🧪 Cron Job Automation Lab

📌 Objective

To build a scheduled automation script, observe its behavior, and learn how to debug failures.

⚙️ Part 1 - Script Setup

Create a working directory:

mkdir -p ~/cronlab
cd ~/cronlab

Create the script:

nano backup.sh

#!/bin/bash

DATE=$(date +"%Y-%m-%d_%H-%M-%S")
LOG_FILE="$HOME/cronlab/backup.log"
SOURCE="$HOME"
BACKUP_DIR="$HOME/cronlab/backups"

exec >> "$LOG_FILE" 2>&1

echo "--- RUN $DATE ---"

mkdir -p "$BACKUP_DIR"

rsync -a --exclude="cronlab" \
"$SOURCE/" "$BACKUP_DIR/home-backup-$DATE/"

echo "Backup finished"

Script

Make it executable:

chmod +x backup.sh

Test manually:

./backup.sh

Check log:

cat ~/cronlab/backup.log

⏰ Part 2 - Schedule with Cron

Open crontab:

crontab -e

Add job (every two minutes):

*/2 * * * * /home/samurai/cronlab/backup.sh

Crontab

Verify cron service:

systemctl status cron

Monitor execution:

tail -f ~/cronlab/backup.log

Monitor

💣 Part 3 — Failure Testing

🔴 Test 1 - Broken Command

Replace rsync with:

cp_not_exist /fake/source /fake/dest

Test1

🔴 Test 2 — Missing Full Path

Edit crontab:

*/2 * * * * backup.sh

Test2

👉 Script will not run.

Reason: cron has no working directory context.

🔴 Test 3 - Remove Execute Permission

chmod -x ~/cronlab/backup.sh

👉 Script does not run at all.

👉 No logs appear (script never starts).

🔴 Test 4 - Silent Failure

Remove logging line:

# exec >> "$LOG_FILE" 2>&1

👉 Cron runs, but no visible output.

📜 Part 4 - Logging Strategy

Key Concept

  • stdout - normal output
  • stderr - errors
  • both redirected into one log file

🧠 Key Takeaways

  • Always use absolute paths
  • Failures can be silent
  • Logging must be explicit
  • Some errors occur before the script even starts