Edit crontab from shell script

Editing crontabs from shell script is not so easy as it sounds first.
You can't simply edit /var/spool/cron/crontabs/user file because you have to install new crontab by running crontab command.

So the right method is:
1. list the current crontabs to a temporary file:
$ crontab -l > crontab.lst


2. Edit/append the created file
$ echo "*/10 * * * * /path/to/new/job" >>crontab.lst

3. Install edited crontab file:
$ crontab crontab.lst