Date: Fri, 21 Aug 1998 15:48:37 -0500 From: "Brian C. Grayson" <bgrayson@marvin.ece.utexas.edu> To: Roman Katsnelson <romank@graphnet.com>, "q's" <freebsd-questions@FreeBSD.ORG> Subject: Re: "clear" curiosity Message-ID: <19980821154837.A10756@marvin.ece.utexas.edu> In-Reply-To: <35DDC102.CE22AD57@graphnet.com>; from Roman Katsnelson on Fri, Aug 21, 1998 at 02:48:34PM -0400 References: <35DDC102.CE22AD57@graphnet.com>
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On Fri, Aug 21, 1998 at 02:48:34PM -0400, Roman Katsnelson wrote:
> Hi,
>
> I did 'cat clear' recently, and saw that all it said was
>
> exec tput clear
>
> I noticed that when I just type that in at the command line, the result
> is entirely different -- it logs me out, clears the screen and gives a
> new login prompt. I like this a lot better than the regular "exit" or
> "Ctrl-D" thing because it clears the screen first. These are my two
> questions:
>
> 1) Why are the results different between the same commands in a shell
> script and at the command line?
Look at execve(2) and exec(3) for info, but here's a
slightly-accurate summary:
The leading `exec' means, ``take the current process,
and _replace_ it with tput.'' So if you type `exec ...' at the
command line, the shell will no longer be running. To see
this in action, do the following:
Pop up two xterms. In one of them, type "echo $$" to get the
shell PID, which I'll call <PID>.
In the other xterm, do "ps <PID>", and it will print your shell info.
Now type "exec sleep 15" in the first xterm.
Quickly do "ps <PID>" in the other xterm -- the PID is no
longer your shell, it is the sleep process. Magic! :)
After 15 seconds, the sleep will finish, and the xterm will close.
If you want to simply clear the screen, skip the exec:
1. At a command line: you could type "tput clear"
2. In a shell script, without exiting the shell script:
"tput clear"
Hope this helps!
Brian
--
"sixty-six sevenths" -- Marty Ross, Math 211 exam
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