Just to start up a single mongrel server on Linux at boot time:
Put this script as /etc/init.d/mongrel, don’t forget to make it executable.
#!/bin/sh
cd /your/app/path
if [ "$1" = start ]
then mongrel_rails start -d
else
mongrel_rails $1
fi
Then link it from the /etc/rc0-6.d directory, depending on the runlevel where you want it to start up or use your preferred system tool to set it up for you.
Deploying mongrel cluster is described at