git pull and friends, was git pull + src/sharefiles.mk

Andreas Eder <andreas_eder at gmx.net> writes:

> Robert Dodier wrote:
>>You wrote some notes about Git which was posted to this list.
>>Can you update the notes to indicate how to deal with pull vs rebase?
>
> Yes, I'd second this. Git is also new to me and sometimes behaves rather
> unexpectedly.
> So, a mini tutorial would be rather welcome. (Yes I know the freely
> available literature about git.)

Ok, I will try to snatch some time in the next week to update the notes.
In the meantime, here are a few pointers (please feel free to correct me
on any misunderstandings):

-a branch X in a git repo R can `track' another a branch X' in another
 repo R'; however, these branches are distinct in that their
 HEAD/history may diverge.

-when branch X tracks X', repo R will contain a branch R'/X'. The
 command

git branch -r

will list these branches.

-git fetch updates the "remote" branches like R'/X', but it does not alter
 the "local" tracking branches like X.

-git merge attempts to merge the history of X and R'/X',
this will include the history of which HEAD is on which
branch at each commit, so a merge preserves as much history
as possible, in some sense. A cost of this is that the history tends
to look like spaghetti after a while.

-git rebase attempts to rewrite the history of X so that 
all commits on X come after any commits on R'/X'. In short
a history that looks like

    ____ X
   /
-------- R'/X'

is changed to a history that looks like

         _____ X
        /
-------- R'/X'

At that point, merging the two branches is trivial (there can be no
conflicts) and git gives this trivial type of merge a name: it's a
fast-forward.

-the command

git pull --rebase

executes git fetch, followed by git rebase. Since rebasing re-writes
history, this is *not* the default for git pull. (The potential harm in
re-writing history is, if you someone else is already basing work on
that history, then when it is re-written you force them to adapt--which
can be quite messy and complicated.)

-the command

git pull --ff-only

will do a git fetch, then a git merge but it will only do a fast-forward
merge (in other words, the merge will succeed only if the histories look
like

     ____ R'/X'
    /
---- X

so the HEAD of X can be fast-forwarded to that of R'/X').



-- 
Leo Butler                <l_butler at users.sourceforge.net>
SDF Public Access UNIX System -   http://sdf.lonestar.org