The program lpd(8), usually invoked at boot time from the /etc/rc file, acts as a master server for coordinating and controlling the spooling queues configured in the printcap file. When lpd is started it makes a single pass through the printcap database restarting any printers that have jobs. In normal operation lpd listens for service requests on multiple sockets, one in the UNIX domain (named ``/dev/printer'') for local requests, and one in the Internet domain (under the ``printer'' service specification) for requests for printer access from off machine; see socket(2) and services(5) for more information on sockets and service specifications, respectively. Lpd spawns a copy of itself to process the request; the master daemon continues to listen for new requests.
Clients communicate with lpd using a simple transaction oriented protocol. Authentication of remote clients is done based on the ``privilege port'' scheme employed by rshd(8C) and rcmd(3X). The following table shows the requests understood by lpd. In each request the first byte indicates the ``meaning'' of the request, followed by the name of the printer to which it should be applied. Additional qualifiers may follow, depending on the request.
Request Interpretation ----------------------------------------------------------------------------------------- ^Aprinter\n check the queue for jobs and print any found ^Bprinter\n receive and queue a job from another machine ^Cprinter [users ...] [jobs ...]\n return short list of current queue state ^Dprinter [users ...] [jobs ...]\n return long list of current queue state ^Eprinter person [users ...] [jobs ...]\n remove jobs from a queue
The lpr(1) command is used by users to enter a print job in a local queue and to notify the local lpd that there are new jobs in the spooling area. Lpd either schedules the job to be printed locally, or if printing remotely, attempts to forward the job to the appropriate machine. If the printer cannot be opened or the destination machine is unreachable, the job will remain queued until it is possible to complete the work.
The lpq(1) program works recursively backwards displaying the queue of the machine with the printer and then the queue(s) of the machine(s) that lead to it. Lpq has two forms of output: in the default, short, format it gives a single line of output per queued job; in the long format it shows the list of files, and their sizes, that comprise a job.
The lprm(1) command deletes jobs from a spooling queue. If necessary, lprm will first kill off a running daemon that is servicing the queue and restart it after the required files are removed. When removing jobs destined for a remote printer, lprm acts similarly to lpq except it first checks locally for jobs to remove and then tries to remove files in queues off-machine.
The lpc(8) program is used by the system administrator to control the operation of the line printer system. For each line printer configured in /etc/printcap, lpc may be used to: