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Re: BASH_COMMAND does not expand correctly in subshells inside traps.

From: Emanuele Torre
Subject: Re: BASH_COMMAND does not expand correctly in subshells inside traps.
Date: Fri, 5 Nov 2021 18:17:08 +0100

I see; I thought this change of behaviour was an unintentional
regression since I noticed that there were some changes to traps.

I actually think this new behaviour makes more sense.

I think that that old DEBUG trap can be ported to bash5 like so:

 debug_prompt () { read -p "[$BASH_SOURCE:$LINENO] $BASH_COMMAND?" _ ;}
 trap 'debug_prompt "$_"' DEBUG

or even like so (to avoid issues with `unset -f debug_prompt'):

 trap '
  debug_prompt () { read -p "[$BASH_SOURCE:$LINENO] $BASH_COMMAND?" _ ;}
  debug_prompt "$_"

It is a bit inconvenient that this solution requires a function, but it
should not be a big deal in most situations.


On 05/11/2021, Chet Ramey <chet.ramey@case.edu> wrote:
> On 11/4/21 5:49 PM, Emanuele Torre wrote:
>> Bash Version: 5.1
>> Patch Level: 8
>> Release Status: release
>> Description:
>>    BASH_COMMAND does not expand to the expected value when used in a
>>    subshell inside a trap.
> This is a variant of
> https://lists.gnu.org/archive/html/help-bash/2021-10/msg00269.html
> In this case, as explained in the above message, the subshell `forgets'
> that it's executing as part of the DEBUG trap. Since we're not in the
> DEBUG trap, the value of BASH_COMMAND doesn't get updated. As a
> consequence, since the expansion of the command happens in the subshell,
> the original value (unexpanded in the parent shell) remains.
> I think I might have to rethink this strategy. What do you think?
> Chet
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/

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