Added all of the MH sources, including RCS files, in

[mmh] / docs / historical / mh-6.8.5 / miscellany / less-177 / less.nro

.TH LESS 1
.SH NAME
less \- opposite of more
.SH SYNOPSIS
.B "less -?"
.br
.B "less [-[+]aBcCdeEfHimMnNqQrsSuUw]"
.br
.B "     [-b \fIbufs\fP] [-h \fIlines\fP] [-j \fIline\fP] [-k \fIkeyfile\fP]"
.br
.B "     [-{oO} \fIlogfile\fP] [-p \fIpattern\fP] [-P \fIprompt\fP] [-t \fItag\fP]"
.br
.B "     [-T \fItagfile\fP] [-x \fItab\fP] [-y \fIlines\fP] [-[z] \fIlines\fP]"
.br
.B "     [+[+]\fIcmd\fP] [\fIfilename\fP]..."

.SH DESCRIPTION
.I Less
is a program similar to 
.I more
(1), but which allows backward movement
in the file as well as forward movement.
Also,
.I less
does not have to read the entire input file before starting,
so with large input files it starts up faster than text editors like
.I vi
(1).
.I Less
uses termcap (or terminfo on some systems),
so it can run on a variety of terminals.
There is even limited support for hardcopy terminals.
(On a hardcopy terminal, lines which should be printed at the top
of the screen are prefixed with an up-arrow.)
.PP
Commands are based on both
.I more
and
.I vi.
Commands may be preceded by a decimal number, 
called N in the descriptions below.
The number is used by some commands, as indicated.

.SH COMMANDS
In the following descriptions, ^X means control-X.
ESC stands for the ESCAPE key; for example ESC-v means the
two character sequence "ESCAPE", then "v".
.IP "h or H"
Help: display a summary of these commands.
If you forget all the other commands, remember this one.
.PP
.IP "SPACE or ^V or f or ^F"
Scroll forward N lines, default one window (see option -z below).
If N is more than the screen size, only the final screenful is displayed.
Warning: some systems use ^V as a special literalization character.
.PP
.IP "z"
Like SPACE, but if N is specified, it becomes the new window size.
.PP
.IP "RETURN or ^N or e or ^E or j or ^J"
Scroll forward N lines, default 1.
The entire N lines are displayed, even if N is more than the screen size.
.PP
.IP "d or ^D"
Scroll forward N lines, default one half of the screen size.
If N is specified, it becomes the new default for 
subsequent d and u commands.
.PP
.IP "b or ^B or ESC-v"
Scroll backward N lines, default one window (see option -z below).
If N is more than the screen size, only the final screenful is displayed.
.PP
.IP "w"
Like ESC-v, but if N is specified, it becomes the new window size.
.PP
.IP "y or ^Y or ^P or k or ^K"
Scroll backward N lines, default 1.
The entire N lines are displayed, even if N is more than the screen size.
Warning: some systems use ^Y as a special job control character.
.PP
.IP "u or ^U"
Scroll backward N lines, default one half of the screen size.
If N is specified, it becomes the new default for 
subsequent d and u commands.
.PP
.IP "r or ^R or ^L"
Repaint the screen.
.PP
.IP R
Repaint the screen, discarding any buffered input.
Useful if the file is changing while it is being viewed.
.PP
.IP "F"
Scroll forward, and keep trying to read when the
end of file is reached.
Normally this command would be used when already at the end of the file.
It is a way to monitor the tail of a file which is growing
while it is being viewed.
(The behavior is similar to the "tail -f" command.)
.PP
.IP "g or < or ESC-<"
Go to line N in the file, default 1 (beginning of file).
(Warning: this may be slow if N is large.)
.PP
.IP "G or > or ESC->"
Go to line N in the file, default the end of the file.
(Warning: this may be slow if N is large,
or if N is not specified and
standard input, rather than a file, is being read.)
.PP
.IP "p or %"
Go to a position N percent into the file.
N should be between 0 and 100.
(This works if standard input is being read, but only if
.I less
has already read to the end of the file.
It is always fast, but not always useful.)
.PP
.IP "{"
If a left curly bracket appears in the top line displayed
on the screen,
the { command will go to the matching right curly bracket.
The matching right curly bracket is positioned on the bottom
line of the screen.
If there is more than one left curly bracket on the top line,
a number N may be used to specify the N-th bracket on the line.
.PP
.IP "}"
If a right curly bracket appears in the bottom line displayed
on the screen,
the } command will go to the matching left curly bracket.
The matching left curly bracket is positioned on the top
line of the screen.
If there is more than one right curly bracket on the top line,
a number N may be used to specify the N-th bracket on the line.
.PP
.IP "("
Like {, but applies to parentheses rather than curly brackets.
.PP
.IP ")"
Like }, but applies to parentheses rather than curly brackets.
.PP
.IP "["
Like {, but applies to square brackets rather than curly brackets.
.PP
.IP "]"
Like }, but applies to square brackets rather than curly brackets.
.PP
.IP "ESC-^F"
Followed by two characters,
acts like {, but uses the two characters as open and close brackets,
respectively.
For example, "ESC ^F < >" could be used to 
go forward to the > which matches the < in the top displayed line.
.IP "ESC-^B"
Followed by two characters,
acts like }, but uses the two characters as open and close brackets,
respectively.
For example, "ESC ^B < >" could be used to 
go backward to the < which matches the > in the bottom displayed line.
.IP m
Followed by any lowercase letter, 
marks the current position with that letter.
.PP
.IP "'"
(Single quote.)
Followed by any lowercase letter, returns to the position which
was previously marked with that letter.
Followed by another single quote, returns to the position at
which the last "large" movement command was executed.
Followed by a ^ or $, jumps to the beginning or end of the
file respectively.
Marks are preserved when a new file is examined,
so the ' command can be used to switch between input files.
.PP
.IP "^X^X"
Same as single quote.
.PP
.IP /pattern
Search forward in the file for the N-th line containing the pattern.
N defaults to 1.
The pattern is a regular expression, as recognized by
.I ed.
The search starts at the second line displayed
(but see the -a and -j options, which change this).
.sp
Certain characters are special
if entered at the beginning of the pattern;
they modify the type of search rather than become part of the pattern:
.RS
.IP !
Search for lines which do NOT match the pattern.
.IP *
Search multiple files.
That is, if the search reaches the end of the current file 
without finding a match,
the search continues in the next file in the command line list.
.IP @
Begin the search at the first line of the first file
in the command line list,
regardless of what is currently displayed on the screen
or the settings of the -a or -j options.
.RE
.PP
.IP ?pattern
Search backward in the file for the N-th line containing the pattern.
The search starts at the line immediately before the top line displayed.
.sp
Certain characters are special as in the / command:
.RS
.IP !
Search for lines which do NOT match the pattern.
.IP *
Search multiple files.
That is, if the search reaches the beginning of the current file 
without finding a match,
the search continues in the previous file in the command line list.
.IP @
Begin the search at the last line of the last file
in the command line list,
regardless of what is currently displayed on the screen
or the settings of the -a or -j options.
.RE
.PP
.IP "ESC-/pattern"
Same as "/*".
.PP
.IP "ESC-?pattern"
Same as "?*".
.PP
.IP n
Repeat previous search, for N-th line containing the last pattern.
If the previous search was modified by !, the search is made for the
N-th line NOT containing the pattern.
If the previous search was modified by *, the search continues
in the next (or previous) file if not satisfied in the current file.
There is no effect if the previous search was modified by @.
.PP
.IP N
Repeat previous search, but in the reverse direction.
.PP
.IP "ESC-n"
Repeat previous search, but crossing file boundaries.
The effect is as if the previous search were modified by *.
.PP
.IP "ESC-N"
Repeat previous search, but in the reverse direction
and crossing file boundaries.
.PP
.IP ":e [filename]"
Examine a new file.
If the filename is missing, the "current" file (see the :n and :p commands
below) from the list of files in the command line is re-examined.
A percent sign (%) in the filename is replaced by the name of the
current file.  
A pound sign (#) is replaced by the name of the previously examined file.
The filename is inserted into the command line list of files
so that it can be seen by subsequent :n and :p commands.
If the filename consists of several files, they are all inserted into
the list of files and the first one is examined.
.PP
.IP "^X^V or E"
Same as :e.
Warning: some systems use ^V as a special literalization character.
.PP
.IP ":n"
Examine the next file (from the list of files given in the command line).
If a number N is specified, the N-th next file is examined.
.PP
.IP ":p"
Examine the previous file in the command line list.
If a number N is specified, the N-th previous file is examined.
.PP
.IP ":x"
Examine the first file in the command line list.
If a number N is specified, the N-th file in the list is examined.
.PP
.IP "= or ^G or :f"
Prints some information about the file being viewed,
including its name
and the line number and byte offset of the bottom line being displayed.
If possible, it also prints the length of the file,
the number of lines in the file
and the percent of the file above the last displayed line.
.PP
.IP \-
Followed by one of the command line option letters (see below),
this will change the setting of that option
and print a message describing the new setting.
If the option letter has a numeric value (such as -b or -h),
or a string value (such as -P or -t),
a new value may be entered after the option letter.
If no new value is entered, a message describing
the current setting is printed and nothing is changed.
.PP
.IP \-+
Followed by one of the command line option letters (see below),
this will reset the option to its default setting
and print a message describing the new setting.
(The "\-+\fIX\fP" command does the same thing
as "\-+\fIX\fP" on the command line.)
This does not work for string-valued options.
.PP
.IP \-\-
Followed by one of the command line option letters (see below),
this will reset the option to the "opposite" of its default setting
and print a message describing the new setting.
(The "\-\-\fIX\fP" command does the same thing
as "\-\fIX\fP" on the command line.)
This does not work for numeric or string-valued options.
.PP
.IP _
(Underscore.)
Followed by one of the command line option letters (see below),
this will print a message describing the current setting of that option.
The setting of the option is not changed.
.PP
.IP +cmd
Causes the specified cmd to be executed each time a new file is examined.
For example, +G causes 
.I less
to initially display each file starting at the end 
rather than the beginning.
.PP
.IP V
Prints the version number of 
.I less 
being run.
.PP
.IP "q or :q or :Q or ZZ or ESC ESC"
Exits
.I less.
.PP
The following 
three
commands may or may not be valid, depending on your particular installation.
.PP
.IP v
Invokes an editor to edit the current file being viewed.
The editor is taken from the environment variable EDITOR,
or defaults to "vi".
See also the discussion of LESSEDIT under the section on PROMPTS below.
.PP
.IP "! shell-command"
Invokes a shell to run the shell-command given.
A percent sign (%) in the command is replaced by the name of the
current file.  
A pound sign (#) is replaced by the name of the previously examined file.
"!!" repeats the last shell command.
"!" with no shell command simply invokes a shell.
In all cases, the shell is taken from the environment variable SHELL,
or defaults to "sh".
.PP
.IP "| <m> shell-command"
<m> represents any mark letter.
Pipes a section of the input file to the given shell command.
The section of the file to be piped is between the first line on
the current screen and the position marked by the letter.
<m> may also be ^ or $ to indicate beginning or end of file respectively.
If <m> is . or newline, the current screen is piped.
.PP
.SH OPTIONS
Command line options are described below.
Most options may be changed while
.I less 
is running, via the "\-" command.
.PP
Options are also taken from the environment variable "LESS".
For example, 
to avoid typing "less -options ..." each time 
.I less 
is invoked, you might tell 
.I csh:
.sp
setenv LESS "-options"
.sp
or if you use 
.I sh:
.sp
LESS="-options"; export LESS
.sp
The environment variable is parsed before the command line,
so command line options override the LESS environment variable.
If an option appears in the LESS variable, it can be reset
to its default on the command line by beginning the command
line option with "-+".
.sp
A dollar sign ($) may be used to signal the end of an option string.
This is important only for options like -P which take a
following string.
.IP -?
This option displays a summary of the commands accepted by
.I less
(the same as the h command).
If this option is given, all other options are ignored, and
.I less
exits after the help screen is viewed.
(Depending on how your shell interprets the question mark,
it may be necessary to quote the question mark, thus: "-\\?".)
.IP -a
Causes searches to start after the last line
displayed on the screen, 
thus skipping all lines displayed on the screen.
By default, searches start at the second line on the screen
(or after the last found line; see the -j option).
.IP -b\fIn\fP
Causes
.I less
to use a non-standard number of buffers.
Buffers are 1K, and by default 10 buffers are used
(except if data in coming from standard input; see the -B option).
The number \fIn\fP specifies a different number of buffers to use.
.IP -B
Disables automatic allocation of buffers,
so that only the default number of buffers are used.
If more data is read than will fit in the buffers, the oldest
data is discarded.
By default, when data is coming from standard input,
buffers are allocated automatically as needed
to avoid loss of data.
.IP -c
Causes full screen repaints to be painted from the top line down.
By default,
full screen repaints are done by scrolling from the bottom of the screen.
.IP -C
The -C option is like -c, but the screen is cleared before it is repainted.
.IP -d
The -d option suppresses the error message
normally displayed if the terminal is dumb;
that is, lacks some important capability,
such as the ability to clear the screen or scroll backward.
The -d option does not otherwise change the behavior of
.I less
on a dumb terminal).
.IP -e
Causes 
.I less 
to automatically exit
the second time it reaches end-of-file.
By default, the only way to exit 
.I less
is via the "q" command.
.IP -E
Causes 
.I less
to automatically exit the first time it reaches end-of-file.
.IP -f
Forces non-regular files to be opened.
(A non-regular file is a directory or a device special file.)
Also suppresses the warning message when a binary file is opened.
By default,
.I less
will refuse to open non-regular files.
.IP -h\fIn\fP
Specifies a maximum number of lines to scroll backward.
If it is necessary to scroll backward more than \fIn\fP lines,
the screen is repainted in a forward direction instead.
(If the terminal does not have the ability to scroll
backward, -h0 is implied.)
.IP -i
Causes searches to ignore case; that is,
uppercase and lowercase are considered identical.
Also, text which is overstruck or underlined can be searched for.
This option is ignored if any uppercase letters
appear in the search pattern.
.IP -j\fIn\fP
Specifies a line on the screen where "target" lines
are to be positioned.
Target lines are the object of text searches, 
tag searches, jumps to a line number,
jumps to a file percentage, and jumps to a marked position.
The screen line is specified by a number: the top line on the screen
is 1, the next is 2, and so on.
The number may be negative to specify a line relative to the bottom
of the screen: the bottom line on the screen is -1, the second
to the bottom is -2, and so on.
If the -j option is used, searches begin at the line immediately
after the target line.
For example, if "-j4" is used, the target line is the
fourth line on the screen, so searches begin at the fifth line
on the screen.
.IP -k\fIfilename\fP
Causes
.I less
to open and interpret the named file as a
.I lesskey
(1) file.
Multiple -k options may be specified.
If a file called .less exists in the user's home directory, this
file is also used as a
.I lesskey
file.
.IP -m
Causes 
.I less
to prompt verbosely (like \fImore\fP),
with the percent into the file.
By default,
.I less
prompts with a colon.
.IP -M
Causes 
.I less
to prompt even more verbosely than 
.I more.
.IP -n
Suppresses line numbers.
The default (to use line numbers) may cause
.I less
to run more slowly in some cases, especially with a very large input file.
Suppressing line numbers with the -n flag will avoid this problem.
Using line numbers means: the line number will be displayed in the verbose
prompt and in the = command,
and the v command will pass the current line number to the editor
(see also the discussion of LESSEDIT in PROMPTS below).
.IP -N
Causes a line number to be displayed at the beginning of
each line in the display.
.IP -o\fIfilename\fP
Causes
.I less
to copy its input to the named file as it is being viewed.
This applies only when the input file is a pipe,
not an ordinary file.
If the file already exists, 
.I less
will ask for confirmation before overwriting it.
.IP -O\fIfilename\fP
The -O option is like -o, but it will overwrite an existing
file without asking for confirmation.
.sp
If no log file has been specified,
the -o and -O options can be used from within 
.I less
to specify a log file.
Without a file name, they will simply report the name of the log file.
The "s" command is equivalent to specifying -o from within
.I less.
.IP -p\fIpattern\fP
The -p option on the command line is equivalent to 
specifying +/\fIpattern\fP;
that is, it tells
.I less
to start at the first occurence of \fIpattern\fP in the file.
.IP -P\fIprompt\fP
Provides a way to tailor the three prompt
styles to your own preference.
This option would normally be put in the LESS environment
variable, rather than being typed in with each 
.I less
command.
Such an option must either be the last option in the LESS variable,
or be terminated by a dollar sign.
-P followed by a string changes the default (short) prompt to that string.
-Pm changes the medium (-m) prompt to the string, and
-PM changes the long (-M) prompt.
Also, -P= changes the message printed by the = command to the given string.
All prompt strings consist of a sequence of 
letters and special escape sequences.
See the section on PROMPTS for more details.
.IP -q
Causes moderately "quiet" operation:
the terminal bell is not rung 
if an attempt is made to scroll past the end of the file
or before the beginning of the file.
If the terminal has a "visual bell", it is used instead.
The bell will be rung on certain other errors,
such as typing an invalid character.
The default is to ring the terminal bell in all such cases.
.IP -Q
Causes totally "quiet" operation:
the terminal bell is never rung.
.IP -r
Causes "raw" control characters to be displayed.
The default is to display control characters using the caret notation;
for example, a control-A (octal 001) is displayed as "^A".
Warning: when the -r flag is used,
.I less
cannot keep track of the actual appearance of the screen
(since this depends on how the screen responds to
each type of control character).
Thus, various display problems may result,
such as long lines being split in the wrong place.
.IP -s
Causes consecutive blank lines to be squeezed into a single blank line.
This is useful when viewing
.I nroff
output.
.IP -S
Causes lines longer than the screen width to be
chopped rather than folded.
That is, the remainder of a long line is simply discarded.
The default is to fold long lines; that is, display the remainder
on the next line.
.IP -t\fItag\fP
The -t option, followed immediately by a TAG,
will edit the file containing that tag.
For this to work, there must be a file called "tags" in the
current directory, which was previously built by the 
.I ctags
(1) command.
This option may also be specified from within 
.I less 
(using the \- command) as a way of examining a new file.
The command ":t" is equivalent to specifying -t from within
.I less.
.IP -T\fItagsfile\fP
Specifies a tags file to be used instead of "tags".
.IP -u
Causes backspaces and carriage returns to be treated as printable characters;
that is, they are sent to the terminal when they appear in the input.
.IP -U
Causes backspaces and carriage returns to be treated as control characters;
that is, they are handled as specified by the -r option.
.sp
By default, if neither -u nor -U is given,
backspaces which appear adjacent to an underscore character
are treated specially:
the underlined text is displayed 
using the terminal's hardware underlining capability.
Also, backspaces which appear between two identical characters
are treated specially: 
the overstruck text is printed 
using the terminal's hardware boldface capability.
Other backspaces are deleted, along with the preceding character.
Carriage returns immediately followed by a newline are deleted.
Other carriage returns are handled as specified by the -r option.
.IP -w
Causes blank lines to be used to represent lines
past the end of the file.
By default,
a tilde character is used.
.IP -x\fIn\fP
Sets tab stops every \fIn\fP positions.
The default for \fIn\fP is 8.
.IP -y\fIn\fP
Specifies a maximum number of lines to scroll forward.
If it is necessary to scroll forward more than \fIn\fP lines,
the screen is repainted instead.
The -c or -C option may be used to repaint from the top of
the screen if desired.
By default, any forward movement causes scrolling.
.IP -[z]\fIn\fP
Changes the default scrolling window size to \fIn\fP lines.
The default is one screenful.
The z and w commands can also be used to change the window size.
The "z" may be omitted, as in "-\fIn\fP" for compatibility with
.I more.
.IP +
If a command line option begins with \fB+\fP,
the remainder of that option is taken to be an initial command to
.I less.
For example, +G tells
.I less
to start at the end of the file rather than the beginning,
and +/xyz tells it to start at the first occurrence of "xyz" in the file.
As a special case, +<number> acts like +<number>g; 
that is, it starts the display at the specified line number
(however, see the caveat under the "g" command above).
If the option starts with ++, the initial command applies to
every file being viewed, not just the first one.
The + command described previously
may also be used to set (or change) an initial command for every file.

.SH "KEY BINDINGS"
You may define your own 
.I less
commands by using the program 
.I lesskey
(1)
to create a file called ".less" in your home directory.
This file specifies a set of command keys and an action
associated with each key.
See the
.I lesskey
manual page for more details.

.SH "NATIONAL CHARACTER SETS"
There are three types of characters in the input file:
.IP "normal characters"
can be displayed directly to the screen.
.IP "control characters"
should not be displayed directly, but are expected to be found
in ordinary text files (such as backspace and tab).
.IP "binary characters"
cannot be displayed directly and are not expected to be found
in text files.
.PP
By default, 
.I less
uses the ASCII character set.
In the ASCII character set, characters
with values between 128 and 255 are treated as binary.
The LESSCHARSET environment variable may be used to select
another character set.
If it is set to the value "latin1",
the ISO 8859/1 character set is assumed.
Latin-1 is the same as ASCII, except characters between 128 and 255 are
treated as normal characters.
The only valid values for LESSCHARSET currently are "ascii" and "latin1".
.PP
In special cases, it may be desired to tailor
.I less
to use a character set other than the ones definable by LESSCHARSET.
In this case, the environment variable LESSCHARDEF can be used
to define a character set.
It should be set to a string where each character in the string represents
one character in the character set.
The character "." is used for a normal character, "c" for control,
and "b" for binary.
A decimal number may be used for repetition.
For example, "bccc4b." would mean character 0 is binary,
1, 2 and 3 are control, 4, 5, 6 and 7 are binary, and 8 is normal.
All characters after the last are taken to be the same as the last,
so characters 9 through 255 would be normal.
(This is an example, and does not necessarily 
represent any real character set.)
.PP
Setting LESSCHARDEF to "8bcccbcc18b95.b" is the same as setting
LESSCHARSET to "ascii".
Setting LESSCHARDEF to "8bcccbcc18b95.33b." is the same as setting
LESSCHARSET to "latin1".
.PP
Control and binary characters are displayed in blinking mode.
Each such character is displayed in caret notation if possible
(e.g. ^A for control-A).  Caret notation is used only if 
inverting the 0100 bit results in a normal printable character.
Otherwise, the character is displayed as an octal number preceded
by a backslash.
This octal format can be changed by 
setting the LESSBINFMT environment variable
to a printf-style format string; the default is '\\%o'.
The blinking mode display of control and binary characters can
be changed or disabled by preceding the LESSBINFMT format 
string with a "*" and one character to select the mode:
"*k" is blinking, "*d" is bold, "*u" is underlined,
and "*n" is normal (no special display attribute).
For example, if LESSBINFMT is "*u[%x]", binary characters
are displayed in underlined hexadecimal surrounded by brackets.

.SH "PROMPTS"
The -P option allows you to tailor the prompt to your preference.
The string given to the -P option replaces the specified prompt string.
Certain characters in the string are interpreted specially.
The prompt mechanism is rather complicated to provide flexibility,
but the ordinary user need not understand the details of constructing
personalized prompt strings.
.sp
A percent sign followed by a single character is expanded
according to what the following character is:
.IP "%b\fIX\fP"
Replaced by the byte offset into the current input file.
The b is followed by a single character (shown as \fIX\fP above)
which specifies the line whose byte offset is to be used.
If the character is a "t", the byte offset of the top line in the
display is used,
an "m" means use the middle line,
a "b" means use the bottom line,
a "B" means use the line just after the bottom line,
and a "j" means use the "target" line, as specified by the -j option.
.IP "%B"
Replaced by the size of the current input file.
.IP "%E"
Replaced by the name of the editor (from the EDITOR environment variable).
See the discussion of the LESSEDIT feature below.
.IP "%f"
Replaced by the name of the current input file.
.IP "%i"
Replaced by the index of the current file in the list of
input files.
.IP "%l\fIX\fP"
Replaced by the line number of a line in the input file.
The line to be used is determined by the \fIX\fP, as with the %b option.
.IP "%L"
Replaced by the line number of the last line in the input file.
.IP "%m"
Replaced by the total number of input files.
.IP "%p\fIX\fP"
Replaced by the percent into the current input file.
The line used is determined by the \fIX\fP as with the %b option.
.IP "%s"
Same as %B.
.IP "%t"
Causes any trailing spaces to be removed.
Usually used at the end of the string, but may appear anywhere.
.IP "%x"
Replaced by the name of the next input file in the list.
.PP
If any item is unknown (for example, the file size if input
is a pipe), a question mark is printed instead.
.PP
The format of the prompt string can be changed
depending on certain conditions.
A question mark followed by a single character acts like an "IF":
depending on the following character, a condition is evaluated.
If the condition is true, any characters following the question mark
and condition character, up to a period, are included in the prompt.
If the condition is false, such characters are not included.
A colon appearing between the question mark and the
period can be used to establish an "ELSE": any characters between
the colon and the period are included in the string if and only if
the IF condition is false.
Condition characters (which follow a question mark) may be:
.IP "?a"
True if any characters have been included in the prompt so far.
.IP "?b\fIX\fP"
True if the byte offset of the specified line is known.
.IP "?B"
True if the size of current input file is known.
.IP "?e"
True if at end-of-file.
.IP "?f"
True if there is an input filename
(that is, if input is not a pipe).
.IP "?l\fIX\fP"
True if the line number of the specified line is known.
.IP "?L"
True if the line number of the last line in the file is known.
.IP "?m"
True if there is more than one input file.
.IP "?n"
True if this is the first prompt in a new input file.
.IP "?p\fIX\fP"
True if the percent into the current input file
of the specified line is known.
.IP "?s"
Same as "?B".
.IP "?x"
True if there is a next input file
(that is, if the current input file is not the last one).
.PP
Any characters other than the special ones
(question mark, colon, period, percent, and backslash)
become literally part of the prompt.
Any of the special characters may be included in the prompt literally
by preceding it with a backslash.
.PP
Some examples:
.sp
?f%f:Standard input.
.sp
This prompt prints the filename, if known;
otherwise the string "Standard input".
.sp
?f%f .?ltLine %lt:?pt%pt\\%:?btByte %bt:-...
.sp
This prompt would print the filename, if known.
The filename is followed by the line number, if known,
otherwise the percent if known, otherwise the byte offset if known.
Otherwise, a dash is printed.
Notice how each question mark has a matching period,
and how the % after the %pt
is included literally by escaping it with a backslash.
.sp
?n?f%f\ .?m(file\ %i\ of\ %m)\ ..?e(END)\ ?x-\ Next\\:\ %x..%t
.sp
This prints the filename if this is the first prompt in a file,
followed by the "file N of N" message if there is more
than one input file.
Then, if we are at end-of-file, the string "(END)" is printed
followed by the name of the next file, if there is one.
Finally, any trailing spaces are truncated.
This is the default prompt.
For reference, here are the defaults for
the other two prompts (-m and -M respectively).
Each is broken into two lines here for readability only.
.nf
.sp
?n?f%f\ .?m(file\ %i\ of\ %m)\ ..?e(END)\ ?x-\ Next\\:\ %x.:
        ?pB%pB\\%:byte\ %bB?s/%s...%t
.sp
?f%f\ .?n?m(file\ %i\ of\ %m)\ ..?ltline\ %lt?L/%L.\ :byte\ %bB?s/%s.\ .
        ?e(END)\ ?x-\ Next\\:\ %x.:?pB%pB\\%..%t
.sp
.fi
And here is the default message produced by the = command:
.nf
.sp
?f%f\ .?m(file\ %i\ of\ %m)\ .?ltline\ %lt?L/%L.\ .
        byte\ %bB?s/%s.\ ?e(END)\ :?pB%pB\\%..%t
.fi
.PP
The prompt expansion features are also used for another purpose:
if an environment variable LESSEDIT is defined, it is used
as the command to be executed when the v command is invoked.
The LESSEDIT string is expanded in the same way as the prompt strings.
The default value for LESSEDIT is:
.nf
.sp
        %E\ ?lm+%lm.\ %f
.sp
.fi
Note that this expands to the editor name, followed by a + and the
line number, followed by the file name.
If your editor does not accept the "+linenumber" syntax, or has other
differences in invocation syntax, the LESSEDIT variable can be 
changed to modify this default.

.SH "ENVIRONMENT VARIABLES"
.IP COLUMNS
Sets the number of columns on the screen.
Takes precedence over the number of columns specified by the TERM variable.
.IP EDITOR
The name of the editor (used for the v command).
.IP HOME
Name of the user's home directory (used to find a .less file).
.IP LESS
Flags which are passed to 
.I less
automatically.
.IP LESSBINFMT
Format for displaying non-printable, non-control characters.
.IP LESSCHARDEF
Defines a character set.
.IP LESSCHARSET
Selects a predefined character set.
.IP LESSEDIT
Editor prototype string (used for the v command).
See discussion under PROMPTS.
.IP LESSHELP
Name of the help file.
.IP LINES
Sets the number of lines on the screen.
Takes precedence over the number of lines specified by the TERM variable.
.IP SHELL
The shell used to execute the ! command, as well as to expand filenames.
.IP TERM
The type of terminal on which
.I less
is being run.

.SH "SEE ALSO"
lesskey(1)

.SH WARNINGS
The = command and prompts (unless changed by -P)
report the line number of the line at the top of the screen,
but the byte and percent of the line at the bottom of the screen.
.PP
If the :e command is used to name more than one file,
and one of the named files has been viewed previously,
the new files may be entered into the list in an unexpected order.
.PP
The handling of national character sets is nonstandard as well as
insufficient for multibyte characters.
It will probably change in a later release.