Login as normal user without password after your installation and create user as follow
use admin
db.createUser(
{
user: "root",
pwd: "root",
roles: [
{ role: "userAdminAnyDatabase", db: "admin" },
{ role: "readWriteAnyDatabase", db: "admin" },
{ role: "dbAdminAnyDatabase", db: "admin" },
{ role: "clusterAdmin", db: "admin" }
]
})
There after restart your db as follow
sudo service mongod stop
sudo service mongod start
Then, can able to login with the username and password. if you are not able to login just restart your machine.
mongo --port 27017 -u "tom" -p "jerry" --authenticationDatabase "admin"