FLOCK(1)

✨ FLOCK(1) ✨

okay bestie, gather round, because flock — a User Commands girly from the util-linux universe — is about to become your whole personality 💅

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🌸 NAME 🌸

flock — manage locks from shell scripts

like… it's giving "I do locking, and I do it from your shell scripts." iconic behavior fr fr.

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🎀 SYNOPSIS 🎀

flock [options] file|directory command [arguments]

flock [options] file|directory -c command

flock [options] number

three whole forms, three whole vibes, all valid, all slaying 👑

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🦋 DESCRIPTION 🦋

So this little utility manages flock(2) locks from inside shell scripts, or straight up from the command line. no cap.

The first and second forms above wrap the lock around the execution of a command, kinda like how su(1) or newgrp(1) do it. They lock a specified file or directory, and if that thing doesn't exist yet, it gets created (assuming you have the right permissions, obviously). By default? If the lock can't be grabbed immediately, flock just… waits. patiently. like a bestie holding your spot in line 🫶

The third form uses an open file by its file descriptor number. Peep the examples below to see how that one gets used 👀

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💖 OPTIONS 💖

• -c, --command command — pass a single command, with no arguments, to the shell using -c.
• -E, --conflict-exit-code number — the exit status used when the -n option is in use and the conflicting lock exists, or when the -w option is in use and the timeout is reached. default is 1. the number has to be in the range of 0 to 255. no exceptions bestie.
• -F, --no-fork — do not fork before executing command. upon execution, the flock process gets replaced by command, which then continues to hold the lock. ⚠️ this option is incompatible with --close, because otherwise there'd be nothing left to hold the lock. like, at all. nothing. gone.
• -e, -x, --exclusive — obtain an exclusive lock, sometimes called a write lock. this is the default, so you're already doing it, main character 🫶
• -n, --nb, --nonblocking — fail rather than wait if the lock can't be immediately acquired. see the -E option for which exit status gets used.
• -o, --close — close the file descriptor the lock is held on before executing command. super useful if command spawns a child process that shouldn't be holding the lock. rude of them tbh.
• -s, --shared — obtain a shared lock, sometimes called a read lock.
• -u, --unlock — drop a lock. usually you don't need this, because a lock gets automatically dropped when the file is closed. but 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. messy of them 💀
• -w, --wait, --timeout seconds — fail if the lock can't be acquired within seconds. decimal fractional values are allowed, so yes, you can be that precise. see the -E option for the exit status used. and heads up: zero 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 the ones applied via flock(2), but — unlike traditional POSIX fcntl(2) locks (F_SETLK, F_SETLKW) — they have semantics matching those of flock(2). ✨

This is only available on kernel versions >= 3.15.

• --start offset — the starting offset for the lock. this option implies --fcntl.
• --length number — the number of bytes to lock. this option implies --fcntl.
• --verbose — report how long it took to acquire the lock, or why the lock could not be obtained. the gossip we all deserve 🍓
• -h, --help — display help text and exit.
• -V, --version — display version and exit.

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⭐ EXIT STATUS ⭐

The command uses <sysexits.h> exit status values for everything, except when you're using either -n or -w — those report a failure to acquire the lock with an exit status given by the -E option, or 1 by default. and 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. the child is the main character, apparently 👑

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🔥 NOTES 🔥

flock does not detect deadlock. see flock(2) for details. so like… be careful, bestie 😭

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. 💀

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🌈 EXAMPLES 🌈

Note that "shell> " in the examples is a command line prompt.

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Set exclusive lock to directory /tmp and the second command will fail.

shell1> flock /tmp -c cat; shell2> flock -w .007 /tmp -c echo; /bin/echo $?

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Set shared lock to directory /tmp and the second command will not fail. Notice that attempting to get an exclusive lock with the second command would fail.

shell1> flock -s /tmp -c cat; shell2> flock -s -w .007 /tmp -c echo; /bin/echo $?

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Grab the exclusive lock "local-lock-file" before running echo with 'a b c'.

shell> flock -x local-lock-file echo 'a b c'

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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.

(; flock -n 9 || exit 1; # ... commands executed under lock ...; )
9>/var/lock/mylockfile

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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. self-locking queen behavior 💅

[ "${FLOCKER}" != "$0" ] && exec env FLOCKER="$0" flock -en "$0" "$0" "$@" || :

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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.

shell> exec 4<>/var/lock/mylockfile; shell> flock -n 4

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👑 AUTHORS 👑

H. Peter Anvin <hpa@zytor.com>

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🎀 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.

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🧚 SEE ALSO 🧚

flock(2), fcntl(2)

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💜 REPORTING BUGS 💜

For bug reports, use the issue tracker <https://github.com/util-linux/util-linux/issues>.

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🦄 AVAILABILITY 🦄

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/>.

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and that's the whole thing, bestie — this page came to you from util-linux 2.42.4, dated 2026-09-21, and it's the FLOCK(1) manual, mwah 💖 now go lock something (responsibly) ✨