my $cmd = 'a_cmd';
my %options = (
  -d => '/home/users',
  -s => '/bin/sh',
  -g => 100,
);

$options{-c} = $realname if $realname;
$options{-d} = $base_homedir if $base_homedir;

my $cmd_str = join ' ', $cmd, map { "$_ $options{$_}" } keys %options;
my $output = qx/$cmd_str/;