Flock

This utility manages flock locks from within shell scripts or the command line.

The first and second forms wraps the lock around the executing a command, in a manner similar to su or newgrp. 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.

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.

By default, if the lock cannot be immediately acquired, flock waits until the lock is available.

Flags

-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 (with an exit code of 1) rather than wait if the lock cannot be immediately acquired. 

-w, --wait, --timeout seconds

   Fail (with an exit code of 1) if the lock cannot be acquired within seconds seconds. Decimal fractional values are allowed. 

-o, --close

   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. 

-c, --command command

   Pass a single command to the shell with -c. 

-h, --help

   Print a help message.

Example

This will prevent rsync from running in the same manner:

   flock -n rsync_lock_file -c "rsync ..."