If you can, you should use
flock(1) for shell script locking
I have in the past worked out and used complicated but portable approaches for doing locking in shell scripts. These approaches get much more complicated if processes can die abruptly without undoing their lock. You can generally arrange things so that your locks are cleared if the entire machine reboots, but that's about it as far as simple approaches go. Sometimes this is what you want, but often it isn't.
As a result of a series of issues with our traditional shell script
locking, I have been more and more moving to using Linux's
flock(1) when I can,
which is to say for scripts that only have to run on our Linux
machines (which is almost all of our machines today).
sufficiently useful and compelling here that I might actually port
it over to other Unixes if we had to integrate such systems into
our current Linux environment.
(Anything we want to use should have
flock(2), and hopefully that's the only
flock program really depends on.)
There are two strongly appealing sides to
flock. The first is
that it provides basically the usage that we want; in normal
operation, it runs something with the lock held and releases the
lock when the thing exits. The second is that it automatically
releases the lock if something goes wrong, because
evaporate when the file descriptor is closed.
(The manpage's description of '
-o' may make you confused about
flock means is that the open file descriptor of the
lock is not inherited by the command
flock runs. Normally you
want the command to inherit the open file descriptor, because it
means that so long as any process involved is still running, the
lock is held, even if
flock itself gets killed for some reason.)
Generally I want to use '
flock -n', because we mostly use locking
for 'only one of these should ever be running at once'; if the lock
is held, a previous cron job or whatever is still active, so the
current one should just give up.
We have one script using a traditional shell script approach to
locking that I very carefully and painfully revised to be more or
less safe in the face of getting killed abruptly. Since it logs
diagnostics if it detects a stale lock, there's a certain amount
of use in having it around, but I definitely don't want to ever
have to do another script like it, and it's a special case in some
other ways that might make it awkward to use with
experience of revising that script is part of what pushed me very
strongly to using
flock for others.