Scheduled Tasks

Scheduled tasks allow you to automatically run commands or scripts at specified times. Ubuntu 26.04 supports two methods: the traditional cron and the more modern systemd timers.

crontab Basics

Managing crontab


# Edit the current user's crontab
crontab -e
 
# View the current user's crontab
crontab -l
 
# Delete the current user's crontab
crontab -r
 
# Edit another user's crontab (requires root)
sudo crontab -u username -e
 
# View another user's crontab
sudo crontab -u username -l

cron Time Syntax

A cron expression consists of 5 time fields:


┌──────── Minute (0-59)
│ ┌────── Hour (0-23)
│ │ ┌──── Day of month (1-31)
│ │ │ ┌── Month (1-12)
│ │ │ │ ┌ Day of week (0-7, both 0 and 7 mean Sunday)
│ │ │ │ │
* * * * * command

Special Characters

Character	Description	Example
`*`	Any value	`* * * * *` every minute
`,`	Multiple values	`1,15,30 * * * *` at minutes 1, 15, and 30
`-`	Range	`1-5 * * * *` minutes 1 through 5
`/`	Step	`/10 * * *` every 10 minutes

Common Examples


# Run every minute
* * * * * /path/to/script.sh
 
# Run every hour (on the hour)
0 * * * * /path/to/script.sh
 
# Run daily at 3:00 AM
0 3 * * * /path/to/script.sh
 
# Run daily at 3:30 AM
30 3 * * * /path/to/script.sh
 
# Run every Monday at 2:00 AM
0 2 * * 1 /path/to/script.sh
 
# Run on the 1st of every month at midnight
0 0 1 * * /path/to/script.sh
 
# Run every 5 minutes
*/5 * * * * /path/to/script.sh
 
# Run at minutes 15 and 45 of every hour
15,45 * * * * /path/to/script.sh
 
# Run weekdays (Monday-Friday) at 9:00 AM
0 9 * * 1-5 /path/to/script.sh
 
# Run every hour from 8 AM to 6 PM daily
0 8-18 * * * /path/to/script.sh
 
# Run every 6 months (1st of January and July)
0 0 1 1,7 * /path/to/script.sh

Special Keywords


# Equivalent shortcuts
@reboot     # Run at system startup
@yearly     # Run yearly = 0 0 1 1 *
@monthly    # Run monthly = 0 0 1 * *
@weekly     # Run weekly = 0 0 * * 0
@daily      # Run daily = 0 0 * * *
@hourly     # Run hourly = 0 * * * *
 
# Usage examples
@reboot /opt/myapp/start.sh
@daily /usr/local/bin/daily-backup.sh

Practical cron Tips

Environment Variables

The cron execution environment differs from the user’s shell environment — variables like PATH may be incomplete:


# Set environment variables at the top of your crontab
SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MAILTO=admin@example.com
 
# Or use full paths in commands
0 3 * * * /usr/bin/rsync -av /source/ /backup/

Logging


# Redirect output to a log file
0 3 * * * /path/to/script.sh >> /var/log/myscript.log 2>&1
 
# Discard all output
0 3 * * * /path/to/script.sh > /dev/null 2>&1
 
# Log only errors
0 3 * * * /path/to/script.sh > /dev/null 2>> /var/log/myscript-error.log

Email Notifications


# Set the email recipient in crontab
MAILTO=admin@example.com
 
# Disable email notifications
MAILTO=""

Viewing cron Logs


# View via journalctl
journalctl -u cron.service
 
# Or check syslog
grep CRON /var/log/syslog

System-Level cron Directories

Ubuntu provides several predefined cron directories:


# Place scripts in the appropriate directory for automatic execution
ls /etc/cron.hourly/     # Hourly
ls /etc/cron.daily/      # Daily
ls /etc/cron.weekly/     # Weekly
ls /etc/cron.monthly/    # Monthly


# Add a script and ensure it has execute permission
sudo cp myscript.sh /etc/cron.daily/
sudo chmod +x /etc/cron.daily/myscript.sh

The system-level cron configuration file /etc/crontab:


cat /etc/crontab


SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

systemd Timers

systemd timers are a modern alternative to cron, offering more precise time control and better logging support.

Viewing Existing Timers


# List all active timers
systemctl list-timers --all
 
# Example output:
# NEXT                        LEFT         LAST                        PASSED    UNIT                     ACTIVATES
# Tue 2026-03-24 06:00:00 CST 2h left      Mon 2026-03-23 06:00:00 CST 22h ago  apt-daily.timer          apt-daily.service

Creating a Custom Timer

Creating a timer requires two files: a .service file and a .timer file.

Step 1: Create the service file


sudo nano /etc/systemd/system/my-backup.service


[Unit]
Description=Daily Backup Job
 
[Service]
Type=oneshot
ExecStart=/usr/local/bin/daily-backup.sh
User=root
StandardOutput=journal
StandardError=journal

Step 2: Create the timer file


sudo nano /etc/systemd/system/my-backup.timer


[Unit]
Description=Run Daily Backup at 3 AM
 
[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true
RandomizedDelaySec=300
 
[Install]
WantedBy=timers.target

Step 3: Enable the timer


sudo systemctl daemon-reload
sudo systemctl enable --now my-backup.timer
 
# Verify
systemctl list-timers | grep my-backup
 
# Manually trigger a test run
sudo systemctl start my-backup.service
 
# View execution logs
journalctl -u my-backup.service

OnCalendar Time Expressions


# Daily at 3 AM
OnCalendar=*-*-* 03:00:00

# Every Monday at 9 AM
OnCalendar=Mon *-*-* 09:00:00

# 1st of every month
OnCalendar=*-*-01 00:00:00

# Weekdays at 8 AM
OnCalendar=Mon..Fri *-*-* 08:00:00

# Every 15 minutes
OnCalendar=*:0/15

# Every hour
OnCalendar=hourly

# Every day
OnCalendar=daily

# Every week
OnCalendar=weekly

Verify time expressions:


# Test an expression and see upcoming trigger times
systemd-analyze calendar "Mon..Fri *-*-* 08:00:00"
systemd-analyze calendar "*:0/15"

Event-Based Timers

In addition to calendar-based scheduling, you can trigger based on events:


[Timer]
# Run 5 minutes after system boot
OnBootSec=5min
 
# Run 1 hour after the last execution completed
OnUnitActiveSec=1h
 
# Combined usage
OnBootSec=10min
OnUnitActiveSec=1h

Transient Timers (One-Time Delayed Execution)


# Run a command after 30 minutes
systemd-run --on-active=30m /usr/local/bin/task.sh
 
# Run at a specific time
systemd-run --on-calendar="2026-03-25 10:00:00" /usr/local/bin/task.sh

cron vs systemd Timers Comparison

Feature	cron	systemd timers
Configuration complexity	Simple, one line	Requires two files
Logging	Manual setup required	Integrated with journalctl
Dependency management	Not supported	Supports service dependencies
Precision	Minimum unit is minutes	Accurate to the second
Missed tasks	Not executed	Persistent=true catches up
Resource control	Not supported	Supports cgroup resource limits
Random delay	Not supported	RandomizedDelaySec

Best Practices

Use cron for simple scheduled tasks, systemd timers for complex ones.
Always log output to make troubleshooting easier.
Use full paths to avoid PATH issues.
Prevent task overlap — long-running tasks should use locking:


# Use flock to prevent task overlap
* * * * * /usr/bin/flock -n /tmp/myjob.lock /path/to/script.sh

Test before deploying — run scripts manually first to confirm they work correctly.