Git: undo, history, and branches

You changed one line. One line. And now the whole site is broken: the prices are wrong, the buttons do nothing, and you cannot remember what that line looked like before. This is the single most common moment in programming, and it has a proper answer. It is not a better memory and it is not Ctrl+Z. It is version control.

The simple version of undo is Ctrl+Z. It works for about a minute, then it runs out: you close the editor, or you save over the file, or you keep going and the undo history is gone. You have also probably made a copy at some point. Be honest about what those copies look like: site-final.zip, site-final-2.zip, site-final-real.zip. Copies do not explain anything. A version control system records every change together with who made it, when, and what they were trying to do. Weeks later you can still ask exactly what that line said, and why it was changed.

A commit is a snapshot of the whole project, saved together with a short message. It is not a difference and not a copy of one file: it is the state of everything at that moment. Each commit points back to the commit before it, so the commits form a chain — this is what people mean by the history of a project.

9f1c2ab
   |
a17d3e0
   |
64b8f21
   |
  (start)

$ git diff 64b8f21 a17d3e0
--- a/cart.js
+++ b/cart.js
-  return net;
+  return net * (1 + VAT);

A branch is a separate line of history that starts from the one you are on. The point is that it is cheap: creating one takes a moment and costs almost nothing, so you can try an idea without betting the working site on it. If the idea works, you merge the branch back into the main line. If it does not, you throw the branch away and nothing that worked has changed. In a team, this is how several people work at the same time without stepping on each other: everybody has their own line and the main line stays in a state that can be released.

A merge conflict happens when two branches changed the same lines. Git refuses to guess and puts both versions in the file, marked with a separator:

<<<<<<< HEAD
price = 19.00
=======
price = 21.00
>>>>>>> experiment