How to create a cron job using Bash automatically without the interactive editor?

You can add to the crontab as follows:

#write out current crontab
crontab -l > mycron
#echo new cron into cron file
echo "00 09 * * 1-5 echo hello" >> mycron
#install new cron file
crontab mycron
rm mycron

Cron line explaination

* * * * * "command to be executed"
- - - - -
| | | | |
| | | | ----- Day of week (0 - 7) (Sunday=0 or 7)
| | | ------- Month (1 - 12)
| | --------- Day of month (1 - 31)
| ----------- Hour (0 - 23)
------------- Minute (0 - 59)

Source nixCraft.

You may be able to do it on-the-fly

crontab -l | { cat; echo "0 0 0 0 0 some entry"; } | crontab -

crontab -l lists the current crontab jobs, cat prints it, echo prints the new command and crontab - adds all the printed stuff into the crontab file. You can see the effect by doing a new crontab -l.

EDIT (fixed overwriting):

cat <(crontab -l) <(echo "1 2 3 4 5 scripty.sh") | crontab -

So, in Debian, Ubuntu, and many similar Debian based distros...

There is a cron task concatenation mechanism that takes a config file, bundles them up and adds them to your cron service running.

You can put a file under the /etc/cron.d/somefilename where somefilename is whatever you want.

sudo echo "0,15,30,45 * * * * ntpdate -u time.nist.gov" >> /etc/cron.d/vmclocksync

Let's disassemble this:

sudo - because you need elevated privileges to change cron configs under the /etc directory

echo - a vehicle to create output on std out. printf, cat... would work as well

" - use a doublequote at the beginning of your string, you're a professional

0,15,30,45 * * * * - the standard cron run schedule, this one runs every 15 minutes

ntpdate -u time.nist.gov - the actual command I want to run

" - because my first double quotes needs a buddy to close the line being output

>> - the double redirect appends instead of overwrites*

/etc/cron.d/vmclocksync - vmclocksync is the filename I've chosen, it goes in /etc/cron.d/

* if we used the > redirect, we could guarantee we only had one task entry. But, we would be at risk of blowing away any other rules in an existing file. You can decide for yourself if possible destruction with > is right or possible duplicates with >> are for you. Alternatively, you could do something convoluted or involved to check if the file name exists, if there is anything in it, and whether you are adding any kind of duplicate-- but, I have stuff to do and I can't do that for you right now.

echo "0 * * * * docker system prune --force >/dev/null 2>&1" | sudo tee /etc/cron.daily/dockerprune

If you're using the Vixie Cron, e.g. on most Linux distributions, you can just put a file in /etc/cron.d with the individual cronjob.

This only works for root of course. If your system supports this you should see several examples in there. (Note the username included in the line, in the same syntax as the old /etc/crontab)

It's a sad misfeature in cron that there is no way to handle this as a regular user, and that so many cron implementations have no way at all to handle this.

My preferred solution to this would be this:

(crontab -l | grep . ; echo -e "0 4 * * * myscript\n") | crontab -

This will make sure you are handling the blank new line at the bottom correctly. To avoid issues with crontab you should usually end the crontab file with a blank new line. And the script above makes sure it first removes any blank lines with the "grep ." part, and then add in a new blank line at the end with the "\n" in the end of the script. This will also prevent getting a blank line above your new command if your existing crontab file ends with a blank line.

Bash script for adding cron job without the interactive editor. Below code helps to add a cronjob using linux files.

#!/bin/bash

cron_path=/var/spool/cron/crontabs/root

#cron job to run every 10 min.
echo "*/10 * * * * command to be executed" >> $cron_path

#cron job to run every 1 hour.
echo "0 */1 * * * command to be executed" >> $cron_path

(2>/dev/null crontab -l ; echo "0 3 * * * /usr/local/bin/certbot-auto renew") | crontab -

cat <(crontab -l 2>/dev/null) <(echo "0 3 * * * /usr/local/bin/certbot-auto renew") | crontab -

#write out current crontab

crontab -l > mycron 2>/dev/null

#echo new cron into cron file

echo "0 3 * * * /usr/local/bin/certbot-auto renew" >> mycron

#install new cron file

crontab mycron

rm mycron

No, there is no option in crontab to modify the cron files.

You have to: take the current cron file (crontab -l > newfile), change it and put the new file in place (crontab newfile).

If you are familiar with perl, you can use this module Config::Crontab.

LLP, Andrea

Say you're logged in as the user "ubuntu", but you want to add a job to a different user's crontab, like "john", for example. You can do the following:

(sudo crontab -l -u john; echo "* * * * * command") | awk '!x[$0]++' | sudo crontab -u john -

Source for most of this solution: https://www.baeldung.com/linux/create-crontab-script

I was having tons of issues trying to add a job to another user's crontab. It kept duplicating crontabs, or just flat-out deleting them. After some testing, though, I'm confident this line of code will append a new job to a specified user's crontab, non-destructively, including not creating a job that already exists.

I wanted to find an example like this, so maybe it helps:

COMMAND="/var/lib/postgresql/backup.sh"
CRON="0 0 * * *"
USER="postgres"
CRON_FILE="postgres-backup"
# At CRON times, the USER will run the COMMAND
echo "$CRON $USER $COMMAND" | sudo tee /etc/cron.d/$CRON_FILE
echo "Cron job created. Remove /etc/cron.d/$CRON_FILE to stop it."