重定向输出
/usr/bin/perl /home/joe/netcheck.pl 1>/home/joe/netcheck-stdout.log 2>/home/joe/netcheck-stderr.log &
http://superuser.com/questions/45874/perlscript-runs-fine-when-executed-manually-but-not-under-cron
crontab 相对路径
切换到相应目录
cd $(dirname $0)
http://blog.enkitec.com/2011/05/make-bash-scripts-crontab-compatible/
http://stackoverflow.com/questions/8899737/crontab-run-in-directory
https://kb.mediatemple.net/questions/1588/Troubleshooting+common+issues+with+cron+jobs#gs
使用crontab 运行sh脚本，然后sh运行程序出现：root: command not found
解决方法：
this begins happening from running sudo crontab /etc/crontab, it then makes a cron file for root in /var/spool/cron/crontabs.
run rm /var/spool/cron/crontabs/root and it is fixed.
http://ubuntuforums.org/archive/index.php/t-1035016.html
参考：
https://kb.mediatemple.net/questions/1588/Troubleshooting+common+issues+with+cron+jobs#gs