CentOS change hostname without reboot

Step 1:
You may also need to add/change the hostname in the /etc/hosts file. By default this would look something like this, using our www.example.com example again:

127.0.0.1 www.example.com localhost localhost.localdomain


Step 2:
The file /etc/sysconfig/network contains the hostname and will look something like this:

NETWORKING="yes"
GATEWAY="10.1.1.1"
HOSTNAME="www.example.com"

Step 3:
This change won't take affect until the next reboot, but you can make the change happen immediately using the hostname command like so:

$ hostname ftp.example.com

Step 4:

service syslog restart

http://shebangme.blogspot.com/2010/10/centos-change-hostname-without-reboot.html