Nessus is a great network security tool but installing it correctly can be a real pain some time even for advance user. I faced few problems and that is why I noted down all the necessary and sufficient steps. The steps below are explained for Ubuntu but I think similar steps can be used for other Unix like OS.

Step1: Download the ubuntu package from the website:http://tenable.com/products/nessus/nessus-download-agreement
and Run it.
Step2: To start nessus, type this command in super user mode: sudo /etc/init.d/nessusd start
Output of this command will result in:

$Starting Nessus : .
takshak@takshak-NB:~$ Missing plugins. Attempting a plugin update…
Your installation is missing plugins. Please register and try again.
To register, please visit http://www.nessus.org/register

Step3. Go on the given link and resgister on the home version, check ur mail get the license and use the command specified in ur mail as super user. It will register the Nessus and will also downlaod the missing plugins.

Step4. Run the nessus by using command: sudo /etc/init.d/nessusd start
Running Nessus command nasl gives an error “nasl : command not found”

(i) Try this command: whereis nessus
output will look something like: nessus: /opt/nessus/bin/nessus

(ii) Check the path see if the mentioned directory is added in the path or not by using command: echo $PATH
If the output of the command is: /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games
then the command is not added to the path

(iii) Add the dirctory to the path by using the command: export PATH=$PATH:/opt/nessus/bin

Step5: Then start nessus by using command: sudo /etc/init.d/nessusd start

Step6: To run a nasl file use: nasl -t target_ip filename.nasl