求教： linux 中 [] 、[[]]和(())区别。循环结构中的返回值

yonka

之前提了几个问题。几位前辈也给了一些指教。
其实论坛里的基础教程什么的我都看过，shell编程的书也在看，不过可能是因为理解不够深刻的原因吧，还是有些混乱...

我看到的表示判断的方式是： test expression或者 [ expression ]。
刚才又在baidu里又看到了 [[ expression ]] 的用法。

之前blackold兄又提到判断不只有[ ]的方式，(())也可以。

我在while循环等处也经常看到 (())、[]、[[]]的交杂使用。但是却没有看到对这些格式的解释。

我以前以为 for (( expr1; expr2; expr3 ))是for循环本身独有的语法结构，这么说来...莫非不是？莫非(( ; ; ))本身即带有判断功能？

我的理解是while、if、for等获取的都是return value而不是value，这点与C不同。不知道各位前辈认为这么理解是否正确。
因为我尝试用while 1结果出错了。


谢谢

reyleon

http://www.tsnc.edu.cn/default/t ... ts.html#DBLBRACKETS

http://www.tsnc.edu.cn/default/t ... html/dblparens.html

看看着两个连接的内容，或许有帮助

waker

楼主,既然精通C可以用cshell

yonka

回复 3# waker


    帽子兄。能否帮忙解答一下呢？

“我在while循环等处也经常看到 (())、[]、[[]]的交杂使用。但是却没有看到对这些格式的解释。

我以前以为 for (( expr1; expr2; expr3 ))是for循环本身独有的语法结构，这么说来...莫非不是？莫非(( ; ; ))本身即带有判断功能？

我的理解是while、if、for等获取的都是return value而不是value，这点与C不同。不知道各位前辈认为这么理解是否正确。
因为我尝试用while 1结果出错了。”


[]  [[]]  (()) 分别是怎么作为判断功能的，Shell中还有其他的判断功能吗？ （除了test）
另外我的这个理解是否正确呢？

谢谢

Shell_HAT

打开ABS，看看第 81 页。

Advanced Bash-Scripting Guide 3.9.1 中文版.pdf
http://bbs.chinaunix.net/thread-1610033-1-1.html

waker

回复 4# yonka

1. 
   
2. 
   
3. Compound Commands
   
4. 
   
5. A compound command is one of the following:
   
6. (list)
   
7. list is executed in a subshell environment (see COMMAND EXECUTION ENVIRONMENT below). Variable assignments and builtin commands that affect the shell's environment do not remain in effect after the command completes. The return status is the exit status of list.
   
8. 
   
9. { list; }
   
10. list is simply executed in the current shell environment. list must be terminated with a newline or semicolon. This is known as a group command. The return status is the exit status of list. Note that unlike the metacharacters ( and ), { and } are reserved words and must occur where a reserved word is permitted to be recognized. Since they do not cause a word break, they must be separated from list by whitespace or another shell metacharacter.
    
11. ((expression))
    
12. The expression is evaluated according to the rules described below under ARITHMETIC EVALUATION. If the value of the expression is non-zero, the return status is 0; otherwise the return status is 1. This is exactly equivalent to let "expression".
    
13. [[ expression ]]
    
14. 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.
    
15. When used with [[, The < and > operators sort lexicographically using the current locale.
    
16. 
    
17. 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 pattern, and 1 otherwise. Any part of the pattern may be quoted to force it to be matched as a string.
    
18. 
    
19. 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 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 parenthesized subexpression.
    
20. 
    
21. Expressions may be combined using the following operators, listed in decreasing order of precedence:
    
22. 
    
23. ( expression )
    
24. Returns the value of expression. This may be used to override the normal precedence of operators.
    
25. 
    
26. ! expression
    
27. True if expression is false.
    
28. 
    
29. expression1 && expression2
    
30. True if both expression1 and expression2 are true.
    
31. 
    
32. expression1 || expression2
    
33. True if either expression1 or expression2 is true.
    
34. 
    
35. The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.
    
36. for name [ [ in [ word ... ] ] ; ] do list ; done
    
37. The list of words following in is expanded, generating a list of items. The variable name is set to each element of this list in turn, and list is executed each time. If the in word is omitted, the for command executes list once for each positional parameter that is set (see PARAMETERS below). The return status is the exit status of the last command that executes. If the expansion of the items following in results in an empty list, no commands are executed, and the return status is 0.
    
38. for (( expr1 ; expr2 ; expr3 )) ; do list ; done
    
39. First, the arithmetic expression expr1 is evaluated according to the rules described below under ARITHMETIC EVALUATION. The arithmetic expression expr2 is then evaluated repeatedly until it evaluates to zero. Each time expr2 evaluates to a non-zero value, list is executed and the arithmetic expression expr3 is evaluated. If any expression is omitted, it behaves as if it evaluates to 1. The return value is the exit status of the last command in list that is executed, or false if any of the expressions is invalid.
    
40. select name [ in word ] ; do list ; done
    
41. The list of words following in is expanded, generating a list of items. The set of expanded words is printed on the standard error, each preceded by a number. If the in word is omitted, the positional parameters are printed (see PARAMETERS below). The PS3 prompt is then displayed and a line read from the standard input. If the line consists of a number corresponding to one of the displayed words, then the value of name is set to that word. If the line is empty, the words and prompt are displayed again. If EOF is read, the command completes. Any other value read causes name to be set to null. The line read is saved in the variable REPLY. The list is executed after each selection until a break command is executed. The exit status of select is the exit status of the last command executed in list, or zero if no commands were executed.
    
42. case word in [ [(] pattern [ | pattern ] ... ) list ;; ] ... esac
    
43. A case command first expands word, and tries to match it against each pattern in turn, using the same matching rules as for pathname expansion (see Pathname Expansion below). The word is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, command substitution, process substitution and quote removal. Each pattern examined is expanded using tilde expansion, parameter and variable expansion, arithmetic substitution, command substitution, and process substitution. If the shell option nocasematch is enabled, the match is performed without regard to the case of alphabetic characters. When a match is found, the corresponding list is executed. If the ;; operator is used, no subsequent matches are attempted after the first pattern match. Using ;& in place of ;; causes execution to continue with the list associated with the next set of patterns. Using ;;& in place of ;; causes the shell to test the next pattern list in the statement, if any, and execute any associated list on a successful match. The exit status is zero if no pattern matches. Otherwise, it is the exit status of the last command executed in list.
    
44. if list; then list; [ elif list; then list; ] ... [ else list; ] fi
    
45. The if list is executed. If its exit status is zero, the then list is executed. Otherwise, each elif list is executed in turn, and if its exit status is zero, the corresponding then list is executed and the command completes. Otherwise, the else list is executed, if present. The exit status is the exit status of the last command executed, or zero if no condition tested true.
    
46. while list; do list; done
    
47. until list; do list; done
    
48. The while command continuously executes the do list as long as the last command in list returns an exit status of zero. The until command is identical to the while command, except that the test is negated; the do list is executed as long as the last command in list returns a non-zero exit status. The exit status of the while and until commands is the exit status of the last do list command executed, or zero if none was executed.
    
49. 
    

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都是大家约定的东西,至于为什么,俺没能力关心