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Full Discussion: Sleep command did not worked
Top Forums UNIX for Beginners Questions & Answers Sleep command did not worked Post 303002763 by Don Cragun on Thursday 31st of August 2017 11:07:28 PM
Old 09-01-2017
Quote:
Originally Posted by bakunin
Either this or you create a semaphore file: consider the following script stub:

Code:
typeset fSemaphore="/path/to/file"

# are we already running?
if [ -e "$fSemaphore" ] ; then
     exit 1
fi

touch "$fSemaphore"

<.... your code here ....>

rm "$fSemaphore"
exit 0

This makes sure the script runs only one instance at a time.

I hope this helps.

bakunin
Not quite. There is a race condition between the time the existence test ([ -e "$fSemaphore" ]) is executed and the time the semaphore file is created (touch "$fSemaphore") that can allow two or more copies of this code to run simultaneously and not realize that another invocation is running.

This can be worked around with the shell's do not clobber option:
Code:
fSemaphore="/path/to/file"
set -C	# Set noclobber option.

# Are we already running?
if ! date "+File $fSemaphore created @ +c by PID $$" > "$fSemaphore"
then	exit 1
fi

# Set a trap to remove the semaphore when we exit.
trap 'rm -f "$fSemaphore"' EXIT
set +C	# Clear noclobber option.

<.... your code here ....>

exit

Note that both of these run a risk of leaving the semaphore file in place if the script is terminated by a kill signal (for the code above) or by any signal (for the code bakunin suggested). If this happens, the semaphore file will have to be manually removed before the script will run again successfully.
 

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LEARN ABOUT OSF1

sem_close

sem_close(3)						     Library Functions Manual						      sem_close(3)

NAME

       sem_close - Deallocates the specified named semaphore (P1003.1b)

LIBRARY

       Realtime Library (librt.so, librt.a)

SYNOPSIS

       #include <semaphore.h>

       int sem_close (
       sem_t *sem) ;

PARAMETERS

       sem	 References the semaphore to be closed.  Use the sem argument returned by the previous call to sem_open.

DESCRIPTION

       The  sem_close function makes a semaphore available for reuse by deallocating any system resources allocated for use by the current process
       for the named semaphore indicated by sem.

       If the semaphore has not been removed with a call to sem_unlink, sem_close does not change the current state of the semaphore.

       If the semaphore has been removed with a call to sem_unlink after the most recent call to sem_open with O_CREAT, the semaphore is no longer
       available after all processes that opened the semaphore close it.

RETURN VALUES

       On  a  successful  call,  a  value of 0 (zero) is returned. Otherwise, a value of -1 is returned and errno is set to indicate that an error
       occurred.

ERRORS

       The sem_close function fails under the following condition:

       [EINVAL]       The sem argument is not a valid semaphore descriptor.

RELATED INFORMATION

       Functions: sem_init(3), sem_open(3), sem_unlink(3)

       Guide to Realtime Programming delim off

																      sem_close(3)
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