sys-utils/flock.1

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  27 .TH FLOCK 1 "September 2011" "util-linux" "User Commands"
  28 .SH NAME
  29 flock \- manage locks from shell scripts
  30 .SH SYNOPSIS
  31 .B flock
  32 [options] <file> -c <command>
  33 .br
  34 .B flock
  35 [options] <directory> -c <command>
  36 .br
  37 .B flock
  38 [options] <file descriptor number>
  39 .SH DESCRIPTION
  40 .PP
  41 This utility manages
  42 .BR flock (2)
  43 locks from within shell scripts or the command line.
  44 .PP
  45 The first and second forms wrap the lock around the executing a command, in
  46 a manner similar to
  47 .BR su (1)
  48 or
  49 .BR newgrp (1).
  50 It locks a specified file or directory, which is created (assuming
  51 appropriate permissions), if it does not already exist.  By default, if the
  52 lock cannot be immediately acquired,
  53 .B flock
  54 waits until the lock is available.
  55 .PP
  56 The third form uses open file by file descriptor number.  See examples how
  57 that can be used.
  58 .SH OPTIONS
  59 .TP
  60 \fB\-s\fP, \fB\-\-shared\fP
  61 Obtain a shared lock, sometimes called a read lock.
  62 .TP
  63 \fB\-x\fP, \fB\-e\fP, \fB\-\-exclusive\fP
  64 Obtain an exclusive lock, sometimes called a write lock.  This is the
  65 default.
  66 .TP
  67 \fB\-u\fP, \fB\-\-unlock\fP
  68 Drop a lock.  This is usually not required, since a lock is automatically
  69 dropped when the file is closed.  However, it may be required in special
  70 cases, for example if the enclosed command group may have forked a background
  71 process which should not be holding the lock.
  72 .TP
  73 \fB\-n\fP, \fB\-\-nb\fP, \fB\-\-nonblock\fP
  74 Fail rather than wait if the lock cannot be
  75 immediately acquired.
  76 See the
  77 .I \-E
  78 option for the exit code used.
  79 .TP
  80 \fB\-w\fP, \fB\-\-wait\fP, \fB\-\-timeout\fP \fIseconds\fP
  81 Fail if the lock cannot be acquired within
  82 .IR seconds .
  83 Decimal fractional values are allowed.
  84 See the
  85 .I \-E
  86 option for the exit code used.
  87 .TP
  88 \fB\-o\fP, \fB\-\-close\fP
  89 Close the file descriptor on which the lock is held before executing
  90 .BR command\ .
  91 This is useful if
  92 .B command
  93 spawns a child process which should not be holding the lock.
  94 .TP
  95 \fB\-E\fP, \fB\-\-conflict\-exit\-code\fP \fInumber\fP
  96 The exit code used when the \fB\-n\fP option is in use, and the
  97 conflicting lock exists, or the \fB\-w\fP option is in use,
  98 and the timeout is reached. The default value is 1.
  99 .TP
 100 \fB\-c\fP, \fB\-\-command\fP \fIcommand\fP
 101 Pass a single
 102 .IR command ,
 103 without arguments, to the shell with
 104 .BR -c .
 105 .TP
 106 \fB\-h\fP, \fB\-\-help\fP
 107 Print a help message.
 108 .IP "\fB\-V, \-\-version\fP"
 109 Show version number and exit.
 110 .SH EXAMPLES
 111 .TP
 112 shell1> flock /tmp -c cat
 113 .TQ
 114 shell2> flock -w .007 /tmp -c echo; /bin/echo $?
 115 Set exclusive lock to directory /tmp and the second command will fail.
 116 .TP
 117 shell1> flock -s /tmp -c cat
 118 .TQ
 119 shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
 120 Set shared lock to directory /tmp and the second command will not fail.
 121 Notice that attempting to get exclusive lock with second command would fail.
 122 .TP
 123 (
 124 .TQ
 125   flock -n 9 || exit 1
 126 .TQ
 127   # ... commands executed under lock ...
 128 .TQ
 129 ) 9>/var/lock/mylockfile
 130 The form is convenient inside shell scripts.  The mode used to open the file
 131 doesn't matter to
 132 .BR flock ;
 133 using
 134 .I >
 135 or
 136 .I >>
 137 allows the lockfile to be created if it does not already exist, however,
 138 write permission is required.  Using
 139 .I <
 140 requires that the file already exists but only read permission is required.
 141 .SH "EXIT STATUS"
 142 The command uses
 143 .B sysexits.h
 144 return values for everything else but an options
 145 .I \-n
 146 or
 147 .I \-w
 148 failures which return either the value given by the
 149 .I \-E
 150 option, or 1 by default.
 151 .SH AUTHOR
 152 .UR hpa@zytor.com
 153 H. Peter Anvin
 154 .UE
 155 .SH COPYRIGHT
 156 Copyright \(co 2003\-2006 H. Peter Anvin.
 157 .br
 158 This is free software; see the source for copying conditions.  There is NO
 159 warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 160 .SH "SEE ALSO"
 161 .BR flock (2)
 162 .SH AVAILABILITY
 163 The flock command is part of the util-linux package and is available from
 164 .UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 165 Linux Kernel Archive
 166 .UE .