The permissions on the parent and lock directory could restrict the access to a specific user and group, but yes, other processes could interfere with this locking if directed to do so.
One condition where this interference is helpful is a crash, where a @reboot entry in the crontab could:
[ -d /your/lockdir ] && rmdir /your/lockdir
You would also not want to place the lock directory in /tmp or otherwise where other users could manipulate (or see) it. In Red Hat, there is a /var/run/lock directory that might be appropriate.
My biggest use case for directory locking in scripts is handling inotify events.
One sure way to get a lock is to make a directory.
No matter how many processes attempt to make the directory, only one will succeed. That works for my scripting, but I have never used it in C.this is great thanks,
was just wondering, could something else remove the dir in between the if and then, before trap?
Just wondering about the atomicity.
The permissions on the parent and lock directory could restrict the access to a specific user and group, but yes, other processes could interfere with this locking if directed to do so.
One condition where this interference is helpful is a crash, where a @reboot entry in the crontab could:
You would also not want to place the lock directory in /tmp or otherwise where other users could manipulate (or see) it. In Red Hat, there is a /var/run/lock directory that might be appropriate.My biggest use case for directory locking in scripts is handling inotify events.
Yes, but that is not a weakness in the locking.
Usually when I read these writeups, I walk away thinking "Wow, $foo was a more complicated problem than I thought".
With this one, it was "Wow, $foo was a simpler problem than I thought and Unix (and thus Linux and OSX) just totally screwed it up for no reason"
Another good read is the SQLite locking module, https://www.sqlite.org/src/artifact/0240c5b547b4cf585c8cac35..., since these guys have to deal with the insanity of locking across different systems in real life.
You know things are bad when the least awful implementation of OS-level locking is the one from Microsoft.
So good in depth post. THANK YOU.