尝试重新安装 mysql-server 时出现此错误。安装前我也遇到过此命令：
sudo apt-get remove --purge mysql-server mysql-server-5.7
Unpacking mysql-server-core-5.7 (5.7.20-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-server-5.7.
Preparing to unpack .../mysql-server-5.7_5.7.20-0ubuntu0.16.04.1_i386.deb ...
Unpacking mysql-server-5.7 (5.7.20-0ubuntu0.16.04.1) ...
Selecting previously unselected package mysql-server.
Preparing to unpack .../mysql-server_5.7.20-0ubuntu0.16.04.1_all.deb ...
Unpacking mysql-server (5.7.20-0ubuntu0.16.04.1) ...
Processing triggers for man-db (2.7.5-1) ...
Processing triggers for systemd (229-4ubuntu21) ...
Processing triggers for ureadahead (0.100.0-19) ...
ureadahead will be reprofiled on next reboot
Setting up mysql-server-core-5.7 (5.7.20-0ubuntu0.16.04.1) ...
Setting up mysql-server-5.7 (5.7.20-0ubuntu0.16.04.1) ...
update-alternatives: warning: alternative /etc/mysql/my.cnf.fallback (part of link group my.cnf) doesn't exist; removing from list of alternatives
update-alternatives: warning: /etc/alternatives/my.cnf is dangling; it will be updated with best choice
update-alternatives: using /etc/mysql/mysql.cnf to provide /etc/mysql/my.cnf (my.cnf) in auto mode
Renaming removed key_buffer and myisam-recover options (if present)
ERROR: Unable to start MySQL server:
mysqld: Can't read dir of '/etc/mysql/conf.d/' (Errcode: 2 - No such file or directory)
mysqld: [ERROR] Fatal error in defaults handling. Program aborted!
Please take a look at https://wiki.debian.org/Teams/MySQL/FAQ for tips on fixing common upgrade issues.
Once the problem is resolved, run apt-get --fix-broken install to retry.
dpkg: error processing package mysql-server-5.7 (--configure):
subprocess installed post-installation script returned error exit status 1
dpkg: dependency problems prevent configuration of mysql-server:
mysql-server depends on mysql-server-5.7; however:
Package mysql-server-5.7 is not configured yet.
dpkg: error processing package mysql-server (--configure):
dependency problems - leaving unconfigured
Processing triggers for systemd (229-4ubuntu21) ...
No apport report written because the error message indicates its a followup error from a previous failure.
Processing triggers for ureadahead (0.100.0-19) ...
Errors were encountered while processing:
mysql-server-5.7
mysql-server
E: Sub-process /usr/bin/dpkg returned an error code (1)
有人能帮助我吗？