PowerShell Write-Host vs Write-Output

Microsoft’s Definition

Reference: https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.utility/write-output?view=powershell-7.2

Write-Host

The Write-Host cmdlet’s primary purpose is to produce for-(host)-display-only output, such as printing colored text like when prompting the user for input in conjunction with Read-Host. Write-Host uses the ToString() method to write the output

Write-Output

Write-Output sends objects to the primary pipeline, also known as the “output stream” or the “success pipeline.”

If Write-Output is the last command in the pipeline, the objects are displayed in the console

How I explain it

I would use Write-Host for any standard output (STDOUT) and will use Write-Output if I want the send the output to the next command.

Bad idea to use Write-Output in a function!

When using Write-Output by itself as the last command will output the object to the console (as explained above), this gives the impression that it works similar to Write-Host. However this will give you a very bad output when used within a function!

Using the following example script:

function MyFunction {
    Write-Output "First print"
    Write-Output "Second print"
    return "my return value"
}

$MyVar = MyFunction

Write-Host "MyVar contains: $($MyVar)"

Write-Host "1st item contains: $($MyVar[0])"
Write-Host "2nd item contains: $($MyVar[1])"
Write-Host "3nd item contains: $($MyVar[2])"

The script above will print out the following output:

MyVar contains: First print Second print my return value
1st item contains: First print
2nd item contains: Second print
3nd item contains: my return value

In most programming languages, only the “return” statement (line 4) should return what’s defined in the function. Therefore when using Write-Output here will give undesired results.

Use Write-Host for any standard output (stdout)!

Since I only want the function to return “my return value” as a string and the other 2 prints should only be a STDOUT, I should be using “Write-Host” instead.

Example:

function MyFunction {
    Write-Host "First print"
    Write-Host "Second print"
    return "my return value"
}

$MyVar = MyFunction

Write-host "This is $($MyVar)"

Script output:

First print
Second print
This is my return value

When to actually use Write-Output?

Write-Output do have its purpose. It is especially very useful when you want to pass an output to the next pipeline. Use the following for example where I want to combine some strings and save it to a file.

$FirstName = "Alex"
$Lastname = "Dinh"

Write-Output "Hello there $($FirstName) $($Lastname)" | Out-File -FilePath .\sample.txt

Example of the “sample.txt” content:

PS> cat .\sample.txt
Hello there Alex Dinh

PowerShell is super friendly and the following would also work, however it is just not as nice to read and easily understood.

$FirstName = "Alex"
$Lastname = "Dinh"

"Hello there $($FirstName) $($Lastname)" | Out-File -FilePath .\sample.txt