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.TH FLOCK 1 "September 2011" "util-linux" "User Commands"
.SH NAME
flock \- manage locks from shell scripts
.SH SYNOPSIS
.B flock
[options] <file> -c <command>
.br
.B flock
[options] <directory> -c <command>
.br
.B flock
[options] <file descriptor number>
.SH DESCRIPTION
.PP
This utility manages
.BR flock (2)
locks from within shell scripts or the command line.
.PP
The first and second forms wrap 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. By default, if the
lock cannot be immediately acquired,
.B flock
waits until the lock is available.
.PP
The third form uses open file by file descriptor number. See examples how
that can be used.
.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
.IR 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
.BR command\ .
This is useful if
.B command
spawns a child process which should not be holding the lock.
.TP
\fB\-c\fP, \fB\-\-command\fP \fIcommand\fP
Pass a single
.IR command ,
without arguments, to the shell with
.BR -c .
.TP
\fB\-h\fP, \fB\-\-help\fP
Print a help message.
.IP "\fB\-V, \-\-version\fP"
Show version number and exit.
.SH EXAMPLES
.TP
shell1> flock /tmp -c cat
.TQ
shell2> flock -w .007 /tmp -c echo; /bin/echo $?
Set exclusive lock to directory /tmp and the second command will fail.
.TP
shell1> flock -s /tmp -c cat
.TQ
shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
Set shared lock to directory /tmp and the second command will not fail.
Notice that attempting to get exclusive lock with second command would fail.
.TP
(
.TQ
flock -n 9 || exit 1
.TQ
# ... commands executed under lock ...
.TQ
) 9>/var/lock/mylockfile
The form is convenient inside shell scripts. The mode used to open the file
doesn't matter to
.BR flock ;
using
.I >
or
.I >>
allows the lockfile to be created if it does not already exist, however,
write permission is required. Using
.I <
requires that the file already exists but only read permission is required.
.SH "EXIT STATUS"
The command uses
.B sysexits.h
return values for everything else but an options
.I \-n
or
.I \-w
failures which return 1.
.SH AUTHOR
.UR hpa@zytor.com
H. Peter Anvin
.UE
.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 package and is available from
.UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
Linux Kernel Archive
.UE .
