= 单等号 和 == 双等号都是用来比较字符串的，它们放在[ ... ]和[[ ... ]]中表现是不一样的，主要体现在有没有对操作数加双引号括起来会不一样的处理。具体看这个post:
[ $a == $b ], specifically used unquoted variable substitution (as of the October 2017 edit). For [...] that is safe for string equality. But if you are going to enumerate alternatives like [[...]], you must inform also that the right-hand-side must be quoted. If not quoted, it is a pattern match! (From the Bash man page: "Any part of the pattern may be quoted to force it to be matched as a string."). Here in Bash, the two statements yielding "yes" are pattern matching, other three are string equality: > rht="A*" > lft="AB" > [ $lft = $rht ] && echo yes #不会执行echo yes > [ $lft == $rht ] && echo yes #不会执行echo yes > [[ $lft = $rht ]] && echo yes #输出 yes > [[ $lft == $rht ]] && echo yes #输出 yes > [[ $lft == "$rht" ]] && echo yes #不会执行echo yes