In each operating system there are must-know command lines, which will significantly improve your effecivness is managing your system. Here are a few chosen ones, which I strongly suggest to memorise by heart 

$ uname -a

Displays kernel version

$ ifconfig

Displays full IP configuration

$ ip addr show eth0

Similarly to ifconfig – shows the ip config of the physical network adapter

$ df -ah

Shows Disk space usage

$ service udev status

Shows services status: Note that this is an old version syntax. You might need to use the next command version

$ systemctl status udev

Shows sevrvices status (new version)

$ du -sh home

Shows disk usage for /Home folder

$ netstat -tulpn

Shows active internet connections. It shows which process / service is listening on which port. IT gives information about Program name and Process ID

$ top

Information about processes

$ htop

Information about processes

$ man

e.g. $ man ifconfig

This will open the manual for any command explaining to you the purpose of a command and its options

$ sudo etc/init.d/ stop

e.g. $sudo etc/init.de/apache2 stop

This will stop any service that you type in. You can use:

stop, start or restart as an option to this command

$ ls

$ ls -l
$ ls -a
$ ls -la

LS lists all the files in a directory.

Parameter -a displays all filess inc. hidden files.

Parameter -l displays all information about the files and folders

$ tcpdump

Listen to traffic on a chosen port.

tcpdump -ni eth0 udp and port 1194