Scheduled tasks

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Table of contents

1. Introduction to cron
2. crontab syntax
3. Examples of common tasks
4. anacron for non-24/7 machines
5. systemd timers
6. Hands-on exercises

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1 - Introduction to cron

What is cron?

cron is the standard task scheduler on Linux. It runs commands at specific times.

Types of crontab

File	Scope	Editing
User crontab	Per user	crontab -e
/etc/crontab	System	direct editing
/etc/cron.d/*	System (files)	direct editing
/etc/cron.daily/	Daily scripts	drop a script
/etc/cron.hourly/	Hourly scripts	drop a script

Manage your crontab

# Éditer son crontab
crontab -e

# Lister son crontab
crontab -l

# Supprimer son crontab
crontab -r

# Éditer le crontab d'un autre utilisateur (root)
sudo crontab -u john -e

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2 - crontab syntax

Format

┌───────────── minute (0 - 59)
│ ┌───────────── heure (0 - 23)
│ │ ┌───────────── jour du mois (1 - 31)
│ │ │ ┌───────────── mois (1 - 12)
│ │ │ │ ┌───────────── jour de la semaine (0 - 7, 0 ou 7 = dimanche)
│ │ │ │ │
* * * * * commande à exécuter

Special characters

Character	Meaning
*	All values
,	List of values
-	Range of values
/	Step (interval)

Scheduling examples

Cron	Meaning
0 * * * *	Every hour (minute 0)
0 0 * * *	Every day at midnight
30 8 * * *	Every day at 8:30
0 0 * * 0	Every Sunday at midnight
0 0 1 * *	The 1st of each month
*/15 * * * *	Every 15 minutes
0 9-18 * * 1-5	Every hour from 9 to 18, Monday-Friday
0 0,12 * * *	At midnight and noon
@reboot	At startup

Shortcuts

Shortcut	Equivalent
@reboot	At startup
@yearly	0 0 1 1 *
@monthly	0 0 1 * *
@weekly	0 0 * * 0
@daily	0 0 * * *
@hourly	0 * * * *

Environment variables

# En haut du crontab
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
[email protected]

# Puis les tâches
0 * * * * /scripts/backup.sh

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3 - Examples of common tasks

Daily backup

# Tous les jours à 2h du matin
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

Log cleanup

# Tous les dimanches à 3h
0 3 * * 0 find /var/log -name "*.log" -mtime +30 -delete

Synchronization

# Toutes les 6 heures
0 */6 * * * rsync -avz /data/ backup@remote:/backup/

Monitoring

# Toutes les 5 minutes
*/5 * * * * /scripts/check_services.sh

Complete backup script

#!/bin/bash
# /usr/local/bin/backup.sh

DATE=$(date +%Y%m%d_%H%M%S)
BACKUP_DIR="/backup"
SOURCE="/var/www"

# Créer le backup
tar -czf "$BACKUP_DIR/www_$DATE.tar.gz" "$SOURCE"

# Garder seulement les 7 derniers
find "$BACKUP_DIR" -name "www_*.tar.gz" -mtime +7 -delete

echo "Backup completed: $DATE"

Best practices

# 1. Toujours rediriger la sortie
0 * * * * /script.sh >> /var/log/script.log 2>&1

# 2. Utiliser des chemins absolus
0 * * * * /usr/bin/python3 /home/user/script.py

# 3. Verrouiller pour éviter les exécutions multiples
*/5 * * * * flock -n /tmp/script.lock /script.sh

tip

Use crontab.guru to visualize and test your cron expressions!

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4 - anacron for non-24/7 machines

The problem with cron

cron does not catch up on missed tasks if the machine was turned off.

The solution: anacron

anacron runs the missed tasks at the next startup.

# /etc/anacrontab
SHELL=/bin/sh
PATH=/sbin:/bin:/usr/sbin:/usr/bin
MAILTO=root

# period  delay  identifier  command
1         5      daily       run-parts /etc/cron.daily
7         10     weekly      run-parts /etc/cron.weekly
@monthly  15     monthly     run-parts /etc/cron.monthly

Field	Description
period	Frequency in days
delay	Delay in minutes after startup
identifier	Unique name
command	Command to run

System directories

Simply drop your scripts into:

/etc/cron.daily/     # Exécutés quotidiennement
/etc/cron.weekly/    # Exécutés hebdomadairement
/etc/cron.monthly/   # Exécutés mensuellement

# Exemple : script de nettoyage
sudo nano /etc/cron.daily/clean-tmp

#!/bin/bash
find /tmp -type f -mtime +7 -delete

# Rendre exécutable
sudo chmod +x /etc/cron.daily/clean-tmp

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5 - systemd timers

A modern alternative to cron

systemd timers offer more features.

Structure

A timer requires two files:

1. A .timer file (scheduling)
2. A .service file (action)

Example: daily backup

The service:

# /etc/systemd/system/backup.service
[Unit]
Description=Daily Backup

[Service]
Type=oneshot
ExecStart=/usr/local/bin/backup.sh

The timer:

# /etc/systemd/system/backup.timer
[Unit]
Description=Run backup daily

[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true

[Install]
WantedBy=timers.target

Enable the timer

# Recharger systemd
sudo systemctl daemon-reload

# Activer et démarrer
sudo systemctl enable --now backup.timer

# Vérifier
sudo systemctl list-timers

OnCalendar syntax

Expression	Meaning
*-*-* 02:00:00	Every day at 2:00
Mon *-*-* 10:00:00	Every Monday at 10:00
*-*-01 00:00:00	The 1st of each month
hourly	Every hour
daily	Every day
weekly	Every week

Useful options

Option	Description
OnCalendar	Absolute scheduling
OnBootSec	After startup
OnUnitActiveSec	After the last run
Persistent=true	Catches up on missed runs
RandomizedDelaySec	Random delay

Useful commands

# Lister les timers
systemctl list-timers

# Statut d'un timer
systemctl status backup.timer

# Logs de l'exécution
journalctl -u backup.service

# Exécuter manuellement
sudo systemctl start backup.service

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6 - Hands-on exercises

Exercise 1: First crontab

# 1. Ouvrez votre crontab
crontab -e

# 2. Ajoutez une tâche de test (toutes les minutes)
* * * * * echo "Test $(date)" >> /tmp/cron_test.log

# 3. Attendez 2 minutes puis vérifiez
cat /tmp/cron_test.log

# 4. Supprimez la tâche
crontab -e
# (supprimez la ligne)

Exercise 2: Monitoring script

# 1. Créez le script
cat << 'EOF' | sudo tee /usr/local/bin/disk_check.sh
#!/bin/bash
USAGE=$(df -h / | awk 'NR==2 {print $5}' | sed 's/%//')
if [ "$USAGE" -gt 80 ]; then
    echo "ALERTE: Disque à $USAGE% - $(date)" >> /var/log/disk_alert.log
fi
EOF

sudo chmod +x /usr/local/bin/disk_check.sh

# 2. Ajoutez au crontab (toutes les heures)
crontab -e
# 0 * * * * /usr/local/bin/disk_check.sh

Exercise 3: Understand the syntax

What do these expressions mean?

1. 30 4 * * *
2. 0 */2 * * *
3. 0 9-17 * * 1-5
4. 0 0 1,15 * *

Answers

1. Every day at 4:30
2. Every 2 hours, at minute 0
3. Every hour from 9 to 17, Monday through Friday
4. The 1st and the 15th of each month at midnight

Quiz

Q1. How do you edit your crontab?

Answer

crontab -e

Q2. What does */15 * * * * mean?

Answer

Every 15 minutes

Q3. What is the advantage of systemd timers over cron?

Answer

Integration with journald (logs), Persistent (catch-up), more flexibility

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Key takeaways

• crontab -e to edit, crontab -l to list
• Format: minute hour day month day_of_week command
• Always redirect the output (>> logfile 2>&1)
• Use absolute paths
• anacron for machines that don't run 24/7
• systemd timers = modern alternative with more features
• Test with crontab.guru

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