Initialize the .ssh dir of a user using credentials from another user (with root privileges)

init_ssh_user.sh

# Copy and paste the following lines into a shell replacing $1 with the actual username.
# It will then ask for the public key to initialize the .ssh dir.
# The key will be appended to the authorized_keys file; current content (if any) will be preserved.

U=$1 ; H=$(eval echo ~$U) ; sudo mkdir -p $H/.ssh \
&& echo "Paste ssh key: (end with ^D)" \
&& cat > /tmp/$U_key.pub \
&& cat /tmp/$U_key.pub | sudo tee -a $H/.ssh/authorized_keys > /dev/null \
&& rm /tmp/$U_key.pub \
&& sudo chown $U:$U $H/.ssh \
&& sudo chmod 700 $H/.ssh \
&& sudo chown $U:$U $H/.ssh/authorized_keys \
&& sudo chmod 600 $H/.ssh/authorized_keys