#034 File Locks
How to implement simple resource semaphores with file locks.
Notes
For when we want to ensure that only one instance of a shell script can be run at the same time.
Operating system locks/semaphores can be used to traffic-light an operation. From a scripting environment, a utility must be used to get a lock - for example flock or man lockfile.
flock / lockfile
The flock utility should be available by default in most Linux distros.
It used to be in MacOS also, but not since at least Yosemite.
On MacOS, lockfile should be available though. The test script will look for either.
TestScript
The lock_n_load.sh script demonstrates locking with either flock
parsing an invented syntax from an environment variable.
$ ./lock_n_load.sh & ./lock_n_load.sh
[27428] Locked
[27428] Doing something interesting here... Mon Oct 9 23:13:27 SGT 2017
[27428] Loaded
[27427] Locked
[27427] Doing something interesting here... Mon Oct 9 23:13:35 SGT 2017
[27427] Loaded
$