问题
Anyone know why this happens? Is this a bug of bash?
x='mnt:[4026532411]'
[[ $x == $x ]] && echo OK
I am expecting result OK, but it did not.
Of course, this works
[[ "$x" == "$x" ]] && echo OK
But as I know, bash [[ ]] have a merit that no need to quote var when compare.
x='a b'
[[ $x == $x ]] && echo OK
works.
Ironical things is
x='mnt:[4026532411]'
[[ $x != $x ]] && echo Oh my god
result is Oh my god
回答1:
The unquoted right-hand side of == and != is treated as a pattern, not a literal string. mnt:[4026532411] will match mnt: followed by exactly one of 0, 1, 2, 3, 4, 5, or 6, since the patterns mnt:[4026532411] and mnt:[0123456] are equivalent. To match the lieral string, you need to quote the expansion.
x='mnt:[4026532411]'
[[ $x == "$x" ]] && echo OK
回答2:
What you are seeing is do do this sentence from the bash man page:
When the == and != operators are used, the string to the right of the operator is considered a pattern and matched according to the rules described below under Pattern Matching, as if the extglob shell option were enabled.
As you may already know, [...] in the shell allows matching from a
group of characters. That is, given the files:
$ ls
fileA fileB fileC fileD
Running ls file[AB] will yield:
fileA fileB
So in your expression, mnt:[1234] is interpreted in a similar
fashion.
来源：https://stackoverflow.com/questions/51002801/bash-a-a-not-true-square-bracket-affect-compare-result