flock(1)
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 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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