Troubleshooting MySQL Configuration Error Caused by Hidden Unicode Space

1 Background Information

The client needed to migrate database data to the DMP platform and added the parameter lower_case_table_names=0 to the MySQL configuration, but the database failed to start.

mysql-error.log

...........
2023-08-23T15:22:01.554471+08:00 0 [Note] Plugin 'FEDERATED' is disabled.
2023-08-23T15:22:01.557909+08:00 0 [Note] Semi-sync replication initialized for transactions.
2023-08-23T15:22:01.557926+08:00 0 [Note] Semi-sync replication enabled on the master.
2023-08-23T15:22:01.557976+08:00 0 [ERROR] unknown variable 'lower_case_table_names
= 0'
2023-08-23T15:22:01.557982+08:00 0 [Note] Starting ack receiver thread
2023-08-23T15:22:01.557987+08:00 0 [ERROR] Aborting
...........

Analysis

Commenting out the added parameter allowed MySQL to start, indicating the problem lay with the parameter itself. Re‑typing the same parameter manually resolved the issue, confirming that the copied text contained hidden characters.

# cat /opt/mysql/etc/4444/my.cnf |tail -n 3
#lower_case_table_names = 0

lower_case_table_names = 0
systemctl restart mysqld_4444.service

2 Local Reproduction

After pasting the parameter into the configuration file and restarting, MySQL failed to start with the same error.

# cat /opt/mysql/etc/4444/my.cnf |tail -n 2
lower_case_table_names = 0
systemctl restart mysqld_4444.service
... (error log shown) ...

Manually typing the parameter and restarting succeeded, proving the hidden character issue.

# cat /opt/mysql/etc/4444/my.cnf |tail -n 2
#lower_case_table_names = 0
lower_case_table_names = 0
systemctl restart mysqld_4444.service
ps -ef |grep mysqld

Method 1: hexdump

Using hexdump reveals a special space (302 240) before the ‘=’ sign, which is a non‑breaking space (UTF‑8) incompatible with ASCII.

Method 2: od

The od command shows the same 302 240 byte sequence, confirming the presence of the non‑breaking space.

Method 3: Editor

Opening the file in Sublime Text with UTF‑8 encoding highlights the special character <0xa0> , identified as a Unicode non‑breaking space.

3 Summary

When adding parameters to MySQL configuration files, avoid copying directly from documents or web pages; hidden non‑breaking spaces can cause startup failures. Prefer using platform tools (e.g., DMP) or manually typing parameters, and always verify the file encoding before restarting.

If large numbers of parameters must be pasted, inspect the pasted content for special characters before restarting.