{"id":"l-git-1-4","courseId":"git-github","lang":"en","kind":"lesson","summary":"Read a repository’s history with git log, git show and git diff, name any commit relative to HEAD, and picture history as a graph of snapshots linked to their parents.","html":"

Your portfolio has a handful of commits now. Six months from now it will have hundreds, and the question you’ll actually ask is never “what’s the history?” but something sharper: when did the footer break, what did this file look like before the redesign, which commit added that colour. Reading history well is how you answer those in seconds.

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The everyday view

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Plain git log prints every commit, newest first, with full IDs and dates. It’s thorough and tiring. This is the version to remember:

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Terminal
git log --oneline --graph --all
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Text
*   e5d81c0 (HEAD -> main) Merge branch 'dark-mode'\n|\\\n| * d4a7f20 (dark-mode) Add dark mode colors\n* | c39be11 Add projects page\n|/\n* b2e04f3 Add navigation bar with base styles\n* a1c9d72 Add home page
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Read it bottom to top for chronology. Each * is a commit; the lines show which commit came from which. In parentheses are labels: main and dark-mode are branches, and HEAD -> main means you’re on main. --all matters: without it, Git shows only commits reachable from where you are, and the dark-mode line would still be there only because it was merged.

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Long output opens in a pager. Use the arrow keys or space to scroll and q to quit.

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Useful filters, all combinable: -n 5 (last five), --stat (which files changed), -p (full diffs), --author="Maya", --since="2 weeks ago", and -- index.html (only commits that touched that file).

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Commits form a graph

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The output above is a drawing of a directed acyclic graph, or DAG. Directed, because each commit points to its parent. Acyclic, because you can’t follow parents and end up where you started; a commit can’t be its own ancestor. Most commits have one parent. The root commit has none. A merge commit has two.

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\n Commits a1c9 and b2e0 in a row, then the line splits: c39b on the main line and d4a7 on the dark-mode line. Both lead to merge commit e5d8, which has two parents. The main label and HEAD point to e5d8; the dark-mode label points to d4a7. Arrows point from each commit to its parent.\n \n a1c9\n \n b2e0\n \n c39b\n \n d4a7\n \n e5d8\n \n \n \n \n \n \n main\n \n HEAD\n \n \n \n dark-mode\n \n
The same history as a commit graph
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Keep this picture in your head. In section 2 you’ll see that a branch is nothing more than one of those labels, and every branching command moves a label or adds a circle.

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Naming commits

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You can name any commit by its ID, and Git accepts the shortest unambiguous prefix, usually 7 characters. IDs are hashes of the commit’s content, metadata and parent, so they never change, and in practice two different commits never share one.

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Counting from where you are is often easier:

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Looking inside commits

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Terminal
git show c39be11              # message, author, date and the diff it introduced\ngit show HEAD~2:styles.css    # the whole file as it was in that snapshot\ngit diff a1c9d72 HEAD         # everything that changed between two snapshots\ngit diff HEAD~3 HEAD -- styles.css   # the same, for one file only
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git diff takes any two commits because each is a complete snapshot. The diff is calculated on demand by comparing them. The same goes for the diff git show prints: it’s this commit’s snapshot compared with its parent’s.

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A real investigation

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Here’s how these commands combine. A friend tells you the footer on your projects page has disappeared, and you’re sure it was there last week. Start by narrowing the history to the file that matters:

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Terminal
git log --oneline --since=\"1 week ago\" -- projects.html
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Text
8f1e2aa Move project cards into a grid\nc39be11 Add projects page
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Two suspects. Look at the newer one in full:

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Terminal
git show 8f1e2aa -- projects.html
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The diff shows the <footer> block in red, removed along with the old list markup. That’s your culprit, found in under a minute. To grab the old footer markup without rewinding anything, print the file as it was in the parent commit and copy what you need:

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Terminal
git show 8f1e2aa~1:projects.html
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Nothing in this investigation changed a single file or commit. Reading history is always safe, so do it freely and often. Most “Git emergencies” I’ve seen were really reading problems: someone panicked before looking at the graph, and the answer was already sitting in git log.

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You’ve seen labels like main and dark-mode sitting on commits. Section 2 starts with what those labels really are and why creating one costs almost nothing.

\n","toc":[{"id":"sec-1","text":"The everyday view","level":2},{"id":"sec-2","text":"Commits form a graph","level":2},{"id":"sec-3","text":"Naming commits","level":2},{"id":"sec-4","text":"Looking inside commits","level":2},{"id":"sec-5","text":"A real investigation","level":2}],"takeaways":["History is a directed acyclic graph, each commit pointing to its parent, and a merge commit pointing to two.","git log --oneline --graph --all draws the graph in the terminal, including branches you’re not on.","git show <commit> prints one commit’s message and changes; git show <commit>:<path> prints a file as it was then.","HEAD~1 is the commit before the one you’re on, HEAD~3 three steps back along first parents.","git diff A B compares any two snapshots, and -- <path> limits it to one file."],"further":[{"title":"Viewing the Commit History (Pro Git)","url":"https://git-scm.com/book/en/v2/Git-Basics-Viewing-the-Commit-History"},{"title":"git log reference","url":"https://git-scm.com/docs/git-log"},{"title":"gitrevisions (naming commits)","url":"https://git-scm.com/docs/gitrevisions"}],"quiz":[{"q":"

git log on main doesn’t show a commit you made on your dark-mode branch. Why?

","options":[{"text":"The commit was lost because you switched branches.","why":"Switching branches never deletes commits. The commit is still there, pointed at by dark-mode."},{"text":"git log only shows commits reachable from HEAD by following parents, and that commit isn’t.","why":"Correct. main doesn’t lead back to it. git log --all or git log dark-mode will show it."},{"text":"git log hides commits until they’re pushed.","why":"Git doesn’t care about pushing when it shows local history. Unpushed commits appear like any others."},{"text":"You need git log -p to see commits from other branches.","why":"-p adds each commit’s diff to the output. It doesn’t change which commits are listed."}],"answer":1},{"q":"

You want to see styles.css exactly as it was two commits ago, without changing any files. Which command does that?

","options":[{"text":"git restore styles.css~2","why":"~2 applies to commits, not file names, and git restore writes into your working tree, which you didn’t want."},{"text":"git log -2 styles.css","why":"This lists the last two commits that touched the file. It shows messages, not the file’s contents."},{"text":"git diff HEAD~2","why":"This shows changes between that commit and your working tree, not the file’s full content."},{"text":"git show HEAD~2:styles.css","why":"Correct. <commit>:<path> names a file inside a snapshot, and git show prints it without touching your files."}],"answer":3},{"q":"

How many parents does a merge commit that joins dark-mode into main have?

","options":[{"text":"Two, the commit main was on and the commit dark-mode was on.","why":"Correct. Two parents is what makes it a merge, and it’s why the graph joins at that point."},{"text":"One, the commit main was on before the merge.","why":"A single parent is an ordinary commit. A merge records both histories it joins."},{"text":"None, because a merge commit starts a new history.","why":"Only the very first commit in a repository (the root commit) has no parent."}],"answer":0},{"q":"

Which command lists only the commits whose changes added or removed the text dark-mode?

","options":[{"text":"git log --grep dark-mode","why":"--grep searches commit messages, not the code changes inside commits."},{"text":"git diff dark-mode","why":"That compares your working tree against the dark-mode branch. It doesn’t list commits."},{"text":"git log -S dark-mode","why":"Correct. The pickaxe option -S finds commits that changed how many times the string appears, which is how you hunt down when something was added or removed."},{"text":"git show dark-mode","why":"That shows the commit the dark-mode branch points to, if such a branch exists."}],"answer":2}],"exercise":{"kind":"choice","multiple":false,"options":["a","b","c","d"],"correct":["c"],"title":"Read the graph","prompt":"

Here is the portfolio’s history as a commit graph, the same shape git log --oneline --graph --all draws in text. Arrows point from each commit to its parent.

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:::figure The portfolio’s commit graph\n

\n Five commits. 1a2b and 3c4d in a row. From 3c4d, one line goes to 5e6f and another to 7a8b. Merge commit 9c0d has parents 5e6f and 7a8b. main and HEAD point to 9c0d; the label hero points to 7a8b.\n \n 1a2b\n \n 3c4d\n \n 5e6f\n \n 7a8b\n \n 9c0d\n \n \n \n \n \n \n main\n \n HEAD\n \n \n hero\n \n\n:::\n

You’re on main. Which commits does git log --oneline hero list?

\n","hints":["git log <name> starts at the commit that name points to.","From there it follows parent arrows only. It never walks forward to children.","Start at the commit labelled hero and follow the arrows until you run out."],"explanation":"

git log hero starts at 7a8b, the commit the hero branch points to, and follows parents: 7a8b → 3c4d → 1a2b. The merge commit 9c0d is a child of 7a8b, and Git never walks from parent to child, so it isn’t listed. 5e6f is on the other side of the fork and isn’t an ancestor of hero either.

\n","checkLabels":{},"optionText":{"a":{"text":"All five commits, because hero was merged into main.","why":"Merging made hero’s commits reachable from main, not the other way round. git log hero starts at hero and only walks backwards."},"b":{"text":"Only 7a8b, the commit hero points to.","why":"git log lists the starting commit and every ancestor, not one commit. git show hero would print only 7a8b."},"c":{"text":"7a8b, 3c4d and 1a2b.","why":"Correct. Start at 7a8b and follow parents: 3c4d, then the root commit 1a2b."},"d":{"text":"9c0d, 7a8b, 3c4d and 1a2b.","why":"9c0d is a child of 7a8b. Commits don’t know their children, so git log can’t reach it from hero."}}},"words":706,"readMinutes":3}