# Debian Scheduling

Run tasks at specific times or intervals using `cron` or `at`.

## Cron

Most common scheduler. Edit user crontab:

```bash
crontab -e
```

Edit root crontab:

```bash
sudo crontab -e
```

List your crons:

```bash
crontab -l
```

## Cron Format

```
minute hour day month day-of-week command
0      2    *   *     *            /path/to/backup.sh
```

Fields:
- `minute` — 0-59
- `hour` — 0-23
- `day` — 1-31
- `month` — 1-12
- `day-of-week` — 0-6 (0=Sunday)

Special values:
- `*` — any value
- `*/5` — every 5 units
- `1-5` — range
- `1,3,5` — specific values

Examples:

```bash
0 2 * * *           # 2 AM daily
*/30 * * * *        # every 30 minutes
0 0 * * 0           # midnight Sunday (weekly)
0 0 1 * *           # 1st of month (monthly)
0 0 1 1 *           # 1 Jan (yearly)
@reboot             # on boot
```

## Cron Permissions

Allow/deny users in:

```bash
sudo nano /etc/cron.allow      # whitelist
sudo nano /etc/cron.deny       # blacklist
```

## Checking Cron Jobs

System cron jobs in:

```bash
ls /etc/cron.d/
cat /etc/crontab
```

List running cron processes:

```bash
ps aux | grep cron
```

View cron logs:

```bash
journalctl -u cron
tail -f /var/log/syslog | grep CRON
```

## At

Run once at a specific time:

```bash
at 3pm tomorrow
at> /path/to/command
at> [Ctrl+D]
```

List scheduled jobs:

```bash
atq
```

Remove job:

```bash
atrm jobid
```

## Systemd Alternative

Modern approach using `systemd-run`:

```bash
systemd-run --on-calendar="daily" /path/to/command
```

Or create a timer unit for complex scheduling.

## Tips

- Redirect output: `0 2 * * * /backup.sh >> /var/log/backup.log 2>&1`
- Use full paths in commands
- Test commands manually before adding to cron
- Cron runs with minimal environment; set `PATH` if needed