为什么expr"i == i"因"无效的裸字"而失败？

Question

1)

% expr "1==1"
1

2)

% expr "i==i"
invalid bareword "i"
in expression "i==i";
should be "$i" or "{i}" or "i(...)" or ...

为什么在步骤2中出现此错误

1) % if {"i" == "i"} {
    puts "hai"
  }
  hai



2) % if {i == "i"} {
   puts "hai"
  }

invalid bareword "i"
in expression "i == "i"";
should be "$i" or "{i}" or "i(...)" or ...

if {"i"=="i"}这是if条件的wotking.

在这里,我发现像expr命令只评估整数,而不是比较字符串,但In"if"条件的一切(整数和字符串)正在评估.

事情在这里如何运作？

Answer 1

答案在expr手册页中.

Operands may be specified in any of the following ways:
...
[4]    As a string enclosed in double-quotes.   The  expression  parser
       will  perform  backslash, variable, and command substitutions on
       the information between the quotes, and use the resulting  value
       as the operand

[5]    As a string enclosed in braces.  The characters between the open
       brace and matching close brace will be used as the operand with?
       out any substitutions.
...

因此,expr可以比较字符串,但您必须将它们用双引号或大括号括起来,具体取决于您是否要执行替换.

因此,在您的示例2中,您必须使用

% expr {"i" == "i"}

要么

% expr {{i} == {i}}

更好地使用字符串比较操作数:

% expr {"i" eq "i"}
% expr {{i} eq {i}}

确保字符串的内容未转换为数值.