cron

Overview

The cron daemon runs scheduled tasks (cron jobs) at specified intervals. It reads crontab files to execute commands automatically.

Syntax

Crontab format:

* * * * * command
- - - - -
| | | | |
| | | | +----- Day of week (0-7)
| | | +------- Month (1-12)
| | +--------- Day of month (1-31)
| +----------- Hour (0-23)
+------------- Minute (0-59)

Common Options

Field	Values	Special Characters
Minute	0-59	, - * /
Hour	0-23	, - * /
Day of Month	1-31	, - * / L W
Month	1-12	, - * /
Day of Week	0-7	, - * / L #

Special Characters

Character	Description
*	Any value
,	Value list separator
-	Range of values
/	Step values
L	Last day
W	Weekday
#	Nth weekday

Key Use Cases

1. Scheduled backups
2. System maintenance
3. Report generation
4. Data processing
5. Automated tasks

Examples with Explanations

Example 1: Every Hour

0 * * * * command

Run at minute 0 of every hour

Example 2: Daily at 3 AM

0 3 * * * command

Run at 3:00 AM daily

Example 3: Every Weekday

0 9 * * 1-5 command

Run at 9:00 AM Monday-Friday

Common Usage Patterns

1. Every minute:

   * * * * * command

2. Every 15 minutes:

   */15 * * * * command

3. Monthly:

   0 0 1 * * command

Security Considerations

1. User permissions
2. Script security
3. Output handling
4. Error logging
5. Resource limits

Related Commands

• crontab - Manage cron jobs
• at - One-time scheduling
• systemd-timer - Systemd timers
• anacron - Periodic command scheduler
• logrotate - Log rotation

Additional Resources

• Cron Manual
• Crontab Guide
• Job Scheduling

Best Practices

1. Document jobs
2. Handle output
3. Use absolute paths
4. Set proper permissions
5. Monitor execution

Environment Setup

1. PATH setting
2. Shell specification
3. Environment variables
4. Working directory
5. User context

Troubleshooting

1. Job timing
2. Permission issues
3. Output handling
4. Script errors
5. Resource conflicts