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Standard Input, Output, And Error
Many of the programs that we have used so far produce output of some kind. This output often consists of two types. First, we have the program's results; that is, the data the pro- gram is designed to produce, and second, we have status and error messages that tell us how the program is getting along. If we look at a command like ls, we can see that it displays its results and its error messages on the screen.
Keeping with the Unix theme of “everything is a file,” programs such as ls actually send their results to a special file called standard output (often expressed as stdout) and their status messages to another file called standard error (stderr). By default, both standard output and standard error are linked to the screen and not saved into a disk file.
In addition, many programs take input from a facility called standard input (stdin) which is, by default, attached to the keyboard.
I/O redirection allows us to change where output goes and where input comes from. Nor- mally, output goes to the screen and input comes from the keyboard, but with I/O redi- rection, we can change that.