Learning the bash Shell, Second Edition

Input and Output

The software field—really, any scientific field—tends to advance most quickly and impressively on those few occasions when someone (i.e., not a committee) comes up with an idea that is small in concept yet enormous in its implications. The standard input and output scheme of UNIX has to be on the short list of such ideas, along with such classic innovations as the LISP language, the relational data model, and object-oriented programming.

The UNIX I/O scheme is based on two dazzlingly simple ideas. First, UNIX file I/O takes the form of arbitrarily long sequences of characters (bytes). In contrast, file systems of older vintage have more complicated I/O schemes (e.g., “block,” “record,” “card image,” etc.). Second, everything on the system that produces or accepts data is treated as a file; this includes hardware devices like disk drives and terminals. Older systems treated every device differently. Both of these ideas have made systems programmers’ lives much more pleasant.

Standard I/O

By convention, each UNIX program has a single way of accepting input called standard input, a single way of producing output called standard output, and a single way of producing error messages called standard error output, usually shortened to standard error. Of course, a program can have other input and output sources as well, as we will see in Chapter 7.

Standard I/O was the first scheme of its kind that was designed specifically for interactive users at terminals, rather than ...