该脚本是先删除已经存在的文件，然后后台执行SQL语句将其执行结果以一定的格式写入文件
#!/bin/bash
if [ -f "/var/lib/mysql/hell.txt" ]; then
rm -f /var/lib/mysql/hell.txt
echo "delete /var/lib/mysql/hell.txt" >> hell.txt
fi
mysql -uroot -plab <<EOF
use home;
select YongHuID,XueYuanID into outfile './hell.txt' fields terminated by '|' lines terminated by '\r\n' from sns_home_user where YongHuID < 20;
EOF