Options, Line editing and Language support for the Less command.
Options Most options can be changed while less is running, via the "-" command. Most options can be given in one of two forms: either a dash followed by a single letter, or two dashes followed by a long option name. A long option name can be abbreviated as long as the abbreviation is unambiguous. For example, --quit-at-eof can be abbreviated --quit, but not --qui, since both --quit-at-eof and --quiet begin with --qui. Some long option names are in uppercase, such as --QUIT-AT-EOF, as distinct from --quit-at-eof. Such option names need only have their first letter capitalized; the remainder of the name can be in either case. For example: --Quit-at-eof is equivalent to --QUIT-AT-EOF. Options are also taken from the environment variable "LESS". For exam- ple, to avoid typing "less -options ..." each time less is invoked, you might tell csh: setenv LESS "-options" or if you use sh: LESS="-options"; export LESS On MS-DOS, you don't need the quotes, but you should replace any per- cent signs in the options string by double percent signs. 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 value on the command line by beginning the command line option with "-+". For options like -P or -D which take a following string, a dollar sign ($) must be used to signal the end of the string. For example, to set two -D options on MS-DOS, you must have a dollar sign between them, like this: LESS="-Dn9.1$-Ds4.1" -? or --help This option displays a summary of the commands accepted by less (the same as the h command). (Depending on how your shell interprets the question mark, it can be necessary to quote the question mark, thus: "-\?".) -a or --search-skip-screen 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). -bn or --buffers=n Specifies the amount of buffer space less will use for each file, in units of kilobytes (1024 bytes). By default 64K of buffer space is used for each file (unless the file is a pipe; see the -B option). The -b option specifies instead that n kilobytes of buffer space should be used for each file. If n is -1, buffer space is unlimited; that is, the entire file is read into memory. -B or --auto-buffers By default, when data is read from a pipe, buffers are allocated automatically as needed. If a large amount of data is read from the pipe, this can cause a large amount of memory to be allo- cated. The -B option disables this automatic allocation of buffers for pipes, so that only 64K (or the amount of space specified by the -b option) is used for the pipe. Warning: use of -B can result in erroneou display, since only the most recently viewed part of the file is kept in memory; any earlier data is lost. -c or --clear-screen 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. -C or --CLEAR-SCREEN The -C option is like -c, but the screen is cleared before it is repainted. -d or --dumb 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 less on a dumb terminal. -Dxcolor or --color=xcolor [MS-DOS only] Sets the color of the text displayed. x is a sin- gle character which selects the type of text whose color is being set: n=normal, s=standout, d=bold, u=underlined, k=blink. color is a pair of numbers separated by a period. The first number selects the foreground color and the second selects the background color of the text. A single number N is the same as N.0. -e or --quit-at-eof Causes less to automatically exit the second time it reaches end-of-file. By default, the only way to exit less is via the "q" command. -E or --QUIT-AT-EOF Causes less to automatically exit the first time it reaches end- of-file. -f or --force Forces non-regular files to be opened. (A non-regular file is a directory or a device special file.) Also suppresses the warn- ing message when a binary file is opened. By default, less will refuse to open non-regular files. -F or --quit-if-one-screen Causes less to automatically exit if the entire file can be dis- played on the first screen. -g or --hilite-search Normally, less will highlight ALL strings which match the last search command. The -g option changes this behavior to high- light only the particular string which was found by the last search command. This can cause less to run somewhat faster than the default. -G or --HILITE-SEARCH The -G option suppresses all highlighting of strings found by search commands. -hn or ---max-back-scroll=n Specifies a maximum number of lines to scroll backward. If it is necessary to scroll backward more than n lines, the screen is repainted in a forward direction instead. (If the terminal does not have the ability to scroll backward, -h0 is implied.) -i or --ignore-case Causes searches to ignore case; that is, uppercase and lowercase are considered identical. This option is ignored if any upper- case letters appear in the search pattern; in other words, if a pattern contains uppercase letters, then that search does not ignore case. -I or --IGNORE-CASE Like -i, but searches ignore case even if the pattern contains uppercase letters. -jn or --jump-target=n Specifies a line on the screen where the "target" line is to be positioned. A target line is the object of a text search, tag search, jump to a line number, jump to a file percentage, or jump 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 can 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. -J or --status-column Displays a status column at the left edge of the screen. The status column shows the lines that matched the current search. The status column is also used if the -w or -W option is in effect. -kfilename or --lesskey-file=filename Causes less to open and interpret the named file as a lesskey (1) file. Multiple -k options can be specified. If the LESSKEY or LESSKEY_SYSTEM environment variable is set, or if a lesskey file is found in a standard place (see KEY BINDINGS), it is also used as a lesskey file. -m or --long-prompt Causes less to prompt verbosely (like more), with the percent into the file. By default, less prompts with a colon. -M or --LONG-PROMPT Causes less to prompt even more verbosely than more. -n or --line-numbers Suppresses line numbers. The default (to use line numbers) can cause less to run more slowly in some cases, especially with a very large input file. Suppressing line numbers with the -n option 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). -N or --LINE-NUMBERS Causes a line number to be displayed at the beginning of each line in the display. -ofilename or --log-file=filename Causes 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, less will ask for confirmation before overwriting it. -Ofilename or --LOG-FILE=filename The -O option is like -o, but it will overwrite an existing file without asking for confirmation. If no log file has been specified, the -o and -O options can be used from within less to specify a log file. Without a file name, they will report the name of the log file. The "s" command is equivalent to specifying -o from within less. -ppattern or --pattern=pattern The -p option on the command line is equivalent to specifying +/pattern; that is, it tells less to start at the first occur- rence of pattern in the file. -Pprompt or --prompt=prompt 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 less command. Such an option must either be the last option in the LESS variable, or be terminated by a dollar sign. -Ps followed by a string changes the default (short) prompt to that string. -Pm changes the medium (-m) prompt. -PM changes the long (-M) prompt. -Ph changes the prompt for the help screen. -P= changes the message printed by the = command. -Pw changes the message printed while waiting for data (in the F command). All prompt strings consist of a sequence of letters and special escape sequences. See the section on PROMPTS for more details. -q or --quiet or --silent 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. -Q or --QUIET or --SILENT Causes totally "quiet" operation: the terminal bell is never rung. -r or --raw-control-chars 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 option is used, 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 dis- play problems can result, such as long lines being split in the wrong place. -R or --RAW-CONTROL-CHARS Like -r, but tries to keep track of the screen appearance where possible. This works only if the input consists of normal text and possibly some ANSI "color" escape sequences, which are sequences of the form: ESC [ ... m where the "..." is zero or more characters other than "m". For the purpose of keeping track of screen appearance, all control characters and all ANSI color escape sequences are assumed to not move the cursor. You can make less think that characters other than "m" can end ANSI color escape sequences by setting the environment variable LESSANSIENDCHARS to the list of charac- ters which can end a color escape sequence. -s or --squeeze-blank-lines Causes consecutive blank lines to be squeezed into a single blank line. This is useful when viewing nroff output. -S or --chop-long-lines Causes lines longer than the screen width to be chopped rather than folded. That is, the remainder of a long line is discarded. The default is to fold long lines; that is, display the remainder on the next line. -ttag or --tag=tag The -t option, followed immediately by a TAG, will edit the file containing that tag. For this to work, tag information must be available; for example, there can be a file in the current directory called "tags", which was previously built by ctags (1) or an equivalent command. If the environment variable LESSGLOB- ALTAGS is set, it is taken to be the name of a command compati- ble with global (1), and that command is executed to find the tag. (See http://www.gnu.org/software/global/global.html). The -t option can also be specified from within less (using the - command) as a way of examining a new file. The command ":t" is equivalent to specifying -t from within less. -Ttagsfile or --tag-file=tagsfile Specifies a tags file to be used instead of "tags". -u or --underline-special Causes backspaces and carriage returns to be treated as print- able characters; that is, they are sent to the terminal when they appear in the input. -U or --UNDERLINE-SPECIAL Causes backspaces, tabs and carriage returns to be treated as control characters; that is, they are handled as specified by the -r option. By default, if neither -u nor -U is given, backspaces which appear adjacent to an underscore character are treated spe- cially: 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 bold- face 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. Text which is overstruck or under- lined can be searched for if neither -u nor -U is in effect. -V or --version Displays the version number of less. -w or --hilite-unread Temporarily highlights the first "new" line after a forward movement of a full page. The first "new" line is the line imme- diately following the line previously at the bottom of the screen. Also highlights the target line after a g or p command. The highlight is removed at the next command which causes move- ment. The entire line is highlighted, unless the -J option is in effect, in which case only the status column is highlighted. -W or --HILITE-UNREAD Like -w, but temporarily highlights the first new line after any forward movement command larger than one line. -xn,... or --tabs=n,... Sets tab stops. If only one n is specified, tab stops are set at multiples of n. If multiple values separated by commas are specified, tab stops are set at those positions, and then con- tinue with the same spacing as the last two. For example, -x9,17 will set tabs at positions 9, 17, 25, 33, etc. The default for n is 8. -X or --no-init Disables sending the termcap initialization and deinitialization strings to the terminal. This is sometimes desirable if the deinitialization string does something unnecessary, like clear- ing the screen. --no-keypad Disables sending the keypad initialization and deinitialization strings to the terminal. This is sometimes useful if the keypad strings make the numeric keypad behave in an undesirable manner. -yn or --max-forw-scroll=n Specifies a maximum number of lines to scroll forward. If it is necessary to scroll forward more than n lines, the screen is repainted instead. The -c or -C option can be used to repaint from the top of the screen if desired. By default, any forward movement causes scrolling. -[z]n or --window=n Changes the default scrolling window size to n lines. The default is one screenful. The z and w commands can also be used to change the window size. The "z" can be omitted for compati- bility with more. If the number n is negative, it indicates n lines less than the current screen size. For example, if the screen is 24 lines, -z-4 sets the scrolling window to 20 lines. If the screen is resized to 40 lines, the scrolling window auto- matically changes to 36 lines. -"cc or --quotes=cc Changes the filename quoting character. This can be necessary if you are trying to name a file which contains both spaces and quote characters. Followed by a single character, this changes the quote character to that character. Filenames containing a space should then be surrounded by that character rather than by double quotes. Followed by two characters, changes the open quote to the first character, and the close quote to the second character. Filenames containing a space should then be preceded by the open quote character and followed by the close quote character. Note that even after the quote characters are changed, this option remains -" (a dash followed by a double quote). -~ or --tilde Normally lines after end of file are displayed as a single tilde (~). This option causes lines after end of file to be displayed as blank lines. -# or --shift Specifies the default number of positions to scroll horizontally in the RIGHTARROW and LEFTARROW commands. If the number speci- fied is zero, it sets the default number of positions to one half of the screen width. -- A command line argument of "--" marks the end of option argu- ments. Any arguments following this are interpreted as file- names. This can be useful when viewing a file whose name begins with a "-" or "+". + If a command line option begins with +, the remainder of that option is taken to be an initial command to less. For example, +G tells 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, +
acts like + 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 can also be used to set (or change) an ini- tial command for every file. LINE EDITING When entering command line at the bottom of the screen (for example, a filename for the :e command, or the pattern for a search command), cer- tain keys can be used to manipulate the command line. Most commands have an alternate form in [ brackets ] which can be used if a key does not exist on a particular keyboard. (The bracketed forms do not work in the MS-DOS version.) Any of these special keys can be entered lit- erally by preceding it with the "literal" character, either ^V or ^A. A backslash itself can also be entered literally by entering two back- slashes. LEFTARROW [ ESC-h ] Move the cursor one space to the left. RIGHTARROW [ ESC-l ] Move the cursor one space to the right. ^LEFTARROW [ ESC-b or ESC-LEFTARROW ] (That is, CONTROL and LEFTARROW simultaneously.) Move the cur- sor one word to the left. ^RIGHTARROW [ ESC-w or ESC-RIGHTARROW ] (That is, CONTROL and RIGHTARROW simultaneously.) Move the cur- sor one word to the right. HOME [ ESC-0 ] Move the cursor to the beginning of the line. END [ ESC-$ ] Move the cursor to the end of the line. BACKSPACE Delete the character to the left of the cursor, or cancel the command if the command line is empty. DELETE or [ ESC-x ] Delete the character under the cursor. ^BACKSPACE [ ESC-BACKSPACE ] (That is, CONTROL and BACKSPACE simultaneously.) Delete the word to the left of the cursor. ^DELETE [ ESC-X or ESC-DELETE ] (That is, CONTROL and DELETE simultaneously.) Delete the word under the cursor. UPARROW [ ESC-k ] Retrieve the previous command line. DOWNARROW [ ESC-j ] Retrieve the next command line. TAB Complete the partial filename to the left of the cursor. If it matches more than one filename, the first match is entered into the command line. Repeated TABs will cycle thru the other matching filenames. If the completed filename is a directory, a "/" is appended to the filename. (On MS-DOS systems, a "\" is appended.) The environment variable LESSSEPARATOR can be used to specify a different character to append to a directory name. BACKTAB [ ESC-TAB ] Like, TAB, but cycles in the reverse direction thru the matching filenames. ^L Complete the partial filename to the left of the cursor. If it matches more than one filename, all matches are entered into the command line (if they fit). ^U (Unix and OS/2) or ESC (MS-DOS) Delete the entire command line, or cancel the command if the command line is empty. If you have changed your line-kill char- acter in Unix to something other than ^U, that character is used instead of ^U. KEY BINDINGS You can define your own less commands by using the program lesskey (1) to create a lesskey file. This file specifies a set of command keys and an action associated with each key. You can also use lesskey to change the line-editing keys (see LINE EDITING), and to set environment variables. If the environment variable LESSKEY is set, less uses that as the name of the lesskey file. Otherwise, less looks in a standard place for the lesskey file: On Unix systems, less looks for a lesskey file called "$HOME/.less". On MS-DOS and Windows systems, less looks for a lesskey file called "$HOME/_less", and if it is not found there, then looks for a lesskey file called "_less" in any directory specified in the PATH environment variable. On OS/2 systems, less looks for a lesskey file called "$HOME/less.ini", and if it is not found, then looks for a lesskey file called "less.ini" in any directory specified in the INIT environment variable, and if it not found there, then looks for a lesskey file called "less.ini" in any directory specified in the PATH environment variable. See the lesskey manual page for more details. A system-wide lesskey file can 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 LESSKEY_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/sysless. (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:\_sys- less. On OS/2 systems, the system-wide lesskey file is c:\sysless.ini. INPUT PREPROCESSOR You can 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 sim- ply an executable program (or shell script), which writes the contents of the file to a different file, called the replacement file. The con- tents of the replacement file are then displayed in place of the con- tents 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 standard output. If the input preprocessor does not output a replace- ment filename, less uses the original file, as normal. The input pre- processor is not called when viewing standard input. To set up an input preprocessor, 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 pro- gram, called the input postprocessor, which can perform any desired clean-up action (such as deleting the replacement file created by LESSOPEN). This program receives two command line arguments, the orig- inal 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 command line which will invoke your input postprocessor. It can 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) uncompress -c $1 >/tmp/less.$$ 2>/dev/null if [ -s /tmp/less.$$ ]; then echo /tmp/less.$$ else rm -f /tmp/less.$$ fi ;; esac lessclose.sh: #! /bin/sh rm $2 To use these scripts, put them both where they can be executed and set LESSOPEN="lessopen.sh %s", and LESSCLOSE="lessclose.sh %s %s". More complex LESSOPEN and LESSCLOSE scripts can be written to accept other types of compressed files, and so on. It is also possible to set up an input preprocessor to pipe the file data directly to less, rather than putting the data into a replacement file. This avoids the need to decompress the entire file before start- ing to view it. An input preprocessor that works this way is called an input pipe. An input pipe, instead of writing the name of a replace- ment file on its standard output, writes the entire contents of the replacement file on its standard output. If the input pipe does not write any characters on its standard output, then there is no replace- ment file and less uses the original file, as normal. To use an input pipe, make the first character in the LESSOPEN environment variable a vertical bar (|) to signify that the input preprocessor is an input pipe. For example, on many Unix systems, this script will work like the pre- vious example scripts: lesspipe.sh: #! /bin/sh case "$1" in *.Z) uncompress -c $1 2>/dev/null ;; esac To use this script, put it where it can be executed and set LESSOPEN="|lesspipe.sh %s". When an input pipe is used, a LESSCLOSE postprocessor can be used, but it is usually not necessary since there is no replacement file to clean up. In this case, the replacement file name passed to the LESSCLOSE postprocessor is "-". NATIONAL CHARACTER SETS There are three types of characters in the input file: normal characters can be displayed directly to the screen. control characters should not be displayed directly, but are expected to be found in ordinary text files (such as backspace and tab). binary characters should not be displayed directly and are not expected to be found in text files. A "character set" is a description of which characters are to be considered normal, control, and binary. The LESSCHARSET environment variable can be used to select a character set. Possible values for LESSCHARSET are: ascii BS, TAB, NL, CR, and formfeed are control characters, all chars with values between 32 and 126 are normal, and all others are binary. iso8859 Selects an ISO 8859 character set. This is the same as ASCII, except characters between 160 and 255 are treated as normal characters. latin1 Same as iso8859. latin9 Same as iso8859. dos Selects a character set appropriate for MS-DOS. ebcdic Selects an EBCDIC character set. IBM-1047 Selects an EBCDIC character set used by OS/390 Unix Services. This is the EBCDIC analogue of latin1. You get similar results by setting either LESSCHARSET=IBM-1047 or LC_CTYPE=en_US in your environment. koi8-r Selects a Russian character set. next Selects a character set appropriate for NeXT computers. utf-8 Selects the UTF-8 encoding of the ISO 10646 character set. In special cases, it might be desired to tailor 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 repre- sents one character in the character set. The character "." is used for a normal character, "c" for control, and "b" for binary. A decimal number can 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 char- acter set.) This table shows the value of LESSCHARDEF which is equivalent to each of the possible values for LESSCHARSET: ascii 8bcccbcc18b95.b dos 8bcccbcc12bc5b95.b. ebcdic 5bc6bcc7bcc41b.9b7.9b5.b..8b6.10b6.b9.7b 9.8b8.17b3.3b9.7b9.8b8.6b10.b.b.b. IBM-1047 4cbcbc3b9cbccbccbb4c6bcc5b3cbbc4bc4bccbc 191.b iso8859 8bcccbcc18b95.33b. koi8-r 8bcccbcc18b95.b128. latin1 8bcccbcc18b95.33b. next 8bcccbcc18b95.bb125.bb If neither LESSCHARSET nor LESSCHARDEF is set, but the string "UTF-8" is found in the LC_ALL, LC_TYPE or LANG environment variables, then the default character set is utf-8. If that string is not found, but your system supports the setlocale interface, less will use setlocale to determine the character set. setlocale is controlled by setting the LANG or LC_CTYPE environment variables. Finally, if the setlocale interface is also not available, the default character set is latin1. Control and binary characters are displayed in standout (reverse video). 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 char- acter is displayed as a hex number in angle brackets. This format can be changed by setting the LESSBINFMT environment variable. LESSBINFMT can begin with a "*" and one character to select the display attribute: "*k" is blinking, "*d" is bold, "*u" is underlined, "*s" is standout, and "*n" is normal. If LESSBINFMT does not begin with a "*", normal attribute is assumed. The remainder of LESSBINFMT is a string which can include one printf-style escape sequence (a % followed by x, X, o, d, etc.). For example, if LESSBINFMT is "*u[%x]", binary characters are displayed in underlined hexadecimal surrounded by brackets. The default if no LESSBINFMT is specified is "*s<%X>". 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 ordi- nary user need not understand the details of constructing personalized prompt strings. A percent sign followed by a single character is expanded according to what the following character is: %bX Replaced by the byte offset into the current input file. The b is followed by a single character (shown as X above) which spec- ifies the line whose byte offset is to be used. If the charac- ter 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 bot- tom 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. %B Replaced by the size of the current input file. %c Replaced by the column number of the text appearing in the first column of the screen. %dX Replaced by the page number of a line in the input file. The line to be used is determined by the X, as with the %b option. %D Replaced by the number of pages in the input file, or equiva- lently, the page number of the last line in the input file. %E Replaced by the name of the editor (from the VISUAL environment variable, or the EDITOR environment variable if VISUAL is not defined). See the discussion of the LESSEDIT feature below. %f Replaced by the name of the current input file. %i Replaced by the index of the current file in the list of input files. %lX Replaced by the line number of a line in the input file. The line to be used is determined by the X, as with the %b option. %L Replaced by the line number of the last line in the input file. %m Replaced by the total number of input files. %pX Replaced by the percent into the current input file, based on byte offsets. The line used is determined by the X as with the %b option. %PX Replaced by the percent into the current input file, based on line numbers. The line used is determined by the X as with the %b option. %s Same as %B. %t Causes any trailing spaces to be removed. Usually used at the end of the string, but can appear anywhere. %x Replaced by the name of the next input file in the list. If any item is unknown (for example, the file size if input is a pipe), a question mark is printed instead. 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 evalu- ated. 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) can be: ?a True if any characters have been included in the prompt so far. ?bX True if the byte offset of the specified line is known. ?B True if the size of current input file is known. ?c True if the text is horizontally shifted (%c is not zero). ?dX True if the page number of the specified line is known. ?e True if at end-of-file. ?f True if there is an input filename (that is, if input is not a pipe). ?lX True if the line number of the specified line is known. ?L True if the line number of the last line in the file is known. ?m True if there is more than one input file. ?n True if this is the first prompt in a new input file. ?pX True if the percent into the current input file, based on byte offsets, of the specified line is known. ?PX True if the percent into the current input file, based on line numbers, of the specified line is known. ?s Same as "?B". ?x True if there is a next input file (that is, if the current input file is not the last one). 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 can be included in the prompt literally by preceding it with a backslash. Some examples: ?f%f:Standard input. This prompt prints the filename, if known; otherwise the string "Stan- dard input". ?f%f .?ltLine %lt:?pt%pt\%:?btByte %bt:-... This prompt would print the filename, if known. The filename is fol- lowed 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. ?n?f%f .?m(file %i of %m) ..?e(END) ?x- Next\: %x..%t This prints the filename if this is the first prompt in a file, fol- lowed 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 refer- ence, here are the defaults for the other two prompts (-m and -M respectively). Each is broken into two lines here for readability only. ?n?f%f .?m(file %i of %m) ..?e(END) ?x- Next\: %x.: ?pB%pB\%:byte %bB?s/%s...%t ?f%f .?n?m(file %i of %m) ..?ltlines %lt-%lb?L/%L. : byte %bB?s/%s. .?e(END) ?x- Next\: %x.:?pB%pB\%..%t And here is the default message produced by the = command: ?f%f .?m(file %i of %m) .?ltlines %lt-%lb?L/%L. . byte %bB?s/%s. ?e(END) :?pB%pB\%..%t 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: %E ?lm+%lm. %f 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. SECURITY When the environment variable LESSSECURE is set to 1, less runs in a "secure" mode. This means these features are disabled: ! the shell command | the pipe command :e the examine command. v the editing command s -o log files -k use of lesskey files -t use of tags files metacharacters in filenames, such as * filename completion (TAB, ^L) Less can also be compiled to be permanently in "secure" mode. ENVIRONMENT VARIABLES Environment variables can be specified either in the system environment as usual, or in a lesskey (1) file. If environment variables are defined in more than one place, variables defined in a local lesskey file take precedence over variables defined in the system environment, which take precedence over variables defined in the system-wide lesskey file. COLUMNS Sets the number of columns on the screen. Takes precedence over the number of columns specified by the TERM variable. (But if you have a windowing system which supports TIOCGWINSZ or WIOCGETD, the window system's idea of the screen size takes precedence over the LINES and COLUMNS environment variables.) EDITOR The name of the editor (used for the v command). HOME Name of the user's home directory (used to find a lesskey file on Unix and OS/2 systems). HOMEDRIVE, HOMEPATH Concatenation of the HOMEDRIVE and HOMEPATH environment vari- ables is the name of the user's home directory if the HOME vari- able is not set (only in the Windows version). INIT Name of the user's init directory (used to find a lesskey file on OS/2 systems). LANG Language for determining the character set. LC_CTYPE Language for determining the character set. LESS Options which are passed to less automatically. LESSANSIENDCHARS Characters which are assumed to end an ANSI color escape sequence (default "m"). LESSBINFMT Format for displaying non-printable, non-control characters. LESSCHARDEF Defines a character set. LESSCHARSET Selects a predefined character set. LESSCLOSE Command line to invoke the (optional) input-postprocessor. LESSECHO Name of the lessecho program (default "lessecho"). The lessecho program is needed to expand metacharacters, such as * and ?, in filenames on Unix systems. LESSEDIT Editor prototype string (used for the v command). See discus- sion under PROMPTS. LESSGLOBALTAGS Name of the command used by the -t option to find global tags. Normally should be set to "global" if your system has the global (1) command. If not set, global tags are not used. LESSKEY Name of the default lesskey(1) file. LESSKEY_SYSTEM Name of the default system-wide lesskey(1) file. LESSMETACHARS List of characters which are considered "metacharacters" by the shell. LESSMETAESCAPE Prefix which less will add before each metacharacter in a com- mand sent to the shell. If LESSMETAESCAPE is an empty string, commands containing metacharacters will not be passed to the shell. LESSOPEN Command line to invoke the (optional) input-preprocessor. LESSSECURE Runs less in "secure" mode. See discussion under SECURITY. LESSSEPARATOR String to be appended to a directory name in filename comple- tion. LINES Sets the number of lines on the screen. Takes precedence over the number of lines specified by the TERM variable. (But if you have a windowing system which supports TIOCGWINSZ or WIOCGETD, the window system's idea of the screen size takes precedence over the LINES and COLUMNS environment variables.) PATH User's search path (used to find a lesskey file on MS-DOS and OS/2 systems). SHELL The shell used to execute the ! command, as well as to expand filenames. TERM The type of terminal on which less is being run. VISUAL The name of the editor (used for the v command).
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 vi.
"To teach is not difficult. But you must be careful not to teach dumb things, that's all. You must not teach them fashion, you must teach them how to create" ~ Wolfgang Weingart
Related macOS commands:
echo - Display message on screen.
tee - Redirect output to multiple files.
The less home page (Mark Nudelman).