DevOps Day 6: Cron's OR Trap and the One-Time EC2 Key Pair Explained

A DevOps practitioner documenting a 100-day learning series tackled two common Linux and AWS automation tasks on Day 6, using the KodeKloud Engineer lab platform. The cron walkthrough highlighted a widely misunderstood scheduling rule: setting both the day-of-month and day-of-week fields to non-wildcard values triggers an OR condition, not AND, causing jobs to fire more often than intended. Additional cron pitfalls covered include its minimal runtime environment requiring absolute binary paths, and the need to manually redirect output to a log file since failed jobs produce no visible error by default. On the AWS side, launching an EC2 instance via the CLI requires pre-gathering four dependencies — an AMI ID, security group, subnet, and key pair — with a warning that AMI IDs are region-specific and change frequently. The write-up also stressed that an EC2 private key is only available for download at the moment of creation, making it a one-time retrieval that cannot be recovered if missed.