add_key.sh

#!/bin/bash
set -e

COPY_PATH=$(mktemp -t add_key)
MOUNT_PATH=/Volumes/mmb

KEY_EXPIRES_AT=$(date -v18H -v0M -v0S +%s)
NOW=$(date +%s)
KEY_LIFETIME=$(($KEY_EXPIRES_AT - $NOW))
if [ "$KEY_LIFETIME" -lt "1" ]; then
  KEY_LIFETIME=1h
fi

cat > $COPY_PATH << EOF
#!/bin/bash
set -e

ssh-add -t $KEY_LIFETIME $MOUNT_PATH/id_rsa
diskutil eject $MOUNT_PATH
rm $COPY_PATH
EOF

chmod u+x $COPY_PATH

exec $COPY_PATH