Command Line Tips

System Monitoring Tools / Network Tools - Each one of these tools have numerous guides so rather than list any specific guide the recommendation is to do a search for guides and have a few to choose from in case a specific guide isn't helpful or just doesn't meet your specific needs. 

System Tools

top

htop

glances

All three of these tools report "Load Average" and this is often misunderstood. The following is the best explanation I have found thus far:

"The system load is a measure of the amount of computational work that a computer system performs. The load average represents the average system load over a period of time. 1.0 on a single core cpu represents 100% utilization. Note that loads can exceed 1.0 this just means that processes have to wait longer for the cpu. 4.0 on a quad core represents 100% utilization. Anything under a 4.0 load average for a quad-core is ok as the load is distributed over the 4 cores." ( credit - https://www.deonsworld.co.za/2012/12/20/understanding-and-using-htop-monitor-system-resources/  ) 

Log Tools

tail - allows you to watch logs live. Here I am checking to make sure a phone is able to get config files via http.

multitail - allows you to view more that one live log. Here I am watching a phone pull a config and register. https://www.vanheusden.com/multitail/

Network Tools 

nmap - useful for seeing if certain ports are open. Here I am checking on UDP 5060 for Chan_pjsip.

netstat - useful for discovering, what exactly, is your PBX using port wise. In this case I want to know what is listening from an overall perspective.