Automatic Update Script using Cron, Exit Codes, and Logging

Automatic Update Script using Cron, Exit Codes, and Logging

I'm managing multiple machines running Debian, Raspian, and Mint. On some of the machines I want to have updating and upgrading automatically. I've drafted a script that I want to do this and log if the update are successful or not.
#!/bin/bash

captains_log=/home/jason/mission_control/captains.log

apt-get update;
if [ $? == 0 ]; then
    apt-get upgrade -y;
    if [ $? == 1 ]; then
        echo `date`": Daily update failed" >> $captains_log
    else
        echo `date`": Daily update successful" >> $captains_log
    fi
else
    echo `date`": Daily update failed" >> $captains_log
fi
I've set the script to run @daily in a root crontab. I run the command manually and it executes as desired. When cron runs the script, I get success in the log but my software is not getting updated.
Can someone tell me where I'm going wrong?