Cron

Cron is a time-based job scheduler in Unix-like computer operating systems. People who set up and maintain software environments use cron to schedule jobs (commands or shell scripts) to run periodically at fixed times, dates, or intervals.

Cron is driven by a crontab (cron table) file, a configuration file that specifies shell commands to run periodically on a given schedule. The crontab files are stored where the lists of jobs and other instructions to the cron daemon are kept. Users can have their own individual crontab files and often there is a system-wide crontab file (usually in /etc or a subdirectory of /etc) that only system administrators can edit.

crontab

A crontab file contains instructions to the cron(8) daemon of the general form: “run this command at this time on this date”. Each user has their own crontab, and commands in any given crontab will be executed as the user who owns the crontab.

Crontab Layout

Each line of a crontab file represents a job, and looks like this:

+-------------- minute (0-59)
| +------------ hour (0-23)
| | +---------- day of the month (1-31)
| | | +-------- month (1-12)
| | | | +------ day of the week (0-6) (Sunday to Saturday)
| | | | |
| | | | |
| | | | |
* * * * * command to execute

Expressions

Field	Required	Allowed values	Allowed special characters
Minutes	Yes	0–59	* , , , -
Hours	Yes	0–23	* , , , -
Day of month	Yes	0-31	* , , , -
Month	Yes	0-12 or JAN–DEC	* , , , -
Day of week	Yes	0–6 or SUN–SAT	* , , , -
Year	No	1970–2099	* , , , -

Asterisk (*)

• asterisk specifies all possible values for a field.

Comma (,)

• Commas define lists.
  • Example: "MON,WED,FRI" in the 5^th field (day of week) means Mondays, Wednesdays and Fridays.

Hyphen (-)

• Hyphens define ranges.
  • Example: 2019–2022 indicates every year between 2019 and 2022, inclusive.

Pound (#)

• Pounds define comments.
  • Example: # This command will initiate a script that does something.

Step Values

• Step values can be used in conjunction with ranges. Following a range with /<number> specifies skips of the number’s value through the range.
  • Example: 0-23/2 indicates every other hour, so 0,2,4,6,8,10,12,14,16,18,20,22

Example Cron File

# run five minutes after midnight, every day
5 0 * * *       $HOME/bin/daily.job >> $HOME/tmp/out 2>&1
# run at 2:15pm on the first of every month -- output mailed to user
15 14 1 * *     $HOME/bin/monthly
# run at 10 pm on weekdays, annoy Joe
0 22 * * 1-5    mail -s "It's 10pm" joe%Joe,%%Where are your kids?%
23 0-23/2 * * * echo "run 23 minutes after midn, 2am, 4am ..., everyday"
5 4 * * sun     echo "run at 5 after 4 every sunday"
# run this as root
* * * * * root touch /tmp/file

General Overview

List the jobs for the current user

crontab -l

Edit your crontab

crontab -e

If this is the first time you are running cron, you will get a prompt to choose your default editor, this list will vary depending on which editors your have installed

no crontab for user - using an empty one
Select an editor. To change later, run 'select-editor'.
 1. /bin/nano <---- easiest
 2. /usr/bin/vim.basic
 3. /usr/bin/vim.tiny
 4. /bin/ed
Choose 1-4 [1]:

Edit someone else’s cron jobs

crontab -u username -e

Remove all cron jobs for the current user

crontab -r

Example crontab use

These are just some examples to get you started.

!!! Important “Keep in mind” There is a difference between running a command every x [window] vs running things every x^th [window].

* Example: Every 15 minutes vs every 15<sup>th</sup> minute.

At minute 15

15 * * * *

Would produce:

next at 2019-12-04 15:15:00
then at 2019-12-04 16:15:00
then at 2019-12-04 17:15:00
then at 2019-12-04 18:15:00
then at 2019-12-04 19:15:00

At every 15^th minute

*/15 * * * *

Would produce:

next at 2019-12-04 14:30:00
then at 2019-12-04 14:45:00
then at 2019-12-04 15:00:00
then at 2019-12-04 15:15:00
then at 2019-12-04 15:30:00

At every minute

* * * * * <command-to-execute>

Would produce:

next at 2019-12-04 14:42:00
then at 2019-12-04 14:43:00
then at 2019-12-04 14:44:00
then at 2019-12-04 14:45:00
then at 2019-12-04 14:46:00

At every 5^th minute

*/5 * * * * <command-to-execute>

Would produce:

next at 2019-12-04 14:10:00
then at 2019-12-04 14:15:00
then at 2019-12-04 14:20:00
then at 2019-12-04 14:25:00
then at 2019-12-04 14:30:00

At minute 0, 5, and 10

0,5,10 * * * * <command-to-execute>

Would produce:

next at 2019-12-04 15:00:00
then at 2019-12-04 15:05:00
then at 2019-12-04 15:10:00
then at 2019-12-04 16:00:00
then at 2019-12-04 16:05:00

At minute 0

0 * * * * <command-to-execute>

Would produce:

next at 2019-12-04 15:00:00
then at 2019-12-04 16:00:00
then at 2019-12-04 17:00:00
then at 2019-12-04 18:00:00
then at 2019-12-04 19:00:00

At minute 0 past every 2^nd hour

0 */2 * * * <command-to-execute>

Would produce:

next at 2019-12-04 16:00:00
then at 2019-12-04 18:00:00
then at 2019-12-04 20:00:00
then at 2019-12-04 22:00:00
then at 2019-12-05 00:00:00

At minute 23 past every 2^nd hour from 0 through 20

23 0-20/2 * * * <command-to-execute>

Would produce:

next at 2020-01-01 00:00:00
then at 2020-04-01 00:00:00
then at 2020-07-01 00:00:00
then at 2020-10-01 00:00:00
then at 2021-01-01 00:00:00

At 00:00 on day-of-month 4 in January, April, July, and October

0 0 4 1,4,7,10 *

Would produce:

next at 2020-01-04 00:00:00
then at 2020-04-04 00:00:00
then at 2020-07-04 00:00:00
then at 2020-10-04 00:00:00
then at 2021-01-04 00:00:00

At 04:05 on Sunday

5 4 * * sun

Would produce:

next at 2019-12-08 04:05:00
then at 2019-12-15 04:05:00
then at 2019-12-22 04:05:00
then at 2019-12-29 04:05:00
then at 2020-01-05 04:05:00

At 22:00 on every day-of-week from Monday through Friday

0 22 * * 1-5

Would produce:

next at 2019-12-04 22:00:00
then at 2019-12-05 22:00:00
then at 2019-12-06 22:00:00
then at 2019-12-09 22:00:00
then at 2019-12-10 22:00:00

At minute 23 past every 2^nd hour from 0 through 20

23 0-20/2 * * *

Would produce:

next at 2019-12-04 16:23:00
then at 2019-12-04 18:23:00
then at 2019-12-04 20:23:00
then at 2019-12-05 00:23:00
then at 2019-12-05 02:23:00

At minute 0 past hour 0 and 12 on day-of-month 1 in every 2^nd month

0 0,12 1 */2 *

Would produce:

next at 2020-01-01 00:00:00
then at 2020-01-01 12:00:00
then at 2020-03-01 00:00:00
then at 2020-03-01 12:00:00
then at 2020-05-01 00:00:00

Non Standards

At 00:00 on day-of-month 1 in January

@yearly

or

@annually

Would produce:

next at 2020-01-01 00:00:00
then at 2021-01-01 00:00:00
then at 2022-01-01 00:00:00
then at 2023-01-01 00:00:00
then at 2024-01-01 00:00:00

At 00:00 on day-of-month 1

@monthly

Would produce:

next at 2020-01-01 00:00:00
then at 2020-02-01 00:00:00
then at 2020-03-01 00:00:00
then at 2020-04-01 00:00:00
then at 2020-05-01 00:00:00

At 00:00 on Sunday

@weekly

Would produce:

next at 2019-12-08 00:00:00
then at 2019-12-15 00:00:00
then at 2019-12-22 00:00:00
then at 2019-12-29 00:00:00
then at 2020-01-05 00:00:00

At 00:00

@daily

Would produce:

next at 2019-12-05 00:00:00
then at 2019-12-06 00:00:00
then at 2019-12-07 00:00:00
then at 2019-12-08 00:00:00
then at 2019-12-09 00:00:00

At minute 0

@hourly

Would produce:

next at 2019-12-04 15:00:00
then at 2019-12-04 16:00:00
then at 2019-12-04 17:00:00
then at 2019-12-04 18:00:00
then at 2019-12-04 19:00:00

At reboot

@reboot

Reference

• crontab.org
• man7.org