A Comprehensive Guide to Cron | Generated by AI
Cron is a powerful time-based job scheduler in Unix-like operating systems that allows users to schedule commands or scripts to run automatically at specified times or intervals. Here’s a complete guide to understanding and using cron effectively.
What is Cron?
Cron is a daemon (background service) that executes scheduled commands. It reads configuration files called “crontabs” (cron tables) to determine when to run particular commands. Cron is ideal for:
- Routine system maintenance
- Automated backups
- Scheduling periodic data processing
- Running scripts at specific times
- Any task that needs to run regularly without manual intervention
Cron Components
The Crontab File
The crontab (cron table) is a configuration file that specifies shell commands to run periodically. Each user can have their own crontab, and there’s also a system-wide crontab.
To edit your personal crontab:
crontab -e
To view your current crontab:
crontab -l
To remove your crontab:
crontab -r
Crontab Syntax
Each line in a crontab file represents a scheduled job and follows this format:
* * * * * command-to-execute
│ │ │ │ │
│ │ │ │ └─── Day of week (0-6, Sunday=0)
│ │ │ └───── Month (1-12)
│ │ └─────── Day of month (1-31)
│ └───────── Hour (0-23)
└─────────── Minute (0-59)
Each position can contain:
- A specific number
- A range (e.g.,
1-5) - A list of values (e.g.,
1,3,5) - An asterisk (
*) meaning “every” - A step value (
*/n) meaning “every n”
After the five time/date fields comes the command to execute.
Common Cron Schedule Examples
Common Cron Schedule Examples
Basic Time Intervals
| Cron Expression | Description | Example |
|---|---|---|
* * * * * |
Run every minute | * * * * * date >> /tmp/date_log.txt |
*/5 * * * * |
Run every 5 minutes | */5 * * * * /scripts/check_server.sh |
0 * * * * |
Run at the start of every hour | 0 * * * * /scripts/hourly_backup.sh |
0 0 * * * |
Run at midnight daily | 0 0 * * * /scripts/daily_backup.sh |
0 0 * * 0 |
Run at midnight on Sundays | 0 0 * * 0 /scripts/weekly_maintenance.sh |
0 0 1 * * |
Run at midnight on the first day of each month | 0 0 1 * * /scripts/monthly_report.sh |
0 0 1 1 * |
Run at midnight on January 1 (yearly) | 0 0 1 1 * /scripts/yearly_cleanup.sh |
Specific Times
| Cron Expression | Description | Example |
|---|---|---|
30 8 * * 1-5 |
Run at 8:30 AM on weekdays (Monday-Friday) | 30 8 * * 1-5 /scripts/workday_start.sh |
0 18 * * 1-5 |
Run at 6:00 PM on weekdays | 0 18 * * 1-5 /scripts/workday_end.sh |
0 9-17 * * * |
Run at the start of every hour during business hours (9 AM - 5 PM) | 0 9-17 * * * /scripts/hourly_check.sh |
0 0,12 * * * |
Run at midnight and noon | 0 0,12 * * * /scripts/twice_daily.sh |
Multiple Time Specifications
| Cron Expression | Description | Example |
|---|---|---|
0 */4 * * * |
Run every 4 hours | 0 */4 * * * /scripts/periodic_task.sh |
0 4,16 * * * |
Run at 4:00 AM and 4:00 PM | 0 4,16 * * * /scripts/twice_daily_at_4.sh |
15,45 * * * * |
Run at 15 and 45 minutes past every hour | 15,45 * * * * /scripts/quarter_hourly.sh |
Special Time Specifications
| Cron Expression | Description | Example |
|---|---|---|
@reboot |
Run once at startup | @reboot /scripts/startup_task.sh |
@yearly or @annually |
Run once a year (0 0 1 1 *) |
@yearly /scripts/annual_task.sh |
@monthly |
Run once a month (0 0 1 * *) |
@monthly /scripts/monthly_task.sh |
@weekly |
Run once a week (0 0 * * 0) |
@weekly /scripts/weekly_task.sh |
@daily or @midnight |
Run once a day (0 0 * * *) |
@daily /scripts/daily_task.sh |
@hourly |
Run once an hour (0 * * * *) |
@hourly /scripts/hourly_task.sh |
Complex Examples
| Cron Expression | Description | Example |
|---|---|---|
0 0 */2 * * |
Run at midnight every other day | 0 0 */2 * * /scripts/every_other_day.sh |
0 3 * * 1,3,5 |
Run at 3:00 AM on Monday, Wednesday, and Friday | 0 3 * * 1,3,5 /scripts/mwf_task.sh |
30 23 * * 2,4 |
Run at 11:30 PM on Tuesday and Thursday | 30 23 * * 2,4 /scripts/tue_thu_night.sh |
45 11 1-7 * * |
Run at 11:45 AM during the first week of each month | 45 11 1-7 * * /scripts/first_week_task.sh |
0 20 * 10,11,12 * |
Run at 8:00 PM during October, November, and December | 0 20 * 10,11,12 * /scripts/q4_evening_task.sh |
0 9-17/2 * * 1-5 |
Run every other hour during business hours on weekdays | 0 9-17/2 * * 1-5 /scripts/biweekly_workday.sh |
Special Cron Strings
Cron also supports several special strings for common time specifications:
@reboot: Run once at startup@yearlyor@annually: Run once a year (equivalent to0 0 1 1 *)@monthly: Run once a month (equivalent to0 0 1 * *)@weekly: Run once a week (equivalent to0 0 * * 0)@dailyor@midnight: Run once a day (equivalent to0 0 * * *)@hourly: Run once an hour (equivalent to0 0 * * *)
Handling Output and Errors
By default, cron sends the output of commands via email to the user who owns the crontab. You can redirect output to a file or discard it:
# Append output to a log file
30 5 * * * /path/to/script.sh >> /path/to/logfile.log 2>&1
# Discard all output
0 12 * * * /path/to/script.sh > /dev/null 2>&1
Environment Variables in Cron
Cron jobs run with a minimal environment, which can cause scripts to fail. You can set variables in the crontab file:
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
SHELL=/bin/bash
MAILTO=your-email@example.com
# Jobs will now use these environment variables
0 5 * * * /path/to/script.sh
Cron Security Considerations
- Use absolute paths for all commands and scripts
- Restrict crontab file permissions
- Use the principle of least privilege when setting up cron jobs
- Log and monitor cron job output for debugging
- Consider using a dedicated user for sensitive cron jobs
Troubleshooting Cron Jobs
Common issues with cron jobs include:
- Job doesn’t run: Check permissions and paths
- Timing issues: Verify the time format is correct
- Environment problems: Ensure necessary variables are set
- Script errors: Test the script manually first
Check system logs for cron-related messages:
grep CRON /var/log/syslog
Cron Alternatives
While cron is prevalent, there are alternatives:
- Anacron: Better for systems that aren’t running 24/7
- Systemd Timers: Modern alternative on systemd-based systems
- Fcron: Enhanced cron with anacron-like features
- Schedule: Higher-level job schedulers often used in web applications
Advanced Cron Usage
Running Jobs As Different Users
To edit the crontab for another user (requires root):
sudo crontab -u username -e
System-Wide Crontabs
System-wide cron jobs can be added to:
/etc/crontab- Files in
/etc/cron.d/ - Directories like
/etc/cron.daily/,/etc/cron.hourly/, etc.
The syntax for system crontabs includes an additional user field:
* * * * * user command-to-execute
Running Cron Jobs with Specific Environment
Create a wrapper script to set up the environment before running your actual command:
#!/bin/bash
source /home/user/.profile
cd /path/to/working/directory
/path/to/actual/command
Then call this wrapper script from cron.
Best Practices for Cron Jobs
- Document your cron jobs: Add comments to explain what each job does
- Use descriptive script names: Makes maintenance easier
- Log outputs: Save job outputs for troubleshooting
- Monitor job execution: Implement monitoring for critical jobs
- Handle failures gracefully: Add error handling to your scripts
- Use lockfiles: Prevent overlapping execution when jobs take longer than expected
- Consider time zones: Be aware of system time zone settings
By following this guide, you should be able to effectively set up, manage, and troubleshoot cron jobs on your Unix-like system.