Running Linux, Third Edition

Managing Print Services

Linux has a fairly complicated printing system, compared to the printing services most personal computers use. It allows many users to print documents at the same time, and each user can send documents from one or more applications without waiting for the previous document to finish printing. The printing system processes the files to be printed correctly on different kinds of printers connected to the computer in different ways. If you print on a network, files can be created on one host and printed out on a printer controlled by another host.

The whole process happens without much fuss, when you press the Print button in an application or issue a command, such as lpr, to print a document. That document does not go directly to the printer, though, because it might already be busy. Instead, the document is stored in a temporary file in a directory called the printer spool directory. As the word “spool” suggests, the documents get taken out of the directory one by one as the printer becomes free. Each printer has its own spool directory.

When Linux starts, it sets up a printer daemon (an independently running process) called lpd. This process waits around checking each spool directory for files that should be printed. When the process finds a file, it makes a copy of itself. The new lpd takes control of the print spool where the file was placed and queues it for printing. It won’t send the next file to that printer until the last file has finished printing. ...