Here are important commands that you should be aware of:

cat - input from stdin or a file and output to stdout or a file
cd - change directory
date - show and set the system date and time
df - find out how much space is free on your disks
disown - removes a process from its parent process, allowing it to continue after the parent exits
du - find out how much space is used on your disks
ethtool - is a network cable connected and which link does it have?
fdisk - partition a hard disk
file - find out the type of a file
hwinfo - find out what harddisk, processor, graphics card and so on you have
ldd - list dependencies of an executable file
ls - list files
lsof - list open files in the system
ps - show running processes
route - manage the network routing table
scp - copy over the network
sleep - waits for a given time
ssh - call a program over the network on another computer
strace - list all syscalls performed by a program
tar - pack and unpack file archives
top - show the top CPU/RAM consuming processes
vmstat - how much I/O is your computer doing?
which - where does a program lodge?
xosview - gives you a nice overview about system load: CPU, disks, swap etc.
zip - pack files so WinZip can extract them

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Commands