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PO$LastExitCode=0 but $?=False in PowerShell. Redirecting stderr to stdout gives NativeCommandError
 

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  1. PO$LastExitCode=0 but $?=False in PowerShell. Redirecting stderr to stdout gives NativeCommandError
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    <p>Why does Powershell show the surprising behaviour in the second example below?</p> <p>First, an example of sane behaviour:</p> <pre><code>PS C:\&gt; &amp; cmd /c "echo Hello from standard error 1&gt;&amp;2"; echo "`$LastExitCode=$LastExitCode and `$?=$?" Hello from standard error $LastExitCode=0 and $?=True </code></pre> <p>No surprises. I print a message to standard error (using <code>cmd</code>'s <code>echo</code>). I inspect the variables <code>$?</code> and <code>$LastExitCode</code>. They equal to True and 0 respectively, as expected. </p> <p>However, if I ask PowerShell to redirect standard error to standard output over the first command, I get a NativeCommandError:</p> <pre><code>PS C:\&gt; &amp; cmd /c "echo Hello from standard error 1&gt;&amp;2" 2&gt;&amp;1; echo "`$LastExitCode=$LastExitCode and `$?=$?" cmd.exe : Hello from standard error At line:1 char:4 + cmd &lt;&lt;&lt;&lt; /c "echo Hello from standard error 1&gt;&amp;2" 2&gt;&amp;1; echo "`$LastExitCode=$LastExitCode and `$?=$?" + CategoryInfo : NotSpecified: (Hello from standard error :String) [], RemoteException + FullyQualifiedErrorId : NativeCommandError $LastExitCode=0 and $?=False </code></pre> <p>My first question, why the NativeCommandError?</p> <p>Secondly, why is <code>$?</code> False when <code>cmd</code> ran successfully and <code>$LastExitCode</code> is 0? PowerShell's documentation <a href="http://technet.microsoft.com/en-us/library/dd347675.aspx">about automatic variables</a> doesn't explicitly define <code>$?</code>. I always supposed it is True if and only if <code>$LastExitCode</code> is 0, but my example contradicts that. </p> <hr> <p>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:</p> <pre><code>cmd /c "echo Hello from standard error 1&gt;&amp;2" if (! $?) { echo "Job failed. Sending email.." exit 1 } # Do something else </code></pre> <p>Running this simply as <code>.\job.ps1</code>, it works fine, and no email is sent. However, I was calling it from another PowerShell script, logging to a file <code>.\job.ps1 2&gt;&amp;1 &gt; log.txt</code>. In this case, an email is sent! What you do outside the script with the error stream affects the internal behaviour of the script. <em>Observing a phenomenon changes the outcome.</em> This feels like quantum physics rather than scripting!</p> <p>[Interestingly: <code>.\job.ps1 2&gt;&amp;1</code> may or not blow up depending on where you run it]</p>
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    1. USColonel Panic
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      1. USColonel Panic
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    1. COpossible duplicate of [Powershell difference between $? and $LastExitCode](http://stackoverflow.com/questions/10666035/powershell-difference-between-and-lastexitcode)
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      1. USRaymond Chen
    2. CORaymond, related maybe, but not really a duplicate. It'd be nice if Jeffrey Snover chimed in here with [Word of God](http://tvtropes.org/pmwiki/pmwiki.php/Main/WordOfGod), though :-)
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      1. USJoey
    3. COI asked the other question to make sure I understood the expected behaviour before exhibiting the unexpected.
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