Redraw screen in terminal

Question

How do some programs edit whats being displayed on the terminal (to pick a random example, the program 'sl')? I'm thinking of the Linux terminal here, it may happen in other OS's too, I don't know. I've always thought once some text was displayed, it stayed there. How do you change it without redrawing the entire screen?

Answer 1

Depending on the terminal you send control seuqences. Common sequences are for example esc[;H to send the cursor to a specific position (e.g. on Ansi, Xterm, Linux, VT100). However, this will vary with the type or terminal the user has ... curses (in conjunction with the terminfo files) will wrap that information for you.

Answer 2

try this shellscript

#!/bin/bash
i=1
while [ true ]
    do
            echo -e -n "\r $i"
            i=$((i+1))
    done

the -n options prevents the newline ... and the \r does the carriage return ... you write again and again into the same line - no scroling or what so ever

Answer 3

Many applications make use of the curses library, or some language binding to it.

For rewriting on a single line, such as updating progress information, the special character "carriage return", often specified by the escape sequence "\r", can return the cursor to the start of the current line allowing subsequent output to overwrite what was previously written there.

Answer 4

If you terminate a line sent to the terminal with a carriage return ('\r') instead of a linefeed ('\n'), it will move the cursor to the beginning of the current line, allowing the program to print more text over top of what it printed before. I use this occasionally for progress messages for long tasks.

If you ever need to do more terminal editing than that, use ncurses or a variant thereof.