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3 .\" Copyright 2003-2006 H. Peter Anvin - All Rights Reserved
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27 .TH FLOCK 1 "July 2014" "util-linux" "User Commands"
29 flock \- manage locks from shell scripts
33 .IR file | "directory command " [ arguments ]
41 .RI [options] " number"
45 locks from within shell scripts or from the command line.
47 The first and second of the above forms wrap the lock around the execution of a
49 in a manner similar to
53 They lock a specified \fIfile\fR or \fIdirectory\fR, which is created (assuming
54 appropriate permissions) if it does not already exist. By default, if the
55 lock cannot be immediately acquired,
57 waits until the lock is available.
59 The third form uses an open file by its file descriptor \fInumber\fR.
60 See the examples below for how that can be used.
63 .BR \-c , " \-\-command " \fIcommand
64 Pass a single \fIcommand\fR, without arguments, to the shell with
67 .BR \-E , " \-\-conflict\-exit\-code " \fInumber
68 The exit status used when the \fB\-n\fP option is in use, and the
69 conflicting lock exists, or the \fB\-w\fP option is in use,
70 and the timeout is reached. The default value is \fB1\fR.
71 The \fInumber\fR has to be in the range of 0 to 255.
73 .BR \-F , " \-\-no\-fork"
74 Do not fork before executing
76 Upon execution the flock process is replaced by
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.
81 .BR \-e , " \-x" , " \-\-exclusive"
82 Obtain an exclusive lock, sometimes called a write lock. This is the
85 .BR \-n , " \-\-nb" , " \-\-nonblock"
86 Fail rather than wait if the lock cannot be
90 option for the exit status used.
92 .BR \-o , " \-\-close"
93 Close the file descriptor on which the lock is held before executing
97 spawns a child process which should not be holding the lock.
99 .BR \-s , " \-\-shared"
100 Obtain a shared lock, sometimes called a read lock.
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.
108 .BR \-w , " \-\-wait" , " \-\-timeout " \fIseconds
109 Fail if the lock cannot be acquired within
111 Decimal fractional values are allowed.
114 option for the exit status used. The zero number of
116 is interpreted as \fB\-\-nonblock\fR.
119 Report how long it took to acquire the lock, or why the lock could not be
122 .BR \-V , " \-\-version"
123 Display version information and exit.
125 .BR \-h , " \-\-help"
126 Display help text and exit.
130 exit status values for everything, except when using either of the options
134 which report a failure to acquire the lock with an exit status given by the
136 option, or 1 by default. The exit status given by
137 .B \-E has to be in the range of 0 to 255.
139 When using the \fIcommand\fR variant, and executing the child worked, then
140 the exit status is that of the child command.
142 Note that "shell> " in examples is a command line prompt.
144 shell1> flock /tmp \-c cat
146 shell2> flock \-w .007 /tmp \-c echo; /bin/echo $?
147 Set exclusive lock to directory /tmp and the second command will fail.
149 shell1> flock \-s /tmp \-c cat
151 shell2> flock \-s \-w .007 /tmp \-c echo; /bin/echo $?
152 Set shared lock to directory /tmp and the second command will not fail.
153 Notice that attempting to get exclusive lock with second command would fail.
155 shell> flock \-x local-lock-file echo 'a b c'
156 Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.
160 flock \-n 9 || exit 1
162 # ... commands executed under lock ...
164 ) 9>/var/lock/mylockfile
165 The form is convenient inside shell scripts. The mode used to open the file
172 allows the lockfile to be created if it does not already exist, however,
173 write permission is required. Using
175 requires that the file already exists but only read permission is required.
177 [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock \-en "$0" "$0" "$@" || :
178 This is useful boilerplate code for shell scripts. Put it at the top of the
179 shell script you want to lock and it'll automatically lock itself on the first
180 run. If the env var $FLOCKER is not set to the shell script that is being run,
181 then execute flock and grab an exclusive non-blocking lock (using the script
182 itself as the lock file) before re-execing itself with the right arguments. It
183 also sets the FLOCKER env var to the right value so it doesn't run again.
185 shell> exec 4<>/var/lock/mylockfile
188 This form is convenient for locking a file without spawning a subprocess.
189 The shell opens the lock file for reading and writing as file descriptor 4,
190 then flock is used to lock the descriptor.
196 Copyright \(co 2003\-2006 H. Peter Anvin.
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.
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/