SSH  linux  Putty  .ppk  OpenSSH 

How to convert a putty private key file (.ppk) to openssh format on Mac operating system.

SSH  linux utilities 

Information and quick-use guide for the linux utility 'ssh_copy_id'

Ubuntu  Ubuntu 16.04  SSH  SFTP  system administration  config files 

How to grant users SFTP access via the /etc/ssh/sshd_config file in Ubuntu 16.04