I want to implement a shell script to run in the background at all times. The script will be added it in startup apps for running. While trying to make sure that only one instance runs at a time, I came across this code:
#!/bin/bash
another_instance()
{
echo “There is another instance running, exiting”
exit 1
}
( flock -n 100 || another_instance DEST=/home/backup/`date +%s` mkdir -p “$DEST” rsync -avz root@myhost:/home/web “$DEST/.” ) 100>/var/lock/dobackup.lock
here, with the explanation:
In this example, we use the file descriptor 100 in the redirection to the lock file. Also, we call another_instance if flock fails, informing there is another instance and then exiting.
It works, but I do not understand the file descriptor 100 part. I tried looking online, but my confusion prevails.
Could someone please explain the flock -n 100 and 100>/var/lock/dobackup.lock?
As far as I understand, flock -n creates/checks for a lock at the given /var/lock.. location, but what's the significance of 100?
Thank You.
flock -n creates/checks for a lock at the given /var/lock.. location
No, flock works on file descriptors.
/var/lock is a typical location for system administrators to put locks for global utilities. Like for example for package managers (pacman, apt-get etc.). You can use (ok, almost) any file anywhere with flock.
Let's say generally, flock is a property associated with a file referenced by a file descriptor. The number 100 is not significant, it has to be any open for writing file descriptor.
100>/var/lock/dobackup.lock redirects file descriptor 100 to file /var/lock/dobackup.lock. flock -n 100 applies flock on the file descriptor 100. When file descriptor 100 is closed (which happens when process terminates) then flock associated with that process is lifted automatically.
See Wikipedia for file descriptor, see man 2 flock and man 2 fcntl.