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Re: Strange behaviour from jobs -p in a subshell


From: Chet Ramey
Subject: Re: Strange behaviour from jobs -p in a subshell
Date: Wed, 14 Nov 2018 10:12:52 -0500
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:60.0) Gecko/20100101 Thunderbird/60.3.0

On 11/14/18 4:48 AM, Christopher Jefferson wrote:
> 
> On 13/11/2018 14:59, Chet Ramey wrote:
>> On 11/13/18 4:28 AM, Christopher Jefferson wrote:
>>> Consider the following script. While the 3 sleeps are running, both jobs
>>> -p and $(jobs -p) will print 3 PIDs. Once the 3 children are finished,
>>> jobs -p will continue to print the 3 PIDs of the done Children, but
>>> $(jobs -p) will only print 1 PID. $(jobs -p) always seems to print at
>>> most 1 PID of a done child.
>> Since the $(jobs -p) is run in a subshell, its knowledge of its parent's
>> jobs is transient. In this case, the subshell deletes knowledge of the
>> jobs it inherits from its parent, but hangs onto the last asynchronous job
>> in case the subshell references $!.
> 
> 
> Is this a case of "works as intended" then? I find the current behaviour 
> very strange -- I could understand if 'jobs -p' showed no information 
> about processes spawned from the parent shell, or all of it, but the 
> current position seems quite inconsistent.

Why? The shell has to keep the value and status of $! because POSIX says
you can always request it. A subshell inherits that pid, so it's `known'
to the subshell, and it will stick around until you get notfied of the
status. You don't restrict the `jobs -p' output to running jobs, so it's
going to show up, and you don't request the status of the job (even jobs
or jobs -l would be sufficient) so it's not going to go away.

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



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