How do I create a crontab through a script

Question

I need to add a cron job thru a script I run to set up a server. I am currently using Ubuntu. I can use crontab -e but that will open an editor to edit the current crontab. I want to do this programmatically.

Is it possible to do so?

Answer 1

I have written a crontab deploy tool in python: https://github.com/monklof/deploycron

pip install deploycron

Install your crontab is very easy, this will merge the crontab into the system's existing crontab.

from deploycron import deploycron
deploycron(content="* * * * * echo hello > /tmp/hello")

Answer 2

In Ubuntu and many other distros, you can just put a file into the /etc/cron.d directory containing a single line with a valid crontab entry. No need to add a line to an existing file.

If you just need something to run daily, just put a file into /etc/cron.daily. Likewise, you can also drop files into /etc/cron.hourly, /etc/cron.monthly, and /etc/cron.weekly.

Answer 3

As a correction to those suggesting crontab -l | crontab -: This does not work on every system. For example, I had to add a job to the root crontab on dozens of servers running an old version SUSE (don't ask why). Old SUSEs prepend comment lines to the output of crontab -l, making crontab -l | crontab - non-idempotent (Debian recognizes this problem in the crontab manpage and patched their version of Vixie Cron to change the default behaviour of crontab -l).

To edit crontabs programmatically on systems where crontab -l adds comments, you can try the following:

EDITOR=cat crontab -e > old_crontab; cat old_crontab new_job | crontab -

EDITOR=cat tells crontab to use cat as an editor (not the usual default vi), which doesn't change the file, but instead copies it to stdout. This might still fail if crontab - expects input in a format different from what crontab -e outputs. Do not try to replace the final crontab - with crontab -e - it will not work.

Answer 4

Well /etc/crontab just an ascii file so the simplest is to just

 echo "*/15 * * * *   root     date" >> /etc/crontab

which will add a job which will email you every 15 mins. Adjust to taste, and test via grep or other means whether the line was already added to make your script idempotent.

On Ubuntu et al, you can also drop files in /etc/cron.* which is easier to do and test for---plus you don't mess with (system) config files such as /etc/crontab.

Answer 5

Even more simple answer to you question would be:

echo "0 1 * * * /root/test.sh" | tee -a /var/spool/cron/root

You can setup cronjobs on remote servers as below:

#!/bin/bash
servers="srv1 srv2 srv3 srv4 srv5"
for i in $servers
  do
  echo "0 1 * * * /root/test.sh" | ssh $i " tee -a /var/spool/cron/root"
done

In Linux, the default location of the crontab file is /var/spool/cron/. Here you can find the crontab files of all users. You just need to append your cronjob entry to the respective user's file. In the above example, the root user's crontab file is getting appended with a cronjob to run /root/test.sh every day at 1 AM.

Answer 6

For user crontabs (including root), you can do something like:

crontab -l -u user | cat - filename | crontab -u user -

where the file named "filename" contains items to append. You could also do text manipulation using sed or another tool in place of cat. You should use the crontab command instead of directly modifying the file.

A similar operation would be:

{ crontab -l -u user; echo 'crontab spec'; } | crontab -u user -

If you are modifying or creating system crontabs, those may be manipulated as you would ordinary text files. They are stored in the /etc/cron.d, /etc/cron.hourly, /etc/cron.daily, /etc/cron.weekly, /etc/cron.monthly directories and in the files /etc/crontab and /etc/anacrontab.