Using Cron
What is Cron?
Cron is a time based job scheduler in Unix-like Operating Systems. People who use and maintain software environments use cron to schedule jobs to run periodically at fixed times, dates, or intervals.
Cron is driven by a crontab (cron table) file that specifies shell commands to run periodically on the given schedules. Crontab files normally reside in the same location as the cron daemon. Individuaals can choose to have their own crontab files. These normally reside in the /etc directory.
Crontab
A crontab file contains instructions for the cron daemon in the form of "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 month (1-31)
| | | +---------- month (1-12)
| | | | +------- day of week (0-6) (Sunday to Saturday)
| | | | |
| | | | |
* * * * * command_to_execute
Expressions
| Field | Required | Allowed Values | Allowed Special Characters |
|---|---|---|---|
| Minutes | Yes | 0-50 | * , , , - |
| Hours | Yes | 0-23 | * , , , - |
| Day of Month | Yes | 1-32 | * , , , - |
| Month | Yes | 1-12 | * , , , - |
| Day of Week | Yes | 0-6 | * , , , - |
Asterisk
The (*) character specifies all possible values for the field.
Example: 10 * 1 1 * will run the command at minute 10 every hour on the first day in January or at 2021-01-01 00:10:00
Comma
The (,) character defines a list.
Example: MON,WED,FRI in the 5th field will tell the command to run on Mondays, Wednesdays and Fridays.
Round
The (#) pound character defines a comment.
Example: # This command does something somewhere.
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 in the 2nd field denotes every other hour.
Example Cron File
# Run five minutes after midnight each day
5 0 * * * /path/to/script
# Run at 10pm on workdays and email John
0 22 * * 1-5 mail -s "Something Important" john
# Run this one as root
5 0 * * * root touch /tmp/file
General Overview
Cron basics
If this in your first time using cron, you will be prompted to choose an editor, the prompt will list your options and it will vary depending on which editor you have installed, for example:
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]:
Remove all cron jobs for the current user
crontab -r
List the jobs for the current user
crontab -l
Edit your crontab file
crontab -e
Edit another user's cron jobs
crontab -u username -e
Example crontab use
Important
There is a difference between running a command every x [window] vs running a command every x<sup>th</sup> [window].
Example: Every 15 minutes vs Every 15th minute.
At minute 15
15 * * * * <command-to-execute>
next at 2020-11-05 22:15:00
then at 2020-11-05 23:15:00
then at 2020-11-06 00:15:00
then at 2020-11-06 01:15:00
then at 2020-11-06 02:15:00
At every 15th minute
*/15 * * * * <command-to-execute>
next at 2020-11-05 21:30:00
then at 2020-11-05 21:45:00
then at 2020-11-05 22:00:00
then at 2020-11-05 22:15:00
then at 2020-11-05 22:30:00
At every minute
* * * * * <command-to-execute>
next at 2020-11-05 21:27:00
then at 2020-11-05 21:28:00
then at 2020-11-05 21:29:00
then at 2020-11-05 21:30:00
then at 2020-11-05 21:31:00
At every 5th minute
*/5 * * * * <command-to-execute>
next at 2020-11-05 21:30:00
then at 2020-11-05 21:35:00
then at 2020-11-05 21:40:00
then at 2020-11-05 21:45:00
then at 2020-11-05 21:50:00
At minute 0, 5, and 10
0,5,10 * * * * <command-to-execute>
next at 2020-11-05 22:00:00
then at 2020-11-05 22:05:00
then at 2020-11-05 22:10:00
then at 2020-11-05 23:00:00
then at 2020-11-05 23:05:00
At minute 0
0 * * * * <command-to-execute>
next at 2020-11-05 22:00:00
then at 2020-11-05 23:00:00
then at 2020-11-06 00:00:00
then at 2020-11-06 01:00:00
then at 2020-11-06 02:00:00
At minute 0 past every 2nd hour
0 */2 * * * <command-to-execute>
next at 2020-11-05 22:00:00
then at 2020-11-06 00:00:00
then at 2020-11-06 02:00:00
then at 2020-11-06 04:00:00
then at 2020-11-06 06:00:00
At minute 23 past every 2nd hour from 0 through 20
23 0-20/2 * * * <command-to-execute>
next at 2020-11-06 00:23:00
then at 2020-11-06 02:23:00
then at 2020-11-06 04:23:00
then at 2020-11-06 06:23:00
then at 2020-11-06 08:23:00
At 00:00 on day-of-month 4 in January, April, July, and October.
0 0 4 1,4,7,10 * <command-to-execute>
next at 2021-01-04 00:00:00
then at 2021-04-04 00:00:00
then at 2021-07-04 00:00:00
then at 2021-10-04 00:00:00
then at 2022-01-04 00:00:00
At 04:05 on Sunday
5 4 * * sun <command-to-execute>
next at 2020-11-08 04:05:00
then at 2020-11-15 04:05:00
then at 2020-11-22 04:05:00
then at 2020-11-29 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 <command-to-execute>
next at 2020-11-05 22:00:00
then at 2020-11-06 22:00:00
then at 2020-11-09 22:00:00
then at 2020-11-10 22:00:00
then at 2020-11-11 22:00:00
At minute 23 past every 2nd hour from 0 through 20
23 0-20/2 * * * <command-to-execute>
next at 2020-11-06 00:23:00
then at 2020-11-06 02:23:00
then at 2020-11-06 04:23:00
then at 2020-11-06 06:23:00
then at 2020-11-06 08:23:00
At minute 0 past hour 0 and 12 on day-of-month 1 in every 2nd month.
0 0,12 1 */2 * <command-to-execute>
next at 2021-01-01 00:00:00
then at 2021-01-01 12:00:00
then at 2021-03-01 00:00:00
then at 2021-03-01 12:00:00
then at 2021-05-01 00:00:00
Non Standards
Yearly or At 00:00 on day-of-month 1 in January
@yearly <command-to-execute>
next 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
then at 2025-01-01 00:00:00
Monthly or At 00:00 on day-of-month 1
@monthly <command-to-execute>
next at 2020-11-01 00:00:00
then at 2021-01-01 00:00:00
then at 2021-02-01 00:00:00
then at 2021-03-01 00:00:00
then at 2021-04-01 00:00:00
Weekly or At 00:00 on Sunday
@weekly <command-to-execute>
next at 2020-11-08 00:00:00
then at 2020-11-15 00:00:00
then at 2020-11-22 00:00:00
then at 2020-11-29 00:00:00
then at 2020-11-06 00:00:00
Daily or At 00:00
@daily <command-to-execute>
next at 2020-11-06 00:00:00
then at 2020-11-07 00:00:00
then at 2020-11-08 00:00:00
then at 2020-11-09 00:00:00
then at 2020-11-10 00:00:00
Hourly or At minute 0
@hourly <command-to-execute>
next at 2020-11-05 21:00:00
then at 2020-11-05 22:00:00
then at 2020-11-05 23:00:00
then at 2020-11-06 00:00:00
then at 2020-11-06 01:00:00
After rebooting
@reboot <command-to-execute>