simple commands definition
Command Definition. A command is an instruction given by a user telling a computer to do something, such a run a single program or a group of linked programs. Commands are generally issued by typing them in at the command line (i.e., the all-text display mode) and then pressing the ENTER key, which passes them to the shell. A shell is a program that reads commands that are typed on a keyboard and then executes (i.e., runs) them.