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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXIT STATUS | EXAMPLES | AUTHORS | COPYRIGHT | SEE ALSO | AVAILABILITY | COLOPHON |
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FLOCK(1) User Commands FLOCK(1)
flock - manage locks from shell scripts
flock [options] file|directory command [arguments]
flock [options] file|directory -c command
flock [options] number
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.
-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.
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.
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.
H. Peter Anvin ⟨hpa@zytor.com⟩
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.
flock(2)
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/⟩.
This page is part of the util-linux (a random collection of Linux
utilities) project. Information about the project can be found
at ⟨https://www.kernel.org/pub/linux/utils/util-linux/⟩. If you
have a bug report for this manual page, send it to
util-linux@vger.kernel.org. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2020-12-18. (At that time, the date of the most recent commit
that was found in the repository was 2020-12-17.) If you
discover any rendering problems in this HTML version of the page,
or you believe there is a better or more up-to-date source for
the page, or you have corrections or improvements to the
information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
util-linux July 2014 FLOCK(1)
Pages that refer to this page: flock(2), losetup(8), lslocks(8)
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