LPRng Reference Manual: 24 Sep 2004 (For LPRng-3.8.28)
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# Chapter 16. Filters

This section gives an overview of how LPRng uses filter programs, and gives a detailed discussion of how the printcap options and filters interact.

# 16.1. Filter Functions

Print filters are one of the most powerful tools in BSD-style printer systems.

In general UNIX terms, a filter is a program that takes its input file(s), does something with it, and sends the result to its standard output. Most UNIX utilities are designed as filters. (But since you are a system manager, you should already know that :))

In the context of a BSD-style print spooler (and also LPRng), the term filter refers to a program that processes file while it is being transferred to a printer.

The filter is executed with STDIN reading from the file to be printed STDOUT to the printer device or a temporary file. STDERR (file handle 2) is redirected to the status file, and file handle3 to an accounting file or program.

A filter can be as simple as a LF to `CR/LF` translator, or it can incorporate a complete accounting system, automatic file type translations, or even redirect the job to another printing system.

The lpf filter supplied as part of the LPRng distribution is a a very simple CR to CR/LF conversion filter. The ifhp filter provides support for more complex PostScript, PCL, and text printers.

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Debugging |  | Filter Exit Codes