Create 'debian-sys-maint' MariaDB user for use of mysqladmin. Just in case you can't use 'root' via 'unix_socket' plugin.

create_debian-sys-maint_for_mysqladmin.sh

#!/bin/sh
MYSQLADMIN_CFG="/etc/mysql/mariadb.conf.d/90-mysqladmin.cnf"
# generate password
PASS=$(perl -e 'print map{("a".."z","A".."Z",0..9)[int(rand(62))]}(1..16)');
# adjust /etc/mysql/debian.cnf (used as defaults file by system scripts)
sed -i "s/^password =.*$/password = ${PASS}/" /etc/mysql/debian.cnf
sed -i "s/^user     =.*$/user     = debian-sys-maint/" /etc/mysql/debian.cnf
# create config file for mysqladmin itself (maybe not needed)
umask 066
cat > ${MYSQLADMIN_CFG} <<EOF
[mysqladmin]
host     = localhost
user     = debian-sys-maint
password = ${PASS}
socket   = /var/run/mysqld/mysqld.sock 
EOF
umask 022
chown 0:0 ${MYSQLADMIN_CFG}; chmod 0600 ${MYSQLADMIN_CFG}
# update credentials
mysql -u root -p -e "GRANT ALL ON *.* TO 'debian-sys-maint'@'localhost' IDENTIFIED BY '${PASS}' WITH GRANT OPTION;"

Google landed me here, useful thanks!