git

Git's undo commands are notoriously confusing because they are usually memorised as recipes. Read against the object model they separate cleanly: each acts on a different one of the three places content lives. This lesson maps them, covers recovery from the accidents that feel unrecoverable, and states what genuinely cannot be undone.

Every operation so far moved a pointer. Merging is the first that has to decide something: two branches changed the same project and one result must come out. This lesson covers the three-way merge and the merge base it depends on, what a conflict actually is, and why rebase produces different commits rather than moving them.

Hashes are unusable by hand, so Git names them. A branch is not a copy of anything, not a directory, and not a container for commits: it is a forty-character file. Once that is clear, checkout, reset, detached HEAD and the reflog stop being separate concepts and become one operation on one kind of file.

Git is usually taught as a set of commands. Underneath it is a key-value store where the key is a hash of the content, and four object types built on that one idea. This lesson derives the hash by hand, shows what a commit actually contains, and explains why the design makes history tamper-evident.

How SVN handles parallel work. Covers the trunk-branches-tags convention, cheap copies that make a branch nearly free, why branches and tags are the same mechanism, merge tracking with svn:mergeinfo, reintegrating a branch, and moving between SVN and Git with git svn.

How Subversion (SVN) works and why it still exists in a Git world. Covers the centralized model, the central repository and working copy, global revision numbers and atomic commits, and the real reasons teams keep SVN: large binaries, path-based access control, and a single source of truth.