Write-Host vs Write-Information in PowerShell 5

The Write-* cmdlets allow you to channel the output of your PowerShell code in a structured way, so you can easily distinguish messages of different severity from each other.

• Write-Host: display messages to an interactive user on the console. Unlike the other Write-* cmdlets this one is neither suitable nor intended for automation/redirection purposes. Not evil, just different.
• Write-Output: write the “normal” output of the code to the default (success) output stream (“STDOUT”).
• Write-Error: write error information to a separate stream (“STDERR”).
• Write-Warning: write messages that you consider warnings (i.e. things that aren’t failures, but something that the user should have an eye on) to a separate stream.
• Write-Verbose: write information that you consider more verbose than “normal” output to a separate stream.
• Write-Debug: write information that you consider relevant for debugging your code to a separate stream.

Write-Information is just a continuation of this approach. It allows you to implement log levels in your output (Debug, Verbose, Information, Warning, Error) and still have the success output stream available for regular output.

As for why Write-Host became a wrapper around Write-Information: I don’t know the actual reason for this decision, but I’d suspect it’s because most people don’t understand how Write-Host actually works, i.e. what it can be used for and what it should not be used for.

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To my knowledge there isn’t a generally accepted or recommended approach to logging in PowerShell. You could for instance implement a single logging function like @JeremyMontgomery suggested in his answer:

function Write-Log {
  Param(
    [Parameter(Mandatory=$true, Position=0)]
    [ValidateNotNullOrEmpty()]
    [string]$Message,
    [Parameter(Mandatory=$false, Position=1)]
    [ValidateSet('Error', 'Warning', 'Information', 'Verbose', 'Debug')]
    [string]$LogLevel="Information"
  )

  switch ($LogLevel) {
    'Error'       { ... }
    'Warning'     { ... }
    'Information' { ... }
    'Verbose'     { ... }
    'Debug'       { ... }
    default       { throw "Invalid log level: $_" }
  }
}

Write-Log 'foo'                    # default log level: Information
Write-Log 'foo' 'Information'      # explicit log level: Information
Write-Log 'bar' 'Debug'

or a set of logging functions (one for each log level):

function Write-LogInformation {
  Param(
    [Parameter(Mandatory=$true, Position=0)]
    [ValidateNotNullOrEmpty()]
    [string]$Message
  )

  ...
}

function Write-LogDebug {
  Param(
    [Parameter(Mandatory=$true, Position=0)]
    [ValidateNotNullOrEmpty()]
    [string]$Message
  )

  ...
}

...

Write-LogInformation 'foo'
Write-LogDebug 'bar'

Another option is to create a custom logger object:

$logger = New-Object -Type PSObject -Property @{
  Filename=""
  Console  = $true
}
$logger | Add-Member -Type ScriptMethod -Name Log -Value {
  Param(
    [Parameter(Mandatory=$true, Position=0)]
    [ValidateNotNullOrEmpty()]
    [string]$Message,
    [Parameter(Mandatory=$false, Position=1)]
    [ValidateSet('Error', 'Warning', 'Information', 'Verbose', 'Debug')]
    [string]$LogLevel="Information"
  )

  switch ($LogLevel) {
    'Error'       { ... }
    'Warning'     { ... }
    'Information' { ... }
    'Verbose'     { ... }
    'Debug'       { ... }
    default       { throw "Invalid log level: $_" }
  }
}
$logger | Add-Member -Type ScriptMethod -Name LogDebug -Value {
  Param([Parameter(Mandatory=$true)][string]$Message)
  $this.Log($Message, 'Debug')
}
$logger | Add-Member -Type ScriptMethod -Name LogInfo -Value {
  Param([Parameter(Mandatory=$true)][string]$Message)
  $this.Log($Message, 'Information')
}
...

Write-Log 'foo'                    # default log level: Information
$logger.Log('foo')                 # default log level: Information
$logger.Log('foo', 'Information')  # explicit log level: Information
$logger.LogInfo('foo')             # (convenience) wrapper method
$logger.LogDebug('bar')

Either way you can externalize the logging code by

• putting it into a separate script file and dot-sourcing that file:

  . 'C:\path\to\logger.ps1'
  

• putting it into a module and importing that module:

  Import-Module Logger