Question

脚本test.sh：

#!/bin/bash
if [[ $# -eq 0 ]]; then
  echo "no arg"
else
  echo "have arg"
fi

当我把它作为

运行时

./test.sh

它说“没有arg”，这是预期的，但如果我以

运行它

sh ./test.sh

它打印“有arg”，但是你打印$＃，在两种情况下都是零。

但是，下面的脚本

#!/bin/bash
if [ $# -eq 0 ]; then
  echo "no arg"
else
  echo "have arg"
fi

在两种情况下都打印“no arg”。

有人可以解释一下吗？为什么[[ ... ]]与$#不同地解释[ ... ]？

我读到的关于[[ ... ]]的解释对此并不十分清楚。

Answer 1

/ bin / sh通常是与bash不同的shell解释器。在我的ubuntu系统上，它是一个破折号的符号链接。不同的shell有不同的语法。

[ foo ]和test foo在bash和dash中都是等效的。

[[ foo ]]是一个类似于[ ]测试的bash表达式，但在man bash中有一些差异。

Dash没有[[命令，导致错误 - exitcode，以及else分支的执行。

   [[ 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.   Condi‐
          tional 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 expres‐
          sion 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
          expression's return value is 2.  If the shell option nocasematch
          is enabled, the match is performed without regard to the case of
          alphabetic characters.  Any part of the pattern may be quoted to
          force it to be matched  as  a  string.   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 expression.  The element of BASH_REMATCH with
          index  n is the portion of the string matching the nth parenthe‐
          sized subexpression.

Answer 2

在我的情况下（ubuntu 9.04）我还看到以下错误行超过“have arg”行：

./ test.sh:6: [[：not found

确实应该如此; / bin / sh是'dash'的符号链接，而不是'bash'。

脚本调用sh时为什么不[[...]]工作，而[...]始终工作？

2 个答案: