using 'printf' in a script in terminal window

[Mike McNally]

Well that's why there's such a thing as the "clear" program: it figures out
what sort of terminal you're printing to and what sort of special character
sequence will cause the screen to be cleared, and it sends that out.

When you're doing standard printing out to any sort of "tty" device in
Linux, with any window manager or desktop, the basic facts are the same:
the software behaves pretty much exactly the same way it would have
behaved 25 years ago when it would have been sending character
through an RS-232 device to a *real* VT-100 (or ADM-3A or PE550 or
any of the other zillion types of terminals, each with its own proprietary
set of control codes).

If the console tty emulator interprets the form-feed character to mean
that the screen should be cleared, that's great. The problem is that that's
just one tty emulator out of many.

Also, for what it's worth, using "printf" to echo out a '\f' character doesn't
cause my console screen to clear. It seems to be treated like a linefeed
character.


On Sat, Nov 21, 2009 at 12:13 PM, Jerry <gesbbb at yahoo.com> wrote:
> On Sat, 21 Nov 2009 11:26:09 -0600
> Mike McNally <emmecinque at gmail.com> replied:
>
>
>> On Sat, Nov 21, 2009 at 11:14 AM, Jerry <gesbbb at yahoo.com> wrote:
>> > On Sat, 21 Nov 2009 13:32:48 +0100
>> > Mike Massonnet <mmassonnet at gmail.com> replied:
>> >
>> >> I tested it on Terminal, Konsole, and finally xterm, none of those
>> >> clear the screen. And if xterm isn't doing pretty much nobody does
>> >> it.
>> >>
>> >> Why don't you use `clear'?
>> >
>> > This would appear to be a "by design" feature; although I cannot
>> > fathom why. Using 'clear' is fine as long as the script is not used
>> > via CRON. 'Clear' produces strange output with scripts run via
>> > CRON. I know that there are ways to program around that; however,
>> > it does complicate things. I prefer the "KISS" approach.
>> >
>> > Although it probably would not make any appreciable difference, I
>> > might appeal to the developers to change this behavior.
>> >
>> There's no truly standard character sequence that clears a terminal
>> display, and there never has been. The '\f' code is a "form feed",
>> which is a printer control to feed out paper up to the next page
>> perforation.
>>
>> Most "terminal" programs (xterm et al) emulate old DEC VT-100
>> terminals, which did not clear the screen in response to a form feed
>> character, as far as I know.
>
> I am not familiar with how it worked historically. I do know that there
> is a very real difference between how the printf "\f" command works at
> a console and how it works within Terminal, xterm, etc. If this was
> Microsoft, I might expect the left hand to be ignorant of what the
> right hand is doing. Here, I would expect better.
>
> The bottom line is that this greatly inhibits my ability to fully test
> scripts that I develop.
>
> --
> Jerry
> gesbbb at yahoo.com
>
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