Cron
Tags: @cron @cronjob @schedule @automation
https://www.computerhope.com/unix/ucrontab.htm
We use cron to schedule tasks to run in the background. It reads comands from a cron table which can be accessed for the current user by doing: $ crontab -e
Once you run the command, you see a file that looks like this (it will be empty the first time you run crontab):
# Each task to run has to be defined through a single line
# indicating with different fields when the task will be run
# and what command to run for the task
#
# To define the time you can provide concrete values for
# minute (m), hour (h), day of month (dom), month (mon),
# and day of week (dow) or use '*' in these fields (for 'any').
#
# Notice that tasks will be started based on the cron's system
# daemon's notion of time and timezones.
#
# Output of the crontab jobs (including errors) is sent through
# email to the user the crontab file belongs to (unless redirected).
#
# For example, you can run a backup of all your user accounts
# at 5 a.m every week with:
# 0 5 * * 1 tar -zcf /var/backups/home.tgz /home/
#
# For more information see the manual pages of crontab(5) and cron(8)
#
# m h dom mon dow command
# Example of job definition:
# .---------------- minute (0 - 59)
# | .------------- hour (0 - 23)
# | | .---------- day of month (1 - 31)
# | | | .------- month (1 - 12) OR jan,feb,mar,apr ...
# | | | | .---- day of week (0 - 6) (Sunday=0 or 7) OR sun,mon,tue,wed,thu,fri,sat
# | | | | |
# * * * * * user-name command to be executed
#If MAILTO is defined but empty (MAILTO=""), no mail is sent. Otherwise, mail is sent to the owner of the crontab.
MAILTO=email@example.com # mails info about job
There are also a few special characters you can include in the schedule component of a cron expression to make scheduling easier:
| Char | Meaning |
|---|---|
| * | In cron expressions, an asterisk is a wildcard variable that represents “all”. Thus, a task scheduled with * * * * * ... will run every minute of every hour of every day of every month. |
| , | Commas break up scheduling values to form a list. If you want to have a task run at the beginning and middle of every hour, rather than writing out two separate tasks (e.g., 0 * * * * ... and 30 * * * * ...), you could achieve the same functionality with one (0,30 * * * * ...). |
| - | A hyphen represents a range of values in the schedule field. Instead of having 30 separate scheduled tasks for a command you want to run for the first 30 minutes of every hour (as in 0 * * * * ..., 1 * * * * ..., 2 * * * * ..., and so on), you could just schedule it as 0-29 * * * * .... |
| / | You can use a forward slash with an asterisk to express a step value. For example, instead of writing out eight separate separate cron tasks to run a command every three hours (as in, 0 0 * * * ..., 0 3 * * * ..., 0 6 * * * ..., and so on), you could schedule it to run like this: 0 */3 * * * .... |
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