Root Cause Analysis of Inconsistent MySQL Client Character Set on RedHat 7

During a routine inspection of newly deployed MySQL 5.7.32 instances on a RedHat 7 server, a client observed that the first batch used the utf8 character set while the second batch defaulted to latin1 . The article explains the investigation process and the root cause.

MySQL charset basics : MySQL distinguishes between server‑level and client‑level character sets. The server charset (controlled by character_set_server ) determines how data is stored, while the client charset (controlled by character_set_client , character_set_connection , and character_set_results ) governs how the client communicates with the server. All three client parameters should be consistent, typically set to utf8 (or utf8mb4 in MySQL 8.0).

Investigation steps :

Verified that the earlier installation indeed used utf8 for the client.

Examined all my.cnf files on the server; both old and new installations only defined character_set_server=utf8mb4 .

Checked the OS locale settings and discovered that the problematic machines were set to en_US while the original ones used en_US.UTF-8 .

Reproduced the issue by changing the server locale to en_US , which caused the MySQL client charset to switch to latin1 .

The MySQL documentation states that client tools have a default charset (latin1 for MySQL 5.7) and that they inherit the OS locale if it maps to a supported MySQL charset; otherwise they fall back to the default. Since en_US maps to latin1 , the client charset changed accordingly.

Fix :

-- Immediate effect (temporary)
export LANG="en_US.UTF-8"

-- Permanent effect
vim /etc/locale.conf
LANG="en_US.UTF-8"
source /etc/locale.conf

After restoring the OS locale to en_US.UTF-8 , the MySQL client charset returned to utf8 .

Conclusion :

The inconsistency was caused by the VM image’s OS locale being set to en_US instead of en_US.UTF-8 .

Server‑level charset is configured via character_set_server .

Client‑level charset depends on three parameters that should match and is also influenced by the server’s OS locale.