Shell脚本未通过crontab运行，但手动运行正常

我有一个脚本来检查pptp vpn是否正在运行，如果没有运行，它会重新连接pptp vpn，当我手动运行脚本时，它会执行得很好，当我设置了cron作业时，它不会运行

I have a script that checks if pptp vpn is running , and if not it reconnect the pptp vpn , when i run the script manually it execute fine , when i do set a cron job , is not running

* * * * * /bin/bash /var/scripts/vpn-check.sh

脚本来了:

#!/bin/sh
/bin/ping -c3 192.168.17.27 > /tmp/pingreport
result=`grep "0 received" /tmp/pingreport`
truncresult="`echo "$result" | sed 's/^\(.................................\).*$$'`"
if [[ $truncresult == "3 packets transmitted, 0 received" ]]; then
/usr/sbin/pppd call home
fi