Torna alla Documentazionesystemd timers are the modern alternative to cron. Each timer is linked to a .service unit and executed by systemd: with complete logs on journald, error handling and the ability to define dependencies between services.
Basic structure
A systemd timer requires two files:
- .service: defines what to execute
- .timer: defines when to execute it
Example: daily backup script
Service file
cat > /etc/systemd/system/backup-daily.service << 'EOF'
[Unit]
Description=Daily server backup
After=network.target
[Service]
Type=oneshot
User=root
ExecStart=/usr/local/bin/backup.sh
StandardOutput=journal
StandardError=journal
EOF
Timer file
cat > /etc/systemd/system/backup-daily.timer << 'EOF'
[Unit]
Description=Run backup every day at 2:00
Requires=backup-daily.service
[Timer]
OnCalendar=*-*-* 02:00:00
Persistent=true
[Install]
WantedBy=timers.target
EOF
Activation
systemctl daemon-reload
systemctl enable --now backup-daily.timer
# Verify
systemctl status backup-daily.timer
OnCalendar syntax
The OnCalendar syntax is more readable than cron:
# Every day at 3:30
OnCalendar=*-*-* 03:30:00
# Every Monday at 8:00
OnCalendar=Mon *-*-* 08:00:00
# First of every month at midnight
OnCalendar=*-*-01 00:00:00
# Every hour
OnCalendar=hourly
# Every day
OnCalendar=daily
# Every week (Monday at 00:00)
OnCalendar=weekly
# Every 15 minutes
OnCalendar=*:0/15
# Every 6 hours
OnCalendar=0/6:00:00
# Monday-Friday at 9:00
OnCalendar=Mon..Fri *-*-* 09:00:00
Verify syntax
# Check when the timer will trigger
systemd-analyze calendar "*-*-* 02:00:00"
systemd-analyze calendar "Mon *-*-* 08:00:00"
Relative timers (OnBootSec, OnUnitActiveSec)
[Timer]
# Run 5 minutes after boot
OnBootSec=5min
# Then every 30 minutes
OnUnitActiveSec=30min
Persistent=true
[Timer]
OnCalendar=daily
Persistent=true
Persistent=true means: if the timer didn't trigger at the scheduled time (server was off), run it immediately at next boot. Equivalent to cron's MAILTO option to not miss executions.
Timer management
# List all active timers
systemctl list-timers
# List all timers (including inactive)
systemctl list-timers --all
# Status of specific timer
systemctl status backup-daily.timer
# Run the service immediately (without waiting for timer)
systemctl start backup-daily.service
# Service logs
journalctl -u backup-daily.service -n 50
# Real-time logs
journalctl -u backup-daily.service -f
# Disable the timer
systemctl disable --now backup-daily.timer
Practical examples
Weekly log cleanup
# /etc/systemd/system/clean-logs.service
[Unit]
Description=Clean old logs
[Service]
Type=oneshot
ExecStart=/usr/bin/find /var/log -name "*.log" -mtime +30 -delete
ExecStart=/usr/bin/journalctl --vacuum-time=30d
# /etc/systemd/system/clean-logs.timer
[Unit]
Description=Weekly log cleanup
[Timer]
OnCalendar=Sun *-*-* 04:00:00
Persistent=true
[Install]
WantedBy=timers.target
Automatic updates
# /etc/systemd/system/auto-update.service
[Unit]
Description=Automatic system updates
[Service]
Type=oneshot
ExecStart=/usr/bin/apt update
ExecStart=/usr/bin/apt upgrade -y
ExecStart=/usr/bin/apt autoremove -y
# /etc/systemd/system/auto-update.timer
[Timer]
OnCalendar=*-*-* 03:00:00
Persistent=true
[Install]
WantedBy=timers.target
Cron vs systemd timer
| Cron | Systemd Timer | |
|---|---|---|
| Logs | ❌ Only via syslog | ✅ integrated journald |
| Retry on error | ❌ No | ✅ With Restart=on-failure |
| Dependencies | ❌ No | ✅ After=, Requires= |
| Persistent | ❌ Skips if offline | ✅ Catches up at boot |
| Simplicity | ✅ One line | ⚠️ Two files |
| Familiar | ✅ Everyone knows it | ⚠️ Learning curve |
For simple tasks without special requirements, cron is still valid. For critical tasks where logs and reliability matter, use systemd timers.
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