PostgreSQL or pl/psSQL equivalent to MS SQL Server's xp_cmdshell?

<p><font face="Arial" size="2">Does anyone know of the PostgreSQL or pl/psSQL equivalent to MS SQL Server's
xp_cmdshell?  This is the command that allows you issue command-line statements from within SQL, e.g., you would do
xp_cmdshell'dir c:\' if you wanted to see the contents of the c: drive.</font><p><font face="Arial"
size="2">TM</font><br/><font face="Arial"
size="2">**************************************************************************</font><br/><font face="Arial"
size="2">Theinformation transmitted herewith is sensitive information intended only for use by the individual or entity
towhich it is addressed. If the reader of this message is not the intended recipient, you are hereby notified that any
review,retransmission, dissemination, distribution, copying or other use of, or taking of any action in reliance upon
thisinformation is strictly prohibited. If you have received this communication in error, please contact the sender and
deletethe material from your computer.</font>