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:
-fcntl
Use "fcntl()" locking on the open mailbox.
-interval
This must be followed by a number, which is a number of seconds; it
sets the interval to sleep between retries (default 3).
-lockfile
Create a lock file before opening the mailbox.
-mbx
Lock the mailbox using MBX rules.
-q
Suppress verification output.
-retries
This must be followed by a number; it sets the number of times to try
to get the lock (default 10).
-timeout
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.
-v
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 writeable; the use
of
-lockfile requires that the directory containing the file be writeable.
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
exim_lock /var/spool/mail/spqr
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 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).
