<tab> - The <tab> key on your keyboard will complete names of commands and files. Start typing a filename and hit tab twice to see all possible acceptable completions. If there is only one possibility, the rest of the name will be filled in for you.
<up> - The <up> key will fill in the last command you ran. Hit it multiple times to cycle through the last commands you've entered.
man <command name> - The man or manual command will provide information on any UNIX command. Try man ls, man grep, or man man.
exit - Exit the terminal session. You can also use <Control-d>
sudo - Run before a command to run it as an administrator. You will need to enter your password, and note that no asterisks or special characters will appear as you type.
su - Become the root user of the system. Your $ prompt will change to a # prompt.
!! - Run the last command. To run the last command you entered with administrative privileges, use sudo !!
* - refer to all files and folders in a directory, i.e., cat *.txt
clear or <Control-l> - Clear the terminal window.
which - Show where a command is stored on your system, i.e. which python
history - See the previous commands you've entered in the terminal. Useful in conjunction with grep
cp - Copy a file. cp file1 file2 will make a copy of file1 named file2. Can be used with the -r flag to copy whole folders.
rm - This command can be dangerous, so use it carefully. Removes a file or files. If used with the wrong flags or in the wrong place, you can delete a lot of important files, so be careful with this one.
rmdir - Remove an empty directory.
. and .. - . Refers to the folder you're in, while .. refers to the folder above.
.hello.txt - Any file with a . in front of its name is a hidden file. You won't see it in ls or in the GUI. To reveal hidden files, use ls -A
ping - Use ping google.com to see if your internet is working.
df -hl - Tells you how much hard drive space you have left.
top - Monitor which processes are using up your memory
kill and killall - Kill a particular process or kill a category of processes by name. Try killall chrome or killall firefox
sudo shutdown -r - Reboot the computer. shutdown -h turns off the computer.
time - Use before a command to find out how long that command takes to run.
uptime - Tells how long your computer has been on.
cal - Show a calendar.
telnet towel.blinkenlights.nl - Watch Star Wars in the terminal. Because.
say "Hello there" - Have your computer talk to you. (Use espeak on Linux.)
Find so much more on the command line:
Bash manual - the no nonsense text descriptions of bash commands.
explain shell - a site that explains commands you paste into the form. This site is fantastic for breaking down commands you find in the wild on the internet.
Easy shell guide - a friendly, styled (pastel!) list of common commands you might want to try out.