Chapter 45. Printing

45.1. Introduction to Printing

This chapter discusses printing, which is a surprisingly complicated subject. To understand why printing is so complicated, though, let's think a little bit about what you might want to print.

First, in the "olden days," we had line printers and their relatives: daisy-wheel printers, dot-matrix printers, and other pieces of equipment that generated typewriter-like output. Printing a simple text file was easy: you didn't need any special processing; you only needed some software to shove the file into the printer. If you wanted, you might add a banner page and do a little simple formatting, but that was really pretty trivial.

The one area of complexity in the printing system was the "spooling system," which had to do several things in addition to force-feeding the printer. Most printers were (and still are) shared devices. This means that many people can send jobs to the printer at the same time. There may also be several printers on which your file gets printed; you may care which one is used, or you may not. The spooling system needs to manage all this: receiving data from users, figuring out whether or not an appropriate printer is in use, and sending the file to the printer (if it's free) or storing the file somewhere (if the printer isn't free).

Historical note: why is this called the "spooling system"? Dave Birnbaum, a Principal Scientist at Xerox, says:

"SPOOL (Simultaneous Printing Off and On Line)" It was written for the early IBM mainframes (of the 3-digit, i.e., 709 kind) and extended to the early 1401 machines. Output for the printer was sent to the spool system, which either printed it directly or queued it (on tape) for later printing (hence the on/off line). There was also a 2nd generation version where the 1401 would act as the printer controller for the (by then) 7094. The two were usually connected by a switchable tape drive that could be driven by either machine." [There's some controversy about exactly what the acronym means, but Dave's is as good as any I've heard. -- JP]

The first few articles in this chapter, Section 45.2, Section 45.3, Section 45.4, and Section 45.5, discuss the basic Unix spooling system and how to work with it as a user.

The next few articles talk about how to format articles for printing -- not the kind of fancy formatting people think of nowadays, but simpler things like pagination, margins, and so on, for text files that are to be sent to a line printer or a printer in line-printer emulation mode. Section 45.6 describes this kind of simple formatting, and Section 45.7 gets a little more complicated on the same subject.

Historical note number two: why is the print spooler called lp or lpr? It typically spooled text to a line printer, a fast printer that used a wide head to print an entire line at a time. These printers are still common in data processing applications, and they can really fly!

In the mid-1970s, lots of Unix people got excited about typesetting. Some typesetters were available that could be connected to computers, most notably the C/A/T phototypesetter. Programs like troff and T_EX were developed to format texts for phototypesetters. Typesetting tools are still with us, and still very valuable, though these days they generally work with laser printers via languages like PostScript. They're discussed in Section 45.10 through Section 45.17, along with the ramifications of fancy printing on Unix.

Finally, Section 45.19 is about the netpbm package. It's a useful tool for people who deal with graphics files. netpbm converts between different graphics formats.

-- ML