title: Jupyter command `jupyter-lab` not found
body: <p>I have tried to install jupyter lab on my Kubuntu machine. 
  If I install jupyter lab with 'pip3 install jupyter jupyterlab' the command 'jupyter notebook' works completly fine. But if I try to run 'jupyter lab' every time I get the message:</p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/local/bin/jupyter", line 11, in &lt;module&gt;
      sys.exit(main())
    File "/usr/local/lib/python3.6/dist-packages/jupyter_core/command.py", line 230, in main
      command = _jupyter_abspath(subcommand)
    File "/usr/local/lib/python3.6/dist-packages/jupyter_core/command.py", line 133, in _jupyter_abspath
      'Jupyter command `{}` not found.'.format(jupyter_subcommand)
  Exception: Jupyter command `jupyter-lab` not found.
  </code></pre>
  
  <p>What is wrong?</p>
  
  <p>I tried to reinstall jupyter and jupyterlab multiple times with the same issue.</p>