Cheatsheet - cron

Show or delete current crontab

# Shwo current cron jobs
$ crontab -l
$ crontab -u username -l

# Delete current cron jobs
$ crontab -r

# Delete job for specific user. Must be run as root user ##
$ crontab -r -u username

Edit the crontab

$ crontab -e

cron job format:

1 2 3 4 5 USERNAME /path/to/command arg1 arg2
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Examples:

# run at 3am every day
0 3 * * * /path/to/command

# run at five minutes after midnight, every day
5 0 * * *  /path/to/command

# run at 2:15pm on the first of every month
15 14 1 * *  /path/to/script.sh

# run at 10 pm on weekdays
0 22 * * 1-5   /path/to/script.sh

# run at 23 minutes after midnight, 2am, 4am …, everyday
23 0-23/2 * * * /path/to/script.sh

# run at 5 after 4 every Sunday
5 4 * * sun     /path/to/script.sh

Operators

• *: specifies all possible values for a field, e.g. every hour or every month.
• , : specifies a list of values, e.g. 1,5,10,15,20,25.
• - : specifies a range of values, e.g. 5-15
• / : specifies a step value, e.g. 0-23/2 or */2 for every two hours.

Special Strings

Special string	Meaning
@reboot	Run once, at startup.
@yearly	Run once a year, 0 0 1 1 *.
@annually	(same as @yearly)
@monthly	Run once a month, 0 0 1 * *.
@weekly	Run once a week, 0 0 * * 0.
@daily	Run once a day, 0 0 * * *.
@midnight	(same as @daily)
@hourly	Run once an hour, 0 * * * *.

e.g.

@hourly /path/to/command

Files

• /etc/crontab: system crontabs file. Usually only used by root user or daemons to configure system wide jobs.
• /var/spool/cron/ or /var/cron/tabs/: personal user crontab files. It must be backup with users home directory. All individual user must must use crontab command to install and edit their jobs.
• /var/log/cron: logs.
• /var/log/syslog: logs grep CRON /var/log/syslog.

Directory	Description
/etc/cron.d/	Put all scripts here and call them from /etc/crontab file.
/etc/cron.daily/	Run all scripts once a day
/etc/cron.hourly/	Run all scripts once an hour
/etc/cron.monthly/	Run all scripts once a month
/etc/cron.weekly/	Run all scripts once a week

/etc/crontab schedules /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly. Scripts in those folders will be executed.

cron is managed by systemd

# Check status
$ sudo systemctl status cron

# Check logs
$ sudo journalctl -u cron

Email alert

To set the email address, define MAILTO variable:

MAILTO="[email protected]"
@daily /path/to/command

# disable email alert, set MATILTO to ""
MAILTO=""
@daily /path/to/command