Linux: How to Add a Line into Crontab from Command Line

The easy method:

newline=@reboot /usr/bin/numlockx
(crontab -l && echo "$newLine") | crontab -

Alternative (longer version) of adding a line to crontab:

crontab -l > tempCron # copy crontab to a new file
echo "$newLine" >> tempCron #echo new cron into cron file
crontab tempCron # Replace crontab with tempCron
rm tempCron

Original (manual) version of installing numlock:

sudo crontab -e
### add this line ###
echo "@reboot /usr/bin/numlockx" >> /usr/bin/crontab