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.
I am in need of some help; think I have confused myself.
Here is the issue I am faced with.
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Discussion started by: pallvi_mahajan
15 Replies
LEARN ABOUT MOJAVE
sem_unlink
SEM_UNLINK(2) BSD System Calls Manual SEM_UNLINK(2)NAME
sem_unlink -- remove a named semaphore
SYNOPSIS
#include <semaphore.h>
int
sem_unlink(const char *name);
DESCRIPTION
The named semaphore named name is removed. If the semaphore is in use by other processes, then name is immediately disassociated with the
semaphore, but the semaphore itself will not be removed until all references to it have been closed. Subsequent calls to sem_open() using
name will refer to or create a new semaphore named name.
If successful, sem_unlink() will return 0. Otherwise, -1 is returned and errno is set, and the state of the semaphore is unchanged.
ERRORS
sem_unlink() succeeds unless:
[EACCES] Permission is denied to be remove the semaphore.
[ENAMETOOLONG] name exceeded PSEMNAMLEN characters.
[ENOENT] The named semaphore does not exist.
SEE ALSO sem_close(2), sem_open(2), semctl(2), semget(2), semop(2)HISTORY
sem_unlink() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995).
Darwin June 8, 2000 Darwin