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I'm using chez-scheme and I can't find a way to clear the screen completely. (If someone knows a better way than printing I'd be interested in that too but it's not my question here)

From what I can find clearing the screen by ^L (control-L) or giving the clear command (in bash at least) is equivalent to outputting ASCII character 12: Form feed. However, printing this does nothing. If I use (display (integer->char 12)) it just prints a newline. Another way to encode this character is \f (analogous to \n for newline), but in Python print("\f") as well as in Scheme (display "\f") is just a newline.

Is my understanding of the meaning of ASCII 12 just wrong, or are implementations lacking?

Is there any way to clear the screen that should work across languages, analogous to \n for a newline?

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  • 1
    Your basic understanding is correct but the available functionality depends on the precise output system calls used by the Scheme implementation as well as the terminal environment etc. Commented Jun 12, 2016 at 14:18
  • The "terminal environment" caveat is critical. The environment variable TERM tells applications which terminal is in use; to be well behaved and portable, they should look up the operation they want to perform in the termcap/terminfo database for that terminal type, rather than hardcoding an escape sequence. Commented Jun 12, 2016 at 16:58

3 Answers 3

Reset to default
108

If you want to clear the screen, the "ANSI" sequence in a printf

\033[2J

clears the entire screen, e.g.,

printf '\033[2J'

The command-line clear program uses this, along with moving the cursor to the "home" position, again an "ANSI" sequence:

\033[H

The program gets the information from the terminal database. For example, for TERM=vt100, it might see this (using \E as \033):

clear=\E[H\E[J$<50>

(the $<50> indicates padding needed for real VT100s). You might notice that the 2 is absent from this string. That is because the cursor is first moved to the home (upper left) position, and the 2 (entire screen) is not necessary. Eliminating that from the string made VT100s a little faster.

On the other hand, if you just want to reset the terminal, you can use the VT100-style RIS:

\033c

but that has side-effects, besides not being in ECMA-48. These bug reports were for side-effects of \033c:

Further reading:

CSI Ps J  Erase in Display (ED).
            Ps = 0  -> Erase Below (default).
            Ps = 1  -> Erase Above.
            Ps = 2  -> Erase All.
            Ps = 3  -> Erase Saved Lines (xterm).
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8

You can print \033c which resets the terminal:

petite -q <<< '(display "\033c")'

\033 is escape and c is literal c.

I can't give you any information about how widely this is supported.

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5 Comments

What does 033 actually refer to?
@Darklightus \033 is an octal escape sequence for escape, see the ascii table for reference: man ascii
It's not different shells you need to test against, but different terminals. The shell has no role in interpreting terminal directives.
Interesting. I'm not familiar with qsh -- does it claim to be POSIX compliant? Were you using printf?
Based on wikipedia it is based on POSIX and X/Open standards
3

For C# and JavaScript (and many others), you have to use \x1b instead of \033 as follows:

Console.WriteLine("\x1b[2J\x1b[H"); // C#

console.log("\x1b[2J\x1b[H"); // Node.JS

Keep in mind that some poorly implemented pseudoterminals will break if you do this.

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