# Bash 的 clear 命令

clear [-x] [-T terminal-type]

clear(1)                         User commands                        clear(1)

NAME
       clear - clear the terminal screen

SYNOPSIS
       clear [-x] [-T terminal-type]

       clear -V

DESCRIPTION
       clear  clears your terminal's screen and its scrollback buffer, if any.
       clear retrieves the terminal type from the environment  variable  TERM,
       then  consults the terminfo terminal capability database entry for that
       type to determine how to perform these actions.

       The capabilities to clear the screen and scrollback  buffer  are  named
       “clear”  and “E3”, respectively.  The latter is a user-defined capabil‐
       ity, applying an extension mechanism introduced in ncurses 5.0 (1999).

OPTIONS
       clear recognizes the following options.

       -T type  produces instructions suitable for the  terminal  type.   Nor‐
                mally,  this  option is unnecessary, because the terminal type
                is inferred from the environment variable TERM.  If  this  op‐
                tion  is  specified,  clear  ignores the environment variables
                LINES and COLUMNS as well.

       -V       reports the version of ncurses associated  with  this  program
                and exits with a successful status.

       -x       prevents clear from attempting to clear the scrollback buffer.

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

       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.

HISTORY
       A clear command using the termcap database and library appeared in 2BSD
       (1979).  Eighth Edition Unix (1985) later included it.

       The commercial Unix arm of AT&T adapted a different BSD program  (tset)
       to  make  a  new  command,  tput, and replaced the clear program with a
       shell script that called “tput clear”.

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

       In 1989, when Keith Bostic revised the BSD tput command to make it sim‐
       ilar to AT&T's tput, he added a clear shell script as well.

           exec tput clear

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

       In 1995, ncurses's clear began by adapting BSD's original clear command
       to use terminfo.  The E3 extension came later.

       •   In June 1999, xterm provided an extension to the  standard  control
           sequence  for  clearing  the screen.  Rather than clearing just the
           visible part of the screen using

               printf '\033[2J'

           one could clear the scrollback buffer as well by using

               printf '\033[3J'

           instead.  “XTerm Control Sequences” documents this feature as orig‐
           inating with xterm.

       •   A few other terminal emulators adopted it, such as PuTTY in 2006.

       •   In April 2011, a Red Hat developer submitted a patch to  the  Linux
           kernel,  modifying  its console driver to do the same thing.  Docu‐
           mentation of this change, appearing in Linux 3.0, did  not  mention
           xterm,  although  that  program was cited in the Red Hat bug report
           (#683733) motivating the feature.

       •   Subsequently, more terminal developers adopted  the  feature.   The
           next  relevant step was to change the ncurses clear program in 2013
           to incorporate this extension.

       •   In 2013, the E3 capability was not exercised by “tput clear”.  That
           oversight was addressed in 2016 by reorganizing tput to  share  its
           logic with clear and tset.

SEE ALSO
       tput(1), xterm(1), terminfo(5)

ncurses 6.4                       2023-12-16                          clear(1)