3.1 Standard I/O

In Chapter 2, we used various commands. Each command is a program. The UNIX program has "one input and two output". These are called standard input, standard output, and standard error, respectively.

Until now, when you type a command, the result is displayed on the screen. For example, typing ls displays a list of files and directories in the current directory. Until now we said "displayed on the screen", but from now on we will say 'the list of files and directories are "outputted to standard output" '.

Figure 3-1: Standard Input, Standard Output, Standard error