Re: How do get a specific version of a particular file?

From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Feb 27, 2007, 16:38 UTC
Message-ID: <Pine.LNX.4.64.0702270830260.12485@woody.linux-foundation.org>
In-Reply-To: <E1HM1XL-00071C-N5@candygram.thunk.org>

On Tue, 27 Feb 2007, Theodore Ts'o wrote:

> 
> git-checkout will only write the output to the working tree.
> git-cat-file and git-show only work on a object identifier (they are
> low-level plumbing commands).

Everybody has already pointed out how easy this actually is, but your fundamental mistake was thinking that "only work on a object identifier" is a bad thing.

Repeat after me: in git, *everything* is a SHA1 object!

So the problem you had was that "only". Remove the "only", and realize that object identifiers will cover _all_ you ever need, and be happy.

That's why

	git show <tree>:<path>

works. All the files are *also* just SHA1 objects, and as a result all the normal SHA1 naming rules apply.

This is also why you can literally do

	git diff v1.4.4:Makefile HEAD~5:Makefile

and it will just magically work and do exactly what the command line would make you think it does - even if you didn't know anything at all about git.

Because "git diff" knows about blob objects too, and "v1.3:Makefile" is the object name for the blob that was the Makefile object at version 1.4.4, and HEAD~5:Makefile is the Makefile in the fifth parent of your current HEAD.

Tell me any other SCM that makes things quite _that_ easy?

So in Unix, we have "everything is a file" (and then we have processes). In git, we have "everything is a SHA1-named object" (and then we have revision walking). There's a really simple underlying logic to it all, and the great thing is how _powerful_ it is.

THIS is what good design is all about. Forget nipples. You just need to internalize the fact that "everything is a SHA1".

> So if it is a matter of we don't have an easy way to do this (as opposed
> to me being stupid or the git documentation just failing to mention it
> in the right places), what's the best way to add it?

So not only do we have it, it's probably not mentioned in the docs because it is *so* fundamental that nobody even thought to mention any of this except in the "how does SHA1 lookup work" sections.

			Linus