Python-Crontab

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Description

Crontab module for reading and create cronotab files and accessing the sytem cron with and easy API

The special character "W", "L", "#", and "?"" is not supported.

Field Name	Mandatory	Allowed Values	Special Characters	Extra Values
Minutes	Yes	0-59	* / , -	< >
Hours	Yes	0-23	* / , -	< >
Day of month	Yes	1-31	* / , -	< >
Month	Yes	1-12 or JAN-DEC	* / , -	< >
Day of week	Yes	0-6 or SUN-SAT	* / , -	< >

Installation

pip install python-crontab

Crontab Syntax

The specific syntax to define the schedule consist of five field

Minute Hours Day Month Day_of_the_Week

as mentioned about the fields can have the following values

┌───────────── minute (0 - 59)
│ ┌───────────── hour (0 - 23)
│ │ ┌───────────── day of month (1 - 31)
│ │ │ ┌───────────── month (1 - 12)
│ │ │ │ ┌───────────── day of week (0 - 6) (Sunday to Saturday;
│ │ │ │ │                                       7 is also Sunday on some systems)
│ │ │ │ │
│ │ │ │ │
* * * * *  command to execute

We can make more complex configuration if we make use of special characters

Character	Meaning
Comma	To separate multiple values
Hyphen	To indicate a range of values
Asterisk	To indicate all possible values
Forward slash	To indicate EVERY

For example: 0 16 1,10,22 * * tells cron to run a task at 4 PM (which is the 16^th hour) on the 1^st, 10^th and 22^nd day of every month.

Access to Crontab

There are 5 ways to access, there are 3 that work just in Linux and the other two will work in windows

Linux 1.

from crontab import CronTab

cron = CronTab(user='username')

2.

from crontab import CronTab

cron = CronTab()

3.

from crontab import CronTab

cron = CronTab(user=True)

Windows and other systems 4.

from crontab import CronTab

cron = CronTab(tabfile='filename.tab')

where filename.tab is a file containing the task.

5.

from crontab import CronTab

cron = CronTab(tab=""" * * * * * command""")

Create A new Job

We can create this jobs in different ways, two main are, with the commands and with commands plus comments

cron.new(command='my command')

now with a comment

cron.new(command='my command',
        comment='my comment')

so a complete example will be

from crontab import CronTab

cron = CronTab(user='username')
job = cron.new(command='python test.py')
job.minute.every(1)
cron.write()

and the test file

from datetime import datetime

my_file = open('append.txt', 'a')
my_file.write(f'append {datetime.now()}')

Setting restrictions

Restriction	Description
job.minute.every(<minutes>)	Set minutes <minutes> (0-59)
job.hour.every(<hours>)	Set hours <hours> (0-23)
job.dow.on(<day_of_the_week>), job.dow.on(<day_of_the_week>,<day_of_the_week>)	Set day of the week <day_of_the_week> ('MON','SAT', etc)
job.month.during(<month>),job.month.during(<month>,<month>)	Set month <month> ('JAN', 'FEB', etc)

Every time we set a time restriction we cancel the previous one thus

job.hour.every(1)
job.hour.every(15)

the value set will be 15 instead of 1 in order to add restriction to a previous one

job.hour.every(1)
job.hour.also.on(15)

In some case we want to set up one filed and make the other field to 0, we can do something like:

job.every(15).hours()

This will set the schedule to 0 */4 * * *.

Similarly for the 'day of the month', 'month' and 'day of the week' fields.

Set job to work every reboot

job.every_reboot()

Clean the restrictions

job.clear()

Enable and disable jobs

To enable the job

job.enable()

To disable the job

job.enable(False)

To check if the task is enable

job.is_enable()

Checking validity

job.is_valid()

Finding a job

Finding by the command name

cron.find_command("command name")

Finding by the comments

cron.find_comment("comment")

Finding according the time

cron.find_time(time schedule)

Remove the jobs

cron.remove(job)

remove base in the comment

cron.remove_all(comment='my comment')

or remove all

cron.remove_all()

Environment Variable

We can define environment variables

job.env['VARIABLE_NAME'] = 'Value'

to get the variables

job.env