exim_lock8

Usage

Overview

The exim_lock utility locks a mailbox file using the same algorithm as Exim. For a discussion of locking issues, see section "appendfile transport", subsection "Operational details for appending" the Exim specification. 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.

-flock

Use “flock()” locking on the open mailbox, provided the operating system supports it.

-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).

-restore_time

This option causes exim_lock to restore the modified and read times to the locked file before exiting. This allows you to access a locked mailbox (for example, to take a backup copy) without disturbing the times that the user subsequently sees.

-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, -flock, -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 or -flock 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 can be used with either or both of -fcntl or -flock. It assumes -fcntl by default. MBX locking 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.