Run Mongodb Server as Service and mongod instance I am using ubuntu and can run mongodb server by the following command

systemctl start mongodb


But, if i want to run Mongodb by mongod 

mongod --dbpath /var/lib/mongodb


it gives me Permission Denied error. 
Adding sudo before the mongod command fix the error and the Server starts.
But now, earlier command

systemctl start mongodb


won't work and says too many files open 

So, I change the owner of the folder /var/log/mongodb and /var/lib/mongodb by the following commands.

sudo chown -R  mongodb:mongodb /var/log/mongodb
sudo chown -R  mongodb:mongodb /var/lib/mongodb


Now, MongoDb server started by systemctl command but again mongod command gives me error.
So, Is there any way to run Mongodb Server by both ways i.e, by using systemctl and mongod command. 

Thanks
The following command has solved my problem

sudo -u mongodb mongod --dbpath /var/lib/mongodb


The -u mongodb after sudo runs the mongod with mongodb user. So,I don't have to change owner of folder /var/lib/mongodb to mongodb.

Now, I am able to start Mongodb Server as Service and also by mongod.