如何在 Bash 中测试一个字符串是否比另一个字符串“更大”？

Question

在 Bash 中我可以编写以下测试

[[ "f" > "a" ]]

这将导致返回 0，即 true。 bash 实际上是如何执行这个字符串比较的？据我了解> 进行整数比较。它会尝试比较操作数的 ASCII 值吗？

Answer 1

来自help test：

  STRING1 > STRING2
                 True if STRING1 sorts after STRING2 lexicographically.

在内部，bash 使用 strcoll() 或 strcmp()：

else if ((op[0] == '>' || op[0] == '<') && op[1] == '\0')
  {
    if (shell_compatibility_level > 40 && flags & TEST_LOCALE)
      return ((op[0] == '>') ? (strcoll (arg1, arg2) > 0) : (strcoll (arg1, arg2) < 0));
    else
      return ((op[0] == '>') ? (strcmp (arg1, arg2) > 0) : (strcmp (arg1, arg2) < 0));
  }

后者实际比较 ASCII 码，前者（在启用区域设置时使用）执行更具体的比较，适合在给定区域设置中排序。

Answer 2

这是一个字母比较（AI​​UI 的排序顺序可能会受到当前语言环境的影响）。它比较每个字符串的第一个字符，如果左边的值较高，则为真，如果较低，则为假；如果它们相同，则比较第二个字符等。

这不与整数比较相同，因为您使用[[ 2 -gt 1 ]] 或(( 2 > 1 ))。为了说明字符串和整数比较之间的区别，请考虑以下所有内容都是“正确的”：

[[ 2 > 10 ]]     # because "2" comes after "1" in ASCII sort order
[[ 10 -gt 2 ]]   # because 10 is a larger number than 2
(( 10 > 2 ))     # ditto

这里还有一些测试，字符串比较是正确的，但整数比较是错误的：

[[ 05 < 5 ]]    # Because "0" comes before "5"
[[ +5 < 0 ]]    # Because "+" comes before the digits
[[ -0 < 0 ]]    # Because "-" comes before the digits
[[ -1 < -2 ]]   # Because "-" doesn't change how the second character is compared

Answer 3

是的，它比较 ascii 值，如果相等，则在下一个字符中重复比较。

/* Copyright (C) 1991, 1996, 1997, 2003 Free Software Foundation, Inc. 
   This file is part of the GNU C Library. 

   The GNU C Library is free software; you can redistribute it and/or 
   modify it under the terms of the GNU Lesser General Public 
   License as published by the Free Software Foundation; either 
   version 2.1 of the License, or (at your option) any later version. 

   The GNU C Library is distributed in the hope that it will be useful, 
   but WITHOUT ANY WARRANTY; without even the implied warranty of 
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU 
   Lesser General Public License for more details. 

   You should have received a copy of the GNU Lesser General Public 
   License along with the GNU C Library; if not, write to the Free 
   Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 
   02111-1307 USA.  */ 

#include <string.h> 
#include <memcopy.h> 

#undef strcmp 

/* Compare S1 and S2, returning less than, equal to or 
   greater than zero if S1 is lexicographically less than, 
   equal to or greater than S2.  */ 
int 
strcmp (p1, p2) 
     const char *p1; 
     const char *p2; 
{ 
  register const unsigned char *s1 = (const unsigned char *) p1; 
  register const unsigned char *s2 = (const unsigned char *) p2; 
  unsigned reg_char c1, c2; 

  do 
    { 
      c1 = (unsigned char) *s1++; 
      c2 = (unsigned char) *s2++; 
      if (c1 == '\0') 
        return c1 - c2; 
    } 
  while (c1 == c2); 

  return c1 - c2; 
}