Standard Input

When programs read or write files, these do not use filenames, but file descriptors that are integer values such as 0, 1, 2, and many more.  The files are used to write output (Standard output) and read in information (Standard Input) and write error messages which are pertinent to the utility (standard error). Their particular file descriptors are 0, 1 or 2.

Generally when a command is run, the standard input is linked to the keyboard, standard output is connected to the show and standard error is also connected to the display screen. Therefore, a command reads from the keyboard (standard input), and writes to the screen (standard output or error).