threads / discuss / 19557

What is the mechanism of a git checkout?

Subject: What is the mechanism of a git checkout?

## tl;dr

4 messages between May 28, 2009 and May 29, 2009.

replies: 3people: 4as markdown or json

mastermindxs· May 28, 2009, 15:21 UTC · lore

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?

thanks -diego www.greyrobot.com

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Peter Harris· May 28, 2009, 16:26 UTC · re: mastermindxs · lore

Re: What is the mechanism of a git checkout?

On Thu, May 28, 2009 at 11:21 AM, mastermindxs wrote:
>
> Can anyone shed more light on the underlying mechanisms of git checkout
> maybe even branch and merge as well?
This may help you understand Git: http://www.youtube.com/watch?v=8dhZ9BXQgc4
Also: http://eagain.net/articles/git-for-computer-scientists/
Peter Harris
Sitaram Chamarty· May 29, 2009, 01:15 UTC · re: mastermindxs · lore

Re: What is the mechanism of a git checkout?

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.
Jakub Narebski· May 29, 2009, 09:05 UTC · re: Sitaram Chamarty · lore

Re: What is the mechanism of a git checkout?

Sitaram Chamarty <sitaramc@gmail.com> writes:
Show 16 quoted lines
> On 2009-05-28 15:21:59, mastermindxs <mastermindxs@gmail.com> wrote:
> >
> > 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.
[...]
Show 12 quoted lines
>   - 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.

In which case (there is file which is different in "before" and "after", and which is modified in working area) you can ask git to try to 'move' (merge) changes from "before" to "after" version of a file with '-m' / --merge option, see git-checkout(1):

  git checkout -m after
-- 
Jakub Narebski
Poland
ShadeHawk on #git

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