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Re: 'declare' does not honor '-e' in command substituted assignments - a


From: Chet Ramey
Subject: Re: 'declare' does not honor '-e' in command substituted assignments - a bug ?
Date: Sun, 10 Aug 2014 16:46:00 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:24.0) Gecko/20100101 Thunderbird/24.6.0

On 8/9/14, 11:34 AM, Jason Vas Dias wrote:
> Good day bash list -
> 
> I don't understand why this emits any output  :
>  $ ( set -e;  declare  v=$(false); echo 'Should not get here'; )
>  Should not get here
>  $
> 
> While this does not:
>  $ ( set -e;   v=$(false); echo 'Should not get here'; )
>  $
> 
> Shouldn't declare / typeset behave like the normal variable assignment 
> statement
> wrt command substitution ?  It does not seem to be documented anywhere if
> it is not.

It's straightforward.  Setting the `-e' option will cause the shell to exit
if a commsnd returns a non-zero exit status.  Look at the exit status
returned by the two commands in question.

The declare builtin returns success unless the assignment itself fails:

        The return value is 0
        unless  an  invalid option is encountered, an attempt is made to
        define a function using ``-f foo=bar'', an attempt  is  made  to
        assign  a  value  to  a readonly variable, an attempt is made to
        assign a value to an array variable without using  the  compound
        assignment  syntax (see Arrays above), one of the names is not a
        valid shell variable name, an attempt is made to turn off  read-
        only  status for a readonly variable, an attempt is made to turn
        off array status for an array variable, or an attempt is made to
        display a non-existent function with -f.

The assignment statement, since there is no command name, usually returns
success.  Posix thought it useful to have a way to discover whether a
command substitution fails even when an assignment succeeds -- probably an
implementation artifact of a historical shell -- so we have

        If  there is a command name left after expansion, execution
        proceeds as described below.  Otherwise, the command exits.  If
        one of  the   expansions  contained a command substitution, the
        exit status of the command is the exit status of the  last
        command  substitution  performed.  If there were no command
        substitutions, the command exits with a status of zero.

(An assignment statement is one of the expansions that is removed before
a command is executed.)

Both of those quotes are from the bash-4.3 manual page.

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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