Re: [QUESTION] What is a tag for?

Alex Bennee <kernel-hacker@bennee.com> writes:

Then maybe its time to change your e-mail address from kernel-hacker to kernel-builder? ;-) ;-) ;-)

Correct.

Tags are just labels. So you can think of them as a shorthand that lets you easily communicate a set of selected well-known versions with others (either people, or git tools). Instead of saying 2664b25051f7ab96b22b199aa2f..., you can say v2.6.16-rc1.

All of the things I demonstrate below, you can substitute v2.6.X with their commit object names and things work the same way. It is more convenient and human friendly to use tags.

Except one thing: signed tags can be verified, if you have GPG public key of the signer (in this case Linus), with "git verify-tag".

<offtopic> Does anybody know where to obtain the public key to verify v2.6.16-rc1 tag? </offtopic>

That is one of the things often done. To build on top of v2.6.16-rc1:

	$ git checkout -b myhack v2.6.16-rc1
	$ hack hack hack
        $ git commit -s -m 'Add support for frotz videocard.

	This adds frotz videocard support.  Blah Blah Blah...'

You could also use them to see what happened during a given timeframe. For example:

	$ git log v2.6.14..v2.6.15 | git shortlog

would list you all the changes between these two releases, for example.

If you know something used to work at a given version, say v2.6.15, and now it does not work, you can use it as good/bad input for the bisection bug hunting:

	$ git bisect good v2.6.15 && git bisect bad master

I do not quite understand. Immediately after the above example checkout, before or after doing the hackhack and commit, git branch would probably say:

	$ git branch
          origin
	  master
        * myhack

Immediately after the example checkout, "git describe myhack" would answer v2.6.16-rc1. OTOH, after the hackhack and commit, it would answer v2.6.16-rc1-gXXXXXXXX where XXXX part is an abbreviated commit object name of the commit you have at the tip of myhack branch.

To see what you did in the branch, the tag, if you remember where you forked from, can be used this way:

	$ git whatchanged -p v2.6.16-rc1..myhack

However, the above is equivalent to saying this:

	$ git whatchanged -p master..myhack

So in that sense tag is not that useful for the purpose of getting list of commits (or, more precicely, naming a development track).

It is useful to name a specific version:

	$ git diff v2.6.16-rc1 myhack

compares two versions; it is not equivalent to "git diff master myhack".

That depends on what you exactly mean by "this", but all of the above are quite cheap. If the "myhack" practice turned out to be useless, you can just:

	$ git checkout master
        $ git branch -D myhack

to come back to the tip of the mainline and delete your side branch.