Quick-and-dirty way to ensure only one instance of a shell script is running at a time

Question

What\'s a quick-and-dirty way to make sure that only one instance of a shell script is running at a given time?

Answer 1

To make locking reliable you need an atomic operation. Many of the above proposals are not atomic. The proposed lockfile(1) utility looks promising as the man-page mentioned, that its "NFS-resistant". If your OS does not support lockfile(1) and your solution has to work on NFS, you have not many options....

NFSv2 has two atomic operations:

• symlink
• rename

With NFSv3 the create call is also atomic.

Directory operations are NOT atomic under NFSv2 and NFSv3 (please refer to the book 'NFS Illustrated' by Brent Callaghan, ISBN 0-201-32570-5; Brent is a NFS-veteran at Sun).

Knowing this, you can implement spin-locks for files and directories (in shell, not PHP):

lock current dir:

while ! ln -s . lock; do :; done

lock a file:

while ! ln -s ${f} ${f}.lock; do :; done

unlock current dir (assumption, the running process really acquired the lock):

mv lock deleteme && rm deleteme

unlock a file (assumption, the running process really acquired the lock):

mv ${f}.lock ${f}.deleteme && rm ${f}.deleteme

Remove is also not atomic, therefore first the rename (which is atomic) and then the remove.

For the symlink and rename calls, both filenames have to reside on the same filesystem. My proposal: use only simple filenames (no paths) and put file and lock into the same directory.