write your command best in a shellscript and add runnable to it
chmod +x /home/scripts/yourscriptfile.sh
add it to /etc/crontab
# /etc/crontab: system-wide crontab # Unlike any other crontab you don't have to run the `crontab' # command to install the new version when you edit this file # and files in /etc/cron.d. These files also have username fields, # that none of the other crontabs do. SHELL=/bin/sh PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin # m h dom mon dow user command 17 * * * * root cd / && run-parts --report /etc/cron.hourly 25 6 * * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.daily ) 47 6 * * 7 root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.weekly ) 52 6 1 * * root test -x /usr/sbin/anacron || ( cd / && run-parts --report /etc/cron.monthly ) # */1 * * * * root /home/scripts/yourscriptfile.sh >> /home/cron_scripts/global_cron_script.log
only use “tab” between fields
*/x means reapeat every x
further details @ http://www.adminschoice.com/crontab-quick-reference/
then restart/reload cronjobs
/etc/init.d/crontab restart
using different editor
env EDITOR=nano crontab -e