THE UNIX SHELL AND OPERATING SYSTEM

There are some interesting similarities between the PostScript language

and the command-line interface to the UNIX operating system. At first

glance, they are quite dissimilar; one is a programming language, the

other is an operating system. But consider that the shell program at which

the user types UNIX commands is in fact an interpreter. The shell

supports a very simple programming framework, with loops and

conditionals, and has a few real operators built in. It is intended only as a

skeletal environment, and the UNIX programs themselves provide most

of the language elements for shell programs.

The PostScript language has an extensible set of operators, each of which

is invoked by name (for example, moveto). The names are looked up, at

run-time, in the dictionary stack of the interpreter. This is exactly

analogous to the UNIX shell, which has an extensible command set and a

name lookup mechanism through which all commands are invoked. The

name lookup is controlled by the concept of a search path, or an ordered

set of directories in which to look for the named program. Either language

can be extended simply by adding another name into the search path, and

using it in the same way that the existing names are used.

TIP

Here’s a trick that you may find very helpful in assimilating the large

space of operators in the PostScript language. (It evolved from my mech-

anism for learning about UNIX.) If you are having a difficult time with a

particular part of your program or you think to yourself that there must be

a better way, the chances are that the designers of the language encoun-

tered the same situation, and designed an operator that exactly solves the

problem. Quickly review a summary list of operators (see the Appendix)

to see if there is some other operator that could make short work of your

task.

Furthermore, both the PostScript language and the UNIX operating

system offer a very large and relatively flat set of operators or commands,

Chapter 2: POSTSCRIPT IS NOT LIKE C

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