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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.unix.shell
Subject: Re: Cleaning up background processes
Date: Fri, 10 May 2024 14:57:41 +0200
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On 10.05.2024 14:35, Geoff Clare wrote:
> Janis Papanagnou wrote:
> 
>> But the pseudo-signal 'EXIT' is non-standard (to my knowledge), so you
>> cannot generally rely on it, depending on your environment, and the OP
>> asked for a "standard POSIX shell idiom".
> 
> Quoting POSIX.2-1992, 3.14.13:
> 
>     The condition can be EXIT; 0 (equivalent to EXIT); or a signal
>     specified using a symbolic name, without the SIG prefix, as listed
>     in Required Signals and Job Control Signals (Table 3-1 and Table 3-2
>     in POSIX.1 {8}). (For example: HUP, INT, QUIT, TERM). Setting a
>     trap for SIGKILL or SIGSTOP produces undefined results.
> 
> It has hardly changed in the current standard (POSIX.1-2017):
> 
>     The condition can be EXIT, 0 (equivalent to EXIT), or a signal
>     specified using a symbolic name, without the SIG prefix, as listed
>     in the tables of signal names in the <signal.h> header defined in
>     XBD Chapter 13; for example, HUP, INT, QUIT, TERM. Implementations
>     may permit names with the SIG prefix or ignore case in signal
>     names as an extension. Setting a trap for SIGKILL or SIGSTOP
>     produces undefined results.
> 

That's good. (And I was obviously misremembering.) Thanks!

BTW, I'm astonished about the "undefined results" for KILL/STOP. Yes,
on OS-level they cannot be caught but why undefined behavior on shell
level; what is the reason or practical rationale for that?

Janis