Quickly update multiple remote Debian/Ubuntu systems.

update-all.sh

#!/bin/bash

# Uncomment the next line to start any pending updates automatically:
#yes="--yes"

for host in host1 host2 # etc.
do
    echo -e "\e[0;33m$host\e[0m"

    ssh "$host" -t "
        # Update package index if it's older than 24 hours.
        test \$((\$(date +%s) - \$(stat -c %Y /var/cache/apt))) -gt $((24*60*60)) && sudo apt-get update;
        # Simulate an update to check if something would be installed. If so, start a real update.
        apt-get -s dist-upgrade | grep -E '[1-9]' -q && sudo apt-get dist-upgrade $yes"

done