Simple one liner to generate an SSH key without a password in the default place with the comment as hostname then print out the public key for copy and paste.

generate-ssh-key

HOSTNAME=`hostname` ssh-keygen -t rsa -C "$HOSTNAME" -f "$HOME/.ssh/id_rsa" -P "" && cat ~/.ssh/id_rsa.pub

To optimise further without piping text or using HOSTNAME variable, I suppose this could also be used if you wanted to override existing key

rm -f "$HOME/.ssh/id_rsa"; ssh-keygen -t rsa -C "`hostname`" -f "$HOME/.ssh/id_rsa" -P "" -q

or this if you didnt want to override:

test -f "$HOME/.ssh/id_rsa" || ssh-keygen -t rsa -C "`hostname`" -f "$HOME/.ssh/id_rsa" -P "" -q

P.S. on my version of ssh-keygen I don't get any interactive prompt requiring y or n when I pass -P ""

Why not auto copy in clipboard ?
You need to install xclip either in linux or mac:

linux: sudo apt-get install xclip

mac: brew install xclip

and then append to any of the above commands:

| xclip -selection clipboard