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Re: Bug: different behavior between "jobs -l" and "builtin jobs -l"


From: Hengyang Zhao
Subject: Re: Bug: different behavior between "jobs -l" and "builtin jobs -l"
Date: Tue, 21 Mar 2017 16:53:59 +0000

Dear Chet,

Thanks for your reply and I apologyze for the mistake I made in the "buggy behavior" section.

I didn't realize that a builtin is forced to run in a subshell until you told me. Yes, as I tried, "jobs" consumes no PID, but even "jobs | true" consumes two. I'll look into the code following the clue you gave me.

But back to the user's perspective, as I looked up "help jobs" or "help builtin", the sematics of "builtin" is only for forcing the shell to use the builtin version, right? Actually, I was writing a script that needs to secure the use of the builtin jobs, but now I need to seek for a reliable walkaround instead of using "builtin". So if we don't treat it as a bug, is it still a good suggestion that we write a caveat info the "builtin" help info?

Thanks again for your detailed reply! It helps me a lot :-)



Sincerely,

Hengyang

On Tue, Mar 21, 2017 at 8:13 AM Chet Ramey <address@hidden> wrote:
On 3/20/17 7:47 PM, Hengyang Zhao wrote:
> Hello,
>
> I am experiencing an unexpected behavior when using "builtin jobs -l". It
> seems that it's output fd is not as same as "jobs -l" without the builtin
> prefix. So when I piped it into wc, I got different results (as seen in the
> code I pasted below).
>
> Bash version: GNU bash, version 4.3.43(1)-release (x86_64-redhat-linux-gnu)
> OS: Fedora 25, kernel 4.9.13-200.fc25.x86_64
>
> Buggy behavior:
> when we have one or more background processes:
> jobs -l | wc        # produces 0 0 0
> builtin -l | wc              # produces non-zero non-zero non-zero

(This is actually the opposite of what happens, as shown by your examples
below.)

> I tried to fix the bug, but after I looked at the code, the jobs printing
> procedure clearly prints to stdout. So I think it's not a easy work to
> track down the bug.

It's not a bug.  The first part of a pipeline is always run in a subshell.
That subshell doesn't really have any jobs, since none of the parent's
jobs are children of that shell -- you can't wait for them, for instance.

There is special-case code that attempts to detect when the `jobs' builtin
is running (execute_cmd.c:execute_subshell_builtin_or_function) and doesn't
remove the jobs from the jobs table in that one case, specifically to allow
the output of `jobs' to be piped.  Running the `builtin' builtin defeats
that.

Chet
--
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, UTech, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/
--
Hengyang Zhao

Ph.D. Candidate, Electrical Engineering
University of California, Riverside

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