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 \-F , " \-\-no\-fork"
  74 Do not fork before executing
  75 .IR command .
  76 Upon execution the flock process is replaced by
  77 .I command
  78 which continues to hold the lock. This option is incompatible with
  79 \fB\-\-close\fR as there would otherwise be nothing left to hold the lock.
  80 .TP
  81 .BR \-e , " \-x" , " \-\-exclusive"
  82 Obtain an exclusive lock, sometimes called a write lock.  This is the
  83 default.
  84 .TP
  85 .BR \-n , " \-\-nb" , " \-\-nonblock"
  86 Fail rather than wait if the lock cannot be
  87 immediately acquired.
  88 See the
  89 .B \-E
  90 option for the exit code used.
  91 .TP
  92 .BR \-o , " \-\-close"
  93 Close the file descriptor on which the lock is held before executing
  94 .IR command .
  95 This is useful if
  96 .I command
  97 spawns a child process which should not be holding the lock.
  98 .TP
  99 .BR \-s , " \-\-shared"
 100 Obtain a shared lock, sometimes called a read lock.
 101 .TP
 102 .BR \-u , " \-\-unlock"
 103 Drop a lock.  This is usually not required, since a lock is automatically
 104 dropped when the file is closed.  However, it may be required in special
 105 cases, for example if the enclosed command group may have forked a background
 106 process which should not be holding the lock.
 107 .TP
 108 .BR \-w , " \-\-wait" , " \-\-timeout " \fIseconds
 109 Fail if the lock cannot be acquired within
 110 .IR seconds .
 111 Decimal fractional values are allowed.
 112 See the
 113 .B \-E
 114 option for the exit code used. The zero number of
 115 .I seconds
 116 is interpreted as \fB\-\-nonblock\fR.
 117 .TP
 118 .B \-\-verbose
 119 Report how long it took to acquire the lock, or why the lock could not be
 120 obtained.
 121 .TP
 122 .BR \-V , " \-\-version"
 123 Display version information and exit.
 124 .TP
 125 .BR \-h , " \-\-help"
 126 Display help text and exit.
 127 .SH EXAMPLES
 128 Note that "shell> " in examples is a command line prompt.
 129 .TP
 130 shell1> flock /tmp \-c cat
 131 .TQ
 132 shell2> flock \-w .007 /tmp \-c echo; /bin/echo $?
 133 Set exclusive lock to directory /tmp and the second command will fail.
 134 .TP
 135 shell1> flock \-s /tmp \-c cat
 136 .TQ
 137 shell2> flock \-s \-w .007 /tmp \-c echo; /bin/echo $?
 138 Set shared lock to directory /tmp and the second command will not fail.
 139 Notice that attempting to get exclusive lock with second command would fail.
 140 .TP
 141 shell> flock \-x local-lock-file echo 'a b c'
 142 Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.
 143 .TP
 144 (
 145 .TQ
 146   flock \-n 9 || exit 1
 147 .TQ
 148   # ... commands executed under lock ...
 149 .TQ
 150 ) 9>/var/lock/mylockfile
 151 The form is convenient inside shell scripts.  The mode used to open the file
 152 doesn't matter to
 153 .BR flock ;
 154 using
 155 .I >
 156 or
 157 .I >>
 158 allows the lockfile to be created if it does not already exist, however,
 159 write permission is required.  Using
 160 .I <
 161 requires that the file already exists but only read permission is required.
 162 .TP
 163 [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock \-en "$0" "$0" "$@" || :
 164 This is useful boilerplate code for shell scripts.  Put it at the top of the
 165 shell script you want to lock and it'll automatically lock itself on the first
 166 run.  If the env var $FLOCKER is not set to the shell script that is being run,
 167 then execute flock and grab an exclusive non-blocking lock (using the script
 168 itself as the lock file) before re-execing itself with the right arguments.  It
 169 also sets the FLOCKER env var to the right value so it doesn't run again.
 170 .TP
 171 .TQ
 172 shell> exec 4<>/var/lock/mylockfile
 173 .TQ
 174 shell> flock -n 4
 175 This form is convenient for locking a file without spawning a subprocess.
 176 The shell opens the lock file for reading and writing as file descriptor 4,
 177 then flock is used to lock the descriptor.
 178 .SH EXIT STATUS
 179 The command uses
 180 .B sysexits.h
 181 return values for everything, except when using either of the options
 182 .B \-n
 183 or
 184 .B \-w
 185 which report a failure to acquire the lock with a return value given by the
 186 .B \-E
 187 option, or 1 by default.
 188 .PP
 189 When using the \fIcommand\fR variant, and executing the child worked, then
 190 the exit status is that of the child command.
 191 .SH AUTHORS
 192 .UR hpa@zytor.com
 193 H. Peter Anvin
 194 .UE
 195 .SH COPYRIGHT
 196 Copyright \(co 2003\-2006 H. Peter Anvin.
 197 .br
 198 This is free software; see the source for copying conditions.  There is NO
 199 warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 200 .SH SEE ALSO
 201 .BR flock (2)
 202 .SH AVAILABILITY
 203 The flock command is part of the util-linux package and is available from
 204 .UR https://\:www.kernel.org\:/pub\:/linux\:/utils\:/util-linux/
 205 Linux Kernel Archive
 206 .UE .