Helpfull linux commands

To change system configuration parameters regarding Firewall use
$ /usr/sbin/setup

To know which ports are open use
$ nmap -sT localhost

To know if a service is running use
/sbin/service postgresql status

or
/etc/init.d/postgresql status

To check the status/add/remove a service from a runlevel
/sbin/chkconfig --list [name]
/sbin/chkconfig --add [name]
/sbin/chkconfig --level [levels] [name]

To find out which rpms are installed, and filter them by a software name use:
rpm -qa |grep [package name]

and to delete a package use
rpm -e [package name]