Fix gwip alias for some BSD flavors of xargs

[ncanceill]

Status: Ready (but testing cannot hurt)

Some BSD flavors of the xargs command (eg on Mac OSX) do not support the -r flag — which is actually a non-standard flag introduced by the GNU flavor of xargs. This breaks the gwip alias in the current git plugin. See #2692 and #2713.

The bug was introduced by a869ec9. The original intention was to take care of the possibility that git ls-files --deleted -z would not send anything in the pipe. However, the author @bdubertret failed to see that, in that case, only the GNU flavor of xarg fails.

This is because the GNU flavor does not follow the xargs standard, and fails on empty input instead of returning 0 silently: it requires the -r flag in order to adopt the standard behavior. Apparently, all BSD flavors abide by the standard, and thus need not the -r flag; some of them (eg FreeBSD) still silently support the flag for the sake of compatibility, but others (eg Mac OSX) do not and fail in case they see it. Hence this PR.

[mcornella]

In my system (debian jessie, xargs 4.4.2) your test makes xargs run indefinitely, until some input is passed (or ^D is pressed).
Also, &>/dev/null at the beginning spits &>/dev/null: command not found on some occasions.
This is how it is run, seen with zsh -xv:

XARGS_OPTS=
+~/.oh-my-zsh/plugins/git/git.plugin.zsh:154> XARGS_OPTS='' 
&>/dev/null xargs -r && XARGS_OPTS+="-r"
+~/.oh-my-zsh/plugins/git/git.plugin.zsh:155> xargs -r

What works for me:

echo | xargs -r &>/dev/null && XARGS_OPTS="-r"

I have tested it with a custom function xargs() that returns 1 and it works ok too, but it should be tested on a non-GNU xargs version.

Finally, (1) the first line could be removed since an undefined variable is empty, and (2) to prevent possible side effects we should unset XARGS_OPTS after the alias.

EDIT: the command not found thing had to do with one function provided by the debian package command-not-found, which is just horribly made. The command is parsed correctly, as far as I've tested, but it seems good practice to put the &>/dev/null thing last.

[ncanceill]

Awesome feedback, will update soon, thanks.

[mcornella]

Minor correction: XARGS_OPTS="-r" 😁

[ncanceill]

You are right, if any other file sets that option there may be side-effects.

EDIT: OK, done.

[mcornella]

This looks ready 👍

[ncanceill]

Obsoleted by #2790