7. How it all works
In order to get printing working well, you need to understand how your spooling software works. All systems work in essentially the same way, although the exact order might vary a bit, and some systems skip a step or two:
The user submits a job along with his selection of options. The job data is usually, but not always, Postscript.
The spooling system copies the job and the options over the network in the general direction of the printer.
The spooling system waits for the printer to be available.
The spooling system applies the user's selected options to the job, and translates the job data into the printer's native language, which is usually not Postscript. This step is called filtering; most of the work in setting things up lies in getting the proper filtering to happen.
The job is done. The spooling system will usually do assorted cleanup things at this point. If there was an error along the way, the spooler will usually notify the user somehow (for example, by email).
7.1. PDQ
Pdq stands for "Print, Don't Queue", and the way it works reflects this design. The following sequence of events happens when you use PDQ to print:
You run pdq or xpdq, specifying a file.
You specify a printer.
You specify the settings for the various options and arguments defined in the printer's PDQ driver file (duplex, copies, print quality, and so forth).
PDQ analyzes the contents of what you printed, and follows the instructions in the PDQ driver file which tell it how to process your data for this printer with your options.
PDQ sends the processed data to the printer according to the interface defined for that printer (straight to /dev/lp0, or to an LPD daemon on the network, over the network to an Apple or Microsoft system, or even to a fax machine).
If PDQ can't send the data to the printer right away, it spawns a background process to wait and try again until it succeeds or hits a time limit.
7.2. LPD
Lpd stands for Line Printer Daemon, and refers in different contexts to both the daemon and the whole collection of programs which run print spooling. These are:
- lpd
The spooling daemon. One of these runs to control everything on a machine, AND one is run per printer while the printer is printing.
- lpr
The user spooling command. Lpr contacts lpd and injects a new print job into the spool.
- lpq
Lists the jobs in a print queue.
- lpc
The Lpd system control command. With lpc you can stop, start, reorder, etc, the print queues.
- lprm
lprm removes a job from the print spool.
So how does it fit together? The following things happen:
At boot time, lpd is run. It waits for connections and manages printer queues.
A user submits a job with the lpr command or, alternatively, with an lpr front-end like GPR, PDQ, etc. Lpr contacts lpd over the network and submits both the user's data file (containing the print data) and a control file (containing user options).
When the printer becomes available, the main lpd spawns a child lpd to handle the print job.
The child lpd executes the appropriate filter(s) (as specified in the if attribute in /etc/printcap) for this job and sends the resulting data on to the printer.
The lp system was originally designed when most printers were line printers - that is, people mostly printed plain ascii. By placing all sorts of magic in the if filter, modern printing needs can be met with lpd (well, more or less; many other systems do a better job).
There are many programs useful for writing LPD filters. Among them are:
- gs
Ghostscript is a host-based Postscript interpreter (aka a Raster Image Processor or RIP). It accepts Postscript and produces output in various printer languages or a number of graphics formats. Ghostscript is covered in Section 10.
- ppdfilt
ppdfilt is a standalone version of a CUPS component. It filters Postscript, executing a few basic transformations on it (n-up printing, multiple copies, etc) and adding in user option statements according to a Postscript Printer Definition (PPD) file usually included with Postscript printers.
ppdfilt is best used together with an option-accepting LPD system (like the VA Linux LPD, or LPRng) and a filter script which parses user-provided options into the equivalent ppdfilt command. VA Linux and HP provide a modified rhs-printfilters package which does exactly this; it produces nice results if you have a Postscript printer. See Section 8.2.2 for information on this system.
- ps2ps
ps2ps is a utility script included with Ghostscript. It filters Postscript into more streamlined Postscript, possibly at a lower Language Level. This is useful if you have an older Postscript printer; most modern software produces modern Postscript.
- mpage
mpage is a utility which accepts text or Postscript, and generates n-up output—that is, output with several page images on each piece of paper. There are actually several programs which do this, including enscript, nenscript, and a2ps.
- a2ps
a2ps, aka any-to-ps, is a program which accepts a variety of file types and converts them to Postscript for printing.