title: How to add a ssh key to remote server?
body: <p>In my VPS, run <code>ssh-add -l</code> and it returns: <code>The agent has no identities.</code>
  but I have run <code>cat ~/.ssh/id_rsa.pub | ssh user@hostname 'cat &gt;&gt; .ssh/authorized_keys'</code> previously</p>
  
  <p>I then run <code>ssh-add ~/.ssh/authorized_keys</code>
  it returns <code>@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
  @         WARNING: UNPROTECTED PRIVATE KEY FILE!          @
  @@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
  Permissions 0644 for '/home/deployer/.ssh/authorized_keys' are too open.
  It is recommended that your private key files are NOT accessible by others.
  This private key will be ignored.</code></p>
  
  <p>and <code>ssh git@github.com</code> still get report <code>Permission denied (publickey).</code></p>