关于$() 和 ` `为什么结果不一样

农民伯伯丶

INPUT_FILE=INPUT_FILE_${BillIndex}_$i                      #BillIndex 和i都是变量

eval INPUT_FILE=`echo \$$INPUT_FILE`

结果 >>>>

改成 eval INPUT_FILE=$(echo \$$INPUT_FILE)
结果 >>>>


$() 和 ` `有什么区别呢？之前一直以为是一样的

lgfang

` ` 更多shell支持，但不如$()好用。后者可以嵌套，不需要额外的转义；而且也不容易写错、看错。好多字体里前者和单引号很难区分。

另外，你的需求不需要用eval + $()，直接用所谓的indirect variable reference就行了

1. 
   
2. # for bash
   
3. name_ref=INPUT_FILE_${BillIndex}_$i
   
4. INPUT_FILE=${!name_ref}
   
5. # for ksh, man nameref
   

复制代码

关于indirect variable reference的更多信息： http://lgfang.github.io/computer ... indirect-reference/

农民伯伯丶

回复 2# lgfang

感谢你的方法，不过从我遇到的问题来看，$() 和``除了使用范围之外，应该还有其他差别的

   

tc1989tc

1. ...skipping...
   
2.               $(command)
   
3.        or
   
4.               `command`
   
5. 
   
6.        Bash performs the expansion by executing command and replacing the command substitu-
   
7.        tion with the standard output of the command, with any  trailing  newlines  deleted.
   
8.        Embedded  newlines  are  not deleted, but they may be removed during word splitting.
   
9.        The command substitution $(cat file) can be replaced by the  equivalent  but  faster
   
10.        $(< file).
    
11. 
    
12.        [color=Red]When  the  old-style  backquote  form of substitution is used, backslash retains its
    
13.        literal meaning except when followed by $, `, or \.  The first  backquote  not  pre-
    
14.        ceded by a backslash terminates the command substitution.[/color] (请问怎么理解反斜杠保留了他的字面意思啊？？) When using the $(command)
    
15.        form, all characters between the parentheses make up the command; none  are  treated
    
16.        specially.
    
17. 
    
18.        Command substitutions may be nested.  To nest when using the backquoted form, escape
    
19.        the inner backquotes with backslashes.
    
20. 
    
21.        If the substitution appears within double quotes, word splitting and pathname expan-
    
22.        sion are not performed on the results.

复制代码

tc1989tc

...skipping...
              $(command)
       or
              `command`

       Bash performs the expansion by executing command and replacing the command substitu-
       tion with the standard output of the command, with any  trailing  newlines  deleted.
       Embedded  newlines  are  not deleted, but they may be removed during word splitting.
       The command substitution $(cat file) can be replaced by the  equivalent  but  faster
       $(< file).

       When  the  old-style  backquote  form of substitution is used, backslash retains its
       literal meaning except when followed by $, `, or \.  The first  backquote  not  pre-
       ceded by a backslash terminates the command substitution. (请问怎么理解反斜杠保留了他的字面意思啊？？) When using the $(command)
       form, all characters between the parentheses make up the command; none  are  treated
       specially.

       Command substitutions may be nested.  To nest when using the backquoted form, escape
       the inner backquotes with backslashes.

       If the substitution appears within double quotes, word splitting and pathname expan-
       sion are not performed on the results.

农民伯伯丶

回复 5# tc1989tc


    谢谢了先，这个是man 什么可以看到的，英语太水了，能帮忙解释下不，拿去谷歌翻译也没看明白

yjh777

应该是 $() 里面的 \ 在第一遍解析展开时不做处理

    $() 里面的所有字符串应该原样传递给sub shell进行解析处理

农民伯伯丶

回复 7# yjh777

感谢分析