Starting MySQL. ERROR! The server quit without updating PID file (/usr/local/mysql-8.0/data//localhost.localdomain.pid).

This error typically occurs when there is an issue with starting the MySQL server. Here are a few steps you can try to troubleshoot the problem:

1. Check the error log: Open the MySQL error log file, which is usually located at `/var/log/mysql/error.log` or `/usr/local/mysql/data/hostname.err`. Look for any error messages or warnings that might provide more information about the issue.

2. Verify file permissions: Ensure that the MySQL data directory and its files are owned by the MySQL user and group. You can use the following command to check and fix the permissions:

   chown -R mysql:mysql /var/lib/mysql

3. Remove the PID file: If the PID file mentioned in the error message exists, try removing it and restarting MySQL. Use the following command to remove the file:

   sudo rm /usr/local/mysql-8.0/data//localhost.localdomain.pid

4. Check disk space: Make sure that there is sufficient disk space available on your server. Use the following command to check disk usage:

   df -h

5. Check for conflicting processes: There might be another instance of MySQL already running or some other process using the same port (usually 3306). Use the following command to check for any running MySQL processes:

   ps aux | grep mysql

If none of these steps resolve the issue, please provide more details about your environment, MySQL version, and any recent changes you made that might have caused this problem.