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Scheduled Tasks: Run Commands Automatically

Scheduled tasks run commands automatically. Daily. Weekly. Whenever you need.

Here's the thing: Scheduled tasks are essential. Learn them. Use them.

Cron: The Classic Scheduler​

Crontab Format​

# Format: minute hour day month weekday command
* * * * * command                  # Every minute
0 * * * * command                  # Every hour
0 0 * * * command                  # Daily at midnight
0 0 * * 0 command                  # Weekly on Sunday
0 0 1 * * command                  # Monthly on 1st

My take: Cron format is: minute hour day month weekday. Learn it.

Edit Crontab​

crontab -e                         # Edit your crontab
crontab -l                         # List your crontabs
crontab -r                         # Remove all crontabs

My take: crontab -e edits your crontab. Use it.

Common Cron Patterns​

# Every 5 minutes
*/5 * * * * /path/to/script.sh

# Daily at 2 AM
0 2 * * * /path/to/backup.sh

# Weekly on Monday at 3 AM
0 3 * * 1 /path/to/cleanup.sh

# Monthly on 1st at midnight
0 0 1 * * /path/to/report.sh

My take: These are common patterns. Use them.

systemd Timers: Modern Alternative​

Create Timer​

# /etc/systemd/system/mytimer.timer
[Unit]
Description=Run Backup Daily
Requires=backup.service

[Timer]
OnCalendar=daily
OnCalendar=02:00

[Install]
WantedBy=timers.target

My take: Timers are modern. More flexible than cron.

Timer Options​

OnCalendar=daily                   # Daily
OnCalendar=weekly                  # Weekly
OnCalendar=monthly                 # Monthly
OnCalendar=02:00                   # At 2 AM
OnCalendar=Mon 03:00             # Monday at 3 AM

My take: Timers use calendar expressions. More readable than cron.

Cron vs Timers​

Cron:

  • Simple
  • Traditional
  • Widely used

Timers:

  • More flexible
  • Better logging
  • Modern

My take: Use cron for simple tasks. Use timers for complex scheduling.

Common Patterns​

Daily Backup​

# Cron
0 2 * * * /usr/local/bin/backup.sh

# Timer
OnCalendar=daily
OnCalendar=02:00

Weekly Cleanup​

# Cron
0 3 * * 0 /usr/local/bin/cleanup.sh

# Timer
OnCalendar=weekly
OnCalendar=Mon 03:00

Common Mistakes (I've Made These)​

  1. Wrong timezone: Cron uses system timezone. Make sure it's correct.

  2. Not setting PATH: Cron has limited PATH. Use full paths in scripts.

  3. Not redirecting output: Cron output goes to email. Redirect to file.

  4. Wrong format: Cron format is strict. One mistake breaks it.

  5. Not testing: Test cron jobs. Make sure they work.

Real-World Examples​

Daily Backup Cron​

0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Systemd Timer​

# Enable and start
sudo systemctl enable mytimer.timer
sudo systemctl start mytimer.timer

What's Next?​

Now that you can schedule tasks, let's talk about Service Troubleshooting when things go wrong.

Personal note: I used cron for everything. Then I learned timers. Now I use timers for complex scheduling. Cron for simple tasks. Both work. Use what fits.