How does clear command work?

Question

I was recently trying to learn more about how the shell works and was looking at how the clear command works. The executable is located in /usr/bin/clear and it seems to print out a bunch of blank lines (equal to the height of the terminal) and puts the cursor at the top-left of the terminal.

The output of the command is always the same, regardless of the size of the terminal:

$ clear | hexdump -C
00000000  1b 5b 48 1b 5b 32 4a                              |.[H.[2J|
00000007

and can be replicated with the echo having the exact same effect:

$ /bin/echo -e "\x1b\x5b\x48\x1b\x5b\x32\x4a\c"

I was really curious how this output of this command translates to clearing the console.

Answer 1

The output of the clear command is console escape codes. The exact codes required depend on the exact terminal you are using, however most use ANSI control sequences. Here is a good link explaining the various codes - http://www.termsys.demon.co.uk/vtansi.htm. The relevant snippets are:

Cursor Home         <ESC>[{ROW};{COLUMN}H

Sets the cursor position where subsequent text will begin. If no row/column
parameters are provided (ie. <ESC>[H), the cursor will move to the home position,
at the upper left of the screen.

And:

Erase Screen        <ESC>[2J

Erases the screen with the background colour and moves the cursor to home.

Where <ESC> is hex 1B or octal 033. Another way to view the characters is with:

clear | sed -n l

Answer 2

It works by issuing certain ANSI escape sequences. Specifically, these two:

Esc[Line;ColumnH          Cursor Position:
Esc[Line;Columnf            Moves the cursor to the specified position (coordinates). If you do not
                                         specify a position, the cursor moves to the home position at the upper-left
                                         corner of the screen (line 0, column 0).

Esc[2J                              Erase Display:
                                         Clears the screen and moves the cursor to the home position
                                         (line 0, column 0).

This is perhaps easier to understand in the output of od -c:

$ clear | od -c
0000000 033   [   H 033   [   2   J
0000007

033 is Esc, so the output above is simply Esc[H and then Esc[2J.

Answer 3

The output sent by clear(1) depends on your terminal type, defined by $TERM in the shell environment. It does the same thing as the command "tput clear", which is looking up the escape code for the current terminal type and sending that string to standard output.

The terminal receiving the escape code from clear/tput interprets it and executes the command sent, such as clearing the local display. "terminal" means the local console or a terminal session (putty, xterm, etc.), possibly via ssh or telnet.

Answer 4

I'm surprised other answers have not mentioned TERMINFO (or TERMCAP)

Use the man pages Luke

man clear says ...

NAME
       clear - clear the terminal screen

SYNOPSIS
       clear

DESCRIPTION
       clear clears your screen if this is possible.  It looks in the environ-
       ment for the terminal type and then in the terminfo database to  figure
       out how to clear the screen.

TERM

The clear command only uses ANSI escape sequences if your $TERM is set to ANSI or some ANSI-based terminal type such as XTERM.

$ TERM=ansi clear | hexdump -C
00000000  1b 5b 48 1b 5b 4a                                 |.[H.[J|
00000006

$ TERM=wy50 clear | hexdump -C
00000000  1b 2b                                             |.+|
00000002

$ TERM=hurd clear | hexdump -C
00000000  1b 63                                             |.c|
00000002

$ TERM=glasstty clear | hexdump -C
00000000  0c                                                |.|
00000001

$ TERM=vt52 clear | hexdump -C
00000000  1b 48 1b 4a                                       |.H.J|
00000004

$ TERM=hpterm clear | hexdump -C
00000000  1b 26 61 30 79 30 43 1b  4a                       |.&a0y0C.J|
00000009

INFOCMP

You can use infocmp to investigate

$ infocmp -L -1 hpterm | grep clear_screen
        clear_screen=\E&a0y0C\EJ,

TPUT

Or you can use tput to view a capability

$ tput -T hpterm clear | hexdump -C
00000000  1b 26 61 30 79 30 43 1b  4a                       |.&a0y0C.J|
00000009


$ tput -T hpterm reset | hexdump -C
00000000  20 20 20 20 20 20 20 20  1b 31 20 20 20 20 20 20  |        .1      |
00000010  20 20 1b 31 20 20 20 20  20 20 20 20 1b 31 20 20  |  .1        .1  |
00000020  20 20 20 20 20 20 1b 31  20 20 20 20 20 20 20 20  |      .1        |
00000030  1b 31 20 20 20 20 20 20  20 20 1b 31 20 20 20 20  |.1        .1    |
00000040  20 20 20 20 1b 31 20 20  20 20 20 20 20 20 1b 31  |    .1        .1|
00000050  20 20 20 20 20 20 20 20  1b 31 20 20 20 20 20 20  |        .1      |
00000060  20 20 1b 31                                       |  .1|
00000064

Answer 5

In addition to all nice answer above, we can do some strace to see what happen:

$ strace -e trace=write echo -e "\x1b\x5b\x48\x1b\x5b\x32\x4a\c"
write(1, "\33[H\33[2J", 7

$ strace -e trace=write clear
write(1, "\33[H\33[2J", 7

You can see, two command provide the same ANSI escape sequences.

Answer 6

Firstly /bin/echo -e "\x1b\x5b\x48\x1b\x5b\x32\x4a\c" doesn't really clear the screen. You can scroll up to see the previous contents.

Secondly, I opened IRB (which is an interactive Ruby shell), and typed:

p `clear`

Or

p %x(clear)

Or:

require 'open3'
p Open3.capture2('clear')[0]

All the codes should return "\e[3J\e[H\e[2J"

Now open your terminal, type echo -e "\e[3J\e[H\e[2J"

It should clear the screen. These are called ANSI escape sequences:

https://en.wikipedia.org/wiki/ANSI_escape_code

You can use these codes to blink a text (\e[5m), colourize a text: (for i in {0..255} ; do printf "\e[38;5;${i}m${i} " ; done ; echo) and many other things!