executing OS programs from pg

<div class="Section1"><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Dear people,</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Does anyone know how to execute an OS command from pgsql. I would like to create a trigger that op
onfiring would run/execute an external program.</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Does such functionality exist or do I have to write my own trigger function in C.</span></font><p
class="MsoNormal"><fontface="Arial" size="2"><span style="font-size:10.0pt; 
font-family:Arial"> </span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial">Reagrds,</span></font><p class="MsoNormal"><font face="Arial" size="2"><span
style="font-size:10.0pt;
font-family:Arial">Gevik.</span></font><p class="MsoNormal"><font face="Arial" size="2"><span style="font-size:10.0pt;
font-family:Arial"> </span></font></div>