Printers and printing

CUPS, and what happens to a job

CUPS is the printing system on essentially every Linux machine. A job submitted with lp or lpr is spooled under /var/spool/cups, filtered into a format the printer understands, and sent to the device by a backend. Printers and classes are defined in /etc/cups/printers.conf and the server itself in /etc/cups/cupsd.conf, though both are normally managed through the web interface on port 631 rather than edited.

CUPS splits its files by purpose in the ordinary way, and a backup question is really a question about that split: CONFIGURATION under /etc/cups, spooled jobs under /var/spool/cups, logs under /var/log/cups. So /etc/cups holds cupsd.conf, printers.conf and a ppd/ directory of the PostScript Printer Description files — one per queue, describing what that model can do — alongside classes.conf, client.conf and cups-files.conf. What is NOT there is error_log, which is a log, or any spool directory. Editing printers.conf by hand is discouraged because the running scheduler rewrites it; lpadmin and a reload are the supported route.

A printer has both a queue and an enabled/disabled state, and they are separate: a queue can accept jobs while the printer is stopped, so work piles up safely, and that separation is exactly what an exam question about "jobs accepted but nothing printing" is testing.

Two vocabularies for the same operations

The System V family is lp to print, lpstat to see status, and cancel to remove a job. The BSD family is lpr, lpq and lprm. CUPS implements both, so either set is a correct answer unless the question names one. lpadmin is the administrative command that adds, modifies and removes printers.

Jobs already queued belong to a destination, and moving them to another one is a scheduler operation rather than a resubmission: lpmove laser laser2 hands everything waiting on laser to laser2, and lpmove takes a single job id in place of the first queue name when only one job is meant. It needs administrative rights when the jobs belong to other users. The pairing to remember is that lpmove rescues what is already there while cupsreject or cupsdisable stops anything new arriving, so a dead printer wants both.

The two switches have two commands each, and CUPS ships both spellings of both. Whether a queue TAKES new jobs is accept and reject, which CUPS also installs as cupsaccept and cupsreject — the same programs under the names that do not collide with anything else on the system. Whether a queue SENDS what it has is cupsenable and cupsdisable. So taking a printer out of service for maintenance and having users told immediately is cupsreject laserjet: the queue refuses submissions at the door, rather than accepting work that will sit there. cupsdisable laserjet is the other choice and does the opposite — it keeps accepting and stops printing, which is what you want when the printer is coming back in an hour.

$ lpstat -p laserjetprinter laserjet is idle.  enabled since Mon 03 Aug 18:40:02$ cupsreject laserjet$ lpr -P laserjet report.pdflpr: Destination "laserjet" is not accepting jobs.# told at submission, rather than piling up$ cupsaccept laserjet