區別使用[和[

Question

什麼之間的精確區別：

if [ $? -ne 0 ]; 

和

if [[ $? -ne 0 ]]; 

Answer 1

如前所述here：

相反[，[[防止變量值的分詞。因此，如果 VAR="var with spaces"，您不需要在測試 中雙引號$VAR - 儘管使用引號仍然是一個好習慣。另外，[[可防止路徑名擴展，因此帶通配符的文字字符串不會嘗試將 擴展爲文件名。使用[[，==和!=將字符串解釋爲 右側，作爲要與左側的值匹配的值，例如：[[ "value" == val* ]]。

Answer 2

有沒有。儘管如此，[[ ... ]]語法還介紹了可以對條件表達式執行的其他操作。從help [[：

Returns a status of 0 or 1 depending on the evaluation of the conditional 
expression EXPRESSION. Expressions are composed of the same primaries used 
by the `test' builtin, and may be combined using the following operators: 

    (EXPRESSION) Returns the value of EXPRESSION 
    ! EXPRESSION    True if EXPRESSION is false; else false 
    EXPR1 && EXPR2 True if both EXPR1 and EXPR2 are true; else false 
    EXPR1 || EXPR2 True if either EXPR1 or EXPR2 is true; else false 

When the `==' and `!=' operators are used, the string to the right of 
the operator is used as a pattern and pattern matching is performed. 
When the `=~' operator is used, the string to the right of the operator 
is matched as a regular expression.