Flock is a command-line utility that allows for the management of file locks from shell scripts. It can be used to ensure that only one process of a command is running at a time. This is particularly useful in situations where multiple processes may be trying to execute the same command simultaneously, and you want to ensure that only one of them succeeds.
The basic usage of flock is to execute a command while holding a lock on a specified file. If the lock is already held by another process, flock will wait until the lock is released before executing the command. For example, the following command would execute the “my-command” command while holding a lock on the file “my-file.lock”:
# flock my-file.lock --command my-command
You can also specify the option to lock the file exclusively so that other processes cannot open the file at the same time. Flock also provides options for controlling the behavior of the lock, such as setting a timeout for acquiring the lock, or non-blocking mode which will exit immediately if the lock cannot be acquired.
Flock is particularly useful in shell scripts where multiple processes may be trying to perform the same action, and you want to ensure that only one of them succeeds. It can also be used to synchronize access to resources such as databases, where multiple processes may be trying to update the same data.
flock Command Examples
1. Run a command with a file lock as soon as the lock is not required by others:
# flock path/to/lock.lock --command "command"
2. Run a command with a file lock, and exit if the lock doesn’t exist:
# flock path/to/lock.lock --nonblock --command "command"
3. Run a command with a file lock, and exit with a specific error code if the lock doesn’t exist:
# flock path/to/lock.lock --nonblock --conflict-exit-code error_code -c "command"