When one uses a subdirectory (e.g. conf.d) for its prospectors, not clear error message will be generated when (group) permissions are incorrect:
[root@host filebeat]# ls -hal conf.d/
total 8.0K
drwxr-xr-x 2 root root 68 Jul 6 10:28 .
drwxr-xr-x. 3 root root 40 Jul 6 10:28 ..
-rw-rw-r-- 1 root root 198 Jul 6 10:26 filebeat-prospector.yml
-rw-rw-r-- 1 root root 194 Jul 6 10:26 logstash-prospector.yml
[root@host filebeat]# /usr/bin/filebeat.sh -v -c /etc/filebeat/filebeat.yml -configtest -d ""
Exiting: No prospectors defined. What files do you want me to watch?
[root@host filebeat]# chmod g-w conf.d/
[root@host filebeat]# /usr/bin/filebeat.sh -v -c /etc/filebeat/filebeat.yml -configtest -d "*"
Config OK
It would save one a lot of time when the error message does says more about why it isn't loading the sub-configuration files.