Re: What is the mechanism of a git checkout?

From: Sitaram Chamarty <sitaramc@gmail.com>
Date: May 29, 2009, 01:15 UTC
Message-ID: <slrnh1udov.6ls.sitaramc@sitaramc.homelinux.net>
In-Reply-To: <23764024.post@talk.nabble.com>

On 2009-05-28 15:21:59, mastermindxs <mastermindxs@gmail.com> wrote:

Show 10 quoted lines

>
> does a git checkout pull the contents of a branch? how does it physically
> separate files in your working directory from other branches? 
>
> The speed of which it works leads me to the logical conclusion that the
> files in the working directory are all there for all branches and git simply
> only shows you the files of the active branch.
>
> Can anyone shed more light on the underlying mechanisms of git checkout
> maybe even branch and merge as well?

I explain it as below when I get asked this question. It's a simplified answer but largely correct, I believe. Corrections from git.gods welcome :-)

When you checkout a branch, you are moving from a "before" branch to an "after" branch. git does this:

  - untracked files are never touched; the rest of this
    discussion does not apply to them

  - files which do NOT show up in "git status" (ie unchanged
    with respect to the "before" branch) are changed as per
    their status in the "after" branch.

    - if they exist in the "after" branch, and are the same
      as in the "before" branch, they are left alone

      This is where all the speed you see comes from!

    - if they exist in the "after" branch, and they are
      **different** than in the "before" branch, they are
      checked out from the "after" branch

      This is the bulk of the disk activity; if there are
      few of these, it's fast enough for you think nothing
      is actually happening ;-)

    - if they do not exist in the "after" branch, they are
      deleted

  - files that do show up in "git status" in the "before"
    branch, are either modified or staged.
    
    - if the "before" branch version is the same as the
      "after" branch version, that also is not touched.  In
      the new branch, running "git status" will show you the
      same files.

    - if the "before" branch version and the "after" branch
      version are *not* the same, git would lose your local
      (uncommitted) changes, so it refuses to do the
      checkout and stays where it is.