run shell command from pl/pgsql

Hello all,

I'm working on a pg database that should be able to run commands on
certain conditions. E.g. whenever a date is stored in column X of table
Y, an email should be sent to user Z. Creating a triggered function is
easy enough but how do I invoke the command? Also, I want to use
variables in the executed command, something like:

m_address := ''me(at)myself(dot)org'';
m_subject := ''this is a test'';
runcommand(''mail -s '' + m_subject + m_address);

I guess the answer is very obvious, but I couldn't find it. TIA for any
help!

--
Jules Alberts.