I'd like to add the following instructions on how to reproduce the problem:
$ apt purge maria* # if mariadb was installed previously. also select "remove all mariadb databases" during purging $ debconf-set-selections <<< "mariadb-server-10.1 mysql-server/root_password password hello" $ debconf-set-selections <<< "mariadb-server-10.1 mysql-server/root_password_again password hello" $ debconf-get-selections |grep maria mariadb-server-10.1 mysql-server/root_password password hello mariadb-server-10.1 mysql-server/root_password_again password hello $ apt install -y mariadb-server-10.1 $mysql mysql <<< "select Host,User,Password,plugin from user;" Host User Password plugin localhost root unix_socket Also, the password remains in the debconf database, which is a security issue: $ debconf-get-selections |grep maria mariadb-server-10.1 mysql-server/root_password password hello mariadb-server-10.1 mysql-server/root_password_again password hello mariadb-server-10.1 mariadb-server-10.1/postrm_remove_databases boolean false mariadb-server-10.1 mariadb-server-10.1/old_data_directory_saved note mariadb-server-10.1 mariadb-server-10.1/nis_warning note The same is valid when using mariadb-server-10.1/root_password as key: $ debconf-get-selections |grep maria mariadb-server-10.1 mariadb-server-10.1/root_password password hello mariadb-server-10.1 mariadb-server-10.1/root_password_again password hello mariadb-server-10.1 mariadb-server-10.1/nis_warning note mariadb-server-10.1 mariadb-server-10.1/postrm_remove_databases boolean false mariadb-server-10.1 mariadb-server-10.1/old_data_directory_saved note