On Jun 09 07:39, Chet Ramey wrote: > > Close. What you call the `implicit' and `explicit' pipelines (subshells) > actually both inherit the trap strings, but not the actual signal > dispositions. Posix says this: > > "The trap command with no operands shall write to standard output a list > of commands associated with each condition. If the command is executed in > a subshell, the implementation does not perform the optional check > described above for a command substitution containing only a single trap > command, and no trap commands with operands have been executed since entry > to the subshell, the list shall contain the commands that were associated > with each condition immediately before the subshell environment was > entered. Otherwise, the list shall contain the commands currently > associated with each condition." > > So the idea is that the output of `trap' can be piped. This came in in > Posix-2008 and bash-4.2.
I've forgot to check POSIX for this, that's a good point. So to get
list of traps corresponding to subshell, some has to get assigned e.g.,
this would work (listing no traps):
#!/bin/bash
trap 'echo EXIT-$BASH_SUBSHELL' EXIT
{ trap - ERR; trap; } | cat
Wouldn't it be beneficial to introduce say an option to a `trap` command
with meaning of "print traps corresponding to current shell/subshell"?
More complexity ... probably, but more clarity too I guess. I wonder if
mentioning this tricky bit in bash(1), maybe with just reference to
POSIX would be of value.
> One exception to this, as you discovered, is that bash clears out the EXIT
> trap's trap string when a (...) subshell is executed. This is one of the
> things bash does for sh compatibility (it goes back to bash-2.0), but it's
> probably not worth it any more.
Is this specific to EXIT trap? I'm not sure I understand original
intention of that, but less exceptions better IMHO.
> Yes, pipelines don't execute the EXIT trap; subshell commands do. All
> Posix shells do this. Even the Bourne shell did.
>
> Subshells started to run group commands also inherit trap strings and run
> the exit trap.
So in other words, if I have an EXIT trap defined say in a function
running as a part of the pipeline (and want to fire that trap), I have
to scope it in explicit subshell like ( ... ).
In fact group command { ... } would work too, as I've showed and you've
confirmed. One last bit here I guess, why is it that the $BASH_SUBSHELL,
would not reflect that is it in fact running in subshell?
Oh, and this behavior to be described in bash(1) in short wouldn't be
bad idea too I think.
Thanks,
Miro
--
Miroslav Koskar
http://mkoskar.com/
signature.asc
Description: PGP signature
