In Linux we have one command "which" which can be used to know the location of executable.
For example
[root@localhost] # which radiusd
/usr/local/sbin/radiusd
[root@localhost] # which pppoe
/usr/sbin/pppoe
For example
[root@localhost] # which radiusd
/usr/local/sbin/radiusd
[root@localhost] # which pppoe
/usr/sbin/pppoe