MySQL Initialization Failure Caused by Missing Newline in Configuration File

Background

During the initialization of a MySQL 5.7.44 instance, the command /opt/mysql/base/5.7.44/bin/mysqld --defaults-file=/u01/my.cnf --datadir=/u01/3306 --basedir=/opt/mysql/base/5.7.44/ --initialize fails with the error "Can't read dir of '/etc/my.cnf.' (Errcode: 2 - No such file or directory)" and aborts.

Steps to Resolve

The error points to a missing /etc/my.cnf. directory. Inspection of my.cnf shows only an !includedir /etc/my.cnf.d directive, and the directory /etc/my.cnf.d does exist.

[root@db1 ~]# cat /u01/my.cnf|grep /etc/my.cnf
!includedir /etc/my.cnf.d

[root@db1 ~]# ls -l /etc/my.cnf.d
total 4
-rw-r--r-- 1 root root 232 May 6 2020 mysql-clients.cnf

Running the server with --help still produces the same error, indicating the problem lies in the configuration file itself. Removing the !includedir /etc/my.cnf.d line allows the server to start.

[root@db1 ~]# cp /u01/my.cnf /u01/my.cnf.bak
[root@db1 ~]# vi /u01/my.cnf.bak
[root@db1 ~]# /opt/mysql/base/5.7.44/bin/mysqld --defaults-file=/u01/my.cnf.bak --help
... (output shows successful start) ...

Adding a newline to the original /u01/my.cnf file resolves the initialization issue:

[root@db1 ~]# echo >> /u01/my.cnf
[root@db1 ~]# tail -1 /u01/my.cnf
!includedir /etc/my.cnf.d
[root@db1 ~]# /opt/mysql/base/5.7.44/bin/mysqld --defaults-file=/u01/my.cnf --datadir=/u01/3306 --basedir=/opt/mysql/base/5.7.44/ --initialize

Root Cause Analysis

Using strace on the failing run shows the server attempts to open "/etc/my.cnf./" (note the trailing dot) and fails, because the last line of the configuration file lacks a newline. The MySQL parser reads the file in 4096‑byte blocks; without a terminating newline, the final !includedir or !include directive is truncated, leading to an incorrect path.

strace -T -tt -s 100 -o /tmp/strace.log /opt/mysql/base/5.7.44/bin/mysqld --defaults-file=/u01/my.cnf ...
... openat(AT_FDCWD, "/etc/my.cnf./", O_RDONLY|O_NONBLOCK|O_CLOEXEC|O_DIRECTORY) = -1 ENOENT ...
... write(2, "mysqld: Can't read dir of '/etc/my.cnf.' ...") = 73 ...

When the final line ends with !includedir or !include and lacks a newline, the parser truncates the last character of the included filename, causing a "file not found" situation. Adding an explicit newline prevents this truncation.

Summary

If the last line of my.cnf is an !includedir directive without a trailing newline, MySQL truncates the final character, reports "Can't read dir of '/etc/my.cnf.'" and aborts initialization.

If the last line is an !include directive without a newline, the referenced filename is truncated but the server may still start.

Therefore, always ensure the configuration file ends with a newline character.

Supplement

Two common ways to create a file without a trailing newline are:

Using echo -n "xx" >> my.cnf

Using printf "xx" >> my.cnf

Any other scenarios that lead to a missing newline are welcome for discussion.