Proxmox VE

Cron Jobs Complete Guide

3 min read

Cron Jobs Complete Guide

Overview

Cron is a time-based job scheduler for automatating scripts and commands. Essential for backups, maintenance, and monitoring tasks.

Basic Cron Syntax

Understanding Cron Expression

┌───────────── minute (0 - 59)
│ ┌─────────── hour (0 - 23)
│ │ ┌──────── day of month (1 - 31)
│ │ │ ┌────── month (1 - 12)
│ │ │ │ ┌──── day of week (0 - 6) (Sunday=0)
│ │ │ │ │
* * * * * command

Cron Examples

Basic Examples

# Every minute
* * * * * /root/script.sh
 
# Every hour
0 * * * * /root/script.sh
 
# Every day at 2 AM
0 2 * * * /root/script.sh
 
# Every Monday at 3 AM
0 3 * * 1 /root/script.sh
 
# Every 15 minutes
*/15 * * * * /root/script.sh

Common Schedules

# Daily at 2 AM
0 2 * * * /root/backup.sh
 
# Weekly Sunday at 2 AM
0 2 * * 0 /root/cleanup.sh
 
# Monthly 1st at 2 AM
0 2 1 * * /root/monthly.sh
 
# Every 5 minutes
*/5 * * * * /root/monitor.sh
 
# Every 30 minutes
*/30 * * * * /root/check.sh

Managing Cron Jobs

Add Cron Job

# Edit crontab
crontab -e
 
# Or use specific file
crontab /root/my-cronjobs
 
# System cron (all users)
nano /etc/crontab

List Cron Jobs

# Current user crontab
crontab -l
 
# Specific user
crontab -u root -l
 
# System cron
cat /etc/crontab

Remove Cron Job

# Remove all
crontab -r
 
# Remove without confirmation
crontab -ir

Cron Environment

PATH Setting

# Set PATH in crontab
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
 
# Or absolute path
0 2 * * * /usr/local/bin/backup.sh

Output Handling

# Redirect output
0 2 * * * /root/script.sh > /root/script.log 2>&1
 
# Send email (if configured)
0 2 * * * /root/script.sh
 
# Discard output
0 2 * * * /root/script.sh > /dev/null 2>&1

Special Characters

Characters Meaning

CharacterMeaning
*Any value
,Value list (1,2,3)
-Range (1-5)
/Step (*/5)

Examples

# 1, 2, or 3
0 2 * * 1,3,5 /root/script.sh
 
# 1 through 5
0 2 * * 1-5 /root/script.sh
 
# Every 2 hours
0 */2 * * * /root/script.sh
 
# Every 30 minutes
0,30 * * * * /root/script.sh

Proxmox Specific Cron

Backup Jobs

# Daily backup at 2 AM
0 2 * * * vzdump --mode suspend --vmid 100 --storage local
 
# Multiple VMs
0 2 * * * for vm in 100 101 102; do vzdump --mode suspend --vmid $vm --storage local; done

Maintenance Jobs

# Clean old logs daily
0 3 * * * find /var/log -name "*.log" -mtime +30 -delete
 
# Update package index
0 4 * * * apt update -y
 
# Check VM status
*/15 * * * * qm list | grep running

Troubleshooting Cron

Not Running

# Check cron service
systemctl status cron
 
# Check logs
grep CRON /var/log/syslog
 
# Check mail
mail

Permission Issues

# Make script executable
chmod +x /root/script.sh
 
# Check ownership
chown root:root /root/script.sh

Best Practices

Script Template

#!/bin/bash
# backup.sh - Backup script
 
# Exit on error
set -e
 
# Log
exec > >(logger -t backup) 2>&1
 
echo "Starting backup..."
 
# Your commands here
vzdump --mode suspend --vmid 100 --storage local
 
echo "Backup complete"

Lock Files

#!/bin/bash
# Prevent multiple instances
 
LOCKFILE=/var/lock/backup.lock
 
if [ -f $LOCKFILE ]; then
    echo "Already running"
    exit 1
fi
 
touch $LOCKFILE
trap "rm -f $LOCKFILE" EXIT
 
# Your commands here

Keywords

cron scheduler automation crontab cronjob

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