Converting Source Files Automagically Within the Spooler (Unix Power Tools, 3rd Edition)

45.17. Converting Source Files Automagically Within the Spooler

Section 45.15 and Section 45.16 showed what sorts of steps are required to get files into a printable form. They seem tedious, however, and computers are really quite good at tedium, so how can we make the spooler do all this for us automatically?

There are a couple of options. One of the more well-known is apsfilter, which is a set of filter scripts designed to work with lpd to automatically convert incoming source files to an appropriate output format before dumping them to the printer. Extensive information is available at http://www.apsfilter.org, and apsfilter has its own automatic setup scripts, but I'll give a quick overview to give you an idea of what configuring lpd's filters looks like.

In Section 45.9, we used an input filter trick to print to a Samba printer by putting a if entry in the printcap for that printer. if stands for "input filter," and there are several other kinds of filters available in standard lpd, including a ditroff filter, a Fortran filter (!), and an output filter.

apsfilter installs itself as the input filter for any printer it manages, and looks at the source file. It decides based on a number of pieces of information what kind of source file it is, automatically processes it with the right set of programs, and poof, you have correct output coming out of your printer. There's a reason this kind of tool is called a "magic filter" (and why the title of this chapter says "Automagically"). Having a magic filter installed makes life so much easier.

If you look at your printcap once apsfilter is installed, you'll notice this entry (or something much like it):

lp|local line printer:\
     ...
     :if=/usr/local/sbin/apsfilter:\
     ...

That's all it takes to hook into lpd and tell the spooler to give apsfilter a shot at the text on the way through. apsfilter looks at the incoming file and its configuration for the printer queue and converts the source into the appropriate printer language using whatever filter or set of filters are needed.

Other magic filters include LPD-O-Matic and magicfilter. http://www.linuxprinting.org has all sorts of information about this and other printing subjects. Don't be fooled by the name -- much of the information it provides can help you with printing on any Unix system, not just Linux.

-- DJPH