Creating Smart Print Queues

This article will help you understand print filters and how to create and install your own personalized filters.
Activating Your New Print Queues

After adding a new queue in /etc/printcap, you will need to tell the print daemon to reread the printcap file. Listing 7 is a script I wrote years ago to take care of the additional work necessary to activate your new printcap entries. You may want to verify the path names to the various directories and commands for your system. You must run it as superuser.

Listing 7

What if you do not always have a printer attached or available, but you wanted to send something to the print queue for printing at a later time? Will the job wait until you attach the printer or connect to the network? Yes, it will if your raw printer is attached to your parallel port because /dev/lp waits for handshake signals from the printer.

Some Debugging Tips

If jobs fail to print, you need to determine if the problem lies with the print system or your filter. The first place to look is in your system log file, /var/log/messages, then look in the log file for the queue. Next, carefully check the syntax in your printcap file. Make sure each line except the last one for each entry ends in a “\”. Turn on some debugging output to check the operation of your filter. If the problem is with the print system, you can try to stop and restart printing. You should have a script in your system startup area that does this properly. In the SuSE distribution, the startup and shutdown script is /sbin/init.d/lpd. As with any System V-style boot script, this may be manually run as /sbin/init.d/lpd start or /sbin/init.d/lpd stop.

A print queue can get stuck when the spool directory's disk gets full. This can happen for a very large job or if your disk partition is too small. You will have to remove the job with lprm to free it up, then clean up unnecessary files on that disk partition or move your spool directories to a larger partition. After copying all the directories and files to a new location, you can remove all the old directories (rm -r lpd) and replace the lpd directory with a symbolic link to the new location where you have copied the old lpd directory tree (ln -s /path/to/new/location/lpd /var/spool/lpd).

Summary

We discussed how BSD printing works and some of the common options in the printcap configuration file. We showed you how to write a filter for altering the file that is sent to the actual printer and how to write a filter that automatically processes different types of input files for printing. We also learned that a filter can redirect the output to different devices or files. Hopefully, this will give you enough information to understand and create your own “smart” printer queues and simplify your printing tasks.

Mark Plimley (markp@blueneptune.com) started home-computing in 1978 with an Imsai 8080 (8-bit, 2MHz Intel 8080). In his former life as a mechanical engineer, he programmed in FORTRAN. He has been employed mainly as a UNIX systems administrator since 1992 and has been using Linux since January, 1995. When not in front of a computer screen, he can be found tinkering around the house, doing some activity with his wife and two teenagers or helping out at church.

All listings referred to in this article are available by anonymous download in the file ftp.linuxjournal.com/pub/lj/listings/issue73/3741.tgz.

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