cron is used for precise scheduling on always-on systems, while anacron ensures periodic tasks run on systems that aren't continuously powered, such as laptops; 1. Use cron for exact timing (e.g., 3 AM daily) via crontab -e with syntax MIN HOUR DOM MON DOW COMMAND; 2. Use anacron for daily, weekly, or monthly jobs on intermittent systems via /etc/anacrontab with period, delay, job-id, and command; 3. System-wide cron jobs can be placed in /etc/cron.daily, /etc/cron.weekly, or /etc/cron.monthly; 4. anacron runs missed jobs after system startup based on recorded last execution in /var/spool/anacron/; 5. Ensure scripts use full paths and redirect output to logs to avoid errors and email spam; 6. Most desktops run anacron hourly through cron to handle missed tasks; always test scripts manually before scheduling; use cron for servers and anacron for laptops to ensure reliability, and avoid duplicating jobs across both systems.

Scheduling tasks on Linux is essential for automation—whether it's running backups, cleaning logs, or updating systems. Two of the most commonly used tools for this are cron and anacron. While they serve similar purposes, they work differently and are suited for different use cases.

Here’s how to use both effectively.

What’s the Difference Between cron and anacron?

• cron runs scheduled tasks at specific times, assuming your system is always on. It checks every minute and runs jobs if the scheduled time matches.
• anacron is designed for systems that aren’t running 24/7, like laptops. It runs jobs based on frequency (daily, weekly, monthly) and ensures missed jobs run when the system is back on.

So:
Use cron for servers or desktops that are always on.
Use anacron for laptops or machines that get turned off or suspended.

Using cron to Schedule Tasks

Cron uses a daemon (crond) that runs in the background and checks special configuration files called crontabs.

1. Edit Your Crontab

Run:

crontab -e

This opens your personal crontab file in the default editor.

2. Crontab Syntax

Each line follows this format:

MIN HOUR DOM MON DOW COMMAND

• MIN: Minute (0–59)
• HOUR: Hour (0–23)
• DOM: Day of Month (1–31)
• MON: Month (1–12)
• DOW: Day of Week (0–7, where both 0 and 7 = Sunday)

Example: Run a script every day at 3:30 AM

30 3 * * * /home/user/backup.sh

Other examples:

# Run every Monday at 6:15 AM
15 6 * * 1 /scripts/cleanup.sh

# Run every 15 minutes (use */15)
*/15 * * * * /scripts/check-status.sh

# Run at midnight on the 1st of every month
0 0 1 * * /scripts/monthly-report.sh

3. System-Wide Cron Jobs

You can also place scripts in:

• /etc/cron.daily/ – runs once per day
• /etc/cron.weekly/ – runs once per week
• /etc/cron.monthly/ – runs once per month

These are automatically handled by the cron daemon if the system is up.

Note: These rely on the system being on at the scheduled time.

Using anacron for Irregularly Running Systems

anacron doesn’t assume the system is always on. Instead of exact times, it uses intervals (in days) and runs jobs when the system boots or at scheduled checks.

1. anacron Syntax

anacron reads from /etc/anacrontab. Example:

# period   delay   job-id          command
1          5       daily-job       /home/user/daily-script.sh
7          10      weekly-job      /home/user/weekly-script.sh
@monthly   15      monthly-job     /home/user/monthly-script.sh

• period: number of days between runs (1 = daily, 7 = weekly). @monthly is special.
• delay: minutes to wait after system startup before running (to avoid slowing boot)
• job-id: name for logging
• command: script or command to run

2. How anacron Works

• When the system starts, anacron checks its job schedule.
• If a job’s period has passed since the last run (recorded in /var/spool/anacron/), it runs the job after the delay.
• So if your laptop was off for 3 days, the daily job runs once when you turn it back on.

3. anacron Is Usually Already Running

Most desktop distributions run anacron via cron. Check /etc/crontab or /etc/cron.d/anacron:

# Run anacron every hour (if system is on)
25 * * * * root test -x /usr/sbin/anacron && /usr/sbin/anacron -s

This line ensures anacron gets a chance to run missed jobs hourly.

Tips and Best Practices

• Always use full paths in cron/anacron scripts. The environment is minimal.

  # Instead of just:
  python script.py
  # Use:
  /usr/bin/python /home/user/scripts/script.py

• Redirect output to avoid email spam or log errors:

  */30 * * * * /scripts/monitor.sh >> /var/log/monitor.log 2>&1

• Test your scripts manually before scheduling.

• Use anacron for laptops, even if you also use cron. It handles missed jobs gracefully.

• Don’t mix cron and anacron unnecessarily. For example, don’t put the same job in both.

---

Summary

• Use cron for precise scheduling on always-on systems.
• Use anacron for periodic jobs on machines that get turned off.
• Combine both if needed: cron can trigger anacron, and system directories like /etc/cron.daily often use anacron under the hood on desktops.

Basically, cron says “run at 3 AM”, while anacron says “run once every day, whenever the system is up.”

Pick the right tool based on your system’s uptime.

The above is the detailed content of How to Schedule Tasks on Linux with Cron and anacron. For more information, please follow other related articles on the PHP Chinese website!