I would like to know what should I know before running a shared webhosting business? Except cPanel, WHM and billing system.

Knowing linux command is a must?

P.S: unmanaged server

No not really you just really need to know your way around whm for the most part. But a managed server always helps.

Server hardening and good support for your clients are must this will help your biz growth a lot

I know how to make a basic secured server - firewall(apf/csf), apache & php tweaks and hardening, installing 3rd party modules(suhosin,eaccelerator, zend and some other), BFD, SIM, Rootkit hunder and chkrootkit, disable direct root access over ssh, mod_deflate, mod_securty and some other.

But another problem. regarding cPanel. When I order a server it comes only with ssh access. When I install cPanel, does it install apache+php+mysql and all these things?

yes, WHM/cpanel installations are including those services again you need to harden apache/php/mysql conf files and other services that you will be using on server

You MUST have good server management skills. Let's imagine that something wrong happens (some kind of server crash, errors in Apache or PHP, etc.). You will surely need to fix this mess yourself if server is not managed

if you plan on runing a web hosting business based of linux servers then it is a MUST that you have good linux cli skills, either that or pay another company to look after your servers. Unless of course you can afford to hire a linux server tech who knows it all, thats the route i took, i have fairly decent knowledge of linux in general, and more in server but would not class that i have all it takes to run a linux server, so i have someone on staff that can, yeah it costs more but its also worth it in my opinion

Level of technical skills required to run the shared hosting,
* level of technical understanding required familiarity with the Internet, working knowledge of its operations, and understanding of how to manage and maintain accounts with the service provider.
* more personalized/individualized services require more technical involvement
*although many automated services are now available, in-depth knowledge of Internet protocols, standards ,concepts etc. is still needed for highly sophisticated Web and e-commerce solutions.

If you are not comfortable with Linux commands in ssh, or can hire someone who is, it's just a matter of time before something bad happens (either you get hacked or have spam issues, usually).

I agree with larwilliams. Spam issues will be #1 trouble for guys who offer shared hosting service.

If you're unsure about spam issues and other troubles it is good if you contact server management companies whom you can hire on monthly basis (there are low cost solution providers).