Running Linux, 5th Edition - Matthias Kalle Dalheimer [273]
Linux Printing Software
Printing in Linux involves the interactions of several different software packages. The most important of these is the Linux printing daemon, which accepts jobs to be printed, keeps them in one or more queues, and sends the jobs to printers in an orderly manner. Additional software includes Ghostscript, which converts between PostScript and a form the printer can understand; Ghostscript printer definitions; and assorted extra tools that can help you create nicely formatted output. Before you can configure Linux printing, you must install all of these components.
Linux printing daemons
A printing daemon's job is to run in the background, accept print jobs from applications, temporarily store those print jobs, and send them to the appropriate printers without interfering with each other. All mainstream Linux distributions ship with at least one printing daemon, and most set them up in at least a minimal way when you install the OS. You may still need to configure your printing daemon so that it knows about your printer, though. This task is described later in this chapter, in "Defining Printers in CUPS."
Traditionally, Linux has used the Berkeley Standard Distribution Line Printer Daemon (BSD LPD) or the updated LPRng package for printing. (For simplicity, both systems are referred to as LPD systems from here on.) Basic LPD systems are fairly simple tools; they accept print jobs, store those jobs in queues, and then send them directly to the printer. These systems can be modified to pipe the print jobs through other programs for additional processing, if desired. Unlike printing systems for Windows, Mac OS, and other OSs, LPD printing systems don't provide a two-way communication path. For instance, an application can't query the LPD system about the page width or color capabilities of a printer. Thus, you must tell each application about a printer's special features. LPD systems are, though, network-enabled, which makes it possible for one computer to share its printers with others, or for a computer to print to a network-capable printer.
In 1999, an experimental new printing system was developed: CUPS. This package uses a new network printing protocol and enables applications to query a printer's capabilities and set printer features in ways that aren't possible with LPD systems. By 2004, all major Linux distributions had either switched to CUPS as the default printing system or offered it as an option on equal footing with BSD LPD or LPRng. For this reason, we describe CUPS in this chapter. Although some of the principles and support software are the same for BSD LPD and LPRng as for CUPS, the details are completely different. If you're using an older printing system, you may want to consider upgrading to CUPS.
In most cases, you can install CUPS (if it's not already installed) by using your distribution's package management tools. Look for a package called cups and install it. If your system is already configured to use BSD LPD or LPRng, you should first remove that package. If you prefer, you can download the original CUPS source code from its web page, http://www.cups.org.
Ghostscript
The traditional LPD printing system passes a file from an application to a printer. In its most basic form, this means that the application must know how to create a file that the printer can understand. This contrasts with printing under most other OSs, such as Windows, in which the application can use OS calls to help prepare a document for printing. As a practical matter, Unix and Linux applications almost always generate one of two types of output:
Plain text
Programs can send plain text to the printer, under the assumption that the printer is either a dumb line printer (that is, a fast printer with few fancy formatting capabilities) or can at least accept the plain text.
PostScript
Adobe's