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fine-grained delay in shell scripts

From Ben Okopnik

Answered By Thomas Adam, Mike Orr, John Karns

So, you're writing a shell script, and you want to add a little pizzazz: you know, a little blinking indicator, or a slow display that prints out "Please wait" one letter at a time or something. You look around, and the only choices you've got are a) "sleep" (a minimum 1-second delay), or various strange things involving loops and "cat"ting large files (which makes you CPU usage shoot up into the 90% range.) Blechhh. What's a poor but honest scripter to do?

Farm the job out, of course.

See attached nap.pl.txt

It doesn't get much simpler. "nap" will give you a delay in milliseconds, plus a tiny machine-dependent fudge factor for starting Perl. Here, as an example, is that famous "rotating dash" indicator, using "nap":

while :; do for x in - \\ \| /; do printf "%c\b" $x; nap 100; done; done

[Thomas Adam] Tut tut Ben. For this kind of use, I always tweak the millisecond usage of the command:

"usleep"

Then I can use a for i in....loop and a usual "echo" in Bash.

Works everytime.

But, I prefer your script!!

OK, I'll admit my ignorance - what's a "usleep"? There's nothing like that on my system, or indeed in the Debian "Contents-i386.gz" file list. Please enlighten me. (I do seem to _vaguely_ remember something like that in C, but that's less than helpful.)

[Thomas] But, I prefer your script!!

Well, you got _something_ useful out of it. That's a plus.

[Mike] ..From "man 3 usleep": "The usleep() function suspends execution of the calling process for usec microseconds."

It looks like it's available only as a C function. Somebody should wrap it up in a command.

<smirk> I did.

[Thomas] ....and they did just that . I believe that on RedHat systems, it was supplied as part of the "initscripts" rpm, thus:

http://www.rpmfind.net//linux/RPM/redhat/7.2/i386/initscripts-6.40-1.i386.html

/sbin/usleep

is where my copy resides (despite the fact im running SuSE 7.1 professional).

Hope that helps

[John K] No such animal on my SuSE 7.2 install ...

jkarns@jkInsp8000:~ > locate usleep /home/jkarns/Dwnlds/Linux/XScreenSavers/xscreensaver-3.32/utils/usleep.c /home/jkarns/Dwnlds/Linux/XScreenSavers/xscreensaver-3.32/utils/usleep.h /home/jkarns/Dwnlds/Linux/XScreenSavers/xscreensaver-3.32/utils/usleep.o /usr/share/doc/packages/mod_php/doc/function.usleep.html /usr/share/doc/packages/phpdoc/manual/function.usleep.html /usr/share/man/allman/man3/usleep.3.gz /usr/share/man/man3/usleep.3.gz

As I'd mentioned, it's not part of Debian - whereas Perl is in every distro. I'm sticking with portability. Besides, when would you ever need microsecond precision in a shell script? Even milliseconds is splitting it frog-hair fine.

[Mike] You don't, but sometimes you want to delay for a quarter second or half a second.

BTW, "usleep" isn't described in "libc.info.gz", either - although there's an interesting-looking "nanosleep".

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