Re: [ANNOUNCE] chronoversion: chronological archiving script with temporary commits

> David Tweed <david.tweed@gmail.com> wrote:
> > A question for those who understand things: I stash the last written
> > _tree_'s hash in a tag and then when a new "commit's" directory tree
> > is written starts look to see if it's the same SHA value. If it is I
> > know I can avoid the commit. At the moment I'm using
> > 
> >    if os.path.exists(lastTreeFile) and
> > tree==open(lastTreeFile,"r").read()[:40]:
> > 
> > to be safe just in case a user, eg, goes mad and manually deletes that
> > record. Clearly this is going to hit trouble if git ever decides to
> > put this tag into a packed refs file.
> > Is there any neat way of using builtin stuff like git-rev-parse to ask
> > if a ref has a given SHA1 value and return an easily parsed yes/no
> > answer?
> 
> The common idiom if you want to compare trees to see if you
> need to make a commit is:
> 
> 	oldc=`git rev-parse $tagname^{commit}`
> 	oldt=`git rev-parse $oldc^{tree}`
> 	newt`git write-tree`

> 	if [ X$oldt = X$newt ]; then
> 		: nothing to save
> 	else
> 		newc=`git commit-tree $newt -p $oldc`
> 		git update-ref $tagname $newc $oldc
> 	fi
> 
> Yes, a little ugly.  But its safe; if another program were to
> alter the value of $tagname (e.g. "git branch -f", "git tag -f")
> while your script is running the update-ref in the else will fail,
> letting you know that $tagname changed in the process.