i run a server on Ubuntu Server 14.04. I want to create a user that is able to connect via sftp, download 1 file and terminate the connection. The user should not be able to do ANYTHING else. Browsing system files or settings nor command line entries, nothing. Log in, download the file, gg. When the file is updated, log in download it, gg. The file is in the same directory every time and has the same name every time but is updated.
EDIT: I'm trying to implement your solution but i've found some problems: On the config file of the ssh on my configuration there is a line with "allow groups sshdusers". I've added sftp_users. But there's another problem ahead. sudo chown root.root /ubuntu/ is not optimal for me because root is not able to ssh and write to the directory the file that "ubuntu" has to download (we will call it user1. I want user1 to be able to write on the directory and ubuntu to read only. If i change sudo chown user1.user1 /ubuntu/ and i try to sftp with the user ubuntu the connection fails. If i leave it root as you do i can connect but i can't see the currentdirectory i see only / and empy on filezilla. i've even added user1 to sftp_users, but still no success.