flock(1)



FLOCK(1)                         User Commands                        FLOCK(1)

NAME
       flock - manage locks from shell scripts

SYNOPSIS
       flock [options] file|directory command [arguments]
       flock [options] file|directory -c command
       flock [options] number

DESCRIPTION
       This  utility  manages flock(2) locks from within shell scripts or from
       the command line.

       The first and second of the above forms wrap the lock around the execu-
       tion  of  a  command,  in a manner similar to su(1) or newgrp(1).  They
       lock a specified file or directory, which is created  (assuming  appro-
       priate  permissions)  if it does not already exist.  By default, if the
       lock cannot be immediately acquired, flock  waits  until  the  lock  is
       available.

       The  third  form  uses an open file by its file descriptor number.  See
       the examples below for how that can be used.

OPTIONS
       -c, --command command
              Pass a single command, without arguments, to the shell with -c.

       -E, --conflict-exit-code number
              The exit code used when the -n option is in use,  and  the  con-
              flicting  lock exists, or the -w option is in use, and the time-
              out is reached.  The default value is 1.

       -F, --no-fork
              Do not fork before executing command.  Upon execution the  flock
              process is replaced by command which continues to hold the lock.
              This option is incompatible with --close as there  would  other-
              wise be nothing left to hold the lock.

       -e, -x, --exclusive
              Obtain  an  exclusive lock, sometimes called a write lock.  This
              is the default.

       -n, --nb, --nonblock
              Fail  rather  than  wait  if  the  lock  cannot  be  immediately
              acquired.  See the -E option for the exit code used.

       -o, --close
              Close  the file descriptor on which the lock is held before exe-
              cuting command.  This  is  useful  if  command  spawns  a  child
              process which should not be holding the lock.

       -s, --shared
              Obtain a shared lock, sometimes called a read lock.

       -u, --unlock
              Drop  a  lock.   This  is  usually not required, since a lock is
              automatically dropped when the file is closed.  However, it  may
              be  required  in special cases, for example if the enclosed com-
              mand group may have forked a background process which should not
              be holding the lock.

       -w, --wait, --timeout seconds
              Fail  if  the  lock  cannot be acquired within seconds.  Decimal
              fractional values are allowed.  See the -E option for  the  exit
              code  used.  The zero number of seconds is interpreted as --non-
              block.

       --verbose
              Report how long it took to acquire the lock,  or  why  the  lock
              could not be obtained.

       -V, --version
              Display version information and exit.

       -h, --help
              Display help text and exit.

EXAMPLES
       shell1> flock /tmp -c cat
       shell2> flock -w .007 /tmp -c echo; /bin/echo $?
              Set exclusive lock to directory /tmp and the second command will
              fail.

       shell1> flock -s /tmp -c cat
       shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?
              Set shared lock to directory /tmp and the  second  command  will
              not  fail.   Notice  that  attempting to get exclusive lock with
              second command would fail.

       shell> flock -x local-lock-file echo 'a b c'
              Grab the exclusive lock "local-lock-file"  before  running  echo
              with 'a b c'.

       (
         flock -n 9 || exit 1
         # ... commands executed under lock ...
       ) 9>/var/lock/mylockfile
              The  form  is convenient inside shell scripts.  The mode used to
              open the file doesn't matter to flock; using > or >> allows  the
              lockfile  to  be  created if it does not already exist, however,
              write permission is required.  Using < requires  that  the  file
              already exists but only read permission is required.

       [  "${FLOCKER}"  != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0"
       "$@" || :
              This is useful boilerplate code for shell scripts.   Put  it  at
              the top of the shell script you want to lock and it'll automati-
              cally lock itself on the first run.  If the env var $FLOCKER  is
              not  set  to  the  shell  script that is being run, then execute
              flock and grab an exclusive non-blocking lock (using the  script
              itself as the lock file) before re-execing itself with the right
              arguments.  It also sets the FLOCKER env var to the right  value
              so it doesn't run again.

EXIT STATUS
       The  command  uses sysexits.h return values for everything, except when
       using either of the options -n or -w which report a failure to  acquire
       the lock with a return value given by the -E option, or 1 by default.

       When  using  the  command variant, and executing the child worked, then
       the exit status is that of the child command.

AUTHOR
       H. Peter Anvin <hpa@zytor.com>

COPYRIGHT
       Copyright (C) 2003-2006 H. Peter Anvin.
       This is free software; see the source for copying conditions.  There is
       NO  warranty;  not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
       PURPOSE.

SEE ALSO
       flock(2)

AVAILABILITY
       The flock command is part of the util-linux package  and  is  available
       from  Linux  Kernel Archive <ftp://ftp.kernel.org/pub/linux/utils/util-
       linux/>.

util-linux                         July 2014                          FLOCK(1)

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