@ reads: this part starts at line 1 and covers 11 lines in the old file, and starts at line 1 and covers 13 lines in the new one. Counting the sample agrees: 11 lines are context or -, and 13 are context or +. With -U1, git kept 1 line of context instead of 3 and split the change into two hunks. The second header, @@ -5,5 +6,6 @@ function total(items) {, ends with the nearest line above the change that looks like a heading, here the function it sits in.

added and removed lines

A - line was removed, a + line was added, and a line that starts with a space is unchanged context. An edited line shows up as both: - return sum; and then + return Math.round(sum * (1 + TAX));. Before the change, receipt() for 2 items at 1250 cents printed Total: $25.00; after it, Total: $27.00.

renames

similarity index 100% with rename from and rename to means utils.js moved to lib/money.js with nothing inside it changed. git diff --staged --name-status printed R100 utils.js lib/money.js. With --no-renames, the same move showed as D utils.js and A lib/money.js: one file deleted and another added.

Good to know

• Made with git 2.55.0 in a test repo with a fixed name and date. In git's output, an unchanged blank line is a single space, and it's kept that way here.
• git diff --staged --stat summed it up as cart.js | 6 ++++--, utils.js => lib/money.js | 0 and 2 files changed, 4 insertions(+), 2 deletions(-).
• --word-diff marks the change inside a line instead of whole lines: return [-sum;-]{+Math.round(sum * (1 + TAX));+}.
• One added line can change more than it looks. Before if (item.qty <= 0) continue;, an item priced 999 with qty: -1 took 999 off the total: total() returned 1501 for it and 2 items at 1250. After the change, the same cart returned 2700: the item is skipped and tax is added.

Watch out: Plain git diff only shows changes you haven't staged yet. Once everything was added with git add -A, git diff printed nothing at all, and only git diff --staged showed the change. Before staging, plain git diff listed utils.js as deleted and left lib/money.js out completely, because git doesn't track a new file until it's added.