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.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\-sxon\fP] [\fB\-w\fP \fItimeout\fP] \fIlockdir\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 and second forms 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 or directory, which is created (assuming appropriate
permissions), if it does not already exist.
.PP
The third 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/.