This utility manages
locks from within shell scripts or the command line.
The first form wraps the lock around the executing a command, in a manner similar to
It locks a specified file, which is created (assuming appropriate
permissions), if it does not already exist.
The second form is conveninent 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, however, write permission is required; using <
requires that the file already exists but only read permission is
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
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
-n, --nb, --nonblock
Fail (with an exit code of 1) rather than wait if the lock cannot be
-w, --wait, --timeoutseconds
Fail (with an exit code of 1) if the lock cannot be acquired within
seconds seconds. Decimal fractional values are allowed.
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 hold ing the lock.