The exim_lock utility locks a mailbox file using the same algorithm as Exim. For a discussion of locking issues, see section 25.2. exim_lock can be used to prevent any modification of a mailbox by Exim or a user agent while investigating a problem. The utility requires the name of the file as its first argument. If the locking is successful, the second argument is run as a command (using C's “system()” function); if there is no second argument, the value of the SHELL environment variable is used; if this is unset or empty, /bin/sh is run. When the command finishes, the mailbox is unlocked and the utility ends. The following options are available:
Use “fcntl()” locking on the open mailbox.
This must be followed by a number, which is a number of seconds; it sets the interval to sleep between retries (default 3).
Create a lock file before opening the mailbox.
Lock the mailbox using MBX rules.
Suppress verification output.
This must be followed by a number; it sets the number of times to try to get the lock (default 10).
This must be followed by a number, which is a number of seconds; it sets a timeout to be used with a blocking “fcntl()” lock. If it is not set (the default), a non-blocking call is used.
Generate verbose output.
If none of -fcntl, -lockfile or -mbx are given, the default is to create a lock file and also use “fcntl()” locking on the mailbox, which is the same as Exim's default. The use of -fcntl requires that the file be writable; the use of -lockfile requires that the directory containing the file be writable. Locking by lock file does not last for ever; Exim assumes that a lock file is expired if it is more than 30 minutes old.
The -mbx option is mutually exclusive with -fcntl. It causes a shared lock to be taken out on the open mailbox, and an exclusive lock on the file /tmp/.n.m where n and m are the device number and inode number of the mailbox file. When the locking is released, if an exclusive lock can be obtained for the mailbox, the file in /tmp is deleted.
The default output contains verification of the locking that takes place. The -v option causes some additional information to be given. The -q option suppresses all output except error messages.
A command such as
runs an interactive shell while the file is locked, whereas
exim_lock -q /var/spool/mail/spqr <<End <some commands> End
runs a specific non-interactive sequence of commands while the file is locked, suppressing all verification output. A single command can be run by a command such as
exim_lock -q /var/spool/mail/spqr "cp /var/spool/mail/spqr /some/where"
Note that if a command is supplied, it must be entirely contained within the second argument - hence the quotes.
This manual page needs a major re-work. If somebody knows better groff than us and has more experience in writing manual pages, any patches would be greatly appreciated.
This manual page was stitched together from spec.txt by Andreas Metzler <ametzler at downhill.at.eu.org>, for the Debian GNU/Linux system (but may be used by others).