Scheduled_tasks

Chapter 20: Cron, at, and Scheduled Tasks

Overview

This chapter covers scheduled task management in Linux using cron, at, and systemd timers.

20.1 Cron System

Cron Basics

                    Cron System Architecture
+------------------------------------------------------------------+
|                                                                   |
|   +---------------+     +---------------+     +---------------+   |
|   |  crontab     |     |   cron        |     |  /etc/cron.d  |   |
|   |  files       |---->|   daemon      |---->|  scripts      |   |
|   +---------------+     +---------------+     +---------------+   |
|           |                       |                       |         |
|           v                       v                       v         |
|   +---------------+     +---------------+     +---------------+   |
|   | User crontabs|     |  cron jobs     |     | System cron   |   |
|   | /var/spool/  |     |  execution     |     | jobs          |   |
|   +---------------+     +---------------+     +---------------+   |
|                                                                   |
+------------------------------------------------------------------+

Cron Syntax

# ┌───────────── minute (0 - 59)
# │ ┌───────────── hour (0 - 23)
# │ │ ┌───────────── day of month (1 - 31)
# │ │ │ ┌───────────── month (1 - 12)
# │ │ │ │ ┌───────────── day of week (0 - 6) (Sunday=0)
# │ │ │ │ │
# * * * * * command

# Examples:
0 5 * * *          # Run at 5:00 AM daily
0 0 * * 0          # Run at midnight on Sundays
30 4 1,15 * *      # Run at 4:30 AM on 1st and 15th
0 8 * * 1-5        # Run at 8 AM on weekdays
*/15 * * * *       # Run every 15 minutes
0 */2 * * *        # Run every 2 hours
@reboot            # Run at reboot
@yearly            # Run once a year (0 0 1 1 *)
@monthly           # Run once a month (0 0 1 * *)
@weekly            # Run once a week (0 0 * * 0)
@daily             # Run once a day (0 0 * * *)
@hourly            # Run every hour (0 * * * *)

Managing Crontabs

# Edit current user's crontab
crontab -e

# List current user's crontab
crontab -l

# Remove current user's crontab
crontab -r

# Edit specific user's crontab (root only)
crontab -u username -e

# List specific user's crontab
crontab -u username -l

System Crontabs

# System cron directories
/etc/cron.d/           # System-wide cron jobs
/etc/cron.daily/        # Scripts run daily
/etc/cron.hourly/      # Scripts run hourly
/etc/cron.monthly/     # Scripts run monthly
/etc/cron.weekly/      # Scripts run weekly

# Example: /etc/cron.d/maintance
# Run maintenance script at 2 AM daily
0 2 * * * root /usr/local/bin/maintenance.sh

20.2 at Command

at Basics

# Install at if needed
sudo pacman -S at

# Schedule a job to run at specific time
at 2:30 PM
at> /path/to/command
# Ctrl+D to save

# Schedule a job for specific date
at 2:30 PM + 2 days
at 14:30 2026-12-25

# List pending jobs
atq
# or
at -l

# Remove a job
atrm job_number

at Examples

# Run a command in 1 hour
at now + 1 hour

# Run a command tomorrow at 10 AM
at 10:00 tomorrow

# Run a script at boot
at now + 1 minute -f /path/to/script.sh

# View job details
at -c job_number

20.3 Systemd Timers

Timer Units

[Unit]
Description=Run daily cleanup

[Timer]
OnCalendar=daily
Persistent=true
RandomizedDelaySec=1h

[Install]
WantedBy=timers.target

[Unit]
Description=Daily cleanup service

[Service]
Type=oneshot
ExecStart=/usr/local/bin/cleanup.sh

Timer Commands

# Start timer
sudo systemctl start mytimer.timer

# Enable timer (start on boot)
sudo systemctl enable mytimer.timer

# List active timers
systemctl list-timers
systemctl list-timers --all

# View timer status
systemctl status mytimer.timer

# View timer next run
systemctl list-timers --all | grep mytimer

Timer Examples

# Run every hour
OnCalendar=hourly
# or
OnCalendar=*:0/1

# Run every 30 minutes
OnCalendar=*:0/30

# Run at specific time daily
OnCalendar=*-*-* 02:00:00

# Run on specific days
OnCalendar=Mon *-*-* 09:00:00

# Run weekly (Sunday at 3 AM)
OnCalendar=Sun *-*-* 03:00:00

# Run monthly (1st of month at midnight)
OnCalendar=*-*-01 00:00:00

20.4 Anacron

Anacron Basics

# Install
sudo pacman -S cronie  # Contains anacron

# format: delay job_identifier command
1       5       cron.daily      run-parts /etc/cron.daily
7       25      cron.weekly     run-parts /etc/cron.weekly
30      60      cron.monthly    run-parts /etc/cron.monthly

# Run anacron manually
sudo anacron -d

20.5 Best Practices

Logging and Monitoring

# Redirect output to log file
0 5 * * * /path/to/script.sh >> /var/log/script.log 2>&1

# Send email on failure
0 5 * * * /path/to/script.sh || echo "Failed" | mail -s "Cron Failed" admin@example.com

Environment Variables

# In crontab, set PATH
PATH=/usr/local/bin:/usr/bin:/bin

# Set HOME
HOME=/root

# Set SHELL
SHELL=/bin/bash

Common Pitfalls

# Problem: Missing PATH in cron
# Solution: Define PATH at top of crontab

# Problem: Commands not found
# Solution: Use full paths: /usr/bin/command

# Problem: Scripts not executing
# Solution: Make script executable: chmod +x script.sh

# Problem: Environment differences
# Solution: Source profile in script: source /etc/profile

20.6 Advanced Scheduling

systemd.time Calendar Format

# Complex calendar expressions
OnCalendar=Mon..Fri 09:00:00      # Weekdays at 9 AM
OnCalendar=*-*-1..7 18:00:00      # First week of month at 6 PM
OnCalendar=*:0/15                 # Every 15 minutes
OnCalendar=Sat,Sun 10:00          # Weekends at 10 AM

# Combined with delays
OnBootSec=5min                    # 5 minutes after boot
OnUnitActiveSec=1h                # 1 hour after last activation
OnUnitInactiveSec=30m             # 30 minutes after deactivation

Summary

In this chapter, you learned:

✅ Cron syntax and management
✅ at command for one-time tasks
✅ systemd timers for modern scheduling
✅ Anacron for systems not running 24/7
✅ Best practices and troubleshooting

Next Chapter

Chapter 21: Networking Fundamentals

Last Updated: February 2026