expire_log_days can't be set in my.cnf

I’m running 5.7.19-17-log and I can’t set expire_log_days in the my.cnf. I’ve tried under [mysqld] and [mysqld-safe]. Is this a bug? error logfile message: 2017-12-06T17:38:26.486560Z 0 [ERROR] unknown variable ‘expire_log_days=5’

hello I think the parameter is expire_logs_days (plural on logs)?
Easily done. There is a bit of a discussion about expire_logs_days in this blog post too https://www.percona.com/blog/2014/01/28/10-mysql-performance-tuning-settings-after-installation/