Linux has a fairly complicated printing system, compared with the printing services most PCs use. Configuring the printing system is usually either very easy (because the semiautomated tools get it right) or very hard (because the semiautomated tools fail or because your printer is poorly supported under Linux). The next few pages describe the Linux printing system, beginning with an overview of the software and moving on to hardware configuration and testing, configuring the Common Unix Printing System (CUPS) to enable its web-based configuration tools and network helpers, defining printers in CUPS, fine-tuning printer definitions, managing printer queues, maintaining LPD compatibility, and troubleshooting printers.
14.2.1. Linux Printing Software
Printing in Linux involves the interactions of several different software packages. The most important of these is the Linux printing daemon, which accepts jobs to be printed, keeps them in one or more queues, and sends the jobs to printers in an orderly manner. Additional software includes Ghostscript, which converts between PostScript and a form the printer can understand; Ghostscript printer definitions; and assorted extra tools that can help you create nicely formatted output. Before you can configure Linux printing, you must install all of these components.
18.104.22.168. Linux printing daemons
A printing daemon's job is to run in the background, accept print jobs from applications, temporarily store those print jobs, and send them to the appropriate printers without interfering with each other. All mainstream Linux distributions ship with at least one printing daemon, and most set them up in at least a minimal way when you install the OS. You may still need to configure your printing daemon so that it knows about your printer, though. This task is described later in this chapter, in "Defining Printers in CUPS."
Traditionally, Linux has used the Berkeley Standard Distribution Line Printer Daemon (BSD LPD) or the updated LPRng package for printing. (For simplicity, both systems are referred to as LPD systems from here on.) Basic LPD systems are fairly simple tools; they accept print jobs, store those jobs in queues, and then send them directly to the printer. These systems can be modified to pipe the print jobs through other programs for additional processing, if desired. Unlike printing systems for Windows, Mac OS, and other OSs, LPD printing systems don't provide a two-way communication path. For instance, an application can't query the LPD system about the page width or color capabilities of a printer. Thus, you must tell each application about a printer's special features. LPD systems are, though, network-enabled, which makes it possible for one computer to share its printers with others, or for a computer to print to a network-capable printer.
In 1999, an experimental new printing system was developed: CUPS. This package uses a new network printing protocol and enables applications to query a printer's capabilities and set printer features in ways that aren't possible with LPD systems. By 2004, all major Linux distributions had either switched to CUPS as the default printing system or offered it as an option on equal footing with BSD LPD or LPRng. For this reason, we describe CUPS in this chapter. Although some of the principles and support software are the same for BSD LPD and LPRng as for CUPS, the details are completely different. If you're using an older printing system, you may want to consider upgrading to CUPS.
In most cases, you can install CUPS (if it's not already installed) by using your distribution's package management tools. Look for a package called cups and install it. If your system is already configured to use BSD LPD or LPRng, you should first remove that package. If you prefer, you can download the original CUPS source code from its web page, http://www.cups.org.
The traditional LPD printing system passes a file from an application to a printer. In its most basic form, this means that the application must know how to create a file that the printer can understand. This contrasts with printing under most other OSs, such as Windows, in which the application can use OS calls to help prepare a document for printing. As a practical matter, Unix and Linux applications almost always generate one of two types of output:
Unfortunately, most printers, and particularly the inexpensive consumer and small business printers that are often paired with Linux, don't understand PostScript. The answer is to pipe output through Ghostscript (http://www.cs.wisc.edu/~ghost/), which is a PostScript interpreter that can reside on the computer rather than in the printer. Ghostscript converts PostScript into formats that most printers can understand. In effect, the combination of PostScript and Ghostscript becomes the Linux equivalent of the Windows printer driver system.
Although CUPS changes many things about the Linux printing system, it still relies on Ghostscript to convert PostScript into printers' native languages. Thus, you must have Ghostscript installed on your system if you expect to print to a non-PostScript printer. Fortunately, all major Linux distributions ship with Ghostscript. You may want to check to be sure it's installed, though; look for a package called ghostscript.
Ghostscript is actually available in two versions. The most recent version of Ghostscript is AFPL Ghostscript, which is available under a license that permits free use for many purposes, but not free redistribution. After a few months, AFPL Ghostscript is released under the GPL as GNU Ghostscript, and it's this version that comes with most Linux distributions. In most cases, being a few months behind the leading edge of Ghostscript development is unimportant. If you absolutely must have the latest version, though, check the Ghostscript home page.
Ghostscript comes with drivers for many common printers , and it can also output many common graphics file formats. You can even generate Adobe Portable Document Format (PDF) files with Ghostscript. (The ps2pdf shell script helps automate this process.) For more flexibility, you can add Ghostscript drivers for assorted printers.
As a point of interest, you should know that Ghostscript treats all printers as graphics devices. That is, if you print a purely textual document, Ghostscript converts that text into a graphics bitmap and sends the bitmap to the printer. This means that Ghostscript cannot take advantage of fonts that are built into the printer. It also means that Ghostscript sometimes prints more slowly than other software, such as Windows drivers, can print to the same printer. (This effect is usually negligible, but it's sometimes dramatic.) On some very old laser printers, another consequence is that Ghostscript (and hence Linux) requires a printer memory upgrade for the printer to print at full resolution. In practice, though, even Windows treats many printers as graphics-only devices, so Ghostscript's doing so may not make any difference.
22.214.171.124. Printer definitions
A standard CUPS installation supports a fairly narrow range of printers, typically PostScript models and some Hewlett-Packard and Epson printers. To support more printers, you must install a printer driver package. (In truth, many of these "drivers" are really just printer descriptions coupled to standard Ghostscript drivers, but in practice they're necessary either way.) Several such driver packages exist:
Most distributions ship with Foomatic or GIMP Print, so check for those packages. Sometimes they're called something else, and occasionally they're bundled with still more printer definitions. If you fail to install any printer definitions, you'll see a very limited set of printers when you configure printing.
Sometimes multiple printer definitions are available for a single printerfor instance, separate definitions in the Foomatic and GIMP Print packages, or even multiple definitions in a single package. In such cases, you may want to try all the available definitions to see which one works best with your printer and the types of documents you print. Sometimes the developers give you a hint by specifying in the configuration menus that one definition is recommended over the others.
126.96.36.199. Additional printing software
CUPS (or an LPD printing system) is a practical necessity for printing from Linux. For most printers, Ghostscript and Ghostscript driver definitions are also practical necessities. Several additional tools are often highly desirable, but may not be necessary in all cases:
Some of these tools are covered elsewhere in this book. Most require little configuration, and all are primarily user-level tools. Be aware that this list is not comprehensive; Linux printing tools are extremely plentiful, and range from simple text formatting tools to sophisticated word processors and graphics applications.
14.2.2. Configuring Printer Hardware
The first order of business when configuring a printer is configuring the hardware . You must verify that your printer is compatible with Linux, check its physical interface to the computer, and verify that the interface is working. Failure to check these details can lead to problems when you try to actually configure the printer in Linux, leading to a wild goose chase as you try to debug problems in the wrong subsystems.
188.8.131.52. Verifying printer compatibility
The main issue with respect to printer compatibility is the language the printer uses. Several languages are common, but many printers use their own unique languages. The more common printer languages include PostScript, HP's Printer Control Language (PCL) , and Epson's ESC/P2 language. (ESC/P2 is most common on old dot matrix printers.) Many manufacturers make printers that use each of these languages, but sometimes they bury this detail in their descriptions or refer to it in another way. One common example is a laser printer that uses PCL; the manufacturer may refer to the printer as being "HP-compatible" (usually with a reference to a specific HP printer model).
The best hope for Linux compatibility is to get a printer that supports PostScript. Such printers are typically mid-range to high-end laser printers. Few inkjet or low-end laser printers support PostScript. With a PostScript printer, you needn't be concerned about whether a Ghostscript driver exists for your printer; just configure CUPS to pass the raw PostScript straight through to the printer.
If you don't have a PostScript printer, your best bet for assessing Linux compatibility is to check the Linux Printing web site, and particularly its printer database, located at http://www.linuxprinting.org/printer_list.cgi. Locate your printer make and model in the drop-down fields on this site and then click the Show button. You'll then see a description of your printer and its Linux compatibility. The description may also tell you where to find drivers or printer descriptions to use with CUPS, so you can check that you have the appropriate support software.
If your printer is listed on the Linux Printing web site as a "paperweight" (meaning it doesn't work at all under Linux), you may want to try doing a web search on the printer's name and Linux. If you're lucky, you'll turn up a new or experimental driver you can try. If not, you may want to look into replacing the printer with one that's more Linux-friendly.
If you have, or are considering buying, a multifunction device (such as one that handles both printing and scanning functions), you should investigate Linux support for all of the device's functions. Sometimes the printer side will work fine but the scanner will be useless, for instance. Typically, support for each function is provided by its own project under Linux, such as Ghostscript for the printer and the Scanner Access Now Easy (SANE ) project for scanner support. Occasionally, a project exists to provide all the necessary drivers in one place, such as the HP Office Jet project (http://hpoj.sourceforge.net) or Epson's drivers (http://www.avasys.jp/english/linux_e). These projects may be independent or sponsored by the device's manufacturer.
184.108.40.206. Printer interfaces
Printers can be connected to computers in several different ways. Four interface methods are common:
You should be aware of which interface method your printer uses. Some testing and configuration features won't work with some interfaces; for instance, you can't easily bypass the printing system to send a file directly to a USB or Ethernet printer. You must also have appropriate ports on your computer to support your printer, and the necessary cabling. If you lack this hardware, you should buy it. You can purchase add-in cards with any of the necessary port types; or if you have USB ports but no open slots in your computer, you can purchase an adapter so that you can connect a parallel, RS-232 serial, or even an Ethernet printer via a USB port. Be sure Linux drivers exist for the adapter, though!
If your printer supports multiple interface types, USB is generally the best to use, followed by a parallel interface and then RS-232 serial. USB provides more information to the computer about the printer, uses less bulky cables, and (in its 2.0 incarnation) can be faster than the parallel port. A network-enabled printer can be a big advantage if you want it to be accessible to several computers, but if it uses an odd protocol it could be more trouble than it's worth, particularly if you use it on only one system. Of course, you should also consider your available ports and cables; it might be more trouble than it's worth to install a USB card in an older system that you're using as a print server, for instance.
220.127.116.11. Verifying basic printer connectivity
Before you set up printer services, be sure the printing devices are online. If you also use another operating system, such as Microsoft Windows, you can exercise the hardware to ensure that it is connected properly and working before loading Linux. Successfully printing a document from another operating system immediately eliminates one major source of woe and head scratching. Similarly, if you are going to use printer services on a network, your system should be on the network and all protocols functioning before proceeding.
If you have a parallel or RS-232 serial printer, you can test basic printer functionality by sending a document directly to the printer device file. For instance, you might test a parallel printer by typing this command:
# cat /etc/fstab > | /dev/lp0
This command copies the /etc/fstab file to /dev/lp0, the most common identifier for a parallel printer. If your printer can print a text file, the result should be a printout of your /etc/fstab file. On some printers, status lights will blink but nothing will emerge. Frequently, pressing a button (marked Form Feed, Continue, or something similar) on the printer will cause the file to print. This happens because the printer didn't receive a full page of data, and so was waiting until it received more data to continue. This shouldn't happen in normal operation, thoughjust when testing the printer. Another problem is "stair-stepping," which is a printout that doesn't reset the line starts, like this:
/dev/hda1 / ext3 defaults 1 1 /dev/hda5 /home ext3 defaults 1 2
You might be able to figure out what went wrong here. Text files in Unix use just a newline (also known as a linefeed, ASCII code 10) to terminate each line. MS-DOS uses both a newline and a carriage return. Your printer was therefore set up to use MS-DOS-style line endings with both newline and carriage-return characters at the end of each line. In practice, CUPS normally filters newlines so that printers configured for DOS can understand them. If you still see this problem once you've fully configured printing, though, you can reconfigure your printer to properly return to the start of the line on receipt of a newline character. Often this is simply a matter of setting a dip switch. Check your printer manual. (Be careful about changing your printer characteristics if you use multiple operating systems.)
If you're using a USB printer, you can look for it with the lsusb command:
$ lsusb Bus 005 Device 004: ID 04b8:0807 Seiko Epson Corp.
In practice, you're likely to see more output than this; however, this output shows a USB printeran Epson RX500, to be precise. It's on USB bus 5, device 4. You can learn more about it by viewing the contents of /proc/bus/usb/devices, but most of the information in that file is likely to look like gibberish to the uninitiated. One feature to look for in this file, though, is Driver=usblp. If a line containing this string is present, it means that Linux has recognized the printer as a printer, which means that CUPS should be able to communicate with it. (Whether you can get output from the printer depends on the status of Ghostscript drivers, though.)
Finally, Ethernet printers can be treated just like other network devices. Specifically, you can use ping to verify that they're connected to the network. More substantial tests of their connectivity depend on the protocols the printer understands. Although low-level diagnostics of these protocols are possible, the simplest approach is usually to try configuring them, as described later in this chapter, in "Defining Printers in CUPS."
14.2.3. Configuring CUPS Security
Before proceeding with configuring CUPS to print, you should review your CUPS security and web-based configuration tool options. Unlike LPD systems, CUPS is best managed through its web-based configuration tool; however, this tool is sometimes disabled or set up in an inconvenient way. (Some distributions provide their own GUI tools for CUPS configuration. When using these distributions, you can either use their GUI tools or use the standard CUPS tools.) Depending on your network configuration, you might want to enable or disable the CUPS browsing feature, which enables CUPS servers to communicate their lists of available printers to one another. Browsing greatly simplifies network printer configuration and maintenance, but some distributions disable it by default. On the other hand, in a high-security environment or if your system is connected directly to the Internet, you might want to disable browsing, but some distributions enable it by default. In any event, you should review these settings and be sure they're set appropriately for your environment and needs.
18.104.22.168. Enabling web-based configuration
The main CUPS configuration file is /etc/cups/cupsd.conf. This file is modeled after the Apache configuration file, so if you're familiar with Apache, you should feel at home with this file. If you're not familiar with the Apache configuration file, the basics are as follows. The file begins with a series of global directives, which take the form of a directive name followed by the directive's value. For instance, to override the default server name (advertised to other systems), you would enter a directive like this one:
This line sets the server name to gutenberg.example.com. CUPS accepts a large number of configuration directives; you should consult its manpage for complete details. Some sets of configuration directives are enclosed in sections delimited by lines that are enclosed in angle brackets:
<Location /admin> AuthType Basic AuthClass System Order Deny,Allow Deny From All Allow From 127.0.0.1 Allow from 192.168.1.0/24 </Location>
This set of lines sets directives that apply only to certain functions of the serverin this case, those handled by the /admin (administrative) subsystem. You can use this feature to selectively modify the CUPS security functions (or other functions). The /admin area is particularly important for controlling access to the CUPS web-based administration tool. The options shown in the preceding example control features you may want to adjust:
To enable web-based configuration, you should be sure that your CUPS configuration has an /admin location defined and that it grants access to the 127.0.0.1 address, as shown in the preceding example. That example also gives administrative access to users on the 192.168.1.0/24 network. Generally speaking, you should activate such access only for print servers that you want to administer remotely; opening this option up increases the security risks associated with running CUPS.
If you want to completely disable the CUPS web-based administrative tools (say, because you intend to use your distribution's own CUPS administrative programs instead), you should remove all the Allow directives and ensure that the configuration includes a Deny from All directive.
22.214.171.124. Enabling or disabling browsing
While you're digging in the /etc/cups/cupsd.conf file, you may want to examine the server's browsing options. In the context of CUPS, browsing refers to automatic network printer discovery. This feature, supported by IPP, enables IPP servers to exchange lists of printers with one another. Each server periodically sends out broadcasts to which other IPP servers respond. The result is that you need configure each printer only once, on the computer to which it's connected. That server then propagates the configuration for the printer to other CUPS/IPP servers. Applications on those remote systems will see the new printer appear once they're restarted. This feature can be a great time-saver, particularly on networks on which printers are frequently added or removed.
A couple of caveats are in order concerning browsing, though. First, like many network features, browsing comes with some security baggage. Systems configured to browse could be tricked into displaying bogus printer information, and it's conceivable that a bug in the CUPS browsing code could lead to more serious system compromises. For this reason, some distributions ship with browsing disabled. This fact is the second caveat: if you do want to use browsing, you may need to enable it. If you fail to do so, your system won't automatically detect other printers on your network. Likewise, if you want to tell your system to enable others to print to its printers you must be sure certain options are enabled. You should check several options on your client and server systems:
These browsing directives are typically set in the global section of the cupsd.conf file. They're sometimes set separately or redundantly in the /printers section, though, so check there if you have problems.
126.96.36.199. Restarting CUPS
If you make changes to your CUPS configuration, you should restart the CUPS daemon. On most distributions, this is done via the SysV startup script file:
# /etc/init.d/cupsd restart
This command (or one like it; you may need to change the path to cupsd) shuts down the CUPS daemon and restarts it. The result should be that the system implements any changes you make to the server's configuration.
14.2.4. Defining Printers in CUPS
Now that you've ensured the CUPS administrative tools are available, you can begin using them. To do so, you'll need a web browser. Any modern browser will work: Mozilla Firefox, Konqueror, Opera, or even the text-mode Lynx, to name just four possibilities. With the help of your web browser, you can add your printer definitions and test your printer configuration.
188.8.131.52. Accessing the printer definition tool
CUPS runs its web-based configuration tool on port 631, so you should be able to access the system by entering http://localhost:631 in your web browser's address field. If you've configured CUPS to accept administrative input from other systems, you should be able to access the printer configuration tool from another computer by entering the server's hostname, as in http://gutenberg.example.com:631.
If CUPS is properly configured and running, the result of accessing a computer's port 631 with a web browser should be a display with a series of links called Do Administration Tasks, Manage Printer Classes, On-Line Help, and so on. If you get an error message from your browser, chances are that something is wrong with your CUPS configuration; review the earlier section "Configuring CUPS Security," and check that the cupsd server is running. If you do see the main CUPS page, you can begin using it to add or reconfigure your printers.
184.108.40.206. Creating a printer definition
To add a printer to a system, you should select the Manage Printers link on the main CUPS configuration page. The result should resemble Figure 14-3, which shows a CUPS configuration with some remote printers already autodetected.
To create a new printer definition, follow these steps:
If you decide to change any of the features of your printer configuration, you can do so by clicking Modify Printer in the printer's area on the main printer configuration page (Figure 14-3). The result is a run through the original configuration options, except that the defaults have changed to whatever you've entered before. (You will be unable to change the printer's name, but you can change other options.)
Figure 14-4. CUPS configured with printer definitions
This procedure (or the GUI tools provided by your distribution) is the simplest way to configure a print queue in CUPS. If you happen to know all the values, though, you can use the lpadmin command-line utility instead. When used to define a queue, this utility takes the following syntax:
lpadmin [-E] [-h server] -p printer option...
The -E option enables the print queue (a desirable option, typically), while -h enables you to modify the configuration of a server other than the local one, and -p sets the name of the queue.
The final, trailing options are the tricky part. Typically, the most important of these options are -i interface, which sets the interface device, and -m model, which points to a PostScript Printer Definition (PPD ) file that defines the printer's model and capabilities. You must know the filename of the PPD file for your printer, which may not be obvious unless you've obtained a PPD file specifically for the printer (say, from the printer's manufacturer). As the name implies, a PPD file describes the capabilities of a PostScript printer, so printer manufacturers normally only make these files available for PostScript models. CUPS printer definitions rely on PPD files for all printers, though, and in fact these are the main part of the Foomatic, GIMP Print, and other Linux printer definition packages.
One type of print queue definition requires special mention: a raw queue. You can create a raw queue by selecting a printer make of Raw and model of Raw Queue. Unlike most CUPS printer queues, a raw queue uses no filteringthat is, CUPS won't try to determine the file's type and pass it through programs such as Ghostscript to produce output that's acceptable for your printer. For most Linux purposes, raw queues aren't very useful; however, there are a few cases where you might want to use one. One such instance is if you want to use Linux as a print server for non-Linux systems, such as Windows computers. You might then install Windows drivers on the Windows clients to have them print to the Linux raw queue. Using a raw queue in this instance guarantees that CUPS won't corrupt the Windows print jobs. (Another option for this scenario is to use PostScript drivers on Windows. Each approach has its plusses and minuses.) A second situation where raw queues can be handy is when you use an application that provides its own printer drivers. The GIMP is one such program, and you might get better results using the GIMP's printer drivers than using the standard Ghostscript drivers for your printer.
If in doubt, you should probably create a regular printer queue, which will try to parse the file type and convert it (via Ghostscript or other filtering programs) to your printer's native format. If this doesn't work or if you know you'll need it, though, a raw queue can be the way to go.
220.127.116.11. Testing your printer definition
Once you've configured your printer queue, you can test it. From the main printer description page (Figure 14-3), click Print Test Page for your queue. CUPS should respond with a message to the effect that a test page has been printed. After a brief delay, the result should be a printer test page being printed. (Precisely how long a delay you'll experience depends on the printer, its configuration, how it's connected to your computer, your computer's CPU speed, and your computer's CPU load.)
The standard CUPS test page includes a color wheel; a circle of fine radial lines (used to assess resolution); information on page size, borders, and nominal resolution; and PostScript interpreter data. Check that the resolution (expressed in dots per inch, or dpi, in the Imageable Area box) is reasonable. If it's not, consult "Fine-Tuning Printer Definitions" for help. Likewise, if a color printer produces merely black-and-white printouts, you will have to adjust your configuration. (This is assuming you know the printer is working correctly; it's entirely possible that the printer's color ink is depleted or clogged!)
If your printer doesn't generate any output at all, consult the section "Printer Troubleshooting," later in this chapter.
14.2.5. Fine-Tuning Printer Definitions
Most modern printers provide options that affect print quality and style. These features include variable resolution, ink- or toner-saving modes, printing modes optimized to particular types of paper, and so on. CUPS provides a way to set defaults for these features for a given queue. From the main print queue list page in the web-based CUPS configuration (Figure 14-3), click Configure Printer in the area for the printer you want to modify. The result is a list of options you can set, as shown in Figure 14-5.
The list of options available for any given printer varies greatly from one printer to another. Some common options you might want to investigate include the following:
Because printer capabilities vary so much, we can't describe all the options here. You may need to examine the options and experiment with them. Change an option and see how it affects output. Some options will affect text more than graphics or graphics more than text. Different types of graphics, such as digital photos versus charts and graphs, may also be affected differently by certain options. As a general rule, if you don't understand an option you should probably leave it alone.
In the past, Linux provided applications with limited or no access to printer options such as those described here. In fact, applications built for the old LPD system may not be able to set these options directly; these programs can only use the printer's defaults. To set such options in such applications, you can create multiple print queues for each printer. For instance, suppose you want to be able to print at 360 dpi for quick printouts or at 1440 dpi for slower but higher-quality printouts. You would create two print queues (say, canon_360 and canon_1440). Both queues would connect to a single printer device, but you'd set different default options for the two queues. You'd then choose your desired print resolution by printing to one queue or the other.
Alternatively, if you're running in X and if the application enables you to enter the printing command, you can use kprinter as the printing command. The result is the standard KDE printing dialog box. When you set it to use CUPS (via the listbox near the bottom of the dialog box) and select a print queue, you can click the Properties button in the upper-right corner of the dialog box. The result is the Configuration dialog box shown in Figure 14-6. This dialog box has multiple tabs that correspond to various CUPS configuration options, so you can adjust the resolution (as shown in Figure 14-6) or other printing options.
Another fine-tuning detail is setting the default print queue. In theory, you should be able to do this from the CUPS interface by clicking the Set as Default option; however, in practice this often produces an error message about an unknown operation. To work around this bug, use the lpadmin command with its -d option, which takes the name of the default queue:
# lpadmin -d hp4500
This command sets the default queue to hp4500. A