实例:-a,-o,!等同于&&,||,! ======>与，或，非
&&:如果&&左侧为假，则&&右侧表达式不执行.
||:如果&&左侧为假，则继续执行右侧表达式。
str1="down"
str2="down"
str3="down"
while [ 1 ]
sleep 1
do
#if [ "up" = "$(str1)" -a "up" = "$(st2)" -o "up" = "$(st3)" ];then
#if [ "up" = "$(str1)" && [["up" = "$(str2)" || "up" = "$(str3)" ]]];then
if [ "up" = "$(str1)" ];then
echo 123456 > ./test.txt
break;
fi
done