⏰ Linux cron & crontab

30 2 * * * /home/user/scripts/daily_backup.sh

0 * * * * /usr/local/bin/hourly_check.sh

*/15 * * * * /home/user/monitor.sh

0 9 * * 1-5 /home/user/weekday_task.sh

0 0 1 * * /home/user/monthly_report.sh

0 8,12,16,20 * * * /home/user/check_status.sh

30 2 * * * /home/user/backup.sh >> /var/log/backup.log 2>&1

@reboot /home/user/startup_script.sh

@hourly   /home/user/hourly.sh
@daily    /home/user/daily.sh
@weekly   /home/user/weekly.sh
@monthly  /home/user/monthly.sh
@yearly   /home/user/yearly.sh
@reboot   /home/user/startup.sh

0 */5 * * * /home/user/five_hour_task.sh

SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com

30 2 * * * /home/user/backup.sh

0 9 * * 1-5 /home/user/business_hours_task.sh

0 9-17/2 * * * /home/user/office_hours_check.sh

0 0 1,15 * * /home/user/twice_monthly.sh

📚 Additional Information

Crontab Field Values

Special Characters Explained

Common Crontab Patterns

# Every minute
* * * * * command

# Every 5 minutes
*/5 * * * * command

# Every hour at minute 30
30 * * * * command

# Every day at 2:30 AM
30 2 * * * command

# Every Sunday at midnight
0 0 * * 0 command

# Every weekday at 6:30 AM
30 6 * * 1-5 command

# Every month on the 1st at midnight
0 0 1 * * command

# Every quarter (Jan, Apr, Jul, Oct) on the 1st
0 0 1 1,4,7,10 * command

# Twice a day (6 AM and 6 PM)
0 6,18 * * * command

# Every 6 hours
0 */6 * * * command

# Every Monday at 9 AM
0 9 * * 1 command

# Last day of month (using a trick with conditional)
0 0 28-31 * * [ "$(date +\%d -d tomorrow)" = "01" ] && command

System-Wide Crontabs

In addition to user crontabs, there are system-wide crontab locations:

• /etc/crontab - System-wide crontab file
• /etc/cron.d/ - Directory for system cron jobs
• /etc/cron.hourly/ - Scripts run every hour
• /etc/cron.daily/ - Scripts run daily
• /etc/cron.weekly/ - Scripts run weekly
• /etc/cron.monthly/ - Scripts run monthly

System Crontab Example

# /etc/crontab format (note the extra 'user' field)
SHELL=/bin/bash
PATH=/sbin:/bin:/usr/sbin:/usr/bin

# m h dom mon dow user  command
17 *    * * *   root    cd / && run-parts --report /etc/cron.hourly
25 6    * * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily )
47 6    * * 7   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly )
52 6    1 * *   root    test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly )

Crontab Commands Summary

Debugging Cron Jobs

• Check cron logs: tail -f /var/log/syslog | grep CRON
• Test with frequent schedule: Set to */1 (every minute) for testing
• Redirect to log file: Add >> /tmp/crontest.log 2>&1 to see output
• Use full paths: Always use absolute paths for commands and files
• Check permissions: Ensure scripts are executable (chmod +x)
• Set PATH: Add PATH=/usr/local/bin:/usr/bin:/bin to your crontab
• Test script manually: Run your script directly to verify it works

Common Cron Issues and Solutions

Best Practices

• Always use absolute paths: /usr/bin/python3 instead of python3
• Add comments: Use # to document what each job does
• Log everything: Redirect output to log files for troubleshooting
• Test scripts first: Run manually before scheduling in cron
• Be timezone aware: Cron uses system timezone, not user timezone
• Avoid overlapping jobs: Ensure previous job completes before next starts
• Use locking: Implement file locks to prevent multiple instances
• Keep scripts simple: Use shell scripts to organize complex tasks
• Monitor cron jobs: Check logs regularly for failures
• Backup your crontab: Run crontab -l > crontab_backup.txt regularly

Environment Variables in Cron

Cron jobs run with a minimal environment. You can set variables at the top of your crontab:

SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com
HOME=/home/user
LANG=en_US.UTF-8

# Your cron jobs below...

Preventing Overlapping Jobs

# Using flock to prevent overlaps
* * * * * /usr/bin/flock -n /tmp/myjob.lock /home/user/script.sh

# Using a PID file
* * * * * [ ! -f /tmp/script.pid ] && echo $$ > /tmp/script.pid && /home/user/script.sh ; rm -f /tmp/script.pid

Real-World Sysadmin Examples

# Daily backup at 2 AM
0 2 * * * /usr/local/bin/backup.sh >> /var/log/backup.log 2>&1

# Clear temp files every hour
0 * * * * find /tmp -type f -mtime +7 -delete

# Monitor disk space every 15 minutes
*/15 * * * * df -h | awk '$5 > 90 {print}' | mail -s "Disk Alert" admin@example.com

# Restart service daily at 3 AM
0 3 * * * systemctl restart myservice

# Update Let's Encrypt certificates weekly
0 3 * * 0 certbot renew --quiet

# Database backup every 6 hours
0 */6 * * * mysqldump -u root mydb | gzip > /backup/mydb_$(date +\%Y\%m\%d_\%H\%M).sql.gz

# Log rotation monthly
0 0 1 * * /usr/sbin/logrotate /etc/logrotate.conf

# System update check daily
0 4 * * * apt update && apt list --upgradable | mail -s "Updates Available" admin@example.com

# Clear old logs weekly
0 2 * * 0 find /var/log -name "*.log" -mtime +30 -delete

# Monitor website uptime every 5 minutes
*/5 * * * * /usr/local/bin/check_website.sh

Advanced Cron Patterns

# Run every weekday at 9:30 AM
30 9 * * 1-5 /home/user/weekday_task.sh

# Run every 2 hours between 8 AM and 6 PM
0 8-18/2 * * * /home/user/business_hours.sh

# Run on the first Monday of every month
0 9 1-7 * 1 /home/user/first_monday.sh

# Run every 90 minutes (use at for this, or creative cron)
0 0,1,3,4,6,7,9,10,12,13,15,16,18,19,21,22 * * * /home/user/90min.sh

# Run at random time between 2-3 AM (prevents thundering herd)
0 2 * * * sleep $((RANDOM \% 3600)) && /home/user/script.sh

Cron vs. Systemd Timers

Modern Linux systems offer systemd timers as an alternative to cron:

Security Considerations

• Restrict cron access: Use /etc/cron.allow and /etc/cron.deny
• Protect credentials: Don't hardcode passwords in crontab; use credential files
• Set proper permissions: Ensure scripts aren't world-writable
• Limit email output: Avoid sending sensitive data via email
• Use sudo carefully: Minimize scripts run as root
• Audit crontabs: Regularly review all user and system crontabs
• Log job execution: Keep audit trail of what runs and when

Access Control Files

• /etc/cron.allow - If exists, only listed users can use crontab
• /etc/cron.deny - If cron.allow doesn't exist, listed users cannot use crontab
• If neither exists, only root can use crontab (default on some systems)
• If only cron.deny exists and is empty, all users can use crontab

Useful Cron Generators

Online tools can help generate cron syntax:

• crontab.guru - Human-readable cron schedule expressions
• crontab-generator.org - Visual cron expression builder
• cronitor.io/cron-job-monitoring - Cron generator with monitoring

Testing Cron Jobs

# Create a test crontab entry that runs every minute
* * * * * echo "Test at $(date)" >> /tmp/crontest.log

# Watch the log file
tail -f /tmp/crontest.log

# After confirming it works, remove the test entry
crontab -e

Backing Up and Restoring Crontabs

# Backup your crontab
crontab -l > ~/crontab_backup_$(date +%Y%m%d).txt

# Backup all user crontabs (as root)
for user in $(cut -f1 -d: /etc/passwd); do
    crontab -u $user -l > /backup/crontab_$user 2>/dev/null
done

# Restore from backup
crontab ~/crontab_backup_20251102.txt

# Restore specific user's crontab (as root)
crontab -u username ~/crontab_backup_username.txt

Fun Facts

• The name "cron" comes from the Greek word "chronos" meaning time
• cron was first implemented in Unix Version 7 by Ken Thompson in 1979
• The anacron command can run jobs on systems that aren't always on (like laptops)
• You can use month and day names: JAN-DEC and SUN-SAT instead of numbers
• Some systems support a 6th field for seconds, but this is non-standard