PowerShell

<#
.SYNOPSIS
    Write-Log -Message [string] -Severity EVER|INFO|WARN|FATAL
	
.DESCRIPTION
    This script/function enables logging within PowerShell scripts.
​
    Global variables which control features of this script are:
    [string]$global:LogFilePath    - The text file into which all messages are getting written
    [string]$global:SubName        - The name of the script calling this Log script
    [int]$global:LogLevel          - The log levels:
									 1 logs everything,
									 2 omits INFOs,
                                     3 omits INFOs and WARNings,
									 4 only logs severity EVER messages
    [bool]$global:WriteToHost      - $FALSE = Only write into logfile; $TRUE = Also write to Host
  
.OUTPUTS
	Write-Log writes into a logfile with name [string]$global:LogFilePath.
	if the user has not given this, the standard filename "$PSScriptRoot\Logs\LogFile.txt" is used.
    The contents of a log file will look like:
    INFO: BERND-PC\Bernd 2020-02-16 12:19:36 [zip_and_copy.ps1] - Log started
    INFO: BERND-PC\Bernd 2020-02-16 12:19:36 [zip_and_copy.ps1] - Log finished
	
	In case the user has set [bool]$global:WriteToHost to $TRUE, WRite-Log also repeats all log
	messages on screen (Host).
​
.PARAMETER Message
    The message you want to log.
​
.PARAMETER Severity
    The severity of the message. This can be 'INFO', 'WARN', 'FATAL', or 'EVER'.
    If global variable LogLevel is 1 then messages with all severities will be logged.
    If LogLevel is 2 then INFO messages will not be logged.
    If LogLevel is 3 then INFO and WARN messages will not be logged.
    If LogLevel is 4 then INFO, WARN and FATAL messages will not be logged.
​
.EXAMPLE
    # Initialize global logging variables and make log script known
    $global:SubName = $MyInvocation.MyCommand.Name
    $global:LogLevel = 1
    $global:LogFilePath = "$PSScriptRoot\Logs\LogFile_" + $SubName + "_" +
        (Get-Date -UFormat "%Y%m%d") + ".txt"
    $global:WriteToHost = $TRUE
    . "$PSScriptRoot\Write-Log.ps1"
​
.EXAMPLE
    # How to call
    Write-Log -Message 'Log started' -Severity INFO
    If(-Not (Test-path $ArchiveFile)) {Write-Log -Message "$ArchiveFile was not created" -Severity FATAL}
​
.NOTES
	https://berndplumhoff.gitbook.io/sulprobil/powershell
    Version Date        Who                   What
    1.0     16-Feb-2020 Bernd Plumhoff        Initial Version
    1.1     16-Jul-2020 Bernd Plumhoff        EVER instead of ALWAYS, and optional Write-Host
#>
​
#Set-ExecutionPolicy -ExecutionPolicy Bypass -Scope CurrentUser -Force
#Clear
#Requires -Version 4
Set-StrictMode -Version Latest
​
function Write-Log {
    [CmdletBinding()]
    param(
        [Parameter()]
        [ValidateNotNullOrEmpty()]
        [string]$Message,
        [Parameter()]
        [ValidateNotNullOrEmpty()]
        [ValidateSet('INFO','WARN','FATAL','EVER')]
        [string]$Severity = 'INFO'
    )
    switch ($Severity)
    {
        'INFO'	{ If ([int]$global:LogLevel -gt 1) {Exit} }
        'WARN'	{ If ([int]$global:LogLevel -gt 2) {Exit} }
        'FATAL'	{ If ([int]$global:LogLevel -gt 3) {Exit} }
        'EVER'	{ <# Will always be logged! #> }
    }
    if (-Not (Test-Path -IsValid $global:LogFilePath)) { $global:LogFilePath = "$PSScriptRoot\Logs\LogFile.txt" }
    [string]$Output = $Severity + ": " + [System.Security.Principal.WindowsIdentity]::GetCurrent().Name +
        " " + (Get-Date -UFormat "%Y-%m-%d %T") + " [" + $global:SubName + "] - " + $Message
    Write-Output $Output | Out-File -append $global:LogFilePath
	If (Test-Path variable:global:WriteToHost) {
		If ($global:WriteToHost) { Write-Host $Output }
	}
}