How can I programmatically create a new cron job?

The best way if you're running as root, is to drop a file into /etc/cron.d

if you use a package manager to package your software, you can simply lay down files in that directory and they are interpreted as if they were crontabs, but with an extra field for the username, e.g.:

Filename: /etc/cron.d/per_minute

Content: * * * * * root /bin/sh /home/root/script.sh

Most of the solutions here are for adding lines to the crontab. If you need more control, you'll want to be able to control the entire contents of the crontab.

You can use piping to do this pretty elegantly.

To completely rewrite the crontab, do

echo "2 2 2 2 2 /bin/echo foobar" |crontab -

This should be easy to combine with other answers described here like

crontab -l | <something> | tee |crontab -

Or, if you have the contents in a file, it is even simpler

cat <file> |crontab -

If you're planning on doing it for a run-once scenario for just wget'ing something, take a look at 'at'

man crontab is also useful:

CRONTAB(1)

NAME

   crontab - manipulate per-user crontabs (Dillon's Cron)

SYNOPSIS

   crontab file [-u user] - replace crontab from file

   crontab - [-u user] - replace crontab from stdin

   crontab -l [user] - list crontab for user

also you can add your tasks to /etc/cron.*/

If you want the task to run as a user:

crontab -l | { cat; echo "@weekly what_you_want_to_execute"; } | crontab -

If you want the task to run with privileges:

sudo crontab -l | { cat; echo "@weekly what_you_want_to_execute"; } | sudo crontab -

and check task (with or without 'sudo'):

crontab -l | sed '/^$/d; /#/d'

Below is what I use in my script

1.

(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

To have flexibility when changing the command in the future you can do this (works best with an update script)

MARK=SOME_UNIQUE_MARK_TEXT
LINE="This is the command # $MARK"
# NOTE: I'm using -e because I might want to avoid weird bash expansions for '*' or '$' and place:
# \x2A instead of *
# \x24 instead of $
( crontab -l | grep -v $MARK ; echo -e "0 * * * *" $LINE ) | crontab -

In this way I can update an old crontab command which no longer serves current purpose.

Basically I get the current crontab, I eliminate the line containing the MARK (grep -v) and replace it with the new one by appending it at the end of the crontab file via echo -e. In this particular case I want it executed every hour at minute 0 0 * * * * as it can be seen from here.

You could also edit the cron table text file directly, but your solution seems perfectly acceptable.

It's always worked well for me.

You should consider a slightly more sophisticated script that can do three things.

1. Append a crontab line; assuring that it didn't exist. Adding when it already exists is bad.

2. Remove the crontab line. Perhaps only warning if it didn't exist.

3. A combination of the above two features to replace the crontab line.