How can I programmatically create a new cron job?

OP's solution has a bug, it might allow entries to be added twice, use below to fix.

(crontab -l ; echo "0 * * * * your_command") | sort - | uniq - | crontab -

To Add something to cron

(crontab -l ; echo "0 * * * * hupChannel.sh") 2>&1 | grep -v "no crontab" | sort | uniq | crontab -

To remove this from cron

(crontab -l ; echo "0 * * * * hupChannel.sh") 2>&1 | grep -v "no crontab" | grep -v hupChannel.sh |  sort | uniq | crontab -

hope would help someone

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'

Assuming that there is already an entry in your crontab, the following command should work relatively well. Note that the $CMD variable is only there for readability. Sorting before filtering duplicates is important, because uniq only works on adjacent lines.

CMD='wget -O - -q http://www.example.com/cron.php"'
(crontab -l ; echo "0 * * * * $CMD") | sort | uniq | crontab -

If you currently have an empty crontab, you will receive the following error to stderr:

no crontab for user

If you want to avoid this, you can add a little bit of complexity add do something like this:

(crontab -l ; echo "0 * * * * $CMD") 2>&1 | sed "s/no crontab for $(whoami)//"  | sort | uniq | crontab -

Simply change the editor to tee command:

export EDITOR="tee"
echo "0 * * * * /bin/echo 'Hello World'" | crontab -e

Here's another one-liner way, that avoids duplicates

(crontab -l 2>/dev/null | fgrep -v "*/1 *  *  *  * your_command"; echo "*/1 *  *  *  * your_command") | crontab -

And here's a way to do JohnZ's answer and avoid no crontab for user message, or if you need to operate in a set -eu type environment and can't have anything return a failure (in which case the 2>/dev/null part is optional):

( (crontab -l 2>/dev/null || echo "")  ; echo "0 * * * * your_command") | sort -u - | crontab -

Or if you want to split things up so that they're more readable:

new_job="0 * * * * your_command"
preceding_cron_jobs=$(crontab -l || echo "")
(echo "$preceding_cron_jobs" ; echo "$new_job") | sort - | uniq - | crontab -

Or optionally remove any references to your_command (ex: if the schedule has changed, you only want it ever cron'ed once). In this case we no longer need uniq (added bonus, insertion order is also preserved):

new_job="0 * * * * your_command"
preceding_cron_jobs=$(crontab -l || echo "")
preceding_cron_jobs=$(echo "$preceding_cron_jobs" | grep -v your_command )
(echo "$preceding_cron_jobs" ; echo "$new_job") | crontab -

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

function cronjob_exists($command){

    $cronjob_exists=false;

    exec('crontab -l', $crontab);


    if(isset($crontab)&&is_array($crontab)){

        $crontab = array_flip($crontab);

        if(isset($crontab[$command])){

            $cronjob_exists=true;

        }

    }
    return $cronjob_exists;
}

function append_cronjob($command){

    if(is_string($command)&&!empty($command)&&cronjob_exists($command)===FALSE){

        //add job to crontab
        exec('echo -e "`crontab -l`\n'.$command.'" | crontab -', $output);


    }

    return $output;
}

    append_cronjob('* * * * * curl -s http://localhost/cron/test.php');

Adding to JohnZ's answer, here's the syntax to schedule as root if you are a sudoer:

(sudo crontab -l ; echo "0 * * * * your_command") | sort - | uniq - | sudo crontab -

This would check to ensure that your command doesn't already exist before adding it.

crontab -l 2>/dev/null | grep -q '/path/to/script' || echo "5 * * * * /path/to/script" | crontab -

Cheers.

Better way is to use Ansible playbook with the following task:

---
- name: Deploy cron
  hosts: all
  gather_facts: false
  tasks:
    - name: Update Cron job
      ansible.builtin.cron:
        name: Run Drupal Cron Job
        user: www-data
        minute: "0"
        job: "/var/www/html/drupal/vendor/bin/drush core:cron"

The above solution uses drush core:cron, since recent Drupal's cron calls require keys (which had to be extracted then hardcoded).

See: Configuring cron jobs using the cron command.

Then use ansible-playbook -i hosts deploy.yml to run it, where hosts file is:

echo "[all]\n\192.168.0.123   ansible_connection=ssh    ansible_user=root" > hosts

where 192.168.0.123 is your host to deploy.

You can use it as part of your deploy scripts or put it in your Jenkinsfile as part of your deploy pipeline.

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

Piping stdout into crontab didn't install the new crontab for me on macOS, so I found this solution instead, using the tee editor in a sub shell:

(EDITOR=tee && (crontab -l ; echo "@daily ~/my-script.sh" ) | uniq - | crontab -e)

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.

I'm adding a solutions similar to @JohnZ's that does not add duplicates while still preserving the existing order of crontab. We have quite a few jobs that are heavily commented and we need to preserve that structure.

$ { \
    crontab -l; \
    echo "*/1  * * * *   echo job one"  &>/dev/null; \
    echo "*/30 * * * *   echo job two"  &>/dev/null; \
  } | awk '(/^#/ || !a[$0]++)' | crontab -

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