Re: git rebase regression: cannot pass a shell expression directly to --exec

From: Jeff King <peff@peff.net>
Date: May 16, 2017, 17:37 UTC
Message-ID: <20170516173727.5zstcg2g5bradhcf@sigill.intra.peff.net>
In-Reply-To: <CA+zRj8XLQiTmiVbsJZ7XjdW4bQvMSJpCtKW8xx5JvMKOhBDmLQ@mail.gmail.com>

On Tue, May 16, 2017 at 10:21:51AM -0700, Eric Rannaud wrote:

Show 12 quoted lines

> On Tue, May 16, 2017 at 9:59 AM, Eric Rannaud <eric.rannaud@gmail.com> wrote:
> > When I use "git rebase --exec <cmd>" I'm basically writing a "foreach
> > commit in range { <cmd> }" in my shell. Same idea with git bisect run.
> >
> > A transparent optimization that tries execve() then falls back to the
> > user's shell sounds like a good idea.
> 
> One issue with the execve-else-shell optimization is that sometimes a
> binary exists that will shadow an exported function or a shell
> builtin:
> 
>   git rebase --exec true master^^  # OK but in fact this runs /usr/bin/true

Yeah, this is the builtin thing I mentioned elsewhere. I think it's pretty rare to run a builtin with no arguments and care about the behavior differences.

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> /usr/bin/time requires an argument. Even though the bash builtin time
> runs fine without argument.
> 
>   $ time
> 
>   real    0m0.000s
>   user    0m0.000s
>   sys     0m0.000s

I've run into the "time" distinction even when running things from the shell, because the time builtin is special. E.g.:

  $ time true
  real	0m0.000s
  user	0m0.000s
  sys	0m0.000s

  $ (echo foo | time true) 2>&1
  0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata 1136maxresident)k
  0inputs+0outputs (0major+61minor)pagefaults 0swaps

So to some degree, depending on builtins versus external commands (especially when you're round-tripping through another program running a second shell) is going to have some surprises.

> But if the optimization is applied to more complex commands, then we
> will have problems. For instance, the builtin echo supports \E, but
> /usr/bin/echo doesn't support it.

No, it shouldn't. If any of

     |&;<>()$`\\\"' \t\n*?[#~=%

appear in the string, we always run the shell.

> In any case, the manpage says --exec <cmd> and "<cmd> will be
> interpreted as one or more shell commands.", it doesn't say "--exec
> <executable>".

Right, it's clearly supposed to use the shell, or behave as if the shell were invoked (within reason).

-Peff