docs: standardize the phrases for --help and --version in all man pages

[thirdparty/util-linux.git] / sys-utils / flock.1

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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|directory> <command> [command args]
.br
.B flock
[options] <file|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 rather than wait if the lock cannot be
immediately acquired.
See the
.I \-E
option for the exit code used.
.TP
\fB\-w\fP, \fB\-\-wait\fP, \fB\-\-timeout\fP \fIseconds\fP
Fail if the lock cannot be acquired within
.IR seconds .
Decimal fractional values are allowed.
See the
.I \-E
option for the exit code used.
.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\-E\fP, \fB\-\-conflict\-exit\-code\fP \fInumber\fP
The exit code used when the \fB\-n\fP option is in use, and the
conflicting lock exists, or the \fB\-w\fP option is in use,
and the timeout is reached. The default value is 1.
.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
Display help text and exit.
.IP "\fB\-V, \-\-version\fP"
Display version information 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 $?
Set shared lock to directory /tmp and the second command will not fail.
Notice that attempting to get exclusive lock with second command would fail.
.TP
shell> flock -x local-lock-file echo 'a b c'
Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.
.TP
(
.TQ
  flock -n 9 || exit 1
.TQ
  # ... commands executed under lock ...
.TQ
) 9>/var/lock/mylockfile
The form is convenient inside shell scripts.  The mode used to open the file
doesn't matter to
.BR flock ;
using
.I >
or
.I >>
allows the lockfile to be created if it does not already exist, however,
write permission is required.  Using
.I <
requires that the file already exists but only read permission is required.
.TP
[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :
This is useful boilerplate code for shell scripts.  Put it at the top of the
shell script you want to lock and it'll automatically lock itself on the first
run.  If the env var $FLOCKER is not set to the shell script that is being run,
then execute flock and grab an exclusive non-blocking lock (using the script
itself as the lock file) before re-execing itself with the right arguments.  It
also sets the FLOCKER env var to the right value so it doesn't run again.
.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 either the value given by the
.I \-E
option, or 1 by default.
.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 .