The caffeinate command prevents the computer to go into sleep mode, just as if you would set it to never sleep in the system preferences. Type caffeinate -d in your terminal and it will prevent your screen to sleep without changing the system preferences.

caffeinate -d
You can also prevent your computer to go sleep while another process is taking place, but turn sleep mode on again when the process is over, for that you use the same code as above, and then type the arguments you want to run and prevent computer to go on sleep mode. For example, say you have a big file on your desktop that you want to md5, and you want to prevent your computer to enter the sleep mode:
caffeinate -d ~/Desktop/bigfile
Doing so, your computer is now prevent to enter on sleep mode while the md5 process is running, and automatically turn the sleep mode on again when the process is completed.