求助：条件判断中[ ] 与[[ ]] 有什么区别？

cheaper

受教了, 一直都没有留意这个问题

dida2000

学习，茅塞顿开...
“字符串比较时，最好使用[[]]  否则可能出现异常”这句话总结的挺好

MeaCulpa

bash 如果真进化到了模糊级别，有【可能】的地步，那真是惊天地，泣鬼神了...

分明就是用户偷懒，开发者不得不妥协的一个鸡肋而已. 窃以为Bash可以用，但最好不要专门学.

adam8157

“字符串比较时，最好使用[[]]  否则可能出现异常”

天啊, 明明有详细的规则的, 竟然"可能异常", nnnd, 诲人不倦的烂书!

ulovko

FROM: Wiley.Linux.Command.Line.and.Shell.Scripting.Bible.May.2008.pdf
The test command (PAGE 267)
The format of the test command is pretty simple:

1. test condition

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The condition is a series of parameters and values that the test command evaluates. When
used in an if-then statement, the test command looks like this:

1. if test condition
   
2. then
   
3. commands
   
4. fi

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The bash shell provides an alternative way of declaring the test command in an if-then
statement:

1. if [ condition ]
   
2. then
   
3. commands
   
4. fi

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The square brackets define the condition that’s used in the test command. Be careful; you must
have a space after the first bracket, and a space before the last bracket or you’ll get an error
message.

There are three classes of conditions the test command can evaluate:
■ Numeric comparisons
■ String comparisons
■ File comparisons

Using double brackets (PAGE 284)
The double bracket command provides advanced features for string comparisons. The double
bracket command format is:

1. [[ expression ]]

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The double bracketed expression uses the standard string comparison used in the test command.
However, it provides an additional feature that the test command doesn’t, pattern matching.

In pattern matching, you can define a regular expression (discussed in detail in Chapter 17) that’s
matched against the string value:

1. $ cat test24
   
2. #!/bin/bash
   
3. # using pattern matching
   
4. if [[ $USER == r* ]]
   
5. then
   
6. echo "Hello $USER"
   
7. else
   
8. echo "Sorry, I don’t know you"
   
9. fi
   
10. $ ./test24
    
11. Hello rich
    
12. $

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The double bracket command matches the $USER environment variable to see if it starts with the
letter r. If so, the comparison succeeds, and the shell executes the then section commands.

下载：http://www.itpub.net/thread-1006351-1-1.html

Hetflyter

这个不错，学习了

Third-Edition 发表于 2012-06-23 20:18
来自ABS
使用[[ ... ]]条件判断结构, 而不是[ ... ], 能够防止脚本中的许多逻辑错误. 比如, &&, ||,  操作 ...

freeterman

路过学习了