Lock Files From Shell Scripts

[David Goodenough]

mercer at ncrcce.StPaul.NCR.COM (Dan Mercer) sez:
> ..... Then I considered using ln,  thinking an error
> condition would be returned if the file existed.  But no,  the file
> was overwritten unconditionally.

Humm - sounds like SYSV and a funky ln. However, if mkdir will overwrite
an already existing directory then I'd say you have a really badly broken
system. So why not:

--- cut here --- cut here --- cut here --- cut here --- cut here ---
lock=/tmp/lock

until mkdir $lock 2>&1 >/dev/null
do
    sleep 10
done

# critical stuff

# clean up the lock directory
rmdir $lock

exit 0
--- cut here --- cut here --- cut here --- cut here --- cut here ---

You could also use umask and touch, except touch doesn't return an error
status, at least BSD touch doesn't (again, if mkdir doesn't you're in
dead trouble anyway)
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