php man page for clear

clear(1)						      General Commands Manual							  clear(1)

NAME
       clear - clear the terminal screen

SYNOPSIS
       clear [-Ttype] [-V] [-x]

DESCRIPTION
       clear clears your screen if this is possible, including its scrollback buffer (if the extended "E3" capability is defined).  clear looks in
       the environment for the terminal type given by the environment variable TERM, and then in the terminfo database to determine how  to  clear
       the screen.

       clear  writes  to  the  standard  output.   You can redirect the standard output to a file (which prevents clear from actually clearing the
       screen), and later cat the file to the screen, clearing it at that point.

OPTIONS
       -T type
	    indicates the type of terminal.  Normally this option is unnecessary, because the default is taken from the environment variable TERM.
	    If -T is specified, then the shell variables LINES and COLUMNS will also be ignored.

       -V   reports the version of ncurses which was used in this program, and exits.  The options are as follows:

       -x   do not attempt to clear the terminal's scrollback buffer using the extended "E3" capability.

HISTORY
       A clear command appeared in 2.79BSD dated February 24, 1979.  Later that was provided in Unix 8th edition (1985).

       AT&T  adapted  a  different BSD program (tset) to make a new command (tput), and used this to replace the clear command with a shell script
       which calls tput clear, e.g.,

	   /usr/bin/tput ${1:+-T$1} clear 2> /dev/null
	   exit

       In 1989, when Keith Bostic revised the BSD tput command to make it similar to the AT&T tput, he added a shell script for the clear command:

	   exec tput clear

       The remainder of the script in each case is a copyright notice.

       The ncurses clear command began in 1995 by adapting the original BSD clear command (with terminfo, of course).

       The E3 extension came later:

       o   In June 1999, xterm provided an extension to the standard control sequence for clearing the screen.	Rather than clearing just the vis-
	   ible part of the screen using

	       printf '33[2J'

	   one could clear the scrollback using

	       printf '33[3J'

	   This is documented in XTerm Control Sequences as a feature originating with xterm.

       o   A few other terminal developers adopted the feature, e.g., PuTTY in 2006.

       o   In April 2011, a Red Hat developer submitted a patch to the Linux kernel, modifying its console driver to do the same thing.  The Linux
	   change, part of the 3.0 release, did not mention xterm, although it was cited in the Red Hat bug report  (#683733)  which  led  to  the
	   change.

       o   Again,  a  few  other terminal developers adopted the feature.  But the next relevant step was a change to the clear program in 2013 to
	   incorporate this extension.

       o   In 2013, the E3 extension was overlooked in tput with the "clear" parameter.  That was addressed in 2016 by reorganizing tput to  share
	   its logic with clear and tset.

PORTABILITY
       Neither IEEE Std 1003.1/The Open  Group	Base  Specifications  Issue  7 (POSIX.1-2008) nor X/Open Curses Issue 7 documents tset or reset.

       The  latter  documents tput, which could be used to replace this utility either via a shell script or by an alias (such as a symbolic link)
       to run tput as clear.

SEE ALSO
       tput(1), terminfo(5)

       This describes ncurses version 6.1 (patch 20180127).

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