01-31-2017
Let me see if I understand the request correctly: You have a NAS mounted on a number of servers sharing a shell script on that NAS. Only one single server is allowed to run one instance of the script at a given point in time, all others should be locked out during that run. The result is server dependent and should be stored locally on the respective server.
As it is many servers, local lock mecanisms won't work. Unless the OS offers lock mechanisms across all servers, you have to create one on your own.
Reading through your posts, I'm not sure if you differentiate between the lock file and the result file. You need both of them. While you can store the results locally, you should create the lock file some place available to all servers. One example - not being everybody's preferred choice - would be the script's path on the NAS, or e.g. a "lock" directory thereunder. You could use a simple file name, or a "personalized" one as Corona688 proposed, and use Scrutinizer's trap to remove it at script exit - not forgetting for the "normal", healthy script end.
No running scripts should be killed by the mechanism, but the nth instance should quit graciously when it sees another instance running, possibly logging such event when quitting.
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SHLOCK(1) BSD General Commands Manual SHLOCK(1)
NAME
shlock -- create or verify a lock file for shell scripts
SYNOPSIS
shlock [-du] [-p PID] -f lockfile
DESCRIPTION
The shlock command can create or verify a lock file on behalf of a shell or other script program. When it attempts to create a lock file, if
one already exists, shlock verifies that it is or is not valid. If valid, shlock will exit with a non-zero exit code. If invalid, shlock
will remove the lock file, and create a new one.
shlock uses the link(2) system call to make the final target lock file, which is an atomic operation (i.e. "dot locking", so named for this
mechanism's original use for locking system mailboxes). It puts the process ID ("PID") from the command line into the requested lock file.
shlock verifies that an extant lock file is still valid by using kill(2) with a zero signal to check for the existence of the process that
holds the lock.
The -d option causes shlock to be verbose about what it is doing.
The -f argument with lockfile is always required.
The -p option with PID is given when the program is to create a lock file; when absent, shlock will simply check for the validity of the lock
file.
The -u option causes shlock to read and write the PID as a binary pid_t, instead of as ASCII, to be compatible with the locks created by
UUCP.
EXIT STATUS
A zero exit code indicates a valid lock file.
EXAMPLES
BOURNE SHELL
#!/bin/sh
lckfile=/tmp/foo.lock
if shlock -f ${lckfile} -p $$
then
# do what required the lock
rm ${lckfile}
else
echo Lock ${lckfile} already held by `cat ${lckfile}`
fi
C SHELL
#!/bin/csh -f
set lckfile=/tmp/foo.lock
shlock -f ${lckfile} -p $$
if ($status == 0) then
# do what required the lock
rm ${lckfile}
else
echo Lock ${lckfile} already held by `cat ${lckfile}`
endif
The examples assume that the file system where the lock file is to be created is writable by the user, and has space available.
HISTORY
shlock was written for the first Network News Transfer Protocol (NNTP) software distribution, released in March 1986. The algorithm was sug-
gested by Peter Honeyman, from work he did on HoneyDanBer UUCP.
AUTHORS
Erik E. Fair <fair@clock.org>
BUGS
Does not work on NFS or other network file system on different systems because the disparate systems have disjoint PID spaces.
Cannot handle the case where a lock file was not deleted, the process that created it has exited, and the system has created a new process
with the same PID as in the dead lock file. The lock file will appear to be valid even though the process is unrelated to the one that cre-
ated the lock in the first place. Always remove your lock files after you're done.
BSD
June 29, 1997 BSD