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<th colspan="3" align="center">LPRng-HOWTO: 1 Apr 2002
(For LPRng-3.8.10)</th>
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<div class="CHAPTER">
<h1><a name="FILTERS">Chapter 16. Filters</a></h1>
<div class="TOC">
<dl>
<dt><b>Table of Contents</b></dt>
<dt>16.1. <a href="filters.htm#AEN7640">Filter
Functions</a></dt>
<dt>16.2. <a href="exitcodes.htm">Filter Exit
Codes</a></dt>
<dt>16.3. <a href="printjobformats.htm">Print Job
Formats</a></dt>
<dt>16.4. <a href="ofdetails.htm">OF Filter</a></dt>
<dt>16.5. <a href="pr.htm"><b class="APPLICATION">lpr</b>
-p format</a></dt>
<dt>16.6. <a href="x7862.htm"><b class=
"APPLICATION">lpr</b> binary (-l) format</a></dt>
<dt>16.7. <a href="chainingfilters.htm">Chaining
Filters</a></dt>
<dt>16.8. <a href="filteroptions.htm">Filter Command Line
Options and Environment Variables</a></dt>
<dt>16.9. <a href="x8279.htm"><b class=
"APPLICATION">LPRng</b> Supported Filters</a></dt>
<dt>16.10. <a href="lpf.htm">lpf</a></dt>
<dt>16.11. <a href="ifhp.htm"><b class=
"APPLICATION">ifhp</b> Filter</a></dt>
</dl>
</div>
<p>This section gives an overview of how <b class=
"APPLICATION">LPRng</b> uses filter programs, and gives a
detailed discussion of how the printcap options and filters
interact.</p>
<div class="SECT1">
<h1 class="SECT1"><a name="AEN7640">16.1. Filter
Functions</a></h1>
<p>Print filters are one of the most powerful tools in
BSD-style printer systems.</p>
<p>In general UNIX terms, a <i class="EMPHASIS">filter</i>
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 :))</p>
<p>In the context of a BSD-style print spooler (and also <b
class="APPLICATION">LPRng</b>), the term <i class=
"EMPHASIS">filter</i> refers to a program that processes
file while it is being transferred to a printer.</p>
<p>The filter is executed with <span class=
"ACRONYM">STDIN</span> reading from the file to be printed
<span class="ACRONYM">STDOUT</span> to the printer device
or a temporary file. <span class="ACRONYM">STDERR</span>
(file handle 2) is redirected to the status file, and file
handle3 to an accounting file or program.</p>
<p>A filter can be as simple as a <span class=
"ACRONYM">LF</span> to <tt class="FILENAME">CR/LF</tt>
translator, or it can incorporate a complete accounting
system, automatic file type translations, or even redirect
the job to another printing system.</p>
<p>The <b class="APPLICATION">lpf</b> filter supplied as
part of the <b class="APPLICATION">LPRng</b> distribution
is a a very simple CR to CR/LF conversion filter. The <b
class="APPLICATION">ifhp</b> filter provides support for
more complex PostScript, PCL, and text printers.</p>
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