ALT Linux Sisyphus

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<H1>Frequently Asked Questions about less</H1>

<H2>Table of Contents</H2>

<OL>
<LI>Overview
<OL>
<LI><A HREF="#what">What is less?</A>
<LI><A HREF="#why">Why would I want to use less?</A>
<LI><A HREF="#history">Where did less come from?</A>
</OL>

<LI>Resources
<OL>
<LI><A HREF="#getit">How can I get a copy of less?</A>
<LI><A HREF="#mail">How can I write to the author of less?</A>
<LI><A HREF="#bugs">What if I find a bug in less?</A>
</OL>

<LI>Technical questions
<OL>
<LI><A HREF="#tricks">Cool less tricks</A>
<LI><A HREF="#build">I can't get less to build!</A>
<LI><A HREF="#unix">Do I need Unix to use less?</A>
<LI><A HREF="#intl">Can less display non-English language characters?</A>
<LI><A HREF="#wrap">Why does less scroll off the screen if there are long lines?</A>
<LI><A HREF="#optfile">How can I view a file whose name looks like a command line option?</A>
<LI><A HREF="#dashr">Why does -r cause display problems?</A>
<LI><A HREF="#dashw">Why is the first line on the screen highlighted?</A>
<LI><A HREF="#tite">Can less leave the screen alone when it quits?</A>
<LI><A HREF="#dashe">Why doesn't less quit when it reaches the end of the file?</A>
<LI><A HREF="#ctrlc">Why doesn't less quit when I type control-C?</A>
<LI><A HREF="#csh">Why does it take so long to start the editor / start a shell?</A>
<LI><A HREF="#csh2">Why does it say
"<CODE>stty: TCGETS: Operation not supported on socket</CODE>"?</A>
<LI><A HREF="#profileout">Why are there problems if my .cshrc, .profile or ENV
files produce output?</A>
<LI><A HREF="#filter">Can less view compressed files?</A>
<LI><A HREF="#searchlt">Why do I get a segmentation fault if I search for a pattern containing "\&lt;"?</A>
<LI><A HREF="#linux269">Why doesn't less work with Linux 2.6.9?</A>
<LI><A HREF="#version">Why are there no dots in the version number?</A>
</OL>
</OL>

This FAQ is Copyright &copy;2007 by Mark Nudelman.
This document may be reproduced freely, provided it is kept
in its entirety and in its original format.

<HR>

<H2>Questions and Answers</H2>

<A NAME="what">
<H3>What is less?</H3>
<P>
Less is a pager.
A pager is a program that displays text files.
Other pagers commonly in use are <CODE>more</CODE> and <CODE>pg</CODE>.
Pagers are often used in command-line environments like
the Unix shell and the MS-DOS command prompt to display files.
Windowed environments like the Windows and Macintosh desktops don't need
pagers as much, since they have other methods for viewing files.
</P>
<P>
Less is not an editor.  
You can't change the contents of the file you're viewing.
Less is not a windowing system.
It doesn't have fancy scroll bars or other 
GUI (graphical user interface) elements.
It was designed to work on simple text-only terminals.
</P>
<P>
Less has been used by thousands of people around the world since its
release in 1985.
</P>

<A NAME="why">
<H3>Why would I want to use less?</H3></A>
<P>
If you're not using a pager now, then you probably don't need less.
But if you're using another pager, you should take a look at less.
Less has some very nice features:
</P>

<DL>
<DT>Backwards movement
<DD>It's hard to believe some people are still using pagers
that don't let you move backwards in the file.
Even pagers that do let you move backwards often don't work
when viewing "standard input"; that is, data that is being 
piped into the pager from another program.

<DT>Searching and highlighting
<DD>Less lets you search for text in the file you're viewing.
You can search forwards and backwards.
Furthermore, less highlights the text that matches your search,
to make it easy to see where the match is.

<DT>Multiple files
<DD>Less allows you to switch between any number of different
files, remembering your position in each file.
You can also do a single search which spans all the files you are working with.

<DT>Bold and underline
<DD>Some text files, notably Unix-style "man pages", 
use overstriking (backspaces) to simulate underlined and bold text.
Less interprets this overstriking and displays the text in real
underlined or bold mode.

<DT>Advanced features
<DD>Less has a large number of fancy features for advanced users.
Once you learn the basics, you'll find you can customize less to
do almost anything you want it to do.
You can change key bindings, set different tab stops,
set up filters to view compressed data or other file types,
customize the prompt, display line numbers, use "tag" files, and more.

<DT>Portability
<DD>Less is available on many different machines, different 
system architectures, and different terminals.
Pretty much wherever you go, you'll find less.
You won't have to learn how to use a different pager on every system you use.
</DL>

<A NAME="history">
<H3>Where did less come from?</H3>
<P>
Back in 1983, I was working for a now-defunct company
called Integrated Office Systems.
Some of our software ran on a Unix system and produced enormous log files
of its transactions.
We wanted to be able to search these log files for error messages.
But the version of "vi" we were using couldn't handle files larger than
a certain size, and our log files often exceeded that size. 
So we were forced to use "more" to view the files.
The problem was, once we found an error message, what we really wanted
to see was the transactions leading up to the error; that is,
the lines in the log file immediately before the error message.
But more didn't allow backward movement in the file.
</P>
<P>
We often complained about this problem.
We said we needed a "backwards more"; someone (it wasn't me,
but unfortunately I don't remember who it was) coined the name "less"
as a joke on the concept of a "backwards more".
It didn't seem to me that
it would be too difficult to write a simple pager that would allow
forward and backward movement.
I wrote the first version in late 1983 and we began using it internally.
I continued to enhance it and finally decided it was a useful enough
tool that I should make it available to the world.
I posted the first version to the newsgroup called (at that time)
<CODE>net.sources</CODE> in May, 1985.
</P>
<P>
Making the program publicly available allowed an enormous number 
of people to use it.
Many people have made their own modifications and donated them
to me for incorporation into the official version, and
many more have reported bugs they've found or
made suggestions about features they'd like to see added.
This was my first experience with the concept of what is now
called "open source" or "free software",
and it has, I believe, produced a very high quality product.
I'd like to thank all the users of less for their invaluable comments
and suggestions over the years.
Less wouldn't be what it is today without you.
</P>

<A NAME="getit">
<H3>How can I get a copy of less?</H3>
<P>
You can <A HREF="download.html">download</A> a copy of less from here.
</P>
<P>
Less is also distributed as a part of the standard software available
with many varieties of Unix and similar systems.
Less is also available from many ftp sites, including the GNU archives
on <A HREF="ftp://ftp.gnu.org/gnu/less">ftp.gnu.org/gnu/less</A>.
</P>

<A NAME="mail">
<H3>How can I write to the author of less?</H3>
<P>
Less was written by Mark Nudelman 
and is currently being maintained by him.
You can write to him at 
<A HREF="mailto:markn@greenwoodsoftware.com">markn@greenwoodsoftware.com</A>.
</P>

<A NAME="bugs">
<H3>What if I find a bug in less?</H3>
<P>
There is a list of known bugs <A HREF="bugs.html">here</A>.
If you find a bug that is not in the list,
please send <A HREF="#mail">email to the author</A>.
Describe the bug in as much detail as possible,
and I'll do what I can to help resolve the problem.
</P>

<A NAME="tricks">
<H3>Cool less tricks</H3>
<P>
New users of less sometimes find that less doesn't behave exactly the
way they expect.
If this is the case, don't despair.
Less has an enormous number of options (perhaps too many)
which let you change the way it behaves.
Chances are, there is a way to make less do what you want.
A complete description of all these features is in the less man page,
but here's a summary of some of the more popular ones.
</P>
<DL>
<DT>Quit at end-of-file
<DD>To make less automatically quit as soon as it reaches the end of the file
(so you don't have to hit "q"), set the -E option.
<DT>Verbose prompt
<DD>To see a more verbose prompt, set the -m or -M option.
You can also design your own prompt; see the man page for details.
<DT>Clear the whole screen
<DD>To make less clear and repaint the screen rather than scrolling
when you move to a new page of text, set the -C option.
<DT>Case-less searches
<DD>To treat upper-case and lower-case letters the same in searches,
set the -I option.
<DT>Start at a specific place in the file
<DD>To start at a specific line number, say line 150, 
use "<CODE>less +150 filename</CODE>".
To start where a specific pattern first appears,
use "<CODE>less +/pattern filename</CODE>".
To start at the end of the file, use "<CODE>less +G filename</CODE>".
<DT>Scan all instances of a pattern in a set of files
<DD>To search multiple files, use "<CODE>/*pattern</CODE>"
instead of just "<CODE>/pattern</CODE>".
To do this from the command line, use "<CODE>less '+/*pattern' ...</CODE>".
Note that you may need to quote the "<CODE>+/*pattern</CODE>" argument 
to prevent your shell from interpreting the "<CODE>*</CODE>".
<DT>Watch a growing file
<DD>Use the F command to go to the end of the file and keep
displaying more text as the file grows.
You can do this from the command line by using "<CODE>less +F ...</CODE>".
<DT>Change keys
<DD>The <CODE>lesskey</CODE> program lets you change the meaning
of any key or sequence of keys.  See the lesskey man page for details.
<DT>Save your favorite options
<DD>If you want certain options to be in effect whenever you run less, 
without needing to type them in every time,
just set your "<CODE>LESS</CODE>" environment variable to the options you want.
(If you don't know how to set an environment variable,
consult the documentation for your system or your shell.)
For example, if your <CODE>LESS</CODE> environment variable 
is set to "<CODE>-IE</CODE>", every time you run less 
it will do case-less searches and quit at end-of-file.
</DL>

<A NAME="build">
<H3>I can't get less to build!</H3>
<P>
There are a number of possible reasons why less 
may not be building on your system.
If you're building on a Unix system, and you ran "sh configure" once,
you might try removing the "config.log" and "config.cache" files and
trying it all from scratch again; that is, run "sh configure" again.
</P>
<P>
If that doesn't help and you're still stumped, send 
<A HREF="#mail">email to the author</A> describing your problem.
If possible, include the messages produced by the configure 
and/or make procedures.
</P>

<A NAME="unix">
<H3>Do I need Unix to use less?</H3>
<P>
Less was originally developed for Unix, but has been ported
to a variety of other systems, including MS-DOS, Windows, OS/2, and OS-9,
as well as Unix variants like Linux, FreeBSD and Solaris.
See the <A HREF="download.html">download</A> page for a
list of supported systems.
</P>

<A NAME="intl">
<H3>Can less display non-English language characters?</H3>
<P>
Less has two ways to display non-English characters.
If your system uses a non-ASCII single-byte character set,
you should set up your system with the correct "locale" settings.
If your system does not support setlocale, you can set the
<CODE>LESSCHARSET</CODE> or <CODE>LESSCHARDEF</CODE>
environment variable to tell less what language you are using.
See the section "NATIONAL CHARACTER SETS" in the man page for details.
</P>
<P>
If your system supports the UTF-8 encoding of Unicode for
non-ASCII text, as many modern systems do,
you should either set your locale to something that includes 
the string "UTF-8" or "UTF8" (either uppercase or lowercase is ok),
or set <CODE>LESSCHARSET</CODE> to "utf-8".
</P>

<A NAME="wrap">
<H3>Why does less scroll off the screen if there are long lines?</H3>
<P>
If you see data scroll off the screen when you view lines longer
than the screen width, something is wrong.
There are two likely possibilities.
First, you may be using the -r option (either explicitly or in
your LESS environment variable).
The man page states that the -r option may cause display problems
(see <A HREF="#dashr">Why does -r cause display problems?</A> for more details).
The other possibility is more difficult to diagnose: you may have
a mismatch between your termcap or terminfo setup and the type of your terminal.
On most systems, you should check that:
<UL>
<LI>your TERM environment variable matches the type of terminal you are using.
<LI>the environment variables LINES and COLUMNS are correct
for your terminal (if they are set).
<LI>"stty rows" and "stty columns" are correct for your terminal
(if they are non-zero).
</UL>
</P>

<A NAME="optfile">
<H3>How can I view a file whose name looks like a command line option?</H3>
<P>
If a filename begins with a dash or a plus sign, you can't just say
<BR>
<CODE> less -funnyfile </CODE>
<BR>
because less will think the name is a command line option.
The best way to view such a file is to use "--" to mark the end of the
real options (if any):
<BR>
<CODE> less -- -funnyfile </CODE>
<BR>
</P>

<A NAME="dashr">
<H3>Why does -r cause display problems?</H3>
<P>
Less needs to keep track of where the cursor is
on the screen as it displays text, so that it knows when it reaches the
end of the line.  
If the screen is 24 lines high, less wants to display
enough text to exactly fill those 24 lines.  
To do this, it needs to know
when it reaches the end of a line on the screen.  
But if it just starts outputting raw control characters (like ESC), 
it won't be able to keep track of this.  
For example, if less outputs four ordinary characters, it
moves the cursor four spaces to the right.
But on a vt220 terminal, if less outputs
the four characters "ESC [ 1 m", it merely switches the terminal to
"bold", but doesn't move the cursor.  
On the other hand, if it outputs "ESC [ 1 H", 
it moves the cursor to the top left corner of the screen!
Obviously less would need to know what every escape sequence does
to the particular terminal being used, which is not practical. 
So by default less does not output ESC or other control characters,
to avoid this problem.
When -r is set, it does output control characters, but then
it is no longer possible to determine when the text wraps to
a new line on the display,
so the display may not be accurate in all cases.
</P>
<P>
One reason you might be using -r is to "pass through" the escape
sequences that change the color of text.
You can use -R instead of -r to pass through these color escape sequences,
and less will understand that such sequences don't move the cursor.
</P>

<A NAME="dashw">
<H3>Why is the first line on the screen highlighted?</H3>
You probably have the -w flag set in your LESS environment variable
or in your lesskey file.
The meaning of the -w flag changed in version 346.
Previously it turned off the tildes displayed after end-of-file,
now it highlights the first unread line on each new screen.
Use the -~ flag to turn off tildes.

<A NAME="tite">
<H3>Can less leave the screen alone when it quits?</H3>
<P>
When less starts, it sends the "ti" termcap string to the terminal,
and when it quits, it sends the "te" string.
Exactly what the "te" string does depends on the system you are using, 
the type of terminal, and your version of termcap.
On some systems, it does nothing; on others it restores the screen
to the state it was in when less started, thus erasing from the screen
the last page of the file which less was viewing.
<P>
If you don't like the behavior of "te" on your system,
you can disable the sending of the "ti" and "te" strings by 
invoking less with the -X option.
Unfortunately, this sometimes has other side effects, because the
"ti"/"te" strings might do other things that are required.
For example, on some terminals, disabling "ti" and "te" causes arrow
keys to cease to function.
<P>
If you want "te" to do something different (for example, restore
the screen if it's not doing that already), you'll have to figure
out how to change the termcap or terminfo for your terminal on your system.
Unfortunately, this is done differently on different systems,
so you'll have to check the documentation for your system.
</P>

<A NAME="dashe">
<H3>Why doesn't less quit when it reaches the end of the file?</H3>
<P>
Normally, less doesn't quit until you type "q".
You can make it quit as soon as it reaches the end of the file
by using the -E option.
However, many (but not all) people prefer to have less remain active until
they explicitly quit, because they might wish to back up and look
at earlier parts of the file again.
</P>

<A NAME="ctrlc">
<H3>Why doesn't less quit when I type control-C?</H3>
<P>
Control-C on Unix systems is an "interrupt" character; it is supposed
to stop whatever the current program is doing.
Many programs react to control-C by just quitting.
But in many interactive programs, such as less and vi,
control-C stops whatever the program was doing and returns the program
to an "idle" state, where it's just waiting for a command.
For example, in less you can use control-C to stop a long search,
or to stop the F command, without quitting the program.
If you really want to quit, just type "q".
If you really really want control-C to cause less to quit,
you can use the -K option.
</P>

<A NAME="csh">
<H3>Why does it take so long to start the editor / start a shell?</H3>
<P>
If you use csh, you may find it takes a long time to start a shell
(using the "!" command) or to start an editor (using the "v" command).
This is usually because your .cshrc file contains many commands.
Csh executes the commands in .cshrc each time less starts a subshell.
If you only need these commands executed once at login, and not
in every subshell, you can move them to .login instead of .cshrc.
Changing your SHELL environment variable to "sh" can also avoid
this problem, although you will then be using sh rather than csh
in programs that use the SHELL variable.
You can also try setting SHELL to "csh -f", which tells
csh not to execute the .cshrc or .login files.
</P>

<A NAME="csh2">
<H3>Why does it say
"<CODE>stty: TCGETS: Operation not supported on socket</CODE>"?</H3>
<P>
If you see this message and you're using csh, you probably have
a "<CODE>stty</CODE>" command in your .cshrc file.
You should either move this to your .login file so it only gets
executed once, when you login;
or set your SHELL variable to "csh -f", which tells
csh not to execute the .cshrc or .login files.
</P>

<A NAME="profileout">
<H3>Why are there problems if my .cshrc, .profile or ENV files produce output?</H3>
<P>
Like some other programs, less invokes the shell to perform certain
operations, such as expanding special characters in filenames.
It does this by invoking a shell program and reading the program's output.
If the shell itself produces output, this interferes with the
output coming from the program that less is trying to read from.
This problem is not unique to less; for example, if you have your
shell set up to produce output from a .cshrc or ENV file and 
then you try the command "<CODE>:n *.c</CODE>" from within vi,
you'll see a similar problem.
</P>
<P>
If you use the LESSOPEN feature, having your shell produce output
can cause other problems.
Since less uses the shell to invoke the LESSOPEN filter, the
output from the shell will appear at the beginning of the output
from the filter program, causing it to appear to be part of the file.
</P>
<P>
In general, it's not a good idea for a .cshrc or ENV file 
to produce output.
If you use csh, tcsh or a variant, you can move the output commands
to your .login file so they only get
executed once, when you login;
or set your SHELL variable to "csh -f", which tells
csh not to execute the .cshrc or .login files.
If you use sh, ksh, bash or a variant, you can move the output commands
from your ENV file to your .profile file so they only get executed once
when you login.
Unfortunately, some older versions of these shells execute the .profile
every time a shell is invoked rather than only at login.
For such shells, it is best to avoid output in the .profile,
or arrange for the output to appear only at login time in some other way.
</P>

<A NAME="filter">
<H3>Can less view compressed files?</H3>
<P>
Less provides a mechanism to <I>preprocess</I> the file being viewed.
The preprocessing can decompress a compressed file, or do other
arbitrary processing on the file contents.
However, each user must specify their own preprocessor.
See the INPUT PREPROCESSOR section of the man page for more details.
Less does not provide any default preprocessing, because the types
of compressed files likely to be viewed depends on the 
work habits of the particular user.
In a future release of less, some automatic preprocessing may be provided.
</P>

<!-- ================================================================== -->
<A NAME="searchlt">
<H3>Why do I get a segmentation fault if I search for a pattern containing "\&lt;"?</H3>
<P>
Less usually uses the pattern matching library supplied by 
the operating system on which it runs.
This pattern matching library is sometimes called the "regular expresson library".
There is a bug in some regular expression libraries which sometimes causes
the program to crash if you search for a pattern that includes a
backslash followed by a less-than sign.
This bug seems to be in glibc 2.2.2, and possibly other versions.
It is known to exist in several versions of Linux.
Less has a built-in regular expression library which does not have
this bug; if you wish to use it, compile with
<BR>
<CODE>
	configure --with-regex=regcomp-local
</CODE>
</P>

<A NAME="linux269">
<H3>Why doesn't less work with Linux 2.6.9?</H3>
<P>
There appears to be a bug in the Linux 2.6.9 release 
which causes <CODE>/dev/tty</CODE> to be created as a directory
instead of a character file as it should be
(it appears that <CODE>/dev/pty</CODE> was inadvertently renamed 
to <CODE>/dev/tty</CODE>).
This causes less to exit immediately after displaying
the first page of the output.
Reportedly, upgrading to the latest <CODE>udev</CODE> package
will fix this.
</P>

<A NAME="version">
<H3>Why are there no dots in the version number?</H3>
<P>
I use a very simple version numbering scheme.
The first version of less was version 1.
The next was version 2.  And so on.
There are no "major" and "minor" releases, so there
are no dots in the version number.
So version 381 is the three hundred and eighty-first version of less.
It's just 381, not 3.81 or 3.8.1.
</P>

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