[hf]

Both scenarios you mentioned would, I believe, benefit from using keychain (see below).

Let's suppose I have an account tests@host which runs the tests (scripts) that need to login to an array of machines.

In order for keychain to be helpful here, you need two prerequisites.

1) You need to be able to interactively login to tests@host once after bootup; after that you don't need to touch the machine again.

2) Then, the test scripts need to say

    . $HOME/.keychain/$HOSTNAME-sh

once before executing any ssh command (the line above simply imports the ssh-agent session variables into the current environment).

edit: I removed the Nagios references as other posters rightly point out that there are more endemic ways to collect information with Nagios.