It should be easy, but when I put these lines:

log_slow_queries = 1
long_query_time = 1
log-slow-queries = /var/log/mysql/slow_query.log
log_queries_not_using_indexes

in [mysqld], MariaDB fails to restart.

I looked at the docs, but could not figure out what is wrong in my configs.

Also , I've set the file permission to /var/log/mysql/slow_query.log

# ls -al /var/log/mysql/slow_query.log 
-rw-rw-r-- 1 mysql mysql 744682 Dec  4 23:46 /var/log/mysql/slow_query.log

However, I can set the parameters in command line:

SET GLOBAL slow_query_log_file = '/var/log/mysql/slow_query.log';
SET GLOBAL LONG_QUERY_TIME = 1;
SET GLOBAL slow_query_log = 'ON';

and see that they are set:

+---------------------+--------------------------------------------------------------------------------------------------------------+
| Variable_name       | Value                                                                                                        |
+---------------------+--------------------------------------------------------------------------------------------------------------+
| log_slow_filter     | admin,filesort,filesort_on_disk,full_join,full_scan,query_cache,query_cache_miss,tmp_table,tmp_table_on_disk |
| log_slow_rate_limit | 1                                                                                                            |
| log_slow_verbosity  |                                                                                                              |
| slow_launch_time    | 2                                                                                                            |
| slow_query_log      | ON                                                                                                           |
| slow_query_log_file | /var/log/mysql/slow_query.log                                                                                |
+---------------------+--------------------------------------------------------------------------------------------------------------+
6 rows in set (0.00 sec)

and even the slow queries are being written to the log. But I want to make the config persistent by writing them to my.cnf. So appreciate your help.