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Subject: Re: [boost] [githelp] Merging two repositories
From: Cox, Michael (mhcox_at_[hidden])
Date: 2013-12-11 17:50:46

On Wed, Dec 11, 2013 at 1:28 AM, Philippe Vaucher <
philippe.vaucher_at_[hidden]> wrote:

> > Learn about git reflog so you know how to reset bad situations, or at
> worse
> >> do some physical backup of the directories if you are unsure and give
> it a
> >> try.
> >>
> >
> > OK, thanks for pointing it out. I might just buy a git book and read it,
> > since I am not patient enough to read everything online.
> >
> Here is a super-quick-micro-crash-course:
> - git commits form a DAG (you can imagine it as a tree if it helps). A
> branch/tag is just a little sticker put on a part of a tree. Creating a
> commit in git is just adding a new part on the tree and moving the
> sticker
> "master" to it. Rewriting a commit is just going back a commit on the
> tree
> and creating a new one, and moving the sticker "master" to it.
> - when you rewrite history, delete commits, whatever.... they are not
> really deleted. They are just not visible anymore, because there are no
> "stickers" attached to them anymore.
> - "git reflog" shows you the history of what you did to the repo. So you
> see all commits you did, all rebases, all checkouts, etc, each time with
> the SHA1.
> - The only really destructive operation in git is "git gc", which
> collects commits/whatever that don't have a sticker attached to them.
> Given this information, you can now guess that "trying out things" in git
> is super safe... you try your stuffs, you verify the results, and if you
> are unhappy you "git reflog", find the SHA1 where stuffs were still ok, and
> "git reset --hard SHA1" and voila, you're back to your previous "good"
> state. There's also "git fsck" which can show you "lost" commits.

Other git commands may automatically garbage collect unreachable commits,
so don't count on them hanging around indefinitely without a ref anchor to
make them reachable. The git-gc man page shows how you can disable gc for
your repo, but that doesn't prevent Github from saving space by gc'ing your
remote repo.


> HIH,
> Philippe
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