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FLOCK(1)                                            User Commands                                            FLOCK(1)



NAME
       flock - manage locks from shell scripts

SYNOPSIS
       flock [options] <file|directory> <command> [command args]
       flock [options] <file|directory> -c <command>
       flock [options] <file descriptor number>

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

       The  first and second forms wrap the lock around the executing a command, in a manner similar to su(1) or new‐
       grp(1).  It locks 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 open file by file descriptor number.  See examples 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  com‐
              mand 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
              Print a help message.

       -V, --version
              Show version number and exit.

EXAMPLES

       (
         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" "[email protected]" || :
              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 else but an options -n or -w failures which return
       either the value given by the -E option, or 1 by default.

AUTHOR
       H. Peter Anvin ⟨[email protected]⟩

COPYRIGHT
       Copyright © 2003-2006 H. Peter Anvin.
       This is free software; see the source for copying conditions.   There  is  NO  warranty;  not  even  for  MER‐
       CHANTABILITY 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                                          September 2011                                           FLOCK(1)