# Fist argument is the names of the service, i.e. a file in /etc/init.d
complete -c service -n "test (count (commandline -poc)) = 1" -xa "(command ls /etc/init.d)" -d "Service name"

#The second argument is what action to take with the service
complete -c service -n "test (count (commandline -poc)) -gt 1" -xa '$__fish_service_commands'