Too many connections” error mysql

$ mysql –u root –p
SHOW STATUS LIKE 'max_used_connections';

$ mysql –u root –p
mysql> SHOW VARIABLES LIKE 'max_connections';
+-----------------+-------+
| Variable_name   | Value |
+-----------------+-------+
| max_connections | 151   |
+-----------------+-------+

# vi /etc/my.cnf
max_connections = 512

MySQL my.cnf file - Found option without preceding group

Just add [mysqld] as first line in the /etc/mysql/my.cnf file.
Example
[mysqld]
max_connections = 512

If on Ubuntu try without the 'd'

systemctl start mysql