if [ $a = "123" ] 與 if [ $a == "123 ]

魔鬼的驚嘆

在有些書中說字符串判斷相等用＝,如if [ $a = "123" ]，我在linux上用==,如:if [ $a == "123 ]依然可以正確運行，請問這是bash的新特性嗎？
bash 版本4.2

jason680

回復(fù) 1# 魔鬼的驚嘆

$ man bash
NAME
       bash - GNU Bourne-Again SHell
...
CONDITIONAL EXPRESSIONS
       Conditional expressions are used by the [[  compound  command  and  the
       test  and [ builtin commands to test file attributes and perform string
       and arithmetic comparisons.  Expressions are formed from the  following
       unary  or  binary  primaries.   If any file argument to one of the pri‐
       maries is of the form /dev/fd/n, then file descriptor n is checked.  If
       the  file  argument  to  one  of  the  primaries  is one of /dev/stdin,
       /dev/stdout, or /dev/stderr, file descriptor 0, 1, or 2,  respectively,
       is checked.
...

       string1 == string2
       string1 = string2
              True  if  the strings are equal.  = should be used with the test
              command for POSIX conformance.

       string1 != string2
              True if the strings are not equal.


   

魔鬼的驚嘆

回復(fù) 2# jason680
用了linux很長時間，貌似好像還沒有好好看過man bash，而且我還發(fā)現(xiàn)了一些新的特性，看的書貌似都有些老了
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