Cron Jobs: Syntax and Practical Examples for Task Automation

Introduction

Cron is the time-based job scheduler in Unix-like operating systems, enabling system administrators to schedule commands or scripts to run automatically at specified times and intervals. Understanding cron is fundamental for any DevOps professional, as it powers countless automated tasks from backups and log rotation to monitoring checks and periodic maintenance.

This comprehensive guide covers everything from basic cron syntax to advanced scheduling patterns, real-world examples, troubleshooting, and best practices for production environments.

Understanding Cron Basics

What is Cron?

Cron is a daemon that runs continuously in the background, checking every minute for scheduled jobs. It reads schedules from cron tables (crontabs) and executes commands at their specified times.

Cron Components

crond: The cron daemon that runs scheduled jobs
crontab: Command to create, edit, list, and remove cron jobs
cron.d: Directory for system cron job files
/etc/crontab: System-wide crontab file
User crontabs: Individual user cron schedules

Cron Syntax

Basic Cron Format

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

Special Characters

* (asterisk): Any value
, (comma): Value list separator
- (dash): Range of values
/ (slash): Step values
@ (at): Special strings

Special Strings

@reboot        # Run once at startup
@yearly        # Run once a year (0 0 1 1 *)
@annually      # Same as @yearly
@monthly       # Run once a month (0 0 1 * *)
@weekly        # Run once a week (0 0 * * 0)
@daily         # Run once a day (0 0 * * *)
@midnight      # Same as @daily
@hourly        # Run once an hour (0 * * * *)

Common Cron Patterns

Every Minute

* * * * * /path/to/script.sh

Every 5 Minutes

*/5 * * * * /path/to/script.sh

Every Hour

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

Every Day at Midnight

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

Every Day at 3:30 AM

30 3 * * * /path/to/script.sh

Every Monday at 9 AM

0 9 * * 1 /path/to/script.sh

First Day of Every Month

0 0 1 * * /path/to/script.sh

Every Weekday at 6 PM

0 18 * * 1-5 /path/to/script.sh

Multiple Times Per Day

0 6,12,18 * * * /path/to/script.sh  # At 6 AM, 12 PM, and 6 PM

Business Hours (9 AM to 5 PM, Weekdays)

0 9-17 * * 1-5 /path/to/script.sh

Every 15 Minutes During Business Hours

*/15 9-17 * * 1-5 /path/to/script.sh

Managing Cron Jobs

Crontab Commands

# List current user's cron jobs
crontab -l

# Edit current user's cron jobs
crontab -e

# Remove all cron jobs for current user
crontab -r

# List cron jobs for specific user (requires root)
sudo crontab -l -u username

# Edit cron jobs for specific user (requires root)
sudo crontab -e -u username

Creating Cron Jobs

# Open crontab editor
crontab -e

# Add your cron job (example: daily backup at 2 AM)
0 2 * * * /usr/local/bin/backup.sh

# Save and exit (vi: :wq, nano: Ctrl+X, Y, Enter)

System-Wide Cron Jobs

# Create system cron job in /etc/cron.d/
sudo nano /etc/cron.d/custom-job

# Content:
# Backup database every day at 1 AM
0 1 * * * root /opt/scripts/db-backup.sh

# Set proper permissions
sudo chmod 644 /etc/cron.d/custom-job

Practical Cron Examples

1. Daily Database Backup

# Cron entry
0 2 * * * /opt/scripts/mysql-backup.sh >> /var/log/mysql-backup.log 2>&1

# Script: /opt/scripts/mysql-backup.sh
#!/bin/bash
set -euo pipefail

BACKUP_DIR="/backup/mysql"
TIMESTAMP=$(date +%Y%m%d_%H%M%S)
RETENTION_DAYS=7

mkdir -p "$BACKUP_DIR"

# Backup all databases
mysqldump --all-databases \
    --single-transaction \
    --routines \
    --triggers | gzip > "${BACKUP_DIR}/all-databases-${TIMESTAMP}.sql.gz"

# Cleanup old backups
find "$BACKUP_DIR" -name "*.sql.gz" -mtime +$RETENTION_DAYS -delete

echo "Backup completed: $(date)"

2. Log Rotation

# Cron entry - Run daily at 11:55 PM
55 23 * * * /opt/scripts/rotate-logs.sh

# Script: /opt/scripts/rotate-logs.sh
#!/bin/bash
set -euo pipefail

LOG_DIRS=(
    "/var/log/nginx"
    "/var/log/application"
)

for dir in "${LOG_DIRS[@]}"; do
    if [ -d "$dir" ]; then
        # Compress logs older than 1 day
        find "$dir" -name "*.log" -mtime +1 ! -name "*.gz" -exec gzip {} \;

        # Delete logs older than 30 days
        find "$dir" -name "*.gz" -mtime +30 -delete
    fi
done

3. System Health Check

# Cron entry - Every 5 minutes
*/5 * * * * /opt/scripts/health-check.sh

# Script: /opt/scripts/health-check.sh
#!/bin/bash
set -euo pipefail

CPU_THRESHOLD=80
MEMORY_THRESHOLD=85
DISK_THRESHOLD=90
ALERT_EMAIL="[email protected]"

# Check CPU
CPU=$(top -bn1 | grep "Cpu(s)" | awk '{print $2}' | cut -d'%' -f1)
if (( $(echo "$CPU > $CPU_THRESHOLD" | bc -l) )); then
    echo "High CPU: ${CPU}%" | mail -s "CPU Alert" "$ALERT_EMAIL"
fi

# Check Memory
MEMORY=$(free | grep Mem | awk '{printf("%.0f", ($3/$2) * 100)}')
if (( MEMORY > MEMORY_THRESHOLD )); then
    echo "High Memory: ${MEMORY}%" | mail -s "Memory Alert" "$ALERT_EMAIL"
fi

# Check Disk
DISK=$(df -h / | tail -1 | awk '{print $5}' | sed 's/%//')
if (( DISK > DISK_THRESHOLD )); then
    echo "High Disk Usage: ${DISK}%" | mail -s "Disk Alert" "$ALERT_EMAIL"
fi

4. SSL Certificate Renewal

# Cron entry - Daily at 3 AM
0 3 * * * /opt/scripts/renew-ssl.sh

# Script: /opt/scripts/renew-ssl.sh
#!/bin/bash
set -euo pipefail

certbot renew --quiet --deploy-hook "systemctl reload nginx"

# Check certificate expiry
for domain in example.com api.example.com; do
    EXPIRY=$(echo | openssl s_client -servername "$domain" \
        -connect "$domain:443" 2>/dev/null | \
        openssl x509 -noout -dates | grep notAfter | cut -d= -f2)

    EXPIRY_EPOCH=$(date -d "$EXPIRY" +%s)
    CURRENT_EPOCH=$(date +%s)
    DAYS_REMAINING=$(( ($EXPIRY_EPOCH - $CURRENT_EPOCH) / 86400 ))

    if (( DAYS_REMAINING < 30 )); then
        echo "Certificate for $domain expires in $DAYS_REMAINING days" | \
            mail -s "SSL Certificate Expiry Warning" [email protected]
    fi
done

5. Website Monitoring

# Cron entry - Every 2 minutes
*/2 * * * * /opt/scripts/monitor-website.sh

# Script: /opt/scripts/monitor-website.sh
#!/bin/bash
set -euo pipefail

URLS=(
    "https://example.com"
    "https://api.example.com/health"
    "https://admin.example.com"
)

LOG_FILE="/var/log/website-monitor.log"

for url in "${URLS[@]}"; do
    if ! curl -f -s -o /dev/null -w "%{http_code}" --max-time 10 "$url" | grep -q "200"; then
        echo "[$(date)] FAILED: $url" | tee -a "$LOG_FILE"
        echo "Website $url is down" | mail -s "Website Down Alert" [email protected]
    else
        echo "[$(date)] OK: $url" >> "$LOG_FILE"
    fi
done

6. Automated Security Updates

# Cron entry - Daily at 4 AM
0 4 * * * /opt/scripts/security-updates.sh

# Script: /opt/scripts/security-updates.sh
#!/bin/bash
set -euo pipefail

LOG_FILE="/var/log/security-updates.log"

{
    echo "Starting security updates: $(date)"

    # Ubuntu/Debian
    if command -v apt-get &>/dev/null; then
        apt-get update
        apt-get upgrade -y --only-upgrade
        apt-get autoremove -y
        apt-get autoclean
    fi

    # CentOS/Rocky Linux
    if command -v yum &>/dev/null; then
        yum update -y --security
        yum autoremove -y
    fi

    echo "Security updates completed: $(date)"
} | tee -a "$LOG_FILE"

# Check if reboot required
if [ -f /var/run/reboot-required ]; then
    echo "Reboot required after updates" | mail -s "Reboot Required" [email protected]
fi

7. Disk Space Cleanup

# Cron entry - Weekly on Sunday at 1 AM
0 1 * * 0 /opt/scripts/cleanup-disk.sh

# Script: /opt/scripts/cleanup-disk.sh
#!/bin/bash
set -euo pipefail

# Remove old temporary files
find /tmp -type f -atime +7 -delete
find /var/tmp -type f -atime +7 -delete

# Clean package manager cache
apt-get clean 2>/dev/null || yum clean all 2>/dev/null

# Remove old log files
find /var/log -name "*.log.*" -mtime +30 -delete
find /var/log -name "*.gz" -mtime +30 -delete

# Clean Docker resources
docker system prune -af --volumes --filter "until=720h" 2>/dev/null || true

# Remove old kernels (Ubuntu/Debian)
if command -v apt-get &>/dev/null; then
    apt-get autoremove --purge -y
fi

echo "Disk cleanup completed: $(date)"

8. Database Optimization

# Cron entry - Weekly on Saturday at 2 AM
0 2 * * 6 /opt/scripts/optimize-database.sh

# Script: /opt/scripts/optimize-database.sh
#!/bin/bash
set -euo pipefail

DB_USER="admin"
DB_PASS="password"

# MySQL/MariaDB optimization
mysql -u"$DB_USER" -p"$DB_PASS" -e "SHOW DATABASES;" | \
    grep -Ev "Database|information_schema|performance_schema|mysql" | \
    while read -r db; do
        echo "Optimizing database: $db"
        mysqlcheck -u"$DB_USER" -p"$DB_PASS" --optimize --skip-write-binlog "$db"
    done

# PostgreSQL vacuum
sudo -u postgres psql -c "VACUUM ANALYZE;"

echo "Database optimization completed: $(date)"

Environment Variables in Cron

Cron runs with a minimal environment. Set required variables:

# Set PATH and other variables at the top of crontab
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
SHELL=/bin/bash
[email protected]

# Then your cron jobs
0 2 * * * /opt/scripts/backup.sh

Output Handling

Redirect Output to Log File

# Append stdout and stderr to log file
0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

# Separate stdout and stderr
0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>> /var/log/backup-error.log

# Discard all output
0 2 * * * /opt/scripts/backup.sh > /dev/null 2>&1

Email Output

# Set MAILTO at top of crontab
[email protected]

# Cron will email output
0 2 * * * /opt/scripts/backup.sh

# Or use mail command in script
0 2 * * * /opt/scripts/backup.sh | mail -s "Backup Report" [email protected]

Troubleshooting Cron Jobs

Check if Cron is Running

# Check cron service status
systemctl status cron       # Debian/Ubuntu
systemctl status crond      # CentOS/Rocky Linux

# Check if cron daemon is running
ps aux | grep cron

# Restart cron if needed
sudo systemctl restart cron

View Cron Logs

# View cron execution log
tail -f /var/log/syslog | grep CRON           # Debian/Ubuntu
tail -f /var/log/cron                         # CentOS/Rocky Linux

# View recent cron activity
grep CRON /var/log/syslog | tail -20          # Debian/Ubuntu
tail -20 /var/log/cron                        # CentOS/Rocky Linux

Common Issues and Solutions

Issue: Cron job not running

# Verify cron syntax
# Use online tools: crontab.guru

# Check cron service
systemctl status cron

# Check user crontab
crontab -l

# Check system logs
grep CRON /var/log/syslog

Issue: Script works manually but not in cron

# Use absolute paths
# Bad:  backup.sh
# Good: /opt/scripts/backup.sh

# Set PATH in crontab
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin

# Use full paths in script
/usr/bin/mysql instead of mysql

Issue: Permissions denied

# Check script permissions
chmod +x /opt/scripts/backup.sh

# Check file ownership
chown user:user /opt/scripts/backup.sh

# Run as specific user
sudo crontab -e -u username

Testing Cron Jobs

# Test script manually
bash -x /opt/scripts/backup.sh

# Test with cron's environment
env -i HOME=$HOME LOGNAME=$LOGNAME PATH=/usr/bin:/bin SHELL=/bin/bash /bin/sh -c '/opt/scripts/backup.sh'

# Temporary frequent execution for testing
* * * * * /opt/scripts/test-script.sh
# After testing, remove or change to desired schedule

Best Practices

1. Use Absolute Paths

# Bad
0 2 * * * backup.sh

# Good
0 2 * * * /opt/scripts/backup.sh

2. Set Environment Variables

PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
SHELL=/bin/bash
[email protected]

3. Implement Logging

0 2 * * * /opt/scripts/backup.sh >> /var/log/backup.log 2>&1

4. Add Comments

# Daily database backup at 2 AM
0 2 * * * /opt/scripts/db-backup.sh

# Clean temporary files every Sunday
0 1 * * 0 /opt/scripts/cleanup.sh

5. Handle Errors

#!/bin/bash
set -euo pipefail

# Error handling in script
trap 'echo "Error occurred"' ERR

# Your script logic

6. Use Locking

Prevent multiple instances:

#!/bin/bash
LOCKFILE="/var/run/backup.lock"

# Check if already running
if [ -f "$LOCKFILE" ]; then
    echo "Script already running"
    exit 1
fi

# Create lock file
touch "$LOCKFILE"

# Cleanup on exit
trap "rm -f $LOCKFILE" EXIT

# Your script logic

7. Monitor Execution

# Log start and end times
#!/bin/bash
echo "Started: $(date)" >> /var/log/script.log

# Your script logic

echo "Completed: $(date)" >> /var/log/script.log

Security Considerations

File Permissions

# Crontab files should not be world-readable
chmod 600 ~/.crontab

# Script files should have appropriate permissions
chmod 750 /opt/scripts/backup.sh

# Restrict directory access
chmod 750 /opt/scripts

Sensitive Data

# Don't put passwords in crontab or scripts
# Use environment files or secret management
source /etc/backup.conf  # Contains DB_PASS

# Or use .my.cnf for MySQL
mysqldump --defaults-file=/root/.my.cnf

User Isolation

# Run jobs as specific users
sudo crontab -e -u backup-user

# Use dedicated service accounts
# Don't run everything as root

Alternative: Anacron

For systems that aren't always on:

# Install anacron
sudo apt-get install anacron

# Edit /etc/anacrontab
1  5  daily-backup  /opt/scripts/backup.sh
7  10 weekly-cleanup  /opt/scripts/cleanup.sh

Conclusion

Cron is an essential tool for automating routine tasks in Linux environments. This guide covered everything from basic syntax to advanced patterns and production-ready examples. Master these concepts to effectively automate your infrastructure maintenance, monitoring, and operational tasks.

Key takeaways:

Understand cron syntax and special characters
Use absolute paths and set environment variables
Implement comprehensive logging
Handle errors gracefully
Test thoroughly before production
Monitor cron job execution
Follow security best practices
Document your scheduled tasks

Remember that while cron is powerful, modern systems may benefit from systemd timers for more advanced scheduling needs. Consider your requirements and choose the right tool for your automation needs.

Cron Jobs: Syntax and Practical Examples for Task Automation

Cron Jobs: Syntax and Practical Examples for Task Automation

Introduction

Understanding Cron Basics

What is Cron?

Cron Components

Cron Syntax

Basic Cron Format

Special Characters

Special Strings

Common Cron Patterns

Every Minute

Every 5 Minutes

Every Hour

Every Day at Midnight

Every Day at 3:30 AM

Every Monday at 9 AM

First Day of Every Month

Every Weekday at 6 PM

Multiple Times Per Day

Business Hours (9 AM to 5 PM, Weekdays)

Every 15 Minutes During Business Hours

Managing Cron Jobs

Crontab Commands

Creating Cron Jobs

System-Wide Cron Jobs

Practical Cron Examples

1. Daily Database Backup

2. Log Rotation

3. System Health Check

4. SSL Certificate Renewal

5. Website Monitoring

6. Automated Security Updates

7. Disk Space Cleanup

8. Database Optimization

Environment Variables in Cron

Output Handling

Redirect Output to Log File

Email Output

Troubleshooting Cron Jobs

Check if Cron is Running

View Cron Logs

Common Issues and Solutions

Testing Cron Jobs

Best Practices

1. Use Absolute Paths

2. Set Environment Variables

3. Implement Logging

4. Add Comments

5. Handle Errors

6. Use Locking

7. Monitor Execution

Security Considerations

File Permissions

Sensitive Data

User Isolation

Alternative: Anacron

Conclusion

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