Obtain a shared lock, sometimes called a read lock.

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 command group may have forked a background process which should not be holding the lock.

Fail rather than wait if the lock cannot be immediately acquired. See the −E option for the exit code used.

Fail if the lock cannot be acquired within seconds. Decimal fractional values are allowed. See the −E option for the exit code used.

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.

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.

Pass a single command, without arguments, to the shell with −c.

Display help text and exit.

Display version information and exit.

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.

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.