解决：error: 'Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)'

在使用

deamon@deamon-H55M-S2:/usr/bin$ mysqladmin -u root -p shutdown

关闭MySQL之后试图通过：

deamon@deamon-H55M-S2:/usr/bin$ mysqld_safe &

来重启服务器，发现重启失败，显示权限不够：

/usr/bin/mysqld_safe: 548: /usr/bin/mysqld_safe: cannot create /var/lib/mysql/mysqld_safe.pid: Permission denied

于是加上sudo:

deamon@deamon-H55M-S2:~$ sudo mysqld_safe&
[4] 7998

[4]+  Stopped                 sudo mysqld_safe
deamon@deamon-H55M-S2:~$ mysql -u root -p
Enter password: 
ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2)

然后就出现了如题所示的错误。

解决方法是将用户切换成root,再重新启动。

deamon@deamon-H55M-S2:~$ su -
Password: 
root@deamon-H55M-S2:~# mysqld_safe &
[1] 8155
root@deamon-H55M-S2:~# 2016-11-29T11:50:50.182755Z mysqld_safe Logging to syslog.
2016-11-29T11:50:50.186438Z mysqld_safe Logging to '/var/log/mysql/error.log'.
2016-11-29T11:50:50.209538Z mysqld_safe Starting mysqld daemon with databases from /var/lib/mysql

root@deamon-H55M-S2:~# mysql -u root -p
Enter password: 
Welcome to the MySQL monitor.  Commands end with ; or g.
Your MySQL connection id is 2
Server version: 5.7.16-0ubuntu0.16.04.1 (Ubuntu)

Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or 'h' for help. Type 'c' to clear the current input statement.

mysql> 

此时将用户切换回原非root用户，mysql还可正常使用。至此该问题解决。