Redirect powershell output and errors to console (in real-time) and to a variable

Question

I would like to redirect the output of a command in PowerShell, following these rules:

• The command is stored to a variable

• Output must be written to the console in real-time (i.e. "ping" results), including errors

• Output must be stored to a variable, including errors (real-time is not mandatory here)

Here are my tests, assuming:

$command = "echo:"

to test errors redirection, and:

$command = "ping 127.0.0.1"

to test real-time output.

1. Output is written in real-time, errors are not redirected at all

   Invoke-Expression $command 2>&1 | Tee-Object -Variable out_content
   

2. Output is written in real-time, errors are only redirected to the console

   Invoke-Expression ($command 2>&1) | Tee-Object -Variable out_content
   Invoke-Expression $command | Tee-Object -Variable out_content 2>&1
   

3. Output is not written in real-time, errors are correctly redirected to both

   (Invoke-Expression $command) 2>&1 | Tee-Object -Variable out_content
   

Is it possible to get those rules working together?

Answer 1

Some general recommendations up front:

• Invoke-Expression should generally be avoided, because it can be a security risk and introduces quoting headaches; there are usually better and safer solutions available; best to form a habit of avoiding Invoke-Expression, unless there is no other solution.

• There is never a reason to use Invoke-Expression to simply execute an external program with arguments, such as ping 127.0.0.1; just invoke it directly - support for such direct invocations is a core feature of any shell, and PowerShell is no exception.

• If you do need to store a command in a variable or pass it as an argument for later invocation, use script blocks ({ ... }); e.g., instead of $command = 'ping 127.0.0.1', use $command = { ping 127.0.0.1 }, and invoke that script block on demand with either &, the call operator, or ., the dot-sourcing operator.
  When calling external programs, the two operators exhibit the same behavior; when calling PowerShell-native commands, & executes the code in a child scope, whereas . (typically) executes in the caller's current scope.

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That Invoke-Expression $command 2>&1 doesn't work as expected looks like a bug (as of PowerShell (Core) 7.4.x) and has been reported in GitHub issue #10476.

As for a workaround for your problem:

PetSerAl, as countless times before, has provided a solution in a comment on the question:

& { Invoke-Expression $command } 2>&1 | Tee-Object -Variable out_content

{ ... } is a script-block literal that contains the Invoke-Expression call, and it is invoked with &, the call operator, which enables applying stream-redirection expression 2>&1 to the & call, which bypasses the bug.

^{If $command contained a PowerShell-native command that you wanted to execute directly in the current scope, such as a function definition, you'd use . instead of &.}

Alternative:

Include the 2>&1 redirection in the string to evaluate:

Invoke-Expression "$command 2>&1" | Tee-Object -Variable out_content