Scheduling Tasks with Cron Jobs on Linux

What is Cron?

Cron is a time-based job scheduler built into every Linux server. You specify when and what to run in a crontab file, and cron handles the rest automatically.

Crontab Syntax

# ┌──── minute (0-59)
# │ ┌──── hour (0-23)
# │ │ ┌──── day of month (1-31)
# │ │ │ ┌──── month (1-12)
# │ │ │ │ ┌──── day of week (0-7, 0=Sunday, 7=Sunday)
# │ │ │ │ │
# * * * * *  command_to_execute

Common Schedule Examples

# Every minute
* * * * * /path/to/script.sh

# Every hour (at :00)
0 * * * * /path/to/script.sh

# Every day at 2:30 AM
30 2 * * * /path/to/backup.sh

# Every Monday at 9:00 AM
0 9 * * 1 /path/to/weekly-report.sh

# Every 5 minutes
*/5 * * * * /path/to/check.sh

# First day of every month at midnight
0 0 1 * * /path/to/monthly.sh

Managing Crontabs

crontab -e    # Edit your crontab (opens in nano/vim)
crontab -l    # List your cron jobs
crontab -r    # Remove ALL your cron jobs (careful!)
crontab -u username -l  # View another user's crontab

Log Cron Output

# Redirect all output to a log file
0 2 * * * /opt/backup.sh >> /var/log/backup.log 2>&1

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

Real-World Backup Script

cat > /opt/daily-backup.sh << 'EOF'
#!/bin/bash
DATE=$(date +%Y-%m-%d)
BACKUP_DIR="/backups"
mkdir -p "$BACKUP_DIR"

# Backup MySQL
mysqldump -u root myapp > "$BACKUP_DIR/myapp-$DATE.sql"

# Keep only last 7 backups
find "$BACKUP_DIR" -name "*.sql" -mtime +7 -delete

echo "Backup complete: $DATE"
EOF
chmod +x /opt/daily-backup.sh

# Schedule at 3 AM daily
(crontab -l; echo "0 3 * * * /opt/daily-backup.sh >> /var/log/backup.log 2>&1") | crontab -