find

https://www.linode.com/docs/tools-reference/tools/find-files-in-linux-using-the-command-line/

find /home/username/ -name "*.err"

find and remove older than https://www.howtogeek.com/howto/ubuntu/delete-files-older-than-x-days-on-linux/
find /path/to/files* -mtime +5 -exec rm {} \;