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.\" -----------------------------------------------------------------------
.\"
.\" Copyright 2003-2006 H. Peter Anvin - All Rights Reserved
.\"
.\" Permission is hereby granted, free of charge, to any person
.\" obtaining a copy of this software and associated documentation
.\" files (the "Software"), to deal in the Software without
.\" restriction, including without limitation the rights to use,
.\" copy, modify, merge, publish, distribute, sublicense, and/or
.\" sell copies of the Software, and to permit persons to whom
.\" the Software is furnished to do so, subject to the following
.\" conditions:
.\"
.\" The above copyright notice and this permission notice shall
.\" be included in all copies or substantial portions of the Software.
.\"
.\" THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
.\" EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
.\" OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
.\" NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
.\" HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
.\" WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
.\" FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
.\" OTHER DEALINGS IN THE SOFTWARE.
.\"
.\" -----------------------------------------------------------------------
.TH FLOCK "1" "4 Feb 2006" "flock utility" "H. Peter Anvin"
.SH NAME
flock \- Manage locks from shell scripts
.SH SYNOPSIS
\fBflock\fP [\fB\-sxon\fP] [\fB\-w\fP \fItimeout\fP] \fIlockfile\fP [\fB\-c\fP] \fIcommand...\fP
.PP
\fBflock\fP [\fB\-sxun\fP] [\fB\-w\fP \fItimeout\fP] \fIfd\fP
.SH DESCRIPTION
.PP
This utility manages
.BR flock (2)
locks from within shell scripts or the command line.
.PP
The first form wraps the lock around the executing a command, in a manner similar to
.BR su (1)
or
.BR newgrp (1).
It locks a specified file, which is created (assuming appropriate
permissions), if it does not already exist.
.PP
The second form is convenient inside shell scripts, and is usually
used the following manner:
.PP
\fC(
.br
flock -s 200
.br
# ... commands executed under lock ...
.br
) 200>/var/lock/mylockfile\fP
.PP
The mode used to open the file doesn't matter to \fBflock\fP; using
\fC>\fP or \fP>>\fP allows the lockfile to be created if it does not
already exist, however, write permission is required; using \fC<\fP
requires that the file already exists but only read permission is
required.
.PP
By default, if the lock cannot be immediately acquired, \fBflock\fP
waits until the lock is available.
.SH OPTIONS
.TP
\fB\-s\fP, \fB\-\-shared\fP
Obtain a shared lock, sometimes called a read lock.
.TP
\fB\-x\fP, \fB\-e\fP, \fB\-\-exclusive\fP
Obtain an exclusive lock, sometimes called a write lock. This is the
default.
.TP
\fB\-u\fP, \fB\-\-unlock\fP
Drop a lock. This is usually not required, since a lock is
automatically dropped when the file is closed. However, it may be
required in special cases, for example if the enclosed command group
may have forked a background process which should not be holding the
lock.
.TP
\fB\-n\fP, \fB\-\-nb\fP, \fB\-\-nonblock\fP
Fail (with an exit code of 1) rather than wait if the lock cannot be
immediately acquired.
.TP
\fB\-w\fP, \fB\-\-wait\fP, \fB\-\-timeout\fP \fIseconds\fP
Fail (with an exit code of 1) if the lock cannot be acquired within
\fIseconds\fP seconds. Decimal fractional values are allowed.
.TP
\fB\-o\fP, \fB\-\-close\fP
Close the file descriptor on which the lock is held before executing
\fIcommand\fP. This is useful if \fIcommand\fP spawns a child process
which should not be hold ing the lock.
.TP
\fB\-c\fP, \fB\-\-command\fP \fIcommand\fP
Pass a single \fIcommand\fP to the shell with \fB\-c\fP.
.TP
\fB\-h\fP, \fB\-\-help\fP
Print a help message.
.SH AUTHOR
Written by H. Peter Anvin <hpa@zytor.com>.
.SH COPYRIGHT
Copyright \(co 2003\-2006 H. Peter Anvin.
.br
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH "SEE ALSO"
.BR flock (2)
.SH AVAILABILITY
The flock command is part of the util-linux-ng package and is available from
ftp://ftp.kernel.org/pub/linux/utils/util-linux-ng/.
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