Re: Difficulties of scripting git

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Jan 9, 2021, 21:42 UTC
Message-ID: <X/ojXdtGOHeP2IhY@camp.crustytoothpaste.net>
In-Reply-To: <X/oAhdZlLwcFRCrR@ACM>

On 2021-01-09 at 19:14:13, Alan Mackenzie wrote:

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> Hello, git.
> 
> I want to write a bash script (more precisely, correct an existing
> script) which uses git, and it is DIFFICULT!!!
> 
> I need a git pull in the script.  Fine, but how does the script know if
> it's worked or not?  I couldn't find any description of a return code in
> the git-pull man page (or in the git-merge man page).
> 
> The big problem is when I have modified, uncommitted files in the target
> repo, and those same files are to be updated in commits coming from the
> source repo.  Sadly, git is unable to merge these changes.  It just
> fails, putting an error message onto stderr, but doesn't tell the
> calling script in any way that I can see.
> 
> One idea would be always to call git stash before doing the pull, then
> git stash pop afterwards.  Trouble is, git stash is unreliable - it
> doesn't always add a new stash entry, so the stash pop at the end would
> sometimes/often pop off an entry it shouldn't.  git stash doesn't have a
> --force argument.  git stash doesn't set a result code, either, that I
> can see.  One way around this would be to do
> 
>     $ git stash list | wc -l
> 
> both before and after the git stash and compare the answers, but really?

git stash doesn't consider there being nothing to stash to be an error, so it doesn't set a nonzero error code. Think of it as, "I want a clean working tree," not, "I want to create a stash." In that sense, what you wanted is already done, so it's not an error.

You are, however, not the only person who's been unhappy with this behavior. A --force option may be a useful option to have if someone wanted to create it.

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> So, next idea, feed the output from git status --porcelain through grep
> before and after the git pull, so as to find out whether there are any
> modified files before the git pull (thus making a stash necessary) and
> any files with conflicts after the git stash pop.  Shouldn't be too
> difficult.

Using "git status --porcelain" before and after the stash is an easy way to find out whether there was anything to stash. If there's a difference, then a stash was created.

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> Except, how does one recognise a file with conflicts from this git
> status output?  The man page says that
> 
>     "   For paths with merge conflicts, `X' and `Y' show the modification
>     states of each side of the merge.  For paths that do not have merge
>     conflicts, `X' shows the status of the index, and `Y' shows the status
>     of the work tree.  For untracked paths, `XY' are `??'.  Other status
>     codes can be interpreted as follows:  ...."
> 
> I've spent nearly an hour trying to make sense of this bit of man page.
> How is one meant to distinguish an XY of a merge conflict from the XY of
> an index/work tree entry?  I can't find that key bit of information
> anywhere.

In the chart, the options in the first section are "normal" cases that occur without a merge conflict. The second section is things that happen when there's a conflict (the "unmerged" states).

So what you're looking for to find conflicts is something maybe a little like this:

  git status --porcelain | grep -E '^(AA|DD|[AUD]U|U[AUD])'

If you believe that you're likely to need to handle names with newlines, then of course you'll need -z as well.

I do agree that this is a little unclear; I went ahead and simulated a merge conflict to verify. I'll try to send a patch making the documentation reflect what I mentioned about the chart above.

-- 
brian m. carlson (he/him or they/them)
Houston, Texas, US