PowerShell一个比较好的特性

PowerShell的cmdlet都带有一个`-WhatIf'，顾名思义，cmdlet后加上这个转换后，并不会实际起作用，而会告诉你实施这个命令后会发生什么，这样当你进行某些不确定，可能带来危害的命令操作前，可以加上`-WhatIf'转换先评估，确定后再进行。

比如，我想重命名当前目录下所有文件，把其中的空格变为下划线'_'

PS D:test
$ ls -name
1 space space.txt
10 space space.txt
2 space space.txt
3 space space.txt
4 space space.txt
5 space space.txt
6 space space.txt
7 space space.txt
8 space space.txt
9 space space.txt

我想的Shell步骤如下，但不能确定是否正确，如果
ls | % { Rename-Item $_ ($_.name -replace " ","_")

这时，-WhatIf就可以派上用场了，让我们来看看，这个Shell步骤到底会做什么么？
PS D:test
$ ls | % { Rename-Item $_ ($_.name -replace " ","_") -WhatIf }
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space sp
ace.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".
What if: Performing operation "Rename File" on Target "Item: D:test space spa
ce.txt Destination: D:test_space_space.txt".

不错，看上去这个重命名步骤没问题，符合预期。接下来，就可以去掉`-WhatIf'，进行实际操作了。
PS D:test
$ ls | % { Rename-Item $_ ($_.name -replace " ","_") }
PS D:test
$ ls -name
10_space_space.txt
1_space_space.txt
2_space_space.txt
3_space_space.txt
4_space_space.txt
5_space_space.txt
6_space_space.txt
7_space_space.txt
8_space_space.txt
9_space_space.txt


而在Linux shell中，并没有这样的测试机制，所以在进行命令操作前，一定要谨慎行事。相同的重命名例子，在bash中，如果要进行测试，就比较困难了，我能做到的仅仅是验证对文件名进行的文本操作是否正确：

for f in *.txt; do new=`echo "$f" | sed 's/ /_/g'`; echo "$f will be renamed to $new";'done

但这个并不是最终结果，因为实际重命名的mv过程并没有测试，比如mv过程中带有空格的$f没有被双引号引用，就会出错，但这一步并没法测试。

上面所说的只是一个很简单的例子，如果碰到更复杂，出错后更具破坏性的情况，-WhatIf的优点就更明显了。