How do I avoid getting data printed to stdout in my return value?

Question

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"
}

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?

score 24 · Accepted Answer · edited May 16 '23 at 11:33

Here are a few different approaches for handling this:

capture the output of the .cmd script:

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

redirect the output to null (throw it away)

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

redirect it to a file

  & external.cmd | Out-File filename.txt

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])

In PowerShell, any output value your code does not capture, is returning the "caller" object (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 the object[] itself as your return value from the function.

The return keyword is really just for clarity, with the exact same effect as just writing the variable alone (as shown), but with an 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

I have also seen people do things like: `& { & "app.exe" } | Out-Default`, but this need more explanation. — not2qubit, May 16 '23 at 11:36
Thanks @not2qubit `& { & "app.exe" } | Out-Default` works for me — Tho Ho, Aug 07 '23 at 07:12

score 17 · Answer 2 · edited Jun 20 '20 at 09:12

I generally prefer to use one of the following two techniques which, in my opinion, make the code more readable:

Cast an expression to void in order to suppress the return value:

[void] (expression)
Assign the output value to the $null variable:

$null = expression

For example:

function foo
{
    # do some work
    return "success"
}

function bar
{
    [void] (foo)  # no output
    $null = foo   # no output
    return "bar"
}

bar # outputs bar

score 7 · Answer 3 · answered Aug 05 '15 at 15:24

7

If you want the output from the command to still print to the powershell command line, you can do a variant of the accepted answer:

& external.cmd | Out-Host

answered Aug 05 '15 at 15:24

nickwesselman

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How do I avoid getting data printed to stdout in my return value?

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