Using the flock Command for Process Locking (with examples)

1: Running a command with a file lock as soon as the lock is not required by others

flock path/to/lock.lock --command "command"

Motivation: Sometimes, it is necessary to ensure that only one instance of a command is running at a time to avoid conflicts or inconsistencies. The “flock” command helps in managing locks from shell scripts, allowing us to ensure exclusive access to a resource.

Explanation:

• flock: The command to manage locks.
• path/to/lock.lock: The path to the lock file. It can be any file name or location. The lock file is used to coordinate and enforce exclusive access.
• --command "command": The command to be executed while holding the lock. Replace “command” with the actual command you want to run.

Example Output:

$ flock /tmp/mylock.lock --command "echo Test"
Test

In the above example, the command “echo Test” is executed while holding the lock /tmp/mylock.lock. If the lock is already held by another process, the command will wait until the lock is released.

2: Running a command with a file lock and exiting if the lock doesn’t exist

flock path/to/lock.lock --nonblock --command "command"

Motivation: In certain situations, it is important to quickly determine the availability of a lock and proceed accordingly. By using the --nonblock option with flock, the lock is checked, and if it doesn’t exist, the command is not executed.

Explanation:

• --nonblock: The option to make flock return immediately if the lock is not available.
• --command "command": The command to be executed while holding the lock. Replace “command” with the actual command you want to run.

Example Output:

$ flock /tmp/mylock.lock --nonblock --command "echo Test"
flock: /tmp/mylock.lock: Resource temporarily unavailable

In the above example, the flock command tries to acquire the lock /tmp/mylock.lock, but if the lock is already held by another process, it immediately returns with an error message.

3: Running a command with a file lock, and exiting with a specific error code if the lock doesn’t exist

flock path/to/lock.lock --nonblock --conflict-exit-code error_code -c "command"

Motivation: When integrating flock into a larger script or workflow, it can be useful to handle different scenarios based on the availability of the lock. By specifying a custom conflict exit code, we can differentiate between cases where the lock is available and where it isn’t.

Explanation:

• --nonblock: The option to make flock return immediately if the lock is not available.
• --conflict-exit-code error_code: The exit code to be used if the lock is already held. Replace “error_code” with the desired value.
• -c "command": The command to be executed while holding the lock. Replace “command” with the actual command you want to run.

Example Output:

$ flock /tmp/mylock.lock --nonblock --conflict-exit-code 127 -c "echo Test"
flock: /tmp/mylock.lock: Resource temporarily unavailable
$ echo $?
127

In the above example, the flock command tries to acquire the lock /tmp/mylock.lock, but if the lock is already held by another process, it returns with the specified exit code (127 in this case). The exit code can be used for conditional handling in the script or workflow where the flock command is used.

Using the flock command provides a convenient way to manage locks from shell scripts, ensuring exclusive access to resources. By utilizing its various options, such as --nonblock and --conflict-exit-code, we can handle different scenarios based on the availability of locks. These examples demonstrate how to effectively utilize the flock command to manage locks and control the execution of commands.