sys-utils/flock.1

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  27 .TH FLOCK 1 "July 2014" "util-linux" "User Commands"
  28 .SH NAME
  29 flock \- manage locks from shell scripts
  30 .SH SYNOPSIS
  31 .B flock
  32 [options]
  33 .IR file | "directory command " [ arguments ]
  34 .br
  35 .B flock
  36 [options]
  37 .IR file | directory
  38 .BI \-c " command"
  39 .br
  40 .B flock
  41 .RI [options] " number"
  42 .SH DESCRIPTION
  43 .PP
  44 This utility manages
  45 .BR flock (2)
  46 locks from within shell scripts or from the command line.
  47 .PP
  48 The first and second of the above forms wrap the lock around the execution of a
  49 .IR command ,
  50 in a manner similar to
  51 .BR su (1)
  52 or
  53 .BR newgrp (1).
  54 They lock a specified \fIfile\fR or \fIdirectory\fR, which is created (assuming
  55 appropriate permissions) if it does not already exist.  By default, if the
  56 lock cannot be immediately acquired,
  57 .B flock
  58 waits until the lock is available.
  59 .PP
  60 The third form uses an open file by its file descriptor \fInumber\fR.
  61 See the examples below for how that can be used.
  62 .SH OPTIONS
  63 .TP
  64 .BR \-c , " \-\-command " \fIcommand
  65 Pass a single \fIcommand\fR, without arguments, to the shell with
  66 .BR \-c .
  67 .TP
  68 .BR \-E , " \-\-conflict-exit-code " \fInumber
  69 The exit code used when the \fB\-n\fP option is in use, and the
  70 conflicting lock exists, or the \fB\-w\fP option is in use,
  71 and the timeout is reached.  The default value is \fB1\fR.
  72 .TP
  73 .BR \-e , " \-x" , " \-\-exclusive"
  74 Obtain an exclusive lock, sometimes called a write lock.  This is the
  75 default.
  76 .TP
  77 .BR \-n , " \-\-nb" , " \-\-nonblock"
  78 Fail rather than wait if the lock cannot be
  79 immediately acquired.
  80 See the
  81 .B \-E
  82 option for the exit code used.
  83 .TP
  84 .BR \-o , " \-\-close"
  85 Close the file descriptor on which the lock is held before executing
  86 .IR command .
  87 This is useful if
  88 .I command
  89 spawns a child process which should not be holding the lock.
  90 .TP
  91 .BR \-s , " \-\-shared"
  92 Obtain a shared lock, sometimes called a read lock.
  93 .TP
  94 .BR \-u , " \-\-unlock"
  95 Drop a lock.  This is usually not required, since a lock is automatically
  96 dropped when the file is closed.  However, it may be required in special
  97 cases, for example if the enclosed command group may have forked a background
  98 process which should not be holding the lock.
  99 .TP
 100 .BR \-w , " \-\-wait" , " \-\-timeout " \fIseconds
 101 Fail if the lock cannot be acquired within
 102 .IR seconds .
 103 Decimal fractional values are allowed.
 104 See the
 105 .B \-E
 106 option for the exit code used.
 107 .TP
 108 .BR \-V , " \-\-version"
 109 Display version information and exit.
 110 .TP
 111 .BR \-h , " \-\-help"
 112 Display help text and exit.
 113 .SH EXAMPLES
 114 .TP
 115 shell1> flock /tmp -c cat
 116 .TQ
 117 shell2> flock -w .007 /tmp -c echo; /bin/echo $?
 118 Set exclusive lock to directory /tmp and the second command will fail.
 119 .TP
 120 shell1> flock -s /tmp -c cat
 121 .TQ
 122 shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
 123 Set shared lock to directory /tmp and the second command will not fail.
 124 Notice that attempting to get exclusive lock with second command would fail.
 125 .TP
 126 shell> flock -x local-lock-file echo 'a b c'
 127 Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.
 128 .TP
 129 (
 130 .TQ
 131   flock -n 9 || exit 1
 132 .TQ
 133   # ... commands executed under lock ...
 134 .TQ
 135 ) 9>/var/lock/mylockfile
 136 The form is convenient inside shell scripts.  The mode used to open the file
 137 doesn't matter to
 138 .BR flock ;
 139 using
 140 .I >
 141 or
 142 .I >>
 143 allows the lockfile to be created if it does not already exist, however,
 144 write permission is required.  Using
 145 .I <
 146 requires that the file already exists but only read permission is required.
 147 .TP
 148 [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :
 149 This is useful boilerplate code for shell scripts.  Put it at the top of the
 150 shell script you want to lock and it'll automatically lock itself on the first
 151 run.  If the env var $FLOCKER is not set to the shell script that is being run,
 152 then execute flock and grab an exclusive non-blocking lock (using the script
 153 itself as the lock file) before re-execing itself with the right arguments.  It
 154 also sets the FLOCKER env var to the right value so it doesn't run again.
 155 .SH "EXIT STATUS"
 156 The command uses
 157 .B sysexits.h
 158 return values for everything, except when using either of the options
 159 .B \-n
 160 or
 161 .B \-w
 162 which report a failure to acquire the lock with a return value given by the
 163 .B \-E
 164 option, or 1 by default.
 165 .PP
 166 When using the \fIcommand\fR variant, and executing the child worked, then
 167 the exit status is that of the child command.
 168 .SH AUTHOR
 169 .UR hpa@zytor.com
 170 H. Peter Anvin
 171 .UE
 172 .SH COPYRIGHT
 173 Copyright \(co 2003\-2006 H. Peter Anvin.
 174 .br
 175 This is free software; see the source for copying conditions.  There is NO
 176 warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 177 .SH "SEE ALSO"
 178 .BR flock (2)
 179 .SH AVAILABILITY
 180 The flock command is part of the util-linux package and is available from
 181 .UR ftp://\:ftp.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 182 Linux Kernel Archive
 183 .UE .