Systems Administration

Cron

What is Cron?

Cron is a job scheduler that runs commands automatically at specified times on Unix-like systems.

Basic Syntax

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

Common Examples

Every minute

* * * * * echo "Hello" >> /tmp/hello.log

Every hour at minute 30

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

Every day at 2:30 AM

30 2 * * * /usr/bin/backup.sh

Every Monday at 9 AM

0 9 * * 1 /home/user/weekly-report.py

Every 5 minutes

*/5 * * * * /usr/local/bin/check-status.sh

How to Use Cron

Edit your crontab

crontab -e

List current cron jobs

crontab -l

Remove all cron jobs

crontab -r

Important Tips

• Use full paths for commands and files
• Test your scripts manually first
• Check logs: grep CRON /var/log/syslog
• For output, redirect to a file: command >> /tmp/output.log 2>&1
• Remember cron runs with minimal environment variables

Quick Reference

Time	Cron Expression
Every minute	* * * * *
Hourly	0 * * * *
Daily at midnight	0 0 * * *
Weekly (Sunday)	0 0 * * 0
Monthly	0 0 1 * *
Yearly	0 0 1 1 *

Example: Weekly Backup

# Edit crontab
crontab -e

# Add this line for weekly backup at 6 AM on Monday
0 6 * * 1 /home/<USERNAME>/.local/bin/dropbox_uploader.sh upload /zata/zippy/<USERNAME> zata_backup

See dropbox-uploader.sh section for more details about this backup tool.