fix

pester-extensions.psm1

@@ -4,30 +4,51 @@ Pester extension that allows to run command and validate exit code
 .EXAMPLE
 "python file.py" | Should -ReturnZeroExitCode
 #>
+
+function Get-CommandResult {
+    param (
+        [Parameter(Mandatory=$true)]
+        [string] $Command,
+        [switch] $Multiline
+    )
+    # Bash trick to suppress and show error output because some commands write to stderr (for example, "python --version")
+    if ($IsWindows) {
+        [string[]]$stdout = & $env:comspec /c "$Command 2>&1"
+    } else {
+        $stdout = & bash -c "$Command 2>&1"
+    }
+    $exitCode = $LASTEXITCODE
+    return @{
+        Output = If ($Multiline -eq $true) { $stdout } else { [string]$stdout }
+        ExitCode = $exitCode
+    }
+}
+
 function ShouldReturnZeroExitCode {
     Param(
-        [Parameter (Mandatory = $true)] [ValidateNotNullOrEmpty()]
-        [String]$ActualValue,
-        [switch]$Negate
+        [String] $ActualValue,
+        [switch] $Negate,
+        [string] $Because # This parameter is unused by we need it to match Pester asserts signature
     )
 
-    Write-Host "Run command '${ActualValue}'"
-    Invoke-Expression -Command $ActualValue | ForEach-Object { Write-Host $_ }
-    $actualExitCode = $LASTEXITCODE
+    $result = Get-CommandResult $ActualValue
 
-    [bool]$succeeded = $actualExitCode -eq 0
+    [bool]$succeeded = $result.ExitCode -eq 0
     if ($Negate) { $succeeded = -not $succeeded }
 
     if (-not $succeeded)
     {
-        $failureMessage = "Command '${ActualValue}' has finished with exit code ${actualExitCode}"
+        $commandOutputIndent = " " * 4
+        $commandOutput = ($result.Output | ForEach-Object { "${commandOutputIndent}${_}" }) -join "`n"
+        $failureMessage = "Command '${ActualValue}' has finished with exit code ${actualExitCode}`n${commandOutput}"
     }
 
-    return New-Object PSObject -Property @{
+    return [PSCustomObject] @{
         Succeeded      = $succeeded
         FailureMessage = $failureMessage
     }
 }
 
-Add-AssertionOperator -Name ReturnZeroExitCode `
-                    -Test  $function:ShouldReturnZeroExitCode
+if (Get-Command -Name Add-AssertionOperator -ErrorAction SilentlyContinue) {
+    Add-AssertionOperator -Name ReturnZeroExitCode -InternalName ShouldReturnZeroExitCode -Test ${function:ShouldReturnZeroExitCode}
+}