.TH FLOCK "1" "November 2004" "flock (util-linux)" "User Commands"
.SH NAME
flock \- acquire a file lock and then execute a command with the lock held
.SH SYNOPSIS
.BR flock
.RB [ \-\-shared ]
.RB [ \-\-timeout=\fP\fIseconds\fP ]
.I lockfile command ...
.SH DESCRIPTION
.\" Add any additional description here
.PP
Acquire a file lock using the flock(2) system call and then execute
the given command with the lock held.  Depending on the options given,
the lock can be either exclusive or shared, and the behavior in the
event of lock contention can be specified as either waiting
indefinitely for the lock to become available (the default), or
failing if the lock does not become available after a specific time,
which can be specified as zero to make the command not wait at all.
.PP
.TP
\fB\-\-shared\fR
Acquire a shared lock.  Acquiring a shared lock does
not stop others from acquiring a shared lock, but it will stop others
from acquiring an exclusive lock.  Conversely, acquiring an exclusive
lock (the default) stops both exclusive and shared attempts to acquire
the lock.  Typically, a shared lock is used if a command is just going
to read the locked data, and an exclusive lock is used if the command
might write to it.
.TP
\fB\-\-timeout=n\fR
Abort if the lock cannot be acquired before \fIn\fR seconds.
For a completely non-blocking attempt to acquire a lock, specify
\fB\-\-timeout=0\fR.
The timer applies only to the attempt to acquire the lock.  As soon
as the lock is acquired, the timeout is cancelled.  The command to
be run is not subject to the timeout.
.PP
.SH "EXAMPLES (invoking some imaginary programs)"
.hl
.PP
flock /etc/passwd read-and-write-to-passwd
.PP
flock \-\-shared /etc/passwd just-read-something-from-passwd
.PP
flock \-\-timeout=0 /sys /usr/local/bin/update-hotplug /sys
.SH AUTHOR
Written by Adam J. Richter