如何避免在返回值中将数据打印到 stdout?

问题描述

在执行一些 Powershell 自动化时，我遇到了自动捕获由 .cmd 文件写入标准输出的数据的方式.我有两个函数可以执行以下操作:

In doing some Powershell automation, I'm having trouble with the way that data written to stdout by a .cmd file is automatically captured. I have two functions that do something like the following:

function a {
    & external.cmd # prints "foo"
    return "bar"
}

function b {
    $val = a
    echo $val # prints "foobar", rather than just "bar"
}

基本上，external.cmd 发送到 stdout 的数据被添加到 a 的返回值中，即使我真的想从 a 是我指定的字符串.我怎样才能防止这种情况?

Basically, data that external.cmd sends to stdout is added to the return value of a, even though all I really want to return from a is the string that I specified. How can I prevent this?

推荐答案

这里有几种不同的处理方法:

Here are a few different approaches for handling this:

• 捕获 .cmd 脚本的输出:

• capture the output of the .cmd script:

$output = & external.cmd # saves foo to $output so it isn't returned from the function

将输出重定向到 null(扔掉)

redirect the output to null (throw it away)

& external.cmd | Out-Null # throws stdout away

重定向到一个文件

redirect it to a file

& external.cmd | Out-File filename.txt

通过在函数返回的对象数组中跳过它，在调用者中忽略它

ignore it in the caller by skipping it in the array of objects that's returned from the function

$val = a
echo $val[1] #prints second object returned from function a (foo is object 1... $val[0])

在 PowerShell 中，您的代码未捕获的任何输出值将返回给调用者(包括 stdout、stderr 等).因此，您必须将其捕获或通过管道传递给不返回值的对象，否则最终会以 object[] 作为函数的返回值.

In PowerShell, any output value your code does not capture is returned the caller (including stdout, stderr, etc). So you have to capture or pipe it to something that doesn't return a value, or you'll end up with an object[] as your return value from the function.

return 关键字实际上只是为了清晰和立即退出 PowerShell 中的脚本块.这样的事情甚至可以工作(不是逐字逐句，只是为了给你这个想法):

The return keyword is really just for clarity and immediate exit of a script block in PowerShell. Something like this would even work (not verbatim but just to give you the idea):

function foo()
{
    "a"
    "b"
    "c"
}

PS> $bar = foo
PS> $bar.gettype()
System.Object[]
PS> $bar
a
b
c

function foobar()
{
    "a"
    return "b"
    "c"
}

PS> $myVar = foobar
PS> $myVar
a
b

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