1FLOCK(1) User Commands FLOCK(1)
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6 flock - manage locks from shell scripts
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9 flock [options] file|directory command [arguments]
10 flock [options] file|directory -c command
12 flock [options] number
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16 This utility manages flock(2) locks from within shell scripts or from
17 the command line.
18 The first and second of the above forms wrap the lock around the
20 execution of a command, in a manner similar to su(1) or newgrp(1). They
21 lock a specified file or directory, which is created (assuming
22 appropriate permissions) if it does not already exist. By default, if
23 the lock cannot be immediately acquired, flock waits until the lock is
24 available.
25 The third form uses an open file by its file descriptor number. See the
27 examples below for how that can be used.
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30 -c, --command command
31 Pass a single command, without arguments, to the shell with -c.
32 -E, --conflict-exit-code number
34 The exit status used when the -n option is in use, and the
35 conflicting lock exists, or the -w option is in use, and the
36 timeout is reached. The default value is 1. The number has to be in
37 the range of 0 to 255.
38 -F, --no-fork
40 Do not fork before executing command. Upon execution the flock
41 process is replaced by command which continues to hold the lock.
42 This option is incompatible with --close as there would otherwise
43 be nothing left to hold the lock.
44 -e, -x, --exclusive
46 Obtain an exclusive lock, sometimes called a write lock. This is
47 the default.
48 -n, --nb, --nonblock
50 Fail rather than wait if the lock cannot be immediately acquired.
51 See the -E option for the exit status used.
52 -o, --close
54 Close the file descriptor on which the lock is held before
55 executing command. This is useful if command spawns a child process
56 which should not be holding the lock.
57 -s, --shared
59 Obtain a shared lock, sometimes called a read lock.
60 -u, --unlock
62 Drop a lock. This is usually not required, since a lock is
63 automatically dropped when the file is closed. However, it may be
64 required in special cases, for example if the enclosed command
65 group may have forked a background process which should not be
66 holding the lock.
67 -w, --wait, --timeout seconds
69 Fail if the lock cannot be acquired within seconds. Decimal
70 fractional values are allowed. See the -E option for the exit
71 status used. The zero number of seconds is interpreted as
72 --nonblock.
73 --verbose
75 Report how long it took to acquire the lock, or why the lock could
76 not be obtained.
77 -h, --help
79 Display help text and exit.
80 -V, --version
82 Print version and exit.
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85 The command uses <sysexits.h> exit status values for everything, except
86 when using either of the options -n or -w which report a failure to
87 acquire the lock with an exit status given by the -E option, or 1 by
88 default. The exit status given by -E has to be in the range of 0 to
89 255.
90 When using the command variant, and executing the child worked, then
92 the exit status is that of the child command.
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95 flock does not detect deadlock. See flock(2) for details.
96 Some file systems (e. g. NFS and CIFS) have a limited implementation of
98 flock(2) and flock may always fail. For details see flock(2), nfs(5)
99 and mount.cifs(8). Depending on mount options, flock can always fail
100 there.
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103 Note that "shell> " in examples is a command line prompt.
104 shell1> flock /tmp -c cat; shell2> flock -w .007 /tmp -c echo;
106 /bin/echo $?
107 Set exclusive lock to directory /tmp and the second command will
108 fail.
109 shell1> flock -s /tmp -c cat; shell2> flock -s -w .007 /tmp -c echo;
111 /bin/echo $?
112 Set shared lock to directory /tmp and the second command will not
113 fail. Notice that attempting to get exclusive lock with second
114 command would fail.
115 shell> flock -x local-lock-file echo 'a b c'
117 Grab the exclusive lock "local-lock-file" before running echo with
118 'a b c'.
119 (; flock -n 9 || exit 1; # ... commands executed under lock ...; )
121 9>/var/lock/mylockfile
122 The form is convenient inside shell scripts. The mode used to open
123 the file doesn’t matter to flock; using > or >> allows the lockfile
124 to be created if it does not already exist, however, write
125 permission is required. Using < requires that the file already
126 exists but only read permission is required.
127 [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0"
129 "$@" || :
130 This is useful boilerplate code for shell scripts. Put it at the
131 top of the shell script you want to lock and it’ll automatically
132 lock itself on the first run. If the environment variable $FLOCKER
133 is not set to the shell script that is being run, then execute
134 flock and grab an exclusive non-blocking lock (using the script
135 itself as the lock file) before re-execing itself with the right
136 arguments. It also sets the FLOCKER environment variable to the
137 right value so it doesn’t run again.
138 shell> exec 4<>/var/lock/mylockfile; shell> flock -n 4
140 This form is convenient for locking a file without spawning a
141 subprocess. The shell opens the lock file for reading and writing
142 as file descriptor 4, then flock is used to lock the descriptor.
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145 H. Peter Anvin <hpa@zytor.com>
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148 Copyright © 2003-2006 H. Peter Anvin. This is free software; see the
149 source for copying conditions. There is NO warranty; not even for
150 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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153 flock(2)
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156 For bug reports, use the issue tracker at
157 https://github.com/util-linux/util-linux/issues.
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160 The flock command is part of the util-linux package which can be
161 downloaded from Linux Kernel Archive
162 <https://www.kernel.org/pub/linux/utils/util-linux/>.
163util-linux 2.38 2022-03-25 FLOCK(1)