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 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.

       -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
              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.

       -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 code 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.

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

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.

util-linux                                          July 2014                                           FLOCK(1)