For multiple screens via one bash interface (edit a file and restart a service without losing place in file)
$ screen
Output content of file to terminal
$ cat
Search a file for a keyword/regex
$ grep
List of current running processes. You can also grep a ps list.
$ ps -aux
$ ps -aux | grep keyword
Disk Space Usage and Free Memory commands
$ df -m
$ free -m
List of incoming/outgoing network connections
$ netstat -ap
List of last user connections with optional user argument
$ last carzy
List of recently used commands
$ history
Run last used command as sudo
$ sudo !!
Show list of running tasks (this updates on the fly too as opposed to ps)
$ top
Show disk usage for file or folder, -h = human readable, -s = summary (size of folder rather than folder and all files in folder)
$ du -hs