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Git from zero to hero: reading diffs and history

Part 2 of the Git guide: how to read git diff, what each diff command compares, good commit messages, git log, and what a commit really contains.

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The recipe book from part 1 has two commits. This part reads what changed and what Git stored.

Reading a diff

Change the cooking time in pasta.md and run git diff:

diff --git a/pasta.md b/pasta.md
index 54af23a..e1f3b27 100644
--- a/pasta.md
+++ b/pasta.md
@@ -1,3 +1,3 @@
 # Pasta
-Boil water. Cook 9 minutes.
+Boil water. Cook 10 minutes.
 Add salt to the water.
  • diff --git a/... b/... names the file. index 54af23a..e1f3b27 100644 holds short hashes of the old and new content plus the file mode; you can ignore it.
  • --- a/... and +++ b/... are the old and new version of the file.
  • @@ -1,3 +1,3 @@: this chunk (a hunk) covers three lines from line 1 in the old file, and three lines from line 1 in the new one.
  • Lines starting with - were removed, with + added, and with a space are unchanged context.

Which two things are compared depends on the flags:

Command Compares
git diff working directory against the staging area (what you haven’t staged yet)
git diff --staged staging area against the last commit (what the next commit will contain)
git diff HEAD working directory against the last commit (everything, staged or not)

Writing a good commit message

A commit message is a note for whoever reads the history later, usually you in six months:

Fix crash when the recipe list is empty

The list view assumed at least one recipe and read the first item
without checking. Show the empty-state message instead.
  • A short first line (about 50 characters) in the imperative mood: “Fix”, “Add”, “Remove”. Read it as “this commit will… fix the crash”.
  • A blank line, then an optional body that explains why. The diff already shows what changed.
  • One idea per commit, so the message can be short and true.

Looking at history

git log --oneline

9c1d4e2 (HEAD -> main) Add salt to pasta
3f2a1b9 Add pasta recipe

Each line is a commit: a short ID (a hash) and the message. Underneath:

Three commits A, B and C drawn left to right with arrows pointing from each commit to its parent. The branch name main points at the newest commit, and HEAD points at main.

  • Every commit points to its parent, the commit before it. History is a chain.
  • A branch (like main) is a label pointing at one commit. When you commit, the label moves forward.
  • HEAD is “where you are now”. Usually it points at a branch.

Everything else in Git is moving these labels around. Useful ways to look:

git log --oneline --graph --all   # a picture of all branches
git show 3f2a1b9                  # what one commit changed
git diff 3f2a1b9 9c1d4e2          # compare two commits

What’s inside a commit

git cat-file -p HEAD prints the real record:

tree 9f1e0c8d2b7a4c3e5d6f708192a3b4c5d6e7f809
parent 3f2a1b94c6d7e8f9a0b1c2d3e4f5a6b7c8d9e0f1
author Dana Levi <dana@example.com> 1791540000 +0300
committer Dana Levi <dana@example.com> 1791540000 +0300

Add salt to pasta
  • tree points at the snapshot of every file at that moment.
  • parent is the commit before it, the arrow in the diagrams. A merge commit has two parent lines.
  • author and committer are who wrote it and who recorded it, with a timestamp. Then the message.

The hash (like 9c1d4e2) is a fingerprint of exactly this content. Change anything (files, message, parent, even the timestamp) and the hash changes. That’s why git commit --amend and git rebase create new commits instead of editing old ones. Seven characters usually name a commit, as long as they’re unique in the repo.

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