flock - manage locks from shell scripts at Linux.org

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FLOCK(1) User Commands FLOCK(1)

NAME
flock - manage locks from shell scripts

SYNOPSIS
flock [options] <file|directory> <command> [command args]
flock [options] <file|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 new‐
grp(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 rather than wait if the lock cannot be immediately acquired. See the -E option for the exit code
used.

-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.

-o, --close
Close the file descriptor on which the lock is held before executing command . This is useful if com‐
mand spawns a child process which should not be holding the lock.

-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.

-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

(
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 else but an options -n or -w failures which return
either the value given by the -E option, or 1 by default.

AUTHOR
H. Peter Anvin ⟨[email protected]⟩