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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
       execution  of  a  command,  in  a manner similar to su(1) or newgrp(1).
       They lock a specified file or directory,  which  is  created  (assuming
       appropriate  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

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

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

       -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
              command group may have forked a background process which  should
              not be holding the lock.

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

       -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.

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

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

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

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

       -V, --version
              Display version information 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
              automatically  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 © 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/⟩.



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