Re: command substitution is stripping set -e from options

[Chet Ramey]

On 10/5/15 5:37 PM, Christoph Gysin wrote:
>> The parent shell (the one that enabled -e) should be the one to make the
>> decision about whether or not the shell exits.  The exit status of the
>> command substitution doesn't make a difference except in one special case,
>> so inheriting errexit and exiting (possibly prematurely) doesn't really
>> help the parent decide whether or not to exit.
> 
> I'm not sure I fully understand.
> 
> The parent shell should be the one to decide if the script is supposed
> to abort on any unsuccessful exit status. Command substitution should
> not change that. The parent shell decided via set -e that it wants to
> exit immediately on error.

Sure, the parent shell should make this decision.  Consider that, other
than one special case, the exit status of a command substitution has no
effect on whether a command succeeds or fails, and whether or not the
parent shell exits.  If you have something like

set -e
command -f $(command that generates a filename) -opts other arguments

and the command substitution inherits the -e option and exits
prematurely, the parent shell will not inspect its exit status and will
happily execute the command, possibly with faulty data.  This is simply
how command substitution works.

> If you don't want to fix this for backwards compatibility, is there
> anyway we could change that behaviour explicitly? I.e. with another
> option? Avoiding command substitution isn't really an option, and this
> essentially disables the whole point of set -e.

I think you're overlooking what I referred to above: that the exit status
of a command substitution doesn't have any effect on whether the parent's
command succeeds or fails except in one case: the right-hand-side of an
assignment statement that is the last assignment in a command consisting
only of assignment statements.  To say that it `disables the whole point
of set -e' is a considerable overstatement.

-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    chet@case.edu    http://cnswww.cns.cwru.edu/~chet/