Re: for newbs = little exercise / tutorial / warmup for windows and other non-sophisticated new Git users :-)

From: Sitaram Chamarty <sitaramc@gmail.com>
Date: Dec 30, 2008, 05:34 UTC
Message-ID: <slrngljcne.n7q.sitaramc@sitaramc.homelinux.net>
In-Reply-To: <gjc7qa$jdj$4@ger.gmane.org>

Hi Conor,

On 2008-12-30, Zorba <cr@altmore.co.uk> wrote:

> so when I reset to A, I've still got the ability to get to B or C again

if this is true...  (call this case 1)

> Now I appreciate that if I commit a new change from versionA (lets call it 
> B1), then HEAD is now at B1, and B, C etc are lost, correct ?

...then this (call this case 2) is not true.

In case 1, *how* do you get back to B and C? However you do it, the same action will get you to B and C *after* you created B1 as well.

It might help to think of these things loosely using a malloc/linked-list analogy. Imagine a singly-linked list, and a variable in your program called "head" pointing to the top element. Each element contains a back-pointer to its parent. You can start from head and walk the list backward, but you can't walk forward -- there are no forward pointers.

A "git commit" is analogous to:
    newblkptr = malloc(...);
    ... store whatever you want in the new block ...
    newblkptr.backptr = head;
    head = newblkptr;

i.e., a classic linked list add.

A "git reset --hard HEAD^" essentially pops the topmost
element off the list:
    head = head.backptr

A "git checkout -f HEAD^" is like walking a linked list backward in a loop, and stopping at some intermediate block to do something -- other the temporary loop-variable, nothing else (no global variable perhaps) points to this block, so this is like a detached head.

Of course, nothing prevents you from permanently saving this
temporary loop variable because you suddenly realise *this*
link in the chain is very important for the rest of your
program:
    some_global_var = temp_blk_ptr;

This is like "git checkout -b newbranch" does.

> Its pertinent to where I am right now, as I've goofed a commit, and want to 
> reset, and commit again but I'm worried about leaveing garbage lying around 
> (the commits for version B and C in the example above).

(1) never worry about leaving garbage around; there are many operations that do that; it's part of the design. Don't sweat it.

(2) when in doubt, try "gitk --all". That's a good GUI. I'm normally a CLI freak, but when something needs to be understood/learnt, a well-designed GUI really helps.

Run "gitk --all" on the side and hit F5 on it after each operation on the command line -- helped me when I was trying to figure out what's happening.

PS: although it doesn't mention detached heads,
http://eagain.net/articles/git-for-computer-scientists/
helped me a lot...