Schedule tasks to run at a set date and time

Automating routine maintenance—like database backups or log rotations—ensures a reliable Linux server. In this guide, you’ll learn how to schedule both recurring and one-off tasks using three core utilities: cron, anacron, and at.

Scheduling Utilities Overview

Utility	Use Case	Configuration File or Command
cron	Repetitive jobs (minutes, hours, days)	`/etc/crontab` & `crontab -e`
anacron	Periodic jobs when system may be off	`/etc/anacrontab`
at	One-time, non-recurring tasks	`at <time>` / `atq` / `atrm`

1. cron

cron runs tasks on a fixed schedule. The system-wide crontab at /etc/crontab also serves as a syntax reference:

# /etc/crontab
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# ┌──────── minute (0 - 59)
# │ ┌────── hour (0 - 23)
# │ │ ┌──── day of month (1 - 31)
# │ │ │ ┌── month (1 - 12 or jan–dec)
# │ │ │ │ ┌ day of week (0 - 6 or sun–sat)
# │ │ │ │ │
35 6 * * * root /bin/some_command --some_options

Field Descriptions

Minute: 0–59
Hour: 0–23 (0 = midnight)
Day of Month: 1–31
Month: 1–12 or jan–dec
Day of Week: 0–6 (Sunday = 0 or 7) or sun–sat

Special operators:

* : every valid value
, : value list (e.g., 15,45)
- : range (e.g., 2-4)
/ : step (e.g., */4 for every 4th unit)

Best Practice

Always use full paths in your cron jobs. For example, find touch with which touch and use /usr/bin/touch.

Edit Your User Crontab

Open the editor:
```
crontab -e
```

Add a job (runs daily at 06:35):

35 6 * * * /usr/bin/touch ~/test_passed

Common cron Examples

Every Sunday at 03:00:

0 3 * * sun /usr/bin/touch weekly_backup

On the 15th of each month at 03:00:

0 3 15 * * /usr/bin/touch midmonth_task

Daily at 03:00:
```
0 3 * * * /usr/bin/touch daily_task
```
Hourly on the hour:
```
0 * * * * /usr/bin/touch hourly_task
```

Managing crontabs

Action	Command
List your crontab	`crontab -l`
List root’s crontab	`sudo crontab -l`
Edit another user’s crontab	`sudo crontab -u username -e`
Remove your crontab	`crontab -r`
Remove another user’s crontab	`sudo crontab -u username -r`

`/etc/cron.*` Directories

Place scripts in these directories to run at fixed intervals:

/etc/cron.hourly/
/etc/cron.daily/
/etc/cron.weekly/
/etc/cron.monthly/

Example—install an hourly script:

touch myscript.sh
sudo cp myscript.sh /etc/cron.hourly/
sudo chmod +x /etc/cron.hourly/myscript.sh

Remove it with:

sudo rm /etc/cron.hourly/myscript.sh

2. anacron

When a system is off during a scheduled job, anacron runs missed tasks at boot. The file /etc/anacrontab uses this format:

# period days   delay minutes   job-identifier   command
1               5               cron.daily       nice run-parts /etc/cron.daily
7              25               cron.weekly      nice run-parts /etc/cron.weekly
@monthly       45               cron.monthly     nice run-parts /etc/cron.monthly
3              10               test_job         /usr/bin/touch /root/anacron_created_this

period days: 1 = daily; 7 = weekly; @monthly
delay minutes: wait before running
job-identifier: unique name for logging
command: full path to execute

Example: Runs test_job every 3 days, 10 minutes after boot.

Verify your configuration without executing jobs:

sudo anacron -T

A silent output means the syntax is correct.

3. at

Use at for one-off tasks. Schedule in 24-hour or relative formats:

# Schedule a one-time job at 15:00 today
at 15:00
# at> /usr/bin/touch file_created_by_at
# at> <Ctrl+D>

Supported time formats:

Absolute: at '2:30 Aug 20 2022'
Relative: at 'now + 30 minutes'
Other: at 'now + 3 days', at 'now + 3 weeks', etc.

Warning

Ensure the atd daemon is running; otherwise at jobs won’t execute.

Managing at Jobs

Action	Command
List pending jobs	`atq`
View job details	`at -c <job-number>`
Remove a job	`atrm <job-number>`

Example—list and remove:

atq
atrm 20

Practice with these tools to automate backups, cleanup tasks, and custom scripts. Proper scheduling keeps your Linux system reliable and maintenance-free.

Links & References

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