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magicfilter - automatic configurable printer filter  


magicfilter config-file [-c] [-n user] [-h host] [-iindent] [--debug] [other-options]  


magicfilter is an extensible and customizable automatic printer filter. It selects an appropriate conversion technique for the input data by seeking for magic numbers, and then utilizing the appropriate conversion utility.

magicfilter is primarily intended for use as the ``input filter'' by the lpd print spooler. The options accepted by magicfilter are exactly the ones passed to the input filter by lpd.  


Typically magicfilter will be invoked by lpd and hence provided the right options automatically. This list is included for reference only.

Note that only the -n and -h options may have spaces between the option letter and the option value.

Copy the input to the output without any conversion whatsoever (used by lpd whenever the -l option is passed to the lpr program).
-nuser, -n user
The login name of the user who submitted the job. Available to subfilters as $LPUSER. If the user has an associated GECOS entry it will be available as $LPUSERNAME.
-hhost, -h host
The host on which the job was submitted. Available to subfilters as $LPHOST.
A numeric option passed by lpd; can be set by the user by the -i option to lpr. Although nominally used for the amount of indentation requested, magicfilter makes it available to subfilters for any useful purpose as $LPINDENT.
LPRng class (priority) name. Available to subfilters as $LPCLASS.
Format letter (passed by LPRng only). When used as input filter (if) this will be f, when used as other filter types it will be the option character corresponding to this filter. Available to subfilters as $LPFORMAT.
The name of the printer job (passed by LPRng only). Available to subfilters as $LPJOB.
Copy count (passed by LPRng only). Available to subfilters as $LPCOPIES.
User name from the banner page (passed by LPRng only). Available to subfilters as $BANNERNAME.
Printer name (passed by LPRng only). Available to subfilters as $PRINTER.
Spool queue name (passed by LPRng only). Available to subfilters as $LPQUEUE.
Accounting information (passed by LPRng only). Available to subfilters as $LPACCT.
LPRng ``Z-options''. The LPRng lpr program supports a -Z option, which can be used to pass arbitrary information to the printer filters. Available to subfilters as $ZOPT.
Write the name of each facility invoked (together with any options) to standard error. This can be useful for debugging complicated configuration files.
other options
Any other options, such as the -w, -l, -x, and -y options typically passed by lpd are ignored.


To run magicfilter from lpd it should be entered as one of the filters in the /etc/printcap file. Typically, it will be the input filter (if). Since most version of lpd do not accept arguments entered as part of the filter name, typically the filter name entered into the /etc/printcap file will simply be the name of the configuration file, which is set executable and starts with the line:

#! /usr/sbin/magicfilter

Most UNIX kernels will then be able to treat the configuration file itself as if it was the actual program.

For systems which do not support the ``#!-hack'', the filter set in the if entry should point to magicfilter directly, and the accounting file (af) entry should point to the configuration file. This, however, is a less general, and hence less desirable solution.

This version of magicfilter supports the enhanced lpd included with the LPRng package from  


The configuration file is used by magicfilter to redirect various types of data to the various conversion utilities. The configuration file is printer-specific, and often system-specific, depending on the available conversion programs. For example, a system which has GhostScript installed would be able to print PostScript to a non-PostScript printer, whereas other systems typically would not.

The configuration file contains a sequence of lines of the form:

offset        magic   facility

where the
offset represents the location of the indentification string in the data format, magic represents the identification string itself, facility represents the type of action to take.

Blank lines and lines with a hash mark (#) as the first nonblank character are ignored. A line may be continued onto the next line by ending the line in a backslash (\).

The offset is a nonnegative integer, which can be represented either in decimal form (default), octal form (preceded by 0), or hexadecimal form (preceded by 0x).

The magic is a string of characters, which may include C-style \-escape sequences. In addition, the sequence \? can be used to represent a ``wildcard'' byte. If the string includes spaces, the spaces have to be preceded by a backslash, or the entire string must be enclosed in double quotation marks.

For ambiguous matches, the first match is used. Hence, the most specific match should always be placed first in the file. In addition, the last line may be of the form:

default        facility

which designates a default action to be used in case no other action matches. This will typically be the action for plain text.  


magicfilter provides the following options for the facility field in the configuration file:
cat [prefix [suffix]]
Copy the input to the output without any conversion, like the cat command. If the optional prefix and suffix strings are specified, they are transmitted to the printer immediately before and after the data itself. The prefix and suffix strings are specified in the same way as the magic string (except that the wildcard sequence \? is not permitted), and like the magic sequence can contain any control character, including nulls (\0). To specify a suffix without a prefix, specify an empty prefix string enclosed in double quotes (i.e. "").
text [prefix [suffix]]
Copy the input to the output, but add carriage return characters before every line feed and form feed character in the file, and a line feed-form feed sequence at end of file. The prefix and suffix arguments are treated the same way as for the cat facility; the suffix, if present, is added after the final line feed-form feed sequence.
Same as the text facility, except add an ASCII EOT (Ctrl-D) character to the end of the data. This lets a PostScript printer know that the end of the job has been reached. This is functionally equivalent to the command
text "" \004
Ignore the job; do not provide any output whatsoever.
reject message
Same as the ignore facility, but attempt to send an email message to the user who submitted the job to inform that a job has been rejected and why.
filter command
Run the given command, feeding it the input data, and sending the output data to the printer. The environment variables LPUSER, LPHOST, and LPINDENT is set to the values of the user, host and indent options passed to magicfilter. Since the command is fed to /bin/sh it may contain shell special characters, and the sequences $LPUSER, $LPHOST, and $LPINDENT can be used to access the values of the passed environment variables. If the lpd daemon on the system is LPRng, the following environment variables are also available, see the OPTIONS section for details: LPCLASS, LPFORMAT, LPJOB, LPCOPIES, BANNERNAME, PRINTER, LPQUEUE, LPACCT, and ZOPT.
pipe command
Same as the filter facility, except that the output data is fed back into magicfilter for reprocessing. This is used for external filter programs which themselves do not produce a format that the printer can accept, but which can be futher processed to obtain such a format.
ffilter command

fpipe command
Same as the filter and pipe facilities, respectively, except that the input is written to a temporary file before being fed to the filter program given by command. This is useful for programs which require seekable input, such as dvips, or which need to do multiple passes over an input file, such as grog. The environment variable FILE is set to the name of the temporary file (and, like the other ones, it can be accessed on the command line as $FILE).


Using the pipe facility together with zcat or gunzip lets you transparently print compressed files.

The pbmplus or netpbm collections of image conversion utilities contain a large number of very useful external filter programs.

You will probably want to examine the sample configuration files included with the magicfilter distribution before creating your own.  


Some data formats cannot be easily identified by a simple fixed-offset magic number check.

Providing large offsets can cause magicfilter to take up lots of memory. Fortunately, large offsets for magic numbers are pretty much unheard of.

Currently, there is no protection against the pipe or fpipe facilities going into an infinite loop.



H. Peter Anvin <>  


printcap(5), lpr(8), dvips(1), grog(1), gs(1), gzip(1), troff(1).




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Time: 01:50:38 GMT, March 05, 2024