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

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

       -E, --conflict-exit-code number
              The exit status 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.
              The number has to be in the range of 0 to 255.

       -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 otherwise 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 status 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.

       -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 command 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 status used. The zero number of seconds is
              interpreted as --nonblock.

       --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.

EXIT STATUS

       The command uses sysexits.h exit status values for everything, except when using either of
       the  options  -n or -w which report a failure to acquire the lock with a exit status given
       by the -E option, or 1 by default.  The exit status given by -E has to be in the range  of
       0 to 255.

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

EXAMPLES

       Note that "shell> " in examples is a command line prompt.

       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.

       shell> exec 4<>/var/lock/mylockfile
       shell> flock -n 4
              This  form  is  convenient  for  locking a file without spawning a subprocess.  The
              shell opens the lock file for reading and writing as file descriptor 4, then  flock
              is used to lock the descriptor.

AUTHORS

       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 ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩.