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.
Here is how to modify cron a entry without directly editing the cron file (which is frowned upon).
crontab -l -u <user> | sed 's/find/replace/g' | crontab -u <user> -
If you want to remove a cron entry, use this:
crontab -l -u <user> | sed '/find/d' | crontab -u <user> -
I realize this is not what gaurav was asking for, but why not have all the solutions in one place?
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.
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 its 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.
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")
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.
Crontab files are simply text files and as such can be treated like any other text file. The purpose of the crontab command is to make editing crontab files safer. When edited through this command, the file is checked for errors and only saved if there are none.
crontab [path to file] can be used to specify a crontab stored in a file. Like crontab -e, this will only install the file if it is error free.
Therefore, a script can either directly write cron tab files, or write them to a temporary file and load them with the crontab [path to temp file] command. Writing directly saves having to write a temporary file, but it also avoids the safety check.
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 $i " tee -a /var/spool/cron/root"
done
In Linux, the default location of the crontab file is /var/spool/cron/. Here you can find the crontab files of all users. You just need to append your cronjob entry to the respective user's file. In the above example, the root user's crontab file is getting appended with a cronjob to run /root/test.sh every day at 1 AM.
It is an approach to incrementally add the cron job:
ssh USER_NAME@$PRODUCT_IP nohup "echo '*/2 * * * * ping -c2 PRODUCT_NAME.com >> /var/www/html/test.html' | crontab -u USER_NAME -"
(I don't have enough reputation to comment, so I'm adding at as an answer: feel free to add it as as comment next to his answer)
Joe Casadonte's one-liner is perfect, except if you run with set -e, i.e. if your script is set to fail on error, and if there are no cronjobs yet. In that case, the one-liner will NOT create the cronjob, but will NOT stop the script. The silent failure can be very misleading.
The reason is that crontab -l returns with a 1 return code, causing the subsequent command (the echo) not to be executed... thus the cronjob is not created. But since they are executed as a subprocess (because of the parenthesis) they don't stop the script.
(Interestingly, if you run the same command again, it will work: once you have executed crontab - once, crontab -l still outputs nothing, but it doesn't return an error anymore (you don't get the no crontab for <user> message anymore). So the subsequent echo is executed and the crontab is created)
In any case, if you run with set -e, the line must be:
(crontab -l 2>/dev/null || true; echo "*/5 * * * * /path/to/job -with args") | crontab -
Here's a one-liner that doesn't use/require the new job to be in a file:
(crontab -l 2>/dev/null; echo "*/5 * * * * /path/to/job -with args") | crontab -
The 2>/dev/null is important so that you don't get the no crontab for username message that some *nixes produce if there are currently no crontab entries.
Source: Stackoverflow.com