title: how to install mongodb on ubuntu 16.04.1?
body: <p>I have used the instructions from the <a href="https://docs.mongodb.com/manual/tutorial/install-mongodb-on-ubuntu/" rel="nofollow noreferrer">docs</a></p>
  
  <p>after this command:</p>
  
  <pre><code>sudo apt-get install -y mongodb-org
  </code></pre>
  
  <p>I get this warning:</p>
  
  <pre><code>WARNING: The following packages cannot be authenticated!
    mongodb-org-shell mongodb-org-server mongodb-org-mongos mongodb-org-tools mongodb-org
  E: There were unauthenticated packages and -y was used without --allow-unauthenticated
  </code></pre>
  
  <p>Its instruction to start and test gives the following:</p>
  
  <pre><code>sudo service mongod start
  
  cat /var/log/mongodb/mongod.log
  cat: /var/log/mongodb/mongod.log: No such file or directory
  </code></pre>