sub _shell_quote_backend {
    my @in = @_;
    my @err = ();
 
    if (0) {
        require RS::Handy;
        print RS::Handy::data_dump(\@in);
    }
 
    return \@err, '' unless @in;
 
    my $ret = '';
    my $saw_non_equal = 0;
    foreach (@in) {
        if (!defined $_ or $_ eq '') {
            $_ = "''";
            next;
        }
 
        if (s/\x00//g) {
            push @err, "No way to quote string containing null (\\000) bytes";
        }
 
        my $escape = 0;
 
        # = needs quoting when it's the first element (or part of a
        # series of such elements), as in command position it's a
        # program-local environment setting
 
        if (/=/) {
            if (!$saw_non_equal) {
                $escape = 1;
            }
        }
        else {
            $saw_non_equal = 1;
        }
 
        if (m|[^\w!%+,\-./:=@^]|) {
            $escape = 1;
        }
 
        if ($escape
                || (!$saw_non_equal && /=/)) {
 
            # ' -> '\''
            s/'/'\\''/g;
 
            # make multiple ' in a row look simpler
            # '\'''\'''\'' -> '"'''"'
            s|((?:'\\''){2,})|q{'"} . (q{'} x (length($1) / 4)) . q{"'}|ge;
 
            $_ = "'$_'";
            s/^''//;
            s/''$//;
        }
    }
    continue {
        $ret .= "$_ ";
    }
 
    chop $ret;
    return \@err, $ret;
}