sshon: add a specified ssh key only when necessary.

sshon.sh

#!/bin/sh
SSH_HOME=$HOME/.ssh
SSH_LOG=${SSH_AGENT_LOG:-$SSH_HOME/ssh-agent.log}
SSH_KEY=${1:-$SSH_HOME/id_ed25519}
KEY_DURATION=1d

IFS=$'\n'
sig=$(ssh-keygen -l -f "$SSH_KEY")
for i in $(ssh-add -l); do
    if [ "$i" = "$sig" ]; then
        echo "$sig"
        exit
    fi
done

ssh-add -t "$KEY_DURATION" "$SSH_KEY" &&
    echo "$(date) added: $SSH_KEY" >>$SSH_LOG;