crontab
http://h50221.www5.hp.com/cgi/service/knavi/production/doc_disp.cgi?category=888&doc=jnav007844
http://park15.wakwak.com/~unixlife/linux/sys-cron.html
http://homepage2.nifty.com/cs/linux_command/re-command/job_12.html
http://www.linux.or.jp/JM/html/netkit/man8/ping.8.html#index
http://x68000.q-e-d.net/~68user/unix/pickup?keyword=%B9%D4+%C9%BD%BC%A8&target=command&partial=on®exp=on&case=off#crontab