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Top Forums Shell Programming and Scripting Conditional execution Post 302853169 by edstevens on Friday 13th of September 2013 10:35:59 AM

09-13-2013

Registered User

Conditional execution

Here's an interesting (to me, anyway) little puzzle.

Background:
I have a process that restores a number of large(ish) files from tape backup. As an individual file is being written, it is owned by root, then the ownership is changed when that file is complete. Since this process can take some time (and operations, who actually runs it) isn't timely about starting it when requested or telling me when it completes, I wrote a little script to watch for the presence of the root-owned files:

Code:

echo "Please enter sleep interval: "
read sleep_interval
while :
do
ls -ltr /mydir |grep root
date
echo Sleeping $sleep_interval seconds
sleep $sleep_interval
done

So when I first kick it off, I set the sleep interval to something large, like 300. Then when I see files who are owned by root showing up, I cancel the script and resubmit with a sleep interval of 10.

So it occurs to me, could I code it to self-determine what the sleep interval should be. Check the outcome of the 'ls', and if nothing is owned by root sleep for 300, else sleep for 10. So, on first starting, there would be no files, so it would sleep for 300. On the first iteration that turns up a file, sleep goes to 10, and on first following iteration with no files owned by root, sleep goes back to 300.

In pseudo code, something like this:

Code:

while :
do
ls -ltr /mydir |grep root|grep -v "lost+found"
#- begin pseudo code
if nothing found
   sleep 300
else
   sleep 10
fi
# - end pseudo code
done

edstevens

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LEARN ABOUT FREEBSD

sleep

SLEEP(1)						    BSD General Commands Manual 						  SLEEP(1)

NAME

     sleep -- suspend execution for an interval of time

SYNOPSIS

     sleep seconds

DESCRIPTION

     The sleep command suspends execution for a minimum of seconds.

     If the sleep command receives a signal, it takes the standard action.  When the SIGINFO signal is received, the estimate of the amount of
     seconds left to sleep is printed on the standard output.

IMPLEMENTATION NOTES

     The SIGALRM signal is not handled specially by this implementation.

     The sleep command allows and honors a non-integer number of seconds to sleep in any form acceptable by strtod(3).	This is a non-portable
     extension, and its use will nearly guarantee that a shell script will not execute properly on another system.

EXIT STATUS

     The sleep utility exits 0 on success, and >0 if an error occurs.

EXAMPLES

     To schedule the execution of a command for x number seconds later (with csh(1)):

	   (sleep 1800; sh command_file >& errors)&

     This incantation would wait a half hour before running the script command_file.  (See the at(1) utility.)

     To reiteratively run a command (with the csh(1)):

	   while (1)
		   if (! -r zzz.rawdata) then
			   sleep 300
		   else
			   foreach i (`ls *.rawdata`)
				   sleep 70
				   awk -f collapse_data $i >> results
			   end
			   break
		   endif
	   end

     The scenario for a script such as this might be: a program currently running is taking longer than expected to process a series of files, and
     it would be nice to have another program start processing the files created by the first program as soon as it is finished (when zzz.rawdata
     is created).  The script checks every five minutes for the file zzz.rawdata, when the file is found, then another portion processing is done
     courteously by sleeping for 70 seconds in between each awk job.

SEE ALSO

     nanosleep(2), sleep(3)

STANDARDS

     The sleep command is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible.

HISTORY

     A sleep command appeared in Version 4 AT&T UNIX.

BSD
								  April 18, 1994							       BSD

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