这句是什么意思呢:[ "$?" = "0" ] || exit 3

这句是什么意思呢:[ "$?" = "0" ] || exit 3

"$?"是指的是什么,
“||”是或的意思吧,在这里到底是表达一个什么意思呢      
the variable of $? represents the exit value of the last command.

|| means or, just like C language.

"[ $? = 0 ] || exit 3" means that if the last command does not exit with 0 then i'll exit with 3      
[ "$?" = "0" ] 判断上一个命令的返回值是否为"0"