8. How to set things up

8.1.1. Drivers and Interfaces

PDQ lets users select a printer to print to. A printer is defined in PDQ as the combination of a "driver" and an "interface". Both drivers and interfaces are, in fact, merely snippets of text in the PDQ configuration file.

A PDQ interface says everything about how to ship data out to a printer. The most common interfaces, which are predefined in the PDQ distribution's example printrc file, are:

A PDQ driver says everything about how to massage print data into a format that a particular printer can handle. For Postscript printers, this will include conversion from ascii into Postscript; for non-Postscript printers this will include conversion from Postscript into the printer's language with Ghostscript.

If one of PDQ's included driver specifications doesn't fit your printer, then read the section below on how to write your own.

8.1.2. Defining Printers

To define a printer in PDQ:

• First check that you've got suitable driver and interface declarations in the system or your personal printrc.

• If you want to define the printer in /etc/printrc (for all users), then su to root.

• Run xpdq, and select Printer->Add printer. This "wizard" will walk you through the selection of a driver and interface.

8.1.3. Creating a PDQ Driver Declaration

Here I'll walk through an example of how to make a PDQ driver declaration. Before you try that, though, there are several places to look for existing driver specs:

• PDQ itself comes with a collection of prewritten driver files.

• The Linux Printing Website's database includes a program called "PDQ-O-Matic" which will generate a PDQ specification from the information in the database. Assuming that the database contains the proper information for your printer and driver, this is the best path if you have a non-Postscript printer.

• I've written a tool called ppdtopdq which takes a Postscript Printer Definition file and converts it into a PDQ driver specification, with about 75% success. This is an option if you have a Postscript printer.

There are several places to look for the information needed to write your own PDQ driver:

• The PDQ driver specification syntax is quite rich, and is fully documented in the printrc(5) man page.

• The PDQ distribution includes a few example files. Look in particular at the Epson Stylus file, which demonstrates the structure of the definition for a Ghostscript-driven printer.

• The Printing HOWTO Database includes raw Linux driver information for over 600 printers. This will tell you what options to give Ghostscript, or what extra program to run on the Ghostscript output.

If you have to create your own driver specification, or if you enhance one from the PDQ distribution or one of the PDQ driver generator programs mentioned above, please share your creation with the world! Send it to me ([email protected]), and I'll make sure that it gets found by future PDQ users with your type of printer.

Now, let's walk through the writing of a driver specification for a printer listed in the Printing HOWTO's database as working, but for which you can't find a PDQ driver spec. I'll use the Canon BJC-210 as the example printer.

First, we look at the database entry for this printer. Note that it is supported "perfectly", so we can expect to get comparable results (or better) to Windows users. The important information is in two places in the entry:

option {
  var = "MODE"
  desc = "Print Mode"
  # default_choice "Color"    # uncomment to default to color
  choice "BW" {
    # The value part assigns to the variable MODE whatever you 
    # want. Here we'll assign the text that varies between the 
    # two Ghostscript option sets for the two modes.
    value = "bj200"
    help = "Fast black printing with the black cartridge."
    desc = "Black-only"
  }
  choice "Color" {
    value = "bjc600"
    help = "Full-color printing."
    desc = "Color"
  }
}

# The first language_driver in the file that matches what file(1) 
# says is what gets used.
language_driver ps {
  # file(1) returns "PostScript document text conforming at..."
  filetype_regx = "postscript"
  convert_exec = { 
    gs -sDEVICE=$MODE -r360x360 \     # gs options from the database
       -q -dNOPAUSE -dBATCH -dSAFER \ # the "usual" Ghostscript options
       -sOutputFile=$OUTPUT $INPUT    # process INPUT into file OUTPUT

    # Those last two lines will often be the same for gs-supported
    # printers.  The gs... line, however, will be different for each
    # printer.      
  }
}

# We declare text after postscript, because the command "file" will
# often describe a postscript file as text (which it is).
language_driver text {
  # No filetype_regx; we match the driver's name: "text"
  convert_exec = {#!/usr/bin/perl
     # a Perl program, just because we can!
     my ($in, $out) = ($ENV{'INPUT'}, $ENV{'OUTPUT'});
     open INPUT, "$in";
     open OUTPUT, ">$out";
     while(<INPUT>) {
        chomp;
        print OUTPUT, "$_\r\n";
     }
  }
}

driver canon-bjc210-0.1 {
  option {
    var = "MODE"
    desc = "Print Mode"
    # default_choice "Color"    # uncomment to default to color
    choice "BW" {
      # The value part assigns to the variable MODE whatever you 
      # want. Here we'll assign the text that varies between the 
      # two Ghostscript option sets for the two modes.
      value = "bj200"
      help = "Fast black printing with the black cartridge."
      desc = "Black-only"
    }
    choice "Color" {
      value = "bjc600"
      help = "Full-color printing."
      desc = "Color"
    }
  }

  # The first language_driver in the file that matches what file(1) 
  # says is what gets used.
  language_driver ps {
    # file(1) returns "PostScript document text conforming at..."
    filetype_regx = "postscript"
    convert_exec = { 
      gs -sDEVICE=$MODE -r360x360 \     # gs options from the database
         -q -dNOPAUSE -dBATCH -dSAFER \ # the "usual" Ghostscript options
         -sOutputFile=$OUTPUT $INPUT    # process INPUT into file OUTPUT

      # Those last two lines will often be the same for gs-supported
      # printers.  The gs... line, however, will be different for each
      # printer.      
    }
  }

  # We declare text after postscript, because the command "file" will
  # often describe a postscript file as text (which it is).
  language_driver text {
    # No filetype_regx; we match the driver's name: "text"
    convert_exec = {#!/usr/bin/perl
       # a Perl program, just because we can!
       my ($in, $out) = ($ENV{'INPUT'}, $ENV{'OUTPUT'});
       open INPUT, "$in";
       open OUTPUT, ">$out";
       while(<INPUT>) {
          chomp;
          print OUTPUT, "$_\r\n";
       }
    }
  }
}

driver generic-ljet4-with-duplex-0.1 {
  # First, two option clauses for the user-selectable things:
  option {
    var = "DUPLEX_MODE"
    desc = "Duplex Mode"
    default_choice = "SIMPLEX"
    choice "SIMPLEX" {
      value = "OFF"
      desc = "One-sided prints"
    }
    choice "DUPLEX" {
      value = "ON"
      desc = "Two-sided prints"
    }
  }

  option { 
    var = "GS_RES"
    desc = "Resolution"
    default_choice = "DPI600"
    choice "DPI300" {
      value = "-r300x300"
      desc = "300 dpi" 
    }
    choice "DPI600" {
      value = "-r600x600"
      desc = "600 dpi" 
    }
  }

  # Now, we handle Postscript input with Ghostscript's ljet4 driver:
  language_driver ps {
    filetype_regx = "postscript"
    convert_exec = { 
       gs -sDEVICE=ljet4 $GS_RES \
          -q -dNOPAUSE -dBATCH -dSAFER \
          -sOutputFile=$OUTPUT $INPUT
    }
  }

  # Finally, we wrap the job in PJL commands:
  filter_exec {
    # requires echo with escape code ability...
    echo -ne '\33%-12345X' > $OUTPUT

    echo "@PJL SET DUPLEX=$DUPLEX_MODE"    >> $OUTPUT
    # You can add additional @PJL commands like the above line here.
    # Be sure to always append (>>) to the output file!

    cat $INPUT >> $OUTPUT
    echo -ne '\33%-12345X' >> $OUTPUT
  }
}

8.2.1. Basic LPD configuration

The minimal setup for lpd results in a system that can queue files and print them. It will not pay any attention to wether or not your printer will understand them, and will probably not let you produce attractive output. But we have to start somewhere.

To add a print queue to lpd, you must add an entry in /etc/printcap, and make the new spool directory under /var/spool/lpd.

An entry in /etc/printcap looks like:

# LOCAL djet500
lp|dj|deskjet:\
        :sd=/var/spool/lpd/dj:\
        :mx#0:\
        :lp=/dev/lp0:\
        :sh:

Go now and read the man page for printcap.

The above looks very simple, but there a catch - unless I send in files a DeskJet 500 can understand, this DeskJet will print strange things. For example, sending an ordinary Unix text file to a deskjet results in literally interpreted newlines, and gets me:

This is line one.
                 This is line two.
                                  This is line three.

Clearly more is needed, and this is the purpose of filtering. The more observant of you who read the printcap man page might have noticed the spool attributes if and of. Well, if, or the input filter, is just what we need here.

If we write a small shell script called filter that adds carriage returns before newlines, the staircasing can be eliminated. So we have to add in an if line to our printcap entry above:

lp|dj|deskjet:\
        :sd=/var/spool/lpd/dj:\
        :mx#0:\
        :lp=/dev/lp0:\
        :if=/var/spool/lpd/dj/filter:\
        :sh:

#!perl
# The above line should really have the whole path to perl
# This script must be executable: chmod 755 filter
while(<STDIN>){chomp $_; print "$_\r\n";};
# You might also want to end with a form feed: print "\f";

The only remaining problem is that printing plain text is really not too hot - surely it would be better to be able to print PostScript and other formatted or graphic types of output. Well, yes, it would, and it's easy to do. The method is simply an extention of the above linefeed-fixing filter.

Such a filter is called a magic filter. It plays the same role as the language filters of PDQ. Don't bother writing one yourself unless you print strange things - there are a good many written for you already, and most have easy-to-use interactive configuration tools. You should simply select a suitable pre-written filter:

There's one catch to such filters: older version of lpd don't run the if filter for remote printers, while most newer ones do (although often with no arguments). The version of LPD shipped with modern Linux and FreeBSD distributions does; most commercial unices that still ship LPD have a version that does not. See the section on network printing later in this document for more information on this. If you only have locally-connected printers, then this won't affect you.

8.2.2. LPD for PostScript Printers

While most versions of LPD don't gracefully handle PostScript (nevermind user options), VA Linux recently modified LPD and Red Hat's filtering software to support PostScript printers fairly well. For the moment, this system works only with Red Hat 6.2, although the packages could be easily adapted for other distributions.

8.2.3. File Permissions

By popular demand, I include below a listing of the permissions on interesting files on my system. There are a number of better ways to do this, ideally using only SGID binaries and not making everything SUID root, but this is how my system came out of the box, and it works for me. (Quite frankly, if your vendor can't even ship a working lpd you're in for a rough ride).

-r-sr-sr-x   1 root     lp    /usr/bin/lpr*
-r-sr-sr-x   1 root     lp    /usr/bin/lprm*
-rwxr--r--   1 root     root  /usr/sbin/lpd*
-r-xr-sr-x   1 root     lp    /usr/sbin/lpc*
drwxrwxr-x   4 root     lp    /var/spool/lpd/
drwxr-xr-x   2 root     lp    /var/spool/lpd/lp/

Lpd must currently be run as root so that it can bind to the low-numbered lp service port. It should probably become UID lp.lp or something after binding, but I don't think it does. This is simply one more reason to avoid the stock BSD LPD.

PDQ uses a different, non-daemon-centric scheme, so it has different programs. The only SUID root programs are the lpd interface programs lpd_cancel, lpd_print, and lpd_status; these are SUID because actual Unix print servers require print requests to originate from a priviledged port. If the only printers for which you use PDQ's bsd-lpd interface are networked print servers (like the HP JetDirect or Lexmark's MarkNet adapters) then you do not need the SUID bit on these programs.