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NAME | SYNOPSIS | DESCRIPTION | OPTIONS | EXIT STATUS | NOTES | EXAMPLES | AUTHORS | COPYRIGHT | SEE ALSO | REPORTING BUGS | AVAILABILITY |
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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.
--fcntl
Instead of flock(2), apply an fcntl(2) open file description
lock (that is, using the F_OFD_SETLK (non-blocking) or
F_OFD_SETLKW (blocking) commands). These locks are independent
of those applied via flock(2), but, unlike traditional POSIX
fcntl() locks (F_SETLK, F_SETLKW), have semantics matching
those of flock(2).
This is only available on kernel versions >= 3.15.
--start offset
The starting offset for lock. Ths option implies --fcntl.
--length number
The number of bytes to lock. Ths option implies --fcntl.
--verbose
Report how long it took to acquire the lock, or why the lock
could not be obtained.
-h, --help
Display help text and exit.
-V, --version
Display version 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 an 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.
flock does not detect deadlock. See flock(2) for details.
Some file systems (e. g. NFS and CIFS) have a limited
implementation of flock(2) and flock may always fail. For details
see flock(2), nfs(5) and mount.cifs(8). Depending on mount
options, flock can always fail there.
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 environment
variable $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 environment variable 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 <[email protected]>
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), fcntl(2)
For bug reports, use the issue tracker
<https://github.com/util-linux/util-linux/issues>.
The flock command is part of the util-linux package which can be
downloaded 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
[email protected]. This page was obtained from the
project's upstream Git repository
⟨git://git.kernel.org/pub/scm/utils/util-linux/util-linux.git⟩ on
2025-08-11. (At that time, the date of the most recent commit that
was found in the repository was 2025-08-05.) 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 [email protected]
util-linux 2.42-start-521-ec46 2025-08-09 FLOCK(1)
Pages that refer to this page: flock(2), losetup(8), lslocks(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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