Switching off a shellcommand with empty list variable

[David Baird]

Hi,

I have a list variable that I define in a control section:, and then use 
it to install ports (this is FreeBSD):

control:
    missing_ports = ( ExecResult(/usr/bin/perl -e blah) )

shellcommands:

  "/usr/local/sbin/portupgrade --new $(missing_ports)"


The perl one-liner returns a colon-separated list of ports that need to 
be installed. The problem is that if there are no missing ports, the 
shellcommand still gets called. Is there any way I can set a class if 
$(missing_ports) is empty?

Thanks,

d.