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flock [-sxon] [-w timeout] lockfile [-c] command...
flock [-sxon] [-w timeout] lockdir [-c] command...
flock [-sxun] [-w timeout] fd
This utility manages flock(2) locks from within shell scripts or the
The first and second forms wraps the lock around the executing a com-
mand, in a manner similar to su(1) or newgrp(1). It locks a specified
file or directory, which is created (assuming appropriate permissions),
if it does not already exist.
The third form is convenient inside shell scripts, and is usually used
the following manner:
flock -s 200
# ... commands executed under lock ...
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, how-
ever, write permission is required; using < requires that the file
already exists but only read permission is required.
By default, if the lock cannot be immediately acquired, flock waits
until the lock is available.
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.
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 com-
mand group may have forked a background process which should not
be holding the lock.
-n, --nb, --nonblock
Fail (with an exit code of 1) rather than wait if the lock can-
not be immediately acquired.
-w, --wait, --timeout seconds
Written by H. Peter Anvin <firstname.lastname@example.org>.
Copyright (C) 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
The flock command is part of the util-linux-ng package and is available
flock utility 4 Feb 2006 FLOCK(1)