if 逻辑表达式

lucash

[[ expression ]]
              Return a status of 0 or 1 depending on the evaluation of the conditional expression expression.  Expressions are composed of  the
              primaries  described  below  under CONDITIONAL EXPRESSIONS.  Word splitting and pathname expansion are not performed on the words
              between the [[ and ]]; tilde expansion, parameter and variable expansion, arithmetic  expansion,  command  substitution,  process
              substitution,  and quote removal are performed.  Conditional operators such as -f must be unquoted to be recognized as primaries.

              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.  If the shell option nocasematch is enabled, the match is performed without
              regard to the case of alphabetic characters.  The return value is 0 if the string matches (==) or does not match  (!=)  the  pat-
              tern, and 1 otherwise.  Any part of the pattern may be quoted to force it to be matched as a string.

              An additional binary operator, =~, is available, with the same precedence as == and !=.  When it is used, the string to the right
              of the operator is considered an extended regular expression and matched accordingly (as in regex(3)).  The return value is 0  if
              the  string  matches the pattern, and 1 otherwise.  If the regular expression is syntactically incorrect, the conditional expres-
              sion’s return value is 2.  If the shell option nocasematch is enabled, the match is performed  without  regard  to  the  case  of
              alphabetic  characters.   Substrings matched by parenthesized subexpressions within the regular expression are saved in the array
              variable BASH_REMATCH.  The element of BASH_REMATCH with index 0 is the portion of the string matching the entire regular expres-
              sion.  The element of BASH_REMATCH with index n is the portion of the string matching the nth parenthesized subexpression.

              Expressions may be combined using the following operators, listed in decreasing order of precedence:

              ( expression )
                     Returns the value of expression.  This may be used to override the normal precedence of operators.
              ! expression
                     True if expression is false.
              expression1 && expression2
                     True if both expression1 and expression2 are true.
              expression1 || expression2
                     True if either expression1 or expression2 is true.

              The  &&  and  || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of
              the entire conditional expression.

xiaobaibbb

[]是shell内建的test,不是/usr/bin/test