UNIX System Administration Handbook - Evi Nemeth [416]
All major vendors use some mutation of the vanilla BSD spooling system (lpd, lpc, lpr and friends), the System V spooling system (lpsched, lpadmin, lp et al.), or a combination of both. Of our reference operating systems, Red Hat and FreeBSD fall into the BSD category, whereas Solaris and HP-UX are descendents of System V.
To determine which printing system you have, check to see which spooler is present (lpd for BSD and lpsched for SysV) rather than looking at the queuing commands. Many vendors provide queueing commands that mimic those of the other system. For example, HP-UX provides lpr, but its printing system is based on SysV.
We start with brief descriptions of printing terminology and the BSD and System V systems, then provide specifics on configuring printers for each of our reference systems. We then discuss LPRng, an alternative to the traditional printing systems. Finally, we conclude with a brief guide to printer debugging, a discussion of common printing software, and some general printing hints.
23.1 MINI-GLOSSARY OF PRINTING TERMS
Although an overview of current printing technology is beyond the scope of this book, we will try to give you enough information to respond when someone begins haranguing you in printer jargon.
spooler
A spooler is a piece of software that receives print jobs, stores them, prioritizes them, and sends them out sequentially to a printer. A user-level command submits jobs to the spooler for printing. A spooler is often called a print server. Some printers have their own internal spoolers.
dpi
Many modern printers are bitmap devices, meaning that the actual output is composed of rows of tiny dots. A printer’s dpi is the number of dots per inch that it can print. Generally speaking, the higher the dpi, the better the print quality. A printer’s resolution is sometimes asymmetrical; a notation such as “300 × 600 dpi” indicates a horizontal resolution of 300 dpi and a vertical resolution of 600 dpi.
PDL
Most printers accept input in one or more “page description languages” that specify the images to be placed on the page in an abstract way. PDL descriptions are more efficient to transmit than raw images, and they are easier for applications to generate. They also have the benefit of being device and resolution-independent. The best-known PDLs are PostScript and PCL.
bitmap
Sometimes you need to print images that are not easily described in a PDL. In these cases, you use a bitmap, which is a set of data that specifies which dots are filled in and which are not (or what color each dot is, in the case of a color or grayscale image).
As with PDLs, there are several competing formats for storing bitmaps. Every PDL supports at least one format. Since bitmaps are usually very large, they are often compressed. Common bitmap formats include JPEG, PNG, TIFF, and GIF.
RIP
A Raster Image Processor (RIP) is a system that accepts documents in one or more PDLs and converts them to a bitmap format appropriate for a particular output device. PDL-to-bitmap conversion is often done by a RIP within the printer.
filters
Filters are programs that modify print jobs en route from the spooler to the printer. Filters translate file formats, do accounting, and often handle communications with the printer. Filters are usually not necessary with simple text printers, but they are essential for sending jobs to printers that require nonstandard PDLs. Some PostScript printers can deal with unfiltered input, but the majority prefer specially filtered print jobs. In the SysV universe, filters are called interfaces. See pages 716 and 726 for more information about filters and interfaces.
PostScript
PostScript is by far the most common PDL found on UNIX systems. It was originally developed by Adobe Systems, and most PostScript printers use an interpreter licensed from Adobe. Almost all page layout programs can