Redirecting output to $null in PowerShell, but ensuring the variable remains set - powershell

Question

I have some code:
$foo = someFunction
This outputs a warning message which I want to redirect to $null:
$foo = someFunction > $null
The problem is that when I do this, while successfully supressing the warning message, it also has the negative side-effect of NOT populating $foo with the result of the function.
How do I redirect the warning to $null, but still keep $foo populated?
Also, how do you redirect both standard output and standard error to null? (In Linux, it's 2>&1.)

Answer 1

I'd prefer this way to redirect standard output (native PowerShell)...
($foo = someFunction) | out-null
But this works too:
($foo = someFunction) > $null
To redirect just standard error after defining $foo with result of "someFunction", do
($foo = someFunction) 2> $null
This is effectively the same as mentioned above.
Or to redirect any standard error messages from "someFunction" and then defining $foo with the result:
$foo = (someFunction 2> $null)
To redirect both you have a few options:
2>&1>$null
2>&1 | out-null
ADDENDUM:
Please note that (Windows) powershell has many more streams than a linux based OS. Here's the list from MS docs:
Thus you can redirect all streams using the wildcard with *>$null, and you can also use a file instead of $null.

Answer 2

This should work.
$foo = someFunction 2>$null

Answer 3

If it's errors you want to hide you can do it like this
$ErrorActionPreference = "SilentlyContinue"; #This will hide errors
$someObject.SomeFunction();
$ErrorActionPreference = "Continue"; #Turning errors back on

Answer 4

Warning messages should be written using the Write-Warning cmdlet, which allows the warning messages to be suppressed with the -WarningAction parameter or the $WarningPreference automatic variable. A function needs to use CmdletBinding to implement this feature.
function WarningTest {
[CmdletBinding()]
param($n)
Write-Warning "This is a warning message for: $n."
"Parameter n = $n"
}
$a = WarningTest 'test one' -WarningAction SilentlyContinue
# To turn off warnings for multiple commads,
# use the WarningPreference variable
$WarningPreference = 'SilentlyContinue'
$b = WarningTest 'test two'
$c = WarningTest 'test three'
# Turn messages back on.
$WarningPreference = 'Continue'
$c = WarningTest 'test four'
To make it shorter at the command prompt, you can use -wa 0:
PS> WarningTest 'parameter alias test' -wa 0
Write-Error, Write-Verbose and Write-Debug offer similar functionality for their corresponding types of messages.

Answer 5

using a function:
function run_command ($command)
{
invoke-expression "$command *>$null"
return $_
}
if (!(run_command "dir *.txt"))
{
if (!(run_command "dir *.doc"))
{
run_command "dir *.*"
}
}
or if you like one-liners:
function run_command ($command) { invoke-expression "$command "|out-null; return $_ }
if (!(run_command "dir *.txt")) { if (!(run_command "dir *.doc")) { run_command "dir *.*" } }

Answer 6

Recently, I had to shut up powershell on a Linux host, this wasn't that obvious to figure out. After back and forth I found out that wrapping a command in $( ) and adding a explicit redirection after the wrapper works.
Anything else I tried, wouldn't - I still don't know why since the PowerShell Docs are of desirable quality (and full of inconsistency...)
To import all modules on startup, I added the following. This produced some stderr output by powershell that couldnt be put to rest by ErrorAction or redirection without using the wrapping...
If anyone could elaborate on why's that would be very appreciated.
# import installed modules on launch
$PsMods = $(Get-InstalledModule);
$($PsMods.forEach({ Import-Module -Name $_.Name -ErrorAction Ignore })) *> /dev/null