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19th chunk of `less.man`
27059a4b6c433d6f2d458465a41bb56f9483bbb836dc7c1f0000000100000fc3
 then looks for a lesskey file called "_lesskey" in any directory specified in the PATH environment variable.  On OS/2 systems, less looks for a lesskey file called "$HOME/lesskey.ini", and if it  is  not  found,  then
       looks  for  a  lesskey file called "lesskey.ini" in any directory specified in the INIT environment variable, and if it not found there, then looks for a lesskey file called "lesskey.ini" in any directory specified in
       the PATH environment variable.  See the lesskey manual page for more details.

       A system‐wide lesskey source file may also be set up to provide key bindings.  If a key is defined in both a local lesskey file and in the system‐wide file, key bindings in the local file take precedence over those in
       the system‐wide file.  If the environment variable LESSKEYIN_SYSTEM is set, less uses that as the name of the system‐wide lesskey file.  Otherwise, less looks in a standard place for the system‐wide lesskey  file:  On
       Unix systems, the system‐wide lesskey file is /usr/local/etc/syslesskey.  (However, if less was built with a different sysconf directory than /usr/local/etc, that directory is where the sysless file is found.)  On MS‐
       DOS and Windows systems, the system‐wide lesskey file is c:\_syslesskey.  On OS/2 systems, the system‐wide lesskey file is c:\syslesskey.ini.

       Previous versions of less (before v582) used lesskey files with a binary format, produced by the lesskey program. It is no longer necessary to use the lesskey program.

INPUT PREPROCESSOR
       You may define an "input preprocessor" for less.  Before less opens a file, it first gives your input preprocessor a chance to modify the way the contents of the file are displayed.  An input preprocessor is simply an
       executable  program  (or  shell script), which writes the contents of the file to a different file, called the replacement file.  The contents of the replacement file are then displayed in place of the contents of the
       original file.  However, it will appear to the user as if the original file is opened; that is, less will display the original filename as the name of the current file.

       An input preprocessor receives one command line argument, the original filename, as entered by the user.  It should create the replacement file, and when finished, print the name of the replacement file to  its  stan‐
       dard output.  If the input preprocessor does not output a replacement filename, less uses the original file, as normal.  The input preprocessor is not called when viewing standard input.  To set up an input preproces‐
       sor,  set the LESSOPEN environment variable to a command line which will invoke your input preprocessor.  This command line should include one occurrence of the string "%s", which will be replaced by the filename when
       the input preprocessor command is invoked.

       When less closes a file opened in such a way, it will call another program, called the input postprocessor, which may perform any desired clean‐up action (such as deleting the replacement file  created  by  LESSOPEN).
       This program receives two command line arguments, the original filename as entered by the user, and the name of the replacement file.  To set up an input postprocessor, set the LESSCLOSE environment variable to a com‐
       mand  line which will invoke your input postprocessor.  It may include two occurrences of the string "%s"; the first is replaced with the original name of the file and the second with the name of the replacement file,
       which was output by LESSOPEN.

       For example, on many Unix systems, these two scripts will allow you to keep files in compressed format, but still let less view them directly:

       lessopen.sh:
            #! /bin/sh
            case "$1" in
            *.Z) TEMPFILE=$(mktemp)
                 uncompress -c $1  >$TEMPFILE  2>/dev/null
                 if [ -s $TEMPFILE ]; then
                      echo $TEMPFILE
Title: Less: System-Wide Key Bindings and Input Preprocessors
Summary
This section details the locations of system-wide lesskey files on Unix, MS-DOS/Windows, and OS/2 systems. It also mentions the deprecation of binary lesskey files. The section then explains how to define an input preprocessor using the `LESSOPEN` environment variable. This allows `less` to display modified versions of files. The preprocessor receives the original filename as an argument, creates a replacement file, and outputs its name. The section also explains how to define an input postprocessor using the `LESSCLOSE` environment variable for cleanup actions, such as deleting replacement files. An example script shows how to view compressed files directly using input pre- and postprocessors.