Python: How to handle 'crontab' command?

Question

We know the crontab command is used for scheduled tasks in Linux.

I want to write a Python script. Its function is to receive some data (these data are related to crontab setting) and execute a 'crontab' command in order to reset the content of the user's crontab file.

I know how to execute external Linux commands in Python. But when you execute the crontab command (e.g. crontab -u xxx -e), you need to interact with an editor to modify the user's crontab file. (Suppose I don't know where the file is. For new users, crontab will generate a new file anyway. And I don't execute the command as the root user).

So the question is, how can I just execute crontab in Python? Is there any way to avoid interacting with an editor to modify the user's crontab file in Python?

My OS is ubuntu 14.01.

• Toolforger: See stackoverflow.com/questions/89228/… for an excellent answer to this question.
• liminche: That is not what I want. I know how to execute linux commands in python. But my situation is different from that. I have re-edited my question.
• tripleee: possible duplicate of How can I programmatically create a new cron job?

Answer 1

You could use python-crontab.

Installation

sudo -H pip install python-crontab

Concepts

• A cron is a time-based job scheduler which is related to a user.
• A job has a command and a comment and is "attached" to a cron.
• A job is executed by the cron it is attached to at given times.

Code examples

List system cron jobs:

from crontab import CronTab
cron = CronTab(tabfile='/etc/crontab', user=False) # system users cron
# cron = CronTab(user=True) # current users cron
# cron = CronTab(user='username') # other users cron
for job in cron:
 print(job)

Create a new job:

job = cron.new(command='/foo/bar', comment='SomeID')

Enable / disable job:

job.enable()
job.enable(False)

Find an existing job by comment:

iter = cron.find_comment('ID or some text')

Remove Items::

cron.remove( job )
cron.remove_all('echo')
cron.remove_all(comment='foo')
cron.remove_all(time='*/2')

Clear entire cron of all jobs::

cron.remove_all()

• 0xc0de: How to list all existing crons (without using regex or names)?
• CS QGB: How to modify a old job? job.command='new-cmd' ?
• Karthic Raghupathi: @0xc0de I'm not sure if this functionality was added after your question, but per the documentation, you can do something like this to get all existing jobs: from crontabs import CronTabs; for cron in CronTabs(): print(repr(cron)). Another alternative is: jobs = CronTabs.all(). All jobs are added to a CronTab object which can further be used as documented per the project.
• MCC: Unfortunately, the only lines other than the command line that python-crontab allows are comment lines. You cannot add line for environment variables which are often necessary for a cron job to run successfully.

Answer 2

As you want it in Python, you can do "something" like this:

import os;
...
cur_cron = os.popen('crontab -l > current_crontab.txt');
cur_cron.read();
cur_cron.close();
fopen_cron = file('current_crontab.txt', 'a');
fopen_cron.write("\n### Comment here if you like");
fopen_cron.write("\n* * * * * Put your command here");
fopen_cron.close();

Hopefully it helps.

Answer 3

With Vixie crontab, you could do something like this (obviously you could check for errors and so on):

import subprocess
cron_in = subprocess.Popen(['crontab', '-l'],
 stdout=subprocess.PIPE)
cur_crontab, _ = cron_in.communicate()
# new_crontab = do_my_magic(cur_crontab)
cron_out = subprocess.Popen(['crontab', '-'],
 stdin=subprocess.PIPE)
cron_out.communicate(input=new_crontab)

Answer 4

You could/should first dump your current crontab with crontab -l, edit it the way you want (e. g. add some lines, or modify) and then install the new one.

This usually works with crontab <filename>, but should as well work with crontab - and then piping the new contents into the process's stdin.

Answer 5

If all you want to do is reset the content of user crontab file , then just remove the crontab file (or overwrite with your default) , and reload the cron service .