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

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(I am using PowerShell v2.)

The ‘$?‘ variable is documented in about_Automatic_Variables:

$?
  Contains the execution status of the last operation

This is referring to the most recent PowerShell operation, as opposed to the last external command, which is what you get in $LastExitCode.

In your example, $LastExitCode is 0, because the last external command was cmd, which was successful in echoing some text. But the 2>&1 causes messages to stderr to be converted to error records in the output stream, which tells PowerShell that there was an error during the last operation, causing $? to be False.

To illustrate this a bit more, consider this:

> java -jar foo; $?; $LastExitCode
Unable to access jarfile foo
False
1

$LastExitCode is 1, because that was the exit code of java.exe. $? is False, because the very last thing the shell did failed.

But if all I do is switch them around:

> java -jar foo; $LastExitCode; $?
Unable to access jarfile foo
1
True

… then $? is True, because the last thing the shell did was print $LastExitCode to the host, which was successful.

Finally:

> &{ java -jar foo }; $?; $LastExitCode
Unable to access jarfile foo
True
1

…which seems a bit counter-intuitive, but $? is True now, because the execution of the script block was successful, even if the command run inside of it was not.

Returning to the 2>&1 redirect…. that causes an error record to go in the output stream, which is what gives that long-winded blob about the NativeCommandError. The shell is dumping the whole error record.

This can be especially annoying when all you want to do is pipe stderr and stdout together so they can be combined in a log file or something. Who wants PowerShell butting in to their log file??? If I do ant build 2>&1 >build.log, then any errors that go to stderr have PowerShell’s nosey $0.02 tacked on, instead of getting clean error messages in my log file.

But, the output stream is not a text stream! Redirects are just another syntax for the object pipeline. The error records are objects, so all you have to do is convert the objects on that stream to strings before redirecting:

From:

> cmd /c "echo Hello from standard error 1>&2" 2>&1
cmd.exe : Hello from standard error
At line:1 char:4
+ cmd &2" 2>&1
    + CategoryInfo          : NotSpecified: (Hello from standard error :String) [], RemoteException
    + FullyQualifiedErrorId : NativeCommandError

To:

> cmd /c "echo Hello from standard error 1>&2" 2>&1 | %{ "$_" }
Hello from standard error

…and with a redirect to a file:

> cmd /c "echo Hello from standard error 1>&2" 2>&1 | %{ "$_" } | tee out.txt
Hello from standard error

…or just:

> cmd /c "echo Hello from standard error 1>&2" 2>&1 | %{ "$_" } >out.txt

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