$ LastExitCode = 0，但$？= False在PowerShell。将stderr重定向到stdout会产生NativeCommandError [英] $LastExitCode=0 but $?=False in PowerShell. Redirecting stderr to stdout gives NativeCommandError

问题描述

为什么Powershell在下面的第二个例子中显示出惊人的行为？

Why does Powershell show the surprising behaviour in the second example below?

首先，一个合理行为的例子：

First, an example of sane behaviour:

PS C:\> & cmd /c "echo Hello from standard error 1>&2"; echo "`$LastExitCode=$LastExitCode and `$?=$?"
Hello from standard error
$LastExitCode=0 and $?=True

没有惊喜。我打印一个消息到标准错误（使用 cmd 的 echo ）。我检查变量 $？和 $ LastExitCode 。它们分别等于True和0，如预期。

No surprises. I print a message to standard error (using cmd's echo). I inspect the variables $? and $LastExitCode. They equal to True and 0 respectively, as expected.

但是，如果我要求PowerShell通过第一个命令将标准错误重定向到标准输出，我会得到一个NativeCommandError：

However, if I ask PowerShell to redirect standard error to standard output over the first command, I get a NativeCommandError:

PS C:\> & cmd /c "echo Hello from standard error 1>&2" 2>&1; echo "`$LastExitCode=$LastExitCode and `$?=$?"
cmd.exe : Hello from standard error
At line:1 char:4
+ cmd <<<<  /c "echo Hello from standard error 1>&2" 2>&1; echo "`$LastExitCode=$LastExitCode and `$?=$?"
    + CategoryInfo          : NotSpecified: (Hello from standard error :String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError

$LastExitCode=0 and $?=False

我第一个问题，为什么是NativeCommandError？

My first question, why the NativeCommandError?

为什么 $？在 cmd 成功运行且 $ LastExitCode 是0？ PowerShell的文档关于自动变量没有明确定义 $？。我总是认为它是真的，如果且仅当 $ LastExitCode 是0，但我的例子矛盾。

Secondly, why is $? False when cmd ran successfully and $LastExitCode is 0? PowerShell's documentation about automatic variables doesn't explicitly define $?. I always supposed it is True if and only if $LastExitCode is 0, but my example contradicts that.

这是我在现实世界中遇到的这种行为（简化）。它真的是FUBAR。我从另一个调用一个PowerShell脚本。内部脚本：

Here's how I came across this behaviour in the real-world (simplified). It really is FUBAR. I was calling one PowerShell script from another. The inner script:

cmd /c "echo Hello from standard error 1>&2"
if (! $?)
{
    echo "Job failed. Sending email.."
    exit 1
}
# Do something else

运行这个只是作为 .\job.ps1 ，它工作正常，电子邮件。但是，我是从另一个PowerShell脚本调用它，日志记录到一个文件。\ job.ps1 2>& 1> log.txt 。在这种情况下，将发送电子邮件！在脚本外使用错误流做什么会影响脚本的内部行为。

Running this simply as .\job.ps1, it works fine, and no email is sent. However, I was calling it from another PowerShell script, logging to a file .\job.ps1 2>&1 > log.txt. In this case, an email is sent! What you do outside the script with the error stream affects the internal behaviour of the script. Observing a phenomenon changes the outcome. This feels like quantum physics rather than scripting!

[有趣的是：。\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

[Interestingly: .\job.ps1 2>&1 may or not blow up depending on where you run it]

推荐答案

PS> nslookup microsoft.com 2>&1 ; echo $?

False

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