Git basics: checkout vs. reset

Today I got a better understanding of how git works, in particular what git checkout and git reset do.

Git basics

• A commit holds a certain state of a directory and a pointer to its antecedent commit.
• A commit is identified by a so-called ref looking something like 7153617ff70e716e229a823cdd205ebb13fa314d.
• HEAD is a pointer that is always pointing at the commit you are currently working on. Usually, it is pointing to a branch which is pointing to that commit.
• Branches are nothing but pointers to commits. You are "on a branch" when HEAD is pointing to a branch.

checkout

Copygit checkout <commit> <paths>

… tells Git to replace the current state of paths with their state in the given commit.

• paths can be one or more files or directories.
• If no branch (or commit hash, see basic facts) is given, Git assumes the HEAD commit.
  • --> git checkout <path> restores path from your last commit. It is a 'filesystem-undo'.
• If no path is given, git moves HEAD to the given commit (thereby changing the commit you're sitting and working on).
  • --> git checkout branch means switching branches.

Example

Copygit checkout HEAD~2 app/models/foo.rb

… drops all modifications of foo.rb and replaces the file with its version from HEAD~2 = two commits before the current.

reset

Copygit reset <commit>

… re-sets the current pointer to the given commit.

• If you are on a branch (you should usually be), HEAD and this branch are moved to commit.
  • Actually HEAD is still pointing to initial the branch and the branch is now pointing towards the commit
• If you are in detached HEAD state, git reset does only move HEAD. To reset a branch, first check it out.

Example

You are currently working upon commit 123abc. After resetting to a previous commit xyz789 (e.g. with git reset HEAD~2), you have no easy access to commit 123abc anymore, because HEAD and the branch are both pointing to xyz789. This will result in staging the commited files from 123abc and keeping the files in the current index.

To move the branch pointer 'back to the front', you can't use git checkout, as it only moves HEAD. You have to reset your branch to that commit: git reset 123abc. (If you didn't save the first commit's hash (123abc), git reflog will help you finding it.)