Debian Scheduling

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

Most common scheduler. Edit user crontab:

crontab -e

Edit root crontab:

sudo crontab -e

List your crons:

crontab -l

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:

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

Allow/deny users in:

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

System cron jobs in:

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

List running cron processes:

ps aux | grep cron

View cron logs:

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

Run once at a specific time:

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

List scheduled jobs:

atq

Remove job:

atrm jobid

Modern approach using systemd-run:

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

Or create a timer unit for complex scheduling.

• 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