> ## Documentation Index
> Fetch the complete documentation index at: https://notes.kodekloud.com/llms.txt
> Use this file to discover all available pages before exploring further.
# Schedule tasks to run at a set date and time
> Learn to automate routine maintenance on Linux by scheduling tasks with cron, anacron, and at utilities.
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**.
` / `atq` / `atrm` |
## 1. cron
`cron` runs tasks on a fixed schedule. The system-wide crontab at `/etc/crontab` also serves as a syntax reference:
```cron theme={null}
# /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)
Always use full paths in your cron jobs. For example, find `touch` with `which touch` and use `/usr/bin/touch`.
### Edit Your User Crontab
1. Open the editor:
```bash theme={null}
crontab -e
```
2. Add a job (runs daily at 06:35):
```cron theme={null}
35 6 * * * /usr/bin/touch ~/test_passed
```
### Common cron Examples
* Every Sunday at 03:00:
```cron theme={null}
0 3 * * sun /usr/bin/touch weekly_backup
```
* On the 15th of each month at 03:00:
```cron theme={null}
0 3 15 * * /usr/bin/touch midmonth_task
```
* Daily at 03:00:
```cron theme={null}
0 3 * * * /usr/bin/touch daily_task
```
* Hourly on the hour:
```cron theme={null}
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:
```bash theme={null}
touch myscript.sh
sudo cp myscript.sh /etc/cron.hourly/
sudo chmod +x /etc/cron.hourly/myscript.sh
```
Remove it with:
```bash theme={null}
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:
```text theme={null}
# 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:
```bash theme={null}
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:
```bash theme={null}
# Schedule a one-time job at 15:00 today
at 15:00
# at> /usr/bin/touch file_created_by_at
# at>
```
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.
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 ` |
| Remove a job | `atrm ` |
Example—list and remove:
```bash theme={null}
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
* [cron Manual Page](https://man7.org/linux/man-pages/man5/crontab.5.html)
* [anacron Documentation](https://man7.org/linux/man-pages/man5/anacrontab.5.html)
* [at Manual Page](https://man7.org/linux/man-pages/man1/at.1.html)