2. Printing Under Linux

This section discusses how to print files, examine the print queue, remove jobs from the print queue, format files before printing them, and configure your printing environment.

2.1 History of Linux Printing

The Linux printing system---the lp system---is a port of the source code written by the Regents of the University of California for the Berkeley Software Distribution version of the UNIX operating system.

2.2 Printing a File Using lpr

By far, the most simplistic way to print in the Linux operating system is to send the file to be printed directly to the printing device. One way to do this is to use the cat command. As the root user, one could do something like

# cat thesis.txt > /dev/lp

In this case, /dev/lp is a symbolic link to the actual printing device---be it a dot-matrix, laser printer, typesetter, or plotter. (See ln(1) for more information on symbolic links.)

For the purpose of security, only the root user and users in the same group as the print daemon are able to write directly to the printer. This is why commands such as lpr, lprm, and lpq have to be used to access the printer.

Because of this, users have to use lpr to print a file. The lpr command takes care of all the initial work needed to print the file, and then it hands control over to another program, lpd, the line printing daemon. The line printing daemon then tells the printer how to print the file.

When lpr is executed, it first copies the specified file to a certain directory (the spool directory) where the file remains until lpd prints it. Once lpd is told that there is a file to print, it will spawn a copy of itself (what we programmers call forking). This copy will print our file while the original copy waits for more requests. This allows for multiple jobs to be queued at once.

The syntax of lpr(1) is a very familiar one,

$ lpr [ options ] [ filename ... ]

If filename is not specified, lpr expects input to come from standard input (usually the keyboard, or another program's output). This enables the user to redirect a command's output to the print spooler. As such,

$ cat thesis.txt | lpr

or,

$ pr -l60 thesis.txt | lpr

The lpr command accepts several command-line arguments that allow a user to control how it works. Some of the most widely used arguments are: -Pprinter specifies the printer to use, -h suppresses printing of the burst page, -s creates a symbolic link instead of copying the file to the spool directory (useful for large files), and -#num specifies the number of copies to print. An example interaction with lpr might be something like

$ lpr -#2 -sP dj thesis.txt

This command will create a symbolic link to the file thesis.txt in the spool directory for the printer named dj, where it would be processed by lpd. It would then print a second copy of thesis.txt.

For a listing of all the options that lpr will recognize, see lpr(1).

2.3 Viewing the Print Queue with lpq

To view the contents of the print queue, use the lpq command. Issued without arguments, it returns the contents of the default printer's queue.

The returned output of lpq can be useful for many purposes.

$ lpq
lp is ready and printing
Rank   Owner      Job  Files                            Total Size
active mwf        31   thesis.txt                       682048 bytes

2.4 Canceling a Print Job Using lprm

Another useful feature of any printing system is the ability to cancel a job that has been previously queued. To do this, use lprm.

$ lprm -

The above command cancels all of the print jobs that are owned by the user who issued the command. A single print job can be canceled by first getting the job number as reported by lpq and then giving that number to lprm. For example,

$ lprm 31

would cancel job 31 (thesis.txt) on the default printer.

2.5 Controlling the lpd program with lpc

The lpc(8) program is used to control the printers that lpd serves. you can enable or disable a printer or its queues, rearrange entries within a queue, and get a status report on the printers and their queues. Lpc is mostly used in a setup where there are multiple printers hanging off one machine.

$ lpc

The above will start the lpc program. By default, this enters you into an interactive mode, and you can begin issuing commands. The other option is to issue an lpc command on the command line.

$ lpc status all

A list of the available commands are in the lpd man page, but here are a few of the major commands you'll want to know about. Any commands marked with option can either be a printer name (lp, print, etc) or the keyword all, which means all printers.

• disable option - prevents any new printer job from being entered
• down option - disables all printing on the printer
• enable option - allow new jobs to enter the print queue
• quit (or exit) - leave lpc
• restart option - restarts lpd for that printer
• status option - print status of printer
• up option - enable everything and start a new lpd

2.6 The RedHat printtool

Just a quick note here on RedHat's amazing printtool program. It seems to do everything that a magicfilter would do. RedHat already installs many of the programs to do the filtering. Here's how I have my printer set up under RH 4.0 with an HP LJ 4L connected to my parallel port (should be the same for other versions of RH as well).

• Become root and fire up printtool (if you su'ed, you remembered to SETENV DISPLAY :0.0 and xhost +, right?)
• Click "Add", and hit "OK" for a local printer.
• Fill in the printer device (/dev/lp1 for me)
• Fill in the input filter - Select a printer type, resolution, and paper size (ljet4, 300x300, and letter)
• Hit "OK" all the way back, and restart the lpd.