How to get the branch HEAD points to using a plumbing command?

How to get the branch HEAD points to using a plumbing command?

[Utku]

If HEAD points to a branch, then `cat .git/HEAD` returns (at least on my Git version):

    ref: refs/heads/2.x

So, to understand which branch HEAD points to, I can:

- Read file contents of `.git/HEAD`.
- Try to match the contents with the regular expression `^ref: refs\/heads\/(.*)$`. If the capturing group is not empty, then HEAD points to the branch indicated by the capturing group. Otherwise, HEAD does not point to a branch.

To do this using only plumbing commands, I came up with:

- Run `git show-ref --head --heads`.
- Parse the output first by newlines, then by spaces and test if the commit hash of HEAD is present in other lines. If it is, then extract the branch name and return it. If not, return empty string.

My question is, is there a "better" (more idiomatic, less "expensive", etc.) way of doing it using only one plumbing command? This solution is OK but it is more work than just matching one line with a regular expression. The reason I want to do this using only plumbing commands is because I think that plumbing command interface would be more stable than the format (organization, structure) of files under the `.git/` directory across Git versions. Please correct me if I'm wrong in this idea as well.

Re: How to get the branch HEAD points to using a plumbing command?

[Ævar Arnfjörð Bjarmason]

On Tue, Jan 05 2021, Utku wrote:

> If HEAD points to a branch, then `cat .git/HEAD` returns (at least on my Git version):
>
>     ref: refs/heads/2.x
>
> So, to understand which branch HEAD points to, I can:
>
> - Read file contents of `.git/HEAD`.
> - Try to match the contents with the regular expression `^ref: refs\/heads\/(.*)$`. If the capturing group is not empty, then HEAD points to the branch indicated by the capturing group. Otherwise, HEAD does not point to a branch.
>
> To do this using only plumbing commands, I came up with:
>
> - Run `git show-ref --head --heads`.
> - Parse the output first by newlines, then by spaces and test if the commit hash of HEAD is present in other lines. If it is, then extract the branch name and return it. If not, return empty string.
>
> My question is, is there a "better" (more idiomatic, less "expensive", etc.) way of doing it using only one plumbing command? This solution is OK but it is more work than just matching one line with a regular expression. The reason I want to do this using only plumbing commands is because I think that plumbing command interface would be more stable than the format (organization, structure) of files under the `.git/` directory across Git versions. Please correct me if I'm wrong in this idea as well.

You'll want to use git-rev-parse for this, e.g.:

    $ git rev-parse --symbolic-full-name HEAD
    refs/heads/master

Re: How to get the branch HEAD points to using a plumbing command?

[Jeff King]

On Tue, Jan 05, 2021 at 04:24:56PM +0100, Ævar Arnfjörð Bjarmason wrote:

> > My question is, is there a "better" (more idiomatic, less
> > "expensive", etc.) way of doing it using only one plumbing command?
> > This solution is OK but it is more work than just matching one line
> > with a regular expression. The reason I want to do this using only
> > plumbing commands is because I think that plumbing command interface
> > would be more stable than the format (organization, structure) of
> > files under the `.git/` directory across Git versions. Please
> > correct me if I'm wrong in this idea as well.
> 
> You'll want to use git-rev-parse for this, e.g.:
> 
>     $ git rev-parse --symbolic-full-name HEAD
>     refs/heads/master

Another option is:

  git symbolic-ref HEAD

which has the benefit of working even when HEAD points to an unborn
branch. It also has a --short option to drop the refs/heads/ part.

It's also worth noting what you'd see on a detached HEAD. With rev-parse
you'll see "HEAD", but symbolic-ref will exit with a non-zero code.

-Peff

Re: How to get the branch HEAD points to using a plumbing command?

[Junio C Hamano]

Utku <ugultopu@gmail.com> writes:

> My question is, is there a "better" (more idiomatic, less
> "expensive", etc.) way of doing it using only one plumbing
> command? This solution is OK but it is more work than just
> matching one line with a regular expression. The reason I want to
> do this using only plumbing commands is because I think that
> plumbing command interface would be more stable than the format
> (organization, structure) of files under the `.git/` directory
> across Git versions. Please correct me if I'm wrong in this idea
> as well.

The "HEAD" pointer can be a symbolic link, not a text file with
"ref: refs/heads/..." in it, and any approach that works directly
with "cat .git/HEAD" would be incorrect, unless it does a stat on
.git/HEAD first to see if it is a symlink.  And such implementation
details can change over time.

So you are right to seek a solution that hides such details from
you, i.e. a Git command, to do so.  It does not have to be plumbing,
though.

The orthodox answer would be to use "git symbolic-ref".  When you
want to learn which underlying ref a symbolic ref is pointing at,
that is the tool designed to answer that question.