aslamnotes

CRON

3 min read

Be aware of Timezone nuances, when you are working with Time

cron = time; tab = table

Time based job scheduler in Unix-like Operating Systems. The crond is the daemon that’s responsible for executing these jobs automatically. systemctl status crond to check the statuses, journalctl -u cron for logs (journalctl -u cron | grep root)

* * * * * command_to_execute
│ │ │ │ │
│ │ │ │ └─── Day of week (0-7, both 0 and 7 are Sunday)
│ │ │ └───── Month (1-12)
│ │ └─────── Day of month (1-31)
│ └───────── Hour (0-23)
└─────────── Minute (0-59)

Installation

The cronie package provides the modern implementation of cron, used on many distros. It provides the crond daemon, and replaces old Vixie Cron. Features PAM, systemd support

Common Time Examples

  • 0 2 * * * - Run at 2:00 AM every day
  • */15 * * * * - Run every 15 minutes
  • 0 0 * * 0 - Run at midnight every Sunday
  • 30 6 * * 1-5 - Run at 6:30 AM on weekdays
  • 0 */4 * * * - Run every 4 hours
  • 0 9 1 * * - Run at 9:00 AM on the first day of every month

System crons

System crons run as root, defined in these areas. journalctl -u cron | grep root provides you with logs of jobs run by the root

  • /etc/cron.hourly/ - Run hourly
  • /etc/cron.daily/ - Run daily
  • /etc/cron.weekly/ - Run weekly
  • /etc/cron.monthly/ - Run monthly

Crontab utility

The crontab utility lets users manage their own time schedule, that is each users gets their own individual schedule, when they are logged in

Edit your crontab

crontab -e

List your crontab

crontab -l

Edit another user crontab as root

crontab -u username -e

Best Practices

  • Ensure your scripts have executable permissions
  • Test before setting the scripts
  • Use Absolute Paths
  • Use anacron for non 24/7 systems
  • Set required environment variables at the top of your crontab (bonus: set mailto for notification)
SHELL=/bin/bash
PATH=/usr/local/bin:/usr/bin:/bin
MAILTO=admin@example.com
 
0 2 * * * /home/user/scripts/backup.sh
  • Redirect output to files
# Log everything
0 2 * * * /path/to/script.sh >> /var/log/myjob.log 2>&1
 
# Separate error logs
0 2 * * * /path/to/script.sh >> /var/log/myjob.log 2>> /var/log/myjob_error.log
 
# Suppress output
0 2 * * * /path/to/script.sh > /dev/null 2>&1
  • Use comments to document
# Daily database backup at 2 AM
0 2 * * * /usr/local/bin/backup_db.sh
 
# Clear temp files every 6 hours
0 */6 * * * /usr/bin/find /tmp -type f -atime +7 -delete

Special Strings

  • @reboot - Run once at startup
  • @yearly or @annually - Run once a year (0 0 1 1 *)
  • @monthly - Run once a month (0 0 1 * *)
  • @weekly - Run once a week (0 0 * * 0)
  • @daily or @midnight - Run once a day (0 0 * * *)
  • @hourly - Run once an hour (0 * * * *) Examples
@reboot /usr/local/bin/startup_task.sh

@yearly /usr/local/bin/yearly_cleanup.sh
@annually /usr/local/bin/yearly_cleanup.sh

@monthly /usr/local/bin/monthly_report.sh

@weekly /usr/local/bin/weekly_backup.sh

@daily /usr/local/bin/daily_sync.sh
@midnight /usr/local/bin/daily_sync.sh

@hourly /usr/local/bin/hourly_healthcheck.sh

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