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Re: [PATCH] Documentation/git-checkout: make doc. of checkout <tree-ish> clearer

From: "Christoph Michelbach" <michelbach94@xxxxxxxxx>
On Sun, 2017-04-16 at 22:25 +0100, Philip Oakley wrote:
From: "Christoph Michelbach" <michelbach94@xxxxxxxxx>
> It's: git checkout [-p|--patch] [<tree-ish>] [--] <pathspec>...
The one I quoted is direct from the Synopsis, which does indicate
there are
potentially more aspects to resolve, such as the influence of using
[-p|--patch] options.

Oh, you are right. I didn't even notice the one in the synopsis doesn't
match the one further down. The one in the synopsis is wrong because
after removing the optional parameters, it's the same as the first one
in the synopsis, yet we expect very different behavior from them.

It maybe that the paragraph / sentence that needs adjusting is;

'git checkout' with <paths> or `--patch` is used to restore modified
deleted paths to their original contents from the index or replace
with the contents from a named <tree-ish> (most often a commit-ish).

and split it at the "or replace paths" option to pick out your

This one is confusing, too: Paths can lead to folders, yet folders whose
contents have been modified are not restored to their original contents
when executing that command. Only files are.

After reading the documentation and having never used the command
before, one would expect

   rm -Rf repo
   mkdir repo
   cd repo
   git init &> /dev/null
   mkdir folder
   echo a > folder/a
   git add -A
   git commit -m "Commit 1." &> /dev/null
   echo b > folder/b
   git add -A
   git commit -m "Commit 2." &> /dev/null
   echo c > folder/c
   git add -A
   git commit -m "Commit 3." &> /dev/null
   git checkout `git log --pretty=format:%H | tail -1` folder
   ls folder

to print `a`. However, it prints `a b c` because all of the files inside
`folder` which have been modified or deleted since (here: none) are
reset to their original state after the first commit, but `folder`
itself isn't. Yet, the only path which was passed to the command in
question is `folder`.

In my opinion, this command needs improved documentation (and the
synopsis needs to be fixed).

I think this example is of a different kind of misunderstanding.

We are at commit 3, with a b c in the working directory and index, and then we ask to checkout certain specific files from the first commit 1, which contains the file a. That old file is extracted and replaces the file from commit 3.

As the file a is identical there is no change and we still have the original b and c files from commit c.

I'd guess that the misunderstanding is that you maybe thought that the whole directory would be reset to it's old state and the files b and c deleted, rather than just the named files present in that old commit being extracted. If we'd created and added a file d just before the checkout, what should have happened to d, and why?

Comming back to the mental model issue with the implicit staging of checked out paths, I suspect this is a because we have both a snapshot and a diff based mental model. Normally the distinction is natural. We have the snapshot from the last commit (or branch checkout) in the index, and then we have the two steps for additional changes we personally made, and then added added, that determine the diff(s).

However in this (git checkout <treeish> -- <paths>) case we don't get that two step option. There is IIUC no 'git copyout <treeish> -- <paths>' which simply copies older files into the current worktree without adding it to the index/staging area.

The confounding of the, both optional, [--patch] and [<paths>] in the same description doesn't make it any easier. Splitting their synopses and descriptions may be the way forward.

I also haven't used the --patch option directly so there may be more issues beneath the surface.