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.TH LESSOPEN 1
.\" NAME should be all caps, SECTION should be 1-8, maybe w/ subsection
.\" other parms are allowed: see man(7), man(1)
.SH NAME
lessfile, lesspipe \- "input preprocessor" for  less.
.SH SYNOPSIS
.B lessfile, lesspipe
.SH "DESCRIPTION"
This manual page documents briefly the
.B lessfile
and
.B lesspipe
commands.
This manual page was written for the Debian GNU/Linux distribution
because the input preprocessor scripts are provided by Debian GNU/Linux and
are not part of the original program.
.PP
.B lessfile
and
.B lesspipe
are programs that can be used to modify the way the contents of a file are
displayed in
.BR less .
What this means is that
.B less
can automatically open
up tar files, uncompress gzipped files, and even display something reasonable
for graphics files.
.PP
.B lesspipe
will toss the contents/info on STDOUT and
.B less
will read them
as they come across.  This means that you do not have to wait for the
decoding to finish before less shows you the file.  This also means that
you will get a 'byte N' instead of an N% as your file position.  You can
seek to the end and back to get the N% but that means you have to wait
for the pipe to finish.
.PP
.B lessfile
will toss the contents/info on a file which
.B less
will then
read.  After you are done,
.B lessfile
will then delete the file.  This means that the process has to finish before
you see it, but you get nice percentages (N%) up front.
.
.SH USAGE
Just put one of the following two commands in your login script (e.g.
~/.bash_profile):
.PP
.I eval $(lessfile)
or
.I eval $(lesspipe)
.SH USER DEFINED FILTERS
It is possible to extend and overwrite the default
.B lesspipe
and
.B lessfile
input processor if you have specialized requirements. Create an executable
program with the name
.I .lessfilter
and put it into your home directory. This can be a shell script or a binary
program.

.PP
It is important that this program returns the correct exit code: return 0 if
your filter handles the input, return 1 if the standard
.B lesspipe/lessfile
filter should handle the input.

.PP
Here is an example script:

.DS
  #!/bin/sh

  case "$1" in
      *.extension)
          extension-handler "$1"
          ;;
      *)
          # We don't handle this format.
          exit 1
  esac

  # No further processing by lesspipe necessary
  exit 0
.DE

.SH FILES
.TP
.I ~/.lessfilter
Executable file that can do user defined processing. See section USER DEFINED
FILTERS for more information.
.SH BUGS
When trying to open compressed 0 byte files,
.B less
displays the actual binary file contents. This is not a bug.
.B less
is designed to do that (see manual page less(1), section INPUT PREPROCESSOR).
This is the answer of Mark Nudelman <markn@greenwoodsoftware.com>:
.IP
"I recognized when I designed it that a
lesspipe filter cannot output an empty file and have less display
nothing in that case; it's a side effect of using the "no output" case
to mean "the filter has nothing to do".  It could have been designed to
have some other mechanism to indicate "nothing to do", but "no output"
seemed the simplest and most intuitive for lesspipe writers."

.PP
Sometimes, less does not display the contents file you want to view but output
that is produced by your login scripts (~/.bashrc or ~/.bash_profile). This
happens because less uses your current shell to run the lesspipe filter. Bash
first looks for the variable $BASH_ENV in the environment expands its value
and  uses the expanded value as the name of a file to read and execute. If
this file produces any output less will display this. A way to solve this
problem is to put the following lines on the top of your login script that
produces output:

.DS
  if [ -z "$PS1" ]; then
      exit
  fi
.DE

This tests whether the prompt variable $PS1 is set and if it isn't (which is
the case for non-interactive shells) it will exit the script.
.SH "SEE ALSO"
.BR file (1),
.BR less (1).
.SH AUTHOR
This manual page was written by Thomas Schoepf <schoepf@debian.org>,
for the Debian GNU/Linux system (but may be used by others). Most of the
text was copied from a description written by Darren Stalder <torin@daft.com>.