1FLOCK(1)                         User Commands                        FLOCK(1)
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NAME

6       flock - manage locks from shell scripts
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SYNOPSIS

9       flock [options] <file|directory> <command> [command args]
10       flock [options] <file|directory> -c <command>
11       flock [options] <file descriptor number>
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DESCRIPTION

14       This  utility  manages  flock(2) locks from within shell scripts or the
15       command line.
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17       The first and second forms wrap the lock around the  executing  a  com‐
18       mand,  in a manner similar to su(1) or newgrp(1).  It locks a specified
19       file or directory, which is created (assuming appropriate permissions),
20       if  it does not already exist.  By default, if the lock cannot be imme‐
21       diately acquired, flock waits until the lock is available.
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23       The third form uses open file by file descriptor number.  See  examples
24       how that can be used.
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OPTIONS

27       -s, --shared
28              Obtain a shared lock, sometimes called a read lock.
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30       -x, -e, --exclusive
31              Obtain  an  exclusive lock, sometimes called a write lock.  This
32              is the default.
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34       -u, --unlock
35              Drop a lock.  This is usually not  required,  since  a  lock  is
36              automatically  dropped when the file is closed.  However, it may
37              be required in special cases, for example if the  enclosed  com‐
38              mand group may have forked a background process which should not
39              be holding the lock.
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41       -n, --nb, --nonblock
42              Fail  rather  than  wait  if  the  lock  cannot  be  immediately
43              acquired.  See the -E option for the exit code used.
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45       -w, --wait, --timeout seconds
46              Fail  if  the  lock  cannot be acquired within seconds.  Decimal
47              fractional values are allowed.  See the -E option for  the  exit
48              code  used.  The zero number of seconds is interpreted as --non‐
49              block.
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51       -o, --close
52              Close the file descriptor on which the lock is held before  exe‐
53              cuting  command .   This  is  useful  if  command spawns a child
54              process which should not be holding the lock.
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56       -E, --conflict-exit-code number
57              The exit code used when the -n option is in use,  and  the  con‐
58              flicting  lock exists, or the -w option is in use, and the time‐
59              out is reached. The default value is 1.
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61       -c, --command command
62              Pass a single command, without arguments, to the shell with -c.
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64       -h, --help
65              Print a help message.
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67       -V, --version
68              Show version number and exit.
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EXAMPLES

71       shell1> flock /tmp -c cat
72       shell2> flock -w .007 /tmp -c echo; /bin/echo $?
73              Set exclusive lock to directory /tmp and the second command will
74              fail.
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76       shell1> flock -s /tmp -c cat
77       shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
78              Set  shared  lock  to directory /tmp and the second command will
79              not fail.  Notice that attempting to  get  exclusive  lock  with
80              second command would fail.
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82       shell> flock -x local-lock-file echo 'a b c'
83              Grab  the  exclusive  lock "local-lock-file" before running echo
84              with 'a b c'.
85
86       (
87         flock -n 9 || exit 1
88         # ... commands executed under lock ...
89       ) 9>/var/lock/mylockfile
90              The form is convenient inside shell scripts.  The mode  used  to
91              open  the file doesn't matter to flock; using > or >> allows the
92              lockfile to be created if it does not  already  exist,  however,
93              write  permission  is  required.  Using < requires that the file
94              already exists but only read permission is required.
95
96       [ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en  "$0"  "$0"
97       "$@" || :
98              This  is  useful  boilerplate code for shell scripts.  Put it at
99              the top of the shell script you want to lock and it'll automati‐
100              cally  lock itself on the first run.  If the env var $FLOCKER is
101              not set to the shell script that  is  being  run,  then  execute
102              flock  and grab an exclusive non-blocking lock (using the script
103              itself as the lock file) before re-execing itself with the right
104              arguments.   It also sets the FLOCKER env var to the right value
105              so it doesn't run again.
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EXIT STATUS

108       The command uses sysexits.h return values for everything  else  but  an
109       options -n or -w failures which return either the value given by the -E
110       option, or 1 by default.
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AUTHOR

113       H. Peter Anvin ⟨hpa@zytor.com⟩
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116       Copyright © 2003-2006 H. Peter Anvin.
117       This is free software; see the source for copying conditions.  There is
118       NO  warranty;  not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
119       PURPOSE.
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SEE ALSO

122       flock(2)
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AVAILABILITY

125       The flock command is part of the util-linux package  and  is  available
126       from  Linux  Kernel Archive ⟨ftp://ftp.kernel.org/pub/linux/utils/util-
127       linux/⟩.
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131util-linux                      September 2011                        FLOCK(1)
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