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Re: Bug when trapping exit from subshell


From: Mark Ferrell
Subject: Re: Bug when trapping exit from subshell
Date: Mon, 19 May 2014 10:39:59 -0700

I stand corrected.  After adding a 'touch' to the script, it does
indeed appear to be calling the exit handler.  So it the redirection
of the output to the error handler only happens if a builtin/function
failed (error on an external command does not result in a
redirection), and this behaviour is only only in observed in bash.

I'm sorry, but the lack of consistency still sounds like it is a bug
in bash.  The behaviour I would expect is functionally equivalent to
'do_cmd > /dev/null 2>&1 || err_handler'.  Further more, this IS the
behaviour seen if the command being executed is NOT a
builtin/function.

The following scripts demonstrate functionally equivalent code which
has different output behaviour due to the error_handler() being
redirected. Please note that these scripts are expected to have the
SAME output, and they simply do not when interpreted by bash (though,
as repeatedly mentioned, they do have the same output when interpreted
by ash, dash, BusyBox's ash, ksh and zsh).

# Apparently builtins have a secret-subshell during 'set -e' resulting in
# behavior which mimcs: ( builtin_false || err_handler ) > /dev/null 2>&1
set -e
exit_handler() { echo "exit code: $?"; }
false() { /bin/false; }
main()
{(
    trap exit_handler 0
    false >> /dev/null 2>&1
)}
main "$@"

# External commands (and builtin/functions in other shells) handle this
# use case as: /bin/false > /dev/null 2>&1 || err_handler
set -e
exit_handler() { echo "exit code: $?"; }
main()
{(
    trap exit_handler 0
    /bin/false >> /dev/null 2>&1
)}
main "$@"

On Mon, May 19, 2014 at 7:00 AM, Chet Ramey <address@hidden> wrote:
> On 5/18/14, 12:56 PM, Mark Ferrell wrote:
>> I believe that if you actually execute the script as intended you will
>> find that your reading of the code is incorrect.  Really though,
>> redirection should not be the problem since:
>>
>>   1. Redirection of the output of one command does not change the
>> redirection of the script as a whole, and so it should never change
>> redirection of the command invoked by the handler. Changing
>> redirection of the script as a whole via 'exec' sure .. but not for a
>> single command.
>
> It depends on the context in which the exit trap is executed.  The `set -e'
> causes the shell to exit as soon as a command fails, and the exit trap is
> run while the shell is exiting, in the context of the failed command.  In
> this case, that's `main', which has its stdout and stderr redirected.  If
> you remove the set -e, you get different behavior.
>
> The exit_handler function is always called: if you add something like
> `touch exit_handler' you'll see that that file is always created.  It's
> not a question of whether or not the trap is executed, it's a question
> of whether or not bash unwinds the redirections on its way out of the
> shell.  When bash executes a builtin or shell function in a subshell, or
> bails because a command is not found, it doesn't bother to do that.
>
> Chet
>
> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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