The minimal setup for lpd results in a system that can queue files and print them. It will not pay any attention to whether or not your printer will understand them, and will probably not let you produce attractive output. But we have to start somewhere.

To add a print queue to lpd, you must add an entry in/etc/printcap, and make the new spool directory under /var/spool/lpd.

An entry in /etc/printcap looks like:

# LOCAL djet500
lp|dj|deskjet:\
        :sd=/var/spool/lpd/dj:\
        :mx#0:\
        :lp=/dev/lp0:\
        :sh:

This defines a spool called lp,dj, or deskjet, spooled in the directory /var/spool/lpd/dj, with no per-job maximum size limit, which prints to the device/dev/lp0, and which does not have a banner page (with the name of the person who printed, etc) added to the front of the print job.

Go now and read the man page for printcap.

The above looks very simple, but there a catch - unless I send in files a DeskJet 500 can understand, this DeskJet will print strange things. For example, sending an ordinary Unix text file to a deskjet results in literally interpreted newlines, and gets me:

This is line one.
	This is line two.
		This is line three.

ad nauseam. Printing a PostScript file to this spool would get a beautiful listing of the PostScript commands, printed out with this "staircase effect", but no useful output.

Clearly more is needed, and this is the purpose of filtering. The more observant of you who read the printcap man page might have noticed the spool attributes if andof. Well, if, or the input filter, is just what we need here.

If we write a small shell script called filter that adds carriage returns before newlines, the staircasing can be eliminated. So we have to add in an if line to our printcap entry above:

lp|dj|deskjet:\
        :sd=/var/spool/lpd/dj:\
        :mx#0:\
        :lp=/dev/lp0:\
        :if=/var/spool/lpd/dj/filter:\
        :sh:

A simple filter script might be:

#!perl
# The above line should really have the whole path to perl
# This script must be executable: chmod 755 filter
while(<STDIN>){chomp $_; print "$_\r\n";};
# You might also want to end with a form feed: print "\f";

If we were to do the above, we'd have a spool to which we could print regular Unix text files and get meaningful results. (Yes, there are four million better ways to write this filter, but few so illustrative. You are encouraged to do this more efficiently.)

The only remaining problem is that printing plain text is really not too hot - surely it would be better to be able to print PostScript and other formatted or graphic types of output. Well, yes, it would, and it's easy to do. The method is simply an extension of the above linefeed-fixing filter.

Such a filter is called a magic filter. Don't bother writing one yourself unless you print strange things - there are a good many written for you already, and most have easy-to-use interactive configuration tools. You should simply select a suitable pre-written filter:

There's one catch to such filters: older version of lpd don't run the if filter for remote printers, while most newer ones do (although often with no arguments). The version of LPD shipped with modern GNU/Linux and FreeBSD distributions does; most commercial Unices that still ship LPD have a version that does not. See the section on network printing later in this document for more information on this. If you only have locally-connected printers, then this won't affect you.

While most versions of LPD don't gracefully handle PostScript (never mind user options), VA Linux modified LPD and Red Hat's filtering software to support PostScript printers fairly well. Because the intention was to donate the code to the gnu project, they called it GNUlpr

By popular demand, I include below a listing of the permissions on interesting files on my system. There are a number of better ways to do this, ideally using only SGID binaries and not making everything SUID root, but this is how my system came out of the box, and it works for me. (Quite frankly, if your vendor can't even ship a working lpd you're in for a rough ride).

-r-sr-sr-x   1 root     lp    /usr/bin/lpr*
-r-sr-sr-x   1 root     lp    /usr/bin/lprm*
-rwxr--r--   1 root     root  /usr/sbin/lpd*
-r-xr-sr-x   1 root     lp    /usr/sbin/lpc*
drwxrwxr-x   4 root     lp    /var/spool/lpd/
drwxr-xr-x   2 root     lp    /var/spool/lpd/lp/

Lpd must currently be run as root so that it can bind to the low-numbered lp service port. It should probably become UID lp.lp or something after binding, but I don't think it does. This is simply one more reason to avoid the stock BSD LPD.