patchgit-prompt.sh: shorter equal upstream branch name

From: Julien Carsique <julien.carsique@gmail.com>

When using the "name" option of GIT_PS1_SHOWUPSTREAM to show the upstream abbrev name, if the upstream name is the same as the local name, then some space could be saved in the prompt. This is especially needed on long branch names.

Replace the upstream name with the sign '=' if equal to the local one.
Example:    [master * u= origin/=]$
instead of: [master * u= origin/master]$

Signed-off-by: Julien Carsique <julien.carsique@gmail.com>
---
 contrib/completion/git-prompt.sh | 7 +++++++
 1 file changed, 7 insertions(+)

On 2014-09-30 11:36, Julien Carsique wrote:

Seems like a good idea to me.

Please add some new tests in t/9903-bash-prompt.sh.  In particular:
  * upstream ref in refs/heads
  * upstream is git-svn
  * branch names containing slashes

To avoid colliding with other stuff, this variable should either be local or prefixed with '__git_ps1'.

This comparison breaks on branches containing a slash (e.g., foo/bar).

Also, how does this interact with git-svn? (I don't use git-svn so I'm not very familiar with how it manages refs.)

Assuming remote names can't contain a slash (which I think is true), a safer approach might be parse the full ref and special-case refs/remotes:

    __git_ps1_upstream_name=$(git rev-parse \
        --abbrev-ref "${upstream}" 2>/dev/null)
    local tmp
    tmp=$(git rev-parse --symbolic-full-name "${upstream}" 2>/dev/null)
    case ${tmp} in
    refs/remotes/*)
        # todo: can ${b} be something other than refs/heads/* here?
        [ "${__git_ps1_upstream_name#*/}" != "${b#refs/heads/}" ] \
            || __git_ps1_upstream_name=${__git_ps1_upstream_name%%/*}/\=
        ;;
    esac

Additional cases could be added to handle git-svn if needed.

  * This could break if ${__head} contains any pattern-special
    characters.

  * While this syntax works in both Bash and Zsh (assuming no
    pattern-special characters), my preference is to stick to POSIX[1]
    when possible.  For example, assuming the upstream name is
    always in refs/remotes (which is not true, but this is an example)
    and remote names can't contain a '/', you could do this:

        __git_ps1_upstream_name=${__git_ps1_upstream_name%%/*}/\=

  * I don't think the CodingGuidelines explicitly prohibit long lines
    for shell code, and this file already contains plenty of long
    lines, but I really dislike lines longer than 80 characters.

[1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html

-Richard

Richard Hansen <rhansen@bbn.com> writes:

Thanks for a review (everything I omitted above looked good to me).

... but I do not think refnames can have *, ? and such so it may not be relevant ;-).

Nah. The existing script is full of bash-isms like local you suggested to add (and other constructs like shell arrays and [[ ]] tests, I suspect), and there is no hope to "fix" them to stick to the bare-minimum POSIX, and there is no need to do so (isn't this bash-prompt script after all?)

Yes, I dislike overlong lines, too. But I also dislike lines that are artificially chopped into shorter pieces without good reason ;-).

On 2014-09-30 18:35, Junio C Hamano wrote:

True.

I don't think it'd be hard to convert it to pure POSIX if there was a desire to do so. The biggest challenge would be 'local', which would require subshells or uniquified prefixed global variables. Both of those are likely to make the code a bit grotesque.

It would be unwise to go to great lengths to avoid Bashisms, but I think it would be smart to use POSIX syntax when it is easy to do so. Rarely is it hard or awkward to use POSIX syntax ('local' and arrays are two major exceptions), so Bashisms like the ${//} expansion in this patch are usually unnecessary divergences from a ubiquitous standard. POSIX is a stable foundation, and it's easy to get POSIX shell code to run consistently on all POSIX-like shells.

One of these days I'll try converting git-prompt.sh to POSIX -- I'm curious to see how bad it would be.

-Richard

Richard Hansen <rhansen@bbn.com> writes:

Not necessarily; if you make it so slow to be usable as a prompt script, that is not a "conversion". Bash-isms in the script is allowed for a reason, unfortunately.

In general, I agree with you. People who know only bash tend to overuse bash-isms where they are not necessary, leaving an unreadable mess.

For the specific purpose of Julien's "if the tail part of this string matches the other string, replace that with an equal sign", ${parameter/pattern/string} is a wrong bash-ism to use. But the right solution to count the length of the other string and take a substring of this string from its beginning would require other bash-isms ${#parameter} and ${parameter:offset:length}.

And that's fine.

Hi,

Thank you both for your feedback!
I'm looking at applying your requests:
- add tests,
- variable renaming,
- use of local,
- fix multiple issues on string parsing
- avoid useless bash-isms? Did you agree on the ones I should remove?

I'll send an updated patch asap. Tell me if I forgot something.

Regards, Julien

On 01/10/2014 19:49, Junio C Hamano wrote:

On 2014-10-07 11:57, Julien Carsique wrote:

I'm guessing the structure of your code will change somewhat when you address the other issues, so I think it may be premature to discuss specific Bashisms right now. (There aren't any particular Bashisms that I think should always be avoided -- I just want people to ponder whether a particular use of a Bashism is truly preferable to a POSIX-conformant alternative.)

Write the code in the way you think is best, and if I see a good way to convert a Bashism to POSIX I'll let you know. And feel free to ignore me -- I'm a member of the Church of POSIX Conformance while Junio is much more grounded in reality. :)

Your list looks complete to me.  Thank you for contributing!

-Richard

Julien Carsique <julien.carsique@gmail.com> writes:

I just re-read the comments in the thread, and a few things look missing:

 - git-svn?
 - conditionally enable this feature?

Other than that the above list looks like a fairly good one.

Thanks.

Julien Carsique <julien.carsique@gmail.com> writes:

When you are on your "xyz" branch that was forked off of "refs/remote/origin/xyz/xyz", $__git_ps1_upstream_name would be "origin/xyz/xyz" and $__head is "xyz" at this point. The latter matches the former after stripping */ maximally from its front (i.e. "xyz"). It is unclear if you really want to make these two match, but as long as you correctly abbreviate you would not lose information, I would imagine. I am guessing that you would want to see "origin/xyz/=" in such a case, and if you started your "xyz" off of "origin/abc/xyz", you would want "origin/abc/=".

Now, what does ${__git_ps1_upstream_name/$__head/=} give us? This should not do regexp substitution in the first place, I would think. You made sure $__head is the tail-matching substring of the longer variable, so you chop that many bytes off of the latter.

Is this new feature something everybody should want unconditionally, or are there valid reasons why some people may not want to see this abbreviation happen (even if the implementation were not buggy)?

Unlike __git_ps1_upstream_name, this __head does not have to be visible outside this function. Wouldn't it be better to declare it as local (this is bash prompt and we can afford to step outside POSIX) and there is no need to "unset" after use if you did so, no?