100 Days of DevOps, Day 6: Cron's Sneaky OR, and the EC2 Key You Only Get Once

Automation is the part of DevOps that actually earns the title. Day 6 was two of the most everyday automation tasks there are, and both had a trap sitting in plain sight. One Linux task, one AWS task. Schedule a recurring job with cron, and launch an EC2 instance from the AWS CLI. The tasks come from the KodeKloud Engineer platform if you want the same lab to work through. Cron: five fields, one rule that trips everyone Cron is the scheduler that has quietly run Linux automation for decades. You hand it a job and a schedule, and its daemon, crond, wakes up every minute to check whether anything is due. If that daemon is not running, nothing fires, so the first move is making sure it is installed and enabled. # Become root sudo su - # Install the cron daemon if it is missing yum install cronie -y # Enable it at boot, start it now, and confirm it is alive systemctl enable crond.service systemctl start crond.service systemctl status crond.service cronie is the package name on RHEL-family distros. enable makes the service survive a reboot, start brings it up right now, and status is how you confirm it is actually running instead of assuming it is. Now edit the crontab: # Edit the current user's crontab crontab -e # minute hour day-of-month month day-of-week command 0 2 * * * /path/to/script.sh # List what is currently scheduled crontab -l That line runs the script every day at 2am. The five fields, in order, are minute, hour, day of month, month, and day of week: minute: 0 to 59 hour: 0 to 23 day of month: 1 to 31 month: 1 to 12 day of week: 0 to 7, where both 0 and 7 mean Sunday Here is the rule that quietly breaks schedules. When you set both the day-of-month field and the day-of-week field to something other than a star, cron treats them as OR, not AND. So 0 0 13 * 5 does not mean midnight on Friday the 13th. It means midnight on every 13th of the month, and also every Friday, whichever lands first. If you actually want the AND, you restrict one field in cron and che