Is it possible to make 'exec' use '$SHELL -c' instead of '/bin/sh -c' in Perl?

Question

In Perl, is it possible to make 'exec', 'system', and 'qx' use a shell other than /bin/sh (without using a construct like 'exec "$SHELL -c ..."', and without recompiling perl)?

EDIT: The motivation for this question is a bash script that does 'export -f foo' and then uses perl in a subshell to invoke the function directly via 'system "foo"'. I am not sure that this technique will work with all sh, and although 'system "/bin/bash -c foo"' may work in that scenario, I wouldn't expect the exported function to propagate through all variants of /bin/sh. But mostly I was just curious, and am now curious about how to extend the solution to qx. Also, since I know nothing about non-unix platforms, I'd like to avoid hard coding the path to an alternate shell in the solution.

Answer 1

exec doesn't use /bin/sh

It just execs the program you specify. No shells.

If you want it to go through a shell you have to do that yourself.

Answer 2

exec and system will use the shell (which will likely not be /bin/sh on non-UNIX systems) if you only pass one argument to it. (Details are described in perlfunc)

You may want to have a look at IPC::Run3 as an alternative to system

Answer 3

You can override exec and system. See perldoc perlsub for the details, but here is roughly what you want (modulo some quoting bugs I don't feel like trying to fix):

#!/usr/bin/perl

use strict;
use warnings;

use subs qw/system/;

sub system {
    #handle one arg version:
    if (@_ == 1) {
     return CORE::system "$ENV{SHELL} -c $_[0]";
    }
    #handle the multi argument version
    return CORE::system @_;
}

print "normal system:\n";
system "perl", "-e", q{system q/ps -ef | grep $$/};

print "overloaded system:\n";
system 'ps -ef | grep $$';

Answer 4

Why don't you want to use 'exec "$SHELL -c ..."'? If you don't want see that code every time you call exec or system, just hide it in a subroutine. That's what they're there for. :)

sub my_exec {
    exec $ENV{SHELL}, '-c', @_;
    }

If you want to do that, however, I suggest somehow sanitizing $ENV{SHELL} so that people don't do odd things to your script by setting weird values. You might want to ensure that the shell is listed in /etc/shells or whatever way your system lists approved login shells. You also need to do a bit more work to make this taint-clean, which you should probably do if you are going to send data to another process.