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⏰ Chapter 14: Cron Jobs & Task Scheduling

Intermediate Chapter 14

📑 Table of Contents

What is Cron?

Cron is a time-based job scheduler. It runs commands or scripts at specified intervals.

🏠 Analogy: Cron is like setting an alarm clock — but instead of waking you up, it runs a command.

Crontab Syntax

┌───────────── minute (0-59)
│ ┌───────────── hour (0-23)
│ │ ┌───────────── day of month (1-31)
│ │ │ ┌───────────── month (1-12)
│ │ │ │ ┌───────────── day of week (0-7, 0 and 7 = Sunday)
│ │ │ │ │
* * * * * command_to_execute

Special Characters

Character Meaning Example
* Every value * * * * * = every minute
, Multiple values 1,15,30 = at 1, 15, 30
- Range 1-5 = 1 through 5
/ Step */15 = every 15
@reboot Run once at startup @reboot /script.sh

Quick Reference

Schedule Cron Expression Description
Every minute * * * * * Runs every minute
Every hour 0 * * * * At minute 0 of every hour
Every day at midnight 0 0 * * * 12:00 AM daily
Every day at 6 AM 0 6 * * * 6:00 AM daily
Every Monday 0 0 * * 1 Monday at midnight
Every weekday 0 9 * * 1-5 Mon-Fri at 9 AM
Every 15 minutes */15 * * * * :00, :15, :30, :45
First of month 0 0 1 * * Midnight, 1st day
Every 6 hours 0 */6 * * * 00:00, 06:00, 12:00, 18:00

Short Names

Shortcut Equivalent
@yearly 0 0 1 1 *
@monthly 0 0 1 * *
@weekly 0 0 * * 0
@daily 0 0 * * *
@hourly 0 * * * *
@reboot At system startup

Managing Crontabs

crontab -e                   # Edit your crontab
crontab -l                   # List your crontab
crontab -r                   # Remove your crontab (careful!)
crontab -u alice -l           # List alice's crontab (root only)
sudo crontab -e              # Edit root's crontab

Practical Examples

# Edit crontab
crontab -e

# Backup home directory every day at 2 AM
0 2 * * * tar -czf /backup/home-$(date +\%Y\%m\%d).tar.gz /home/sovon

# Check disk space every hour, alert if > 90%
0 * * * * df -h | awk '$5 > 90 {print}' | mail -s "Disk Alert" admin@example.com

# Rotate logs every Sunday at 3 AM
0 3 * * 0 /usr/sbin/logrotate /etc/logrotate.conf

# Update packages every day at 4 AM
0 4 * * * apt update && apt upgrade -y >> /var/log/auto-update.log 2>&1

# Clean /tmp every day at midnight
0 0 * * * find /tmp -type f -mtime +7 -delete

# Send a report every Friday at 5 PM
0 17 * * 5 /home/sovon/scripts/weekly-report.sh

# Run at reboot
@reboot /home/sovon/scripts/startup.sh

Logging Cron Output

# Log stdout and stderr to file
0 * * * * /script.sh >> /var/log/myjob.log 2>&1

# Discard all output
0 * * * * /script.sh > /dev/null 2>&1

# Send output as email (if mail is configured)
MAILTO="sovon@example.com"
0 6 * * * /script.sh

Environment in Cron

# Cron runs with a minimal environment! Set paths explicitly:
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
SHELL=/bin/bash
HOME=/home/sovon
MAILTO=""

# Or use full paths in your commands
0 * * * * /usr/bin/python3 /home/sovon/scripts/myscript.py

⚠️ Common Gotcha: Cron doesn't load your .bashrc. Commands may fail because the PATH is different. Always use full paths!

System Cron Directories

# Scripts placed here run automatically:
/etc/cron.d/           # Custom cron files
/etc/cron.hourly/      # Run every hour
/etc/cron.daily/       # Run every day
/etc/cron.weekly/      # Run every week
/etc/cron.monthly/     # Run every month

# Just drop an executable script in the directory:
sudo cp my-backup.sh /etc/cron.daily/
sudo chmod +x /etc/cron.daily/my-backup.sh

# System-wide crontab
cat /etc/crontab

The at Command — One-Time Tasks

at schedules a command to run once at a specific time.

sudo apt install at

# Schedule for a specific time
at 10:00 PM
> /home/sovon/scripts/cleanup.sh
> <Ctrl+D>

# Schedule relative to now
at now + 30 minutes
> echo "Reminder: meeting!" | mail -s "Meeting" sovon
> <Ctrl+D>

# Schedule for a specific date
at 2:00 PM Feb 25
> /backup/run.sh
> <Ctrl+D>

# List pending jobs
atq

# Remove a job
atrm 3              # Remove job #3

# View job details
at -c 3

Systemd Timers

Modern alternative to cron with better logging and dependency management.

Create a Timer

# /etc/systemd/system/backup.service
[Unit]
Description=Daily Backup

[Service]
Type=oneshot
ExecStart=/home/sovon/scripts/backup.sh
User=sovon
# /etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily at 2 AM

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true

[Install]
WantedBy=timers.target
# Enable and start the timer
sudo systemctl daemon-reload
sudo systemctl enable backup.timer
sudo systemctl start backup.timer

# Check timer status
systemctl list-timers
systemctl status backup.timer

# Manually trigger the service
sudo systemctl start backup.service

Timer vs Cron

Feature Cron Systemd Timer
Logging Manual journalctl
Dependencies None Full systemd deps
Missed runs Lost Persistent=true
Resource control None cgroups
Randomized delay None RandomizedDelaySec

Anacron — For Desktops/Laptops

Anacron ensures daily/weekly/monthly jobs run even if the machine was off at the scheduled time.

cat /etc/anacrontab
# period  delay  job-identifier   command
1          5      cron.daily       run-parts /etc/cron.daily
7         10      cron.weekly      run-parts /etc/cron.weekly
@monthly  15      cron.monthly     run-parts /etc/cron.monthly

🏋️ Practice Exercises

  1. Cron: Create a cron job that logs the date to a file every 5 minutes
  2. Schedule: Set up a cron job to clean /tmp files older than 7 days at midnight
  3. at: Schedule a one-time command using at for 10 minutes from now
  4. Timer: Create a systemd timer that runs a script every hour
  5. List: View all scheduled cron jobs and systemd timers
  6. Log: Set up a cron job with proper output redirection
  7. Verify: Check cron logs to confirm your job ran: grep CRON /var/log/syslog

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