FLOCK(1)

NAME
flock - manage locks from shell scripts

SYNOPSIS
flock [options] <file> -c <command>
flock [options] <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 newgrp(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 (with an exit code of 1) rather than wait if the lock cannot be immediately acquired.

-w, --wait, --timeout seconds
    Fail (with an exit code of 1) if the lock cannot be acquired within seconds. Decimal fractional values are allowed.

-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.

-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

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.

(
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.

EXIT STATUS
The command uses sysexits.h return values for everything else but an options -n or -w failures which return 1.

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/).