title: Create virtual users (users that do not exist in system) in ubuntu for SSH
body: <p>I have a situation where people can login in SSH to run a specific application. The ssh-username is the username for the application to start. If is logged in to a user that exist in the system (/etc/passwd): then SSH drop them to a command shell (for example root get a root shell)</p>
  
  <p>I want to let users login in ssh, but the user which they use to login does not exist in the system. If a user try to login (before password or keyfile authentication) with a non-existent user a bash script is started (root). That bash script can read the login username and can set a list of accepted authorized keys. (no password authentication is allowed). After the login is successful, again a bash script is started (this time it may not be root) in the terminal that they CANNOT exit or cancel. This bash script CAN also read which login username is used. The bash script runs the application (with as argument the used username) under a specific system user (make sure that the application is under that user!) in the SSH shell. I promise this is not a duplicate.</p>
  
  <p>No users must be created in bash scripts.</p>