Time travel

Goal#

Read history, see exactly what a commit changed, and undo one old commit safely.

Read the past#

git log --oneline          # one line per commit, newest first
git show 21683d2           # what that commit changed
git diff eb1bf5a HEAD      # everything between then and now
git log -p README.md       # every change to one file

In quest 03, git log --oneline shows this. The hashes are the same on every machine, because HEAD the cat wrote the history at fixed times:

9baebff Add the nap plan
21683d2 Tidy the README
36d0e1b Add the toy list
eb1bf5a Write the house rules

A short hash like 21683d2 is enough to name a commit. HEAD~1 means "one before HEAD", and HEAD~2 two before.

Two ways to bring something back#

Revert a commit. git revert <commit> makes a new commit that does the opposite. History stays as it was, plus one honest commit that says "this was undone". Safe on shared branches.

Restore a file. git restore --source <commit> README.md puts one file back as it was in that commit, in your working tree. Then you commit it like any change.

Never "go back" by deleting history on a branch other people use. Chapter 07 covers when rewriting is fine.

Quest 03#

quest start 3

Somebody "tidied" the README, and rule 3 vanished. Find the commit that removed it, and bring it back with a new commit. Keep naps.txt and the rest of history as they are. Then quest check.

Check it works#

• README.md has rule 3 again, and git log --oneline shows all four old commits plus yours.
• quest check passes.

Exercise#

Do the quest twice: once with git revert, once with git restore --source. Start over with quest start 3 between them. Compare the two results with git log -p -1. When would you choose each?

More to try

• git switch --detach eb1bf5a lets you look around an old commit. Run git status: git calls this "detached HEAD". Come back with git switch main.
• git log -S "keyboard" finds commits that added or removed that word. Try it on the quest.