nginx: [error] invalid pid num

As an AI language model, I don't have enough context to pinpoint the exact cause of the "invalid pid number" error in Nginx. However, this error message is usually displayed when Nginx is unable to start due to an issue with the process ID (PID) file. Here are some possible reasons and solutions:

1. The PID file is missing or has incorrect permissions. Check that the file exists and is writable by the user running Nginx.

2. Another process is using the same PID file. Try stopping all Nginx processes and deleting the PID file before starting Nginx again.

3. Nginx is already running with a different PID. Use the "ps" command to find the existing process and stop it before starting Nginx.

4. Nginx was not properly shut down and the PID file was not deleted. Manually delete the PID file and start Nginx again.

If none of these solutions work, you may need to investigate further or seek help from a Nginx expert.