5.7. Printing services

The GNU/Linux [Gt] [Smi02] printing server derives from UNIX's BSD variant; this system was called LPD (line printer daemon). This is a very powerful printing system, because it integrates the capacity to manage both local and network printers. And it provides this service within the system for both the client and the printing server.

LPD is a system that is quite old, as its origins date back to UNIX's BSD branch (mid 1980s). Consequently, LPD usually lacks support for modern devices, given that the system was not originally conceived for the type of printing that takes place now. The LPD system was not designed as a system based on device drivers, as it was typical to produce only printers in series or in parallel for writing text characters.

Currently, the LPD system combines with another common software, such as the Ghostscript system, which offers a postscript type output for a very wide range of printers for which it has the right drivers. At the same time, they are usually combined with filtering software, which, depending on the type of document that must be printed, selects the appropriate filters. Normally, the procedure that should be followed is (basically):

1) The work is started by a command in the LPD system.

2) The filtering system identifies the type of job (or file) that must be used and transforms the job into an outgoing postscript file, which is the one sent to the printer. In GNU/Linux and UNIX, most of the applications assume that the job will be sent to a postscript printer and many of them directly generate a postscript output, which is why the following step needs to be taken.

3) The Ghostscript has to interpret the postscript file it receives, and, depending on the driver of the printer to which the file has been sent, it performs the transformation to the driver's own format. If the printer is a postscript type printer, the printing process is direct; if not, it has to "translate" the job. The job is sent to the printing queue.

Apart from the LPD printing system (that originated with UNIX's BSD), there is also the system known as System V (originally in the other System V branch of UNIX). Normally, for compatibility reasons, most UNIX systems integrate both systems, so that either one or the other is used as the main one and the other emulates the main one. In the case of GNU/Linux, a similar process occurs, depending on the installation that we have, we can have only the LPD commands of the printing system, but it will also be common to have the System V commands. A simple way of identifying the two systems (BSD or System V) is using the main printing command (which sends the jobs to the system), in BSD, it is Ipr, and it is Ip in System V.

This is the initial situation for the GNU/Linux printing systems, although over the last few years, more systems have appeared, which provide more flexibility and make more drivers available for the printers. The two main systems are CUPS and, to a lesser extent, LPRng. In fact, recently, CUPS is GNU/Linux's de facto standard, although the other systems must be supported for compatibility with the existing UNIX systems.

Both (both CUPS and LPRng) are a type of higher-level system, but they are not all that perceptibly different for average users, with regard to the standard BSD and System V systems; for example, the same client commands (or compatible commands in the options) are used for printing. There are perceptible differences for the administrator, because the configuration systems are different. In one way, we can consider LPRng and CUPS as new architectures for printing systems, which are compatible for users with regard to the old commands.

In the current GNU/Linux distributions, we can find different printing systems. If the distribution is old, it may only incorporate the BSD LPD system; in the current distributions: both Debian and Fedora/Red Hat use CUPS. In older versions of Red Hat, there was a tool, Print switch, which made it possible to change the system, switching the printing system, although recently only CUPS is available. In Debian, it is possible to install both systems, but they are mutually exclusive: only one may be used for printing.

In the case of Fedora Core, the default printing system is CUPS (as LPRng disappeared in Fedora Core 4), and the Print Switch tool no longer exists, as it is no longer necessary: system-config-printer is used to configure devices. By default, Debian uses BSD LPD, but it is common to install CUPS (and we can expect it to continue to be the default option in future new versions) and LPRng may also be used. In addition, we must remember that we also had the possibility (seen in the unit on migration) of interacting with Windows systems through the Samba protocols, which allowed you to share printers and access to these printers.

Regarding each of the [Gt] systems:

• BSD LPD: this is one of UNIX's standards, and some applications assume that the commands and the printing system will be available, for which both LPRng and CUPS emulate the functions and commands of BDS LPD. The LPD system is usable but not very configurable, especially with regard to access control, which is why the distributions have been moved to other, more modern, systems.

• LPRng: basically it was designed to replace BSD, and therefore, most of the configuration is similar and only some of the configuration files are different.

• CUPS: it is the biggest deviation from the original BSD and the configuration is the same. Information is provided to the applications on the available printers (also in LPRng). In CUPS, both the client and the server have to have CUPS software.

The two systems emulate the printing commands of System V.

For GNU/Linux printing, various aspects have to be taken into account:

• Printing system that is used: BSD, LPRng or CUPS.

• Printing device (printer): it may have a local connection to a machine or be on the network. The current printers may be connected to a machine using local connections, through interfaces in series, in parallel, USB etc. Or they may simply be on the network, as another machine, or with special ownership protocols. Those connected to the network can normally act themselves as a printing server (for example, many HP laser printers are BSD LPD servers) or they can be connected to a machine that acts as a printing server for them.

• Communication protocols used with the printer or the printing system: whether it is direct TCP/IP connection (for example, an HP with LPD) or high level ones based on TCP/IP, such as IPP (CUPS), JetDirect (some HP printers) etc. This parameter is important, as we have to know it so as to install the printer in a system.

• Filtering systems used: each printing system supports one or more.

• Printer drivers: in GNU/Linux, there are quite a few different types; we might mention, for example CUPS drivers, the system's or third parties' (for example, HP and Epson provide them); Gimp, the image editing program also has drivers optimised for printing images; Foomatic is a driver management system that works with most systems (CUPS, LPD, LPRng and others); Ghostscript drivers etc. In almost all printers, there are one or more of the drivers in these sets.

With regard to the client part of the system, the basic commands are the same for the different systems and these are the BSD system commands (each system supports emulation of these commands):

• lpr: a job is sent to the default printing queue (or the one that is selected), and the printing daemon (lpd) then sends it to the corresponding queue and assigns a job number, which will be used with the other commands. Normally, the default printer would be indicated by the PRINTER system variable or the first defined and existing one will be used or, in some systems, the Ip queue will be used (as the default name).

• lpq: This allows us to examine the jobs in the queue.

This command shows us the jobs in the queue, with the respective order and sizes; the files may appear with different names, as this depends on whether we have sent them with Ipr or with another application that might change the names when it sends them or if any filters have had to be used when converting them.

• lprm: eliminates jobs from the queue and we can specify a job number or the user, to cancel these operations.

With regard to the administrative side (in BSD), the main command would be lpc; this command can be used to activate or deactivate queues, move jobs in the queue order and activate or deactivate the printers (jobs may be received in the queues but they are not sent to the printers).

We should also point out that, in the case of System V, the printing commands are usually also available, normally simulated on the basis of the BSD commands. In the client's case, the commands are: lp, lpstat, cancel and, for administrative subjects, lpadmin, accept, reject, lpmove, enable, disable, lpshut.

In the following sections we will see that it is necessary to configure a printer server for the three main systems. These servers may be used both for local printing and for the network clients' prints (if they are enabled).

# Local printer input 
lp|epson|Epson C62:\
 :lp=/dev/lp1:sd=/var/spool/lpd/epson:\ 
 :sh:pw#80:pl#72:px#1440:mx#0:\ 
 :if = /etc/magicfilter/StylusColor@720dpi-filter:\filter 
 :af = /var/log/lp-acct:lf = /var/log/lp-errs:
# Remote printer input 
hpremote|hpr|remote hp of the department|:\
 :lp = :\ 
 :rm = server:rp = queuehp:\ 
 :lf = /var/adm/lpd_rem_errs:\log file. 
 :sd = /var/spool/lpd/hpremote:local associated spool