# Cronjobs – Scheduling Tasks Automatically

Cronjobs – Scheduling Tasks Automatically – Server administration and Linux knowledge from NexoraHost.

With cronjobs you automate recurring tasks on your server – from backups to updates to deleting old logs. This guide shows you how to set up and manage cronjobs.

**Note:** Detailed guides on server administration can be found in the left navigation.

## What is a cronjob?

A cronjob is a time-controlled task that the cron daemon runs in the background. You define when and how often a command or script is executed – once or regularly. Cron runs on every Linux server and is active by default.

## Understanding cronjob syntax

A cronjob consists of five time fields and the command to execute:

`* * * * * /path/to/command
│ │ │ │ │
│ │ │ │ └── Day of week (0-7, 0 and 7 = Sunday)
│ │ │ └──── Month (1-12)
│ │ └────── Day of month (1-31)
│ └──────── Hour (0-23)
└────────── Minute (0-59)`

## Time fields – examples

    
        Schedule
        Syntax
        Meaning
    

    
        Every minute
        `* * * * *`
        Runs 1440 times a day
    

    
        Every hour
        `0 * * * *`
        At the top of the hour
    

    
        Daily at 3 AM
        `0 3 * * *`
        Good for backups and maintenance
    

    
        Every Sunday at 4 AM
        `0 4 * * 0`
        Weekly tasks
    

    
        1st of every month
        `0 0 1 * *`
        Monthly reports
    

    
        Every 15 minutes
        `*/15 * * * *`
        Frequent checks
    

    
        Weekdays at 9 AM
        `0 9 * * 1-5`
        Monday to Friday
    

## Setting up cronjobs

### For the current user

`# Open crontab
crontab -e`
The first time you'll be asked for an editor. Choose **nano** (easier) or **vim** (advanced).

### For root (system tasks)

`sudo crontab -e`

### For a specific user

`sudo crontab -u username -e`

## Practical examples

### Backup automation

`# Daily database backup at 2:30 AM
30 2 * * * mysqldump -u root -p'Password' database > /backup/db_$(date +\%Y\%m\%d).sql

# Weekly server backup every Sunday at 4 AM
0 4 * * 0 tar -czf /backup/server_$(date +\%Y\%m\%d).tar.gz /var/www`

### Log management

`# Delete logs older than 30 days
0 5 * * * find /var/log -name "*.log" -mtime +30 -delete

# Clean up Docker logs
0 6 * * 0 docker system prune -af`

### Updates and maintenance

`# System updates every Monday at 3 AM
0 3 * * 1 apt update && apt upgrade -y

# Restart a server once daily (for game servers)
0 4 * * * systemctl restart minecraft`

### Monitoring

`# Disk space check with email warning
0 8 * * * df -h | awk '{if ($5+0 > 90) print "Warning: " $6 " is " $5 " full"}' | mail -s "Disk Space Warning" admin@domain.com`

## Managing cronjobs

`# List all cronjobs of the current user
crontab -l

# Delete all cronjobs (Caution!)
crontab -r

# Show all cronjobs of all users
for user in $(cut -f1 -d: /etc/passwd); do echo "=== $user ==="; sudo crontab -u $user -l 2>/dev/null; done`

## Output and logging

By default, the output of a cronjob is sent via email to the user. You can control this:

`# Write output to file
0 3 * * * /path/to/script.sh >> /var/log/my-cron.log 2>&1

# Discard output (silent)
0 3 * * * /path/to/script.sh > /dev/null 2>&1

# Only log errors
0 3 * * * /path/to/script.sh >> /var/log/my-cron.log 2>> /var/log/my-cron-error.log`

## System-wide cronjobs (/etc/crontab)

In addition to user-specific crontabs, there are system-wide cronjobs. These have an additional field for the user:

`# In /etc/crontab
# m h dom mon dow user command
0 3 * * * root /usr/local/bin/backup.sh`

Pre-configured directories for scripts:

    - **/etc/cron.hourly/** – Runs every hour
- **/etc/cron.daily/** – Runs daily
- **/etc/cron.weekly/** – Runs weekly
- **/etc/cron.monthly/** – Runs monthly

Simply place an executable script in the corresponding directory.

## Common problems

**Cronjob not running:**

    - Check if the cron daemon is running: `sudo systemctl status cron`
- Use absolute paths – cron doesn't know your PATH environment
- Check the cron logs: `grep CRON /var/log/syslog`

**Percent signs (%) in commands:**

    - Percent signs have special meaning in crontabs. Escape them with a backslash: `\%`

**Script is executable but doesn't run via cron:**

    - Cron has a minimal environment. Define SHELL and PATH at the top of your crontab:
        `SHELL=/bin/bash
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

0 3 * * * /path/to/script.sh`
    

*Detailed guides on scripts and automation can be found in the articles in the left navigation.*