Richard Siddaway's Blog

PowerShell pause – how can you pause a PowerShell script?

Two ways come to mind.

First if you just want the script to pause for a specified time period then you can use Start-Sleep

1..10 |
foreach {
  $PSItem
  if ($PSItem -eq 5) {
    Write-Warning -Message "Starting sleep"
    Start-Sleep -Seconds 5
  }
}

Run this and you’ll see the numbers 1-5 output then then warning message. After the delay you’ll see the numbers 6-10 output.

But what if you want to control the pause manually? Not sure if there are advantages to this approach but if you do need to do this you can use Read-Host

1..10 |
foreach {
  $PSItem
  if ($PSItem -eq 5) {
    Read-Host -Prompt "Press Enter key to continue"
  }
}

You’ll see the numbers 1-5 output then the message

Press Enter key to continue:

After pressing the enter key the script continues and outputs 6-10

Don’t know why you’d want to do this in an automation scenario but the technique is there if you need it – I don’t recommend the approach.

There are also a few cmdlets that can be used under specific circumstances:

Wait-Event
Wait-Job
Wait-Process
Wait-VM

Also check the –wait parameter on Restart-Computer

Loops are a construction seen in most scripting and programming languages. A loop is used to repeat a set of statements a set number of times or until a specific criterion is met or while a specific criterion is true. In this post I’ll describe the PowerShell for loop.

For loops are found in many languages. A for loop is sometimes referred to as a counting loop as it will have a counter that starts at a pre-set value and counts up to a specific value. The counter is usually incremented by 1 for each iteration of the loop.

A PowerShell for loop looks like this

for ($i=1; $i -le 10; $i++){$i}

The counter – $i – is initialised to 1. The loop will execute while $i is less than or equal to 10 and $i is incremented by 1 for each turn round the loop. In this case the loop lists the value of the counter.  You can see the results like this

PS> $results = for ($i=1; $i -le 10; $i++){$i}
PS> "$results"
1 2 3 4 5 6 7 8 9 10

You can also run loops where the counter decreases

PS> $results = for ($i=10; $i -ge 1; $i--){$i}
PS> "$results"
10 9 8 7 6 5 4 3 2 1

A for loop is great when you need to perform the loop and exact number of times but if your loop depends on a specific criterion you’re better off using a while loop or a do loop which I’ll cover in another post

PowerShell enables you to work with the file system on your machine – one question that often comes up is how to create a directory.

When working interactively you can use md

PS> md c:\testf1


    Directory: C:\


 Mode                LastWriteTime         Length Name 
 ----                -------------         ------ ---- 
 d-----       19/08/2017     14:24                testf1

md doesn’t look like a PowerShell command – more like an old style DOS command.

Its actually an alias for mkdir

PS> Get-Command md

CommandType     Name                                               Version    Source 
 -----------     ----                                               -------    ------ 
 Alias           md –> mkdir

Which raises the question – what’s mkdir?

PS> Get-Command mkdir

CommandType     Name                                               Version    Source 
 -----------     ----                                               -------    ------ 
 Function        mkdir

Its a function that PowerShell creates for you

Digging into the function

PS> Get-ChildItem -Path function:\mkdir | select  -ExpandProperty  Definition

<# 
 .FORWARDHELPTARGETNAME New-Item 
 .FORWARDHELPCATEGORY Cmdlet 
 #>

[CmdletBinding(DefaultParameterSetName='pathSet', 
    SupportsShouldProcess=$true, 
    SupportsTransactions=$true, 
    ConfirmImpact='Medium')] 
    [OutputType([System.IO.DirectoryInfo])] 
param( 
    [Parameter(ParameterSetName='nameSet', Position=0, ValueFromPipelineByPropertyName=$true)] 
    [Parameter(ParameterSetName='pathSet', Mandatory=$true, Position=0, ValueFromPipelineByPropertyName=$true)] 
    [System.String[]] 
    ${Path},

    [Parameter(ParameterSetName='nameSet', Mandatory=$true, ValueFromPipelineByPropertyName=$true)] 
    [AllowNull()] 
    [AllowEmptyString()] 
    [System.String] 
    ${Name},

    [Parameter(ValueFromPipeline=$true, ValueFromPipelineByPropertyName=$true)] 
    [System.Object] 
    ${Value},

    [Switch] 
    ${Force},

    [Parameter(ValueFromPipelineByPropertyName=$true)] 
    [System.Management.Automation.PSCredential] 
    ${Credential} 
 )

begin {

    try { 
        $wrappedCmd = $ExecutionContext.InvokeCommand.GetCommand('New-Item', [System.Management.Automation.CommandTypes] 
 ::Cmdlet) 
        $scriptCmd = {& $wrappedCmd -Type Directory @PSBoundParameters } 
        $steppablePipeline = $scriptCmd.GetSteppablePipeline() 
        $steppablePipeline.Begin($PSCmdlet) 
    } catch { 
        throw 
    }

}

shows that its based on New-Item

PS> New-Item -Path c:\ -Name testf2 -ItemType Directory


    Directory: C:\


 Mode                LastWriteTime         Length Name 
 ----                -------------         ------ ---- 
 d-----       19/08/2017     14:32                testf2

The default for New-Item in the filesystem is to create a file so you need to use –ItemType Directory to create the folder.

If the folder you’re creating is a subfolder of a non-existent folder you can create the hierarchy is one go

PS> New-Item -Path c:\ -Name testf3\tests1 -ItemType Directory


    Directory: C:\testf3


 Mode                LastWriteTime         Length Name 
 ----                -------------         ------ ---- 
 d-----       19/08/2017     14:33                tests1


 PS> Get-ChildItem -Path c:\testf3 -Recurse


    Directory: C:\testf3


 Mode                LastWriteTime         Length Name 
 ----                -------------         ------ ---- 
 d-----       19/08/2017     14:33                tests1

This can get complicated if you try to nest too many levels so I recommend explicitly creating each level of your folder hierarchy. Its much easier to maintain and modify

PowerShell has a number of ways to perform a loop – I recently counted seven distinct methods. If you can’t list them all don’t worry one is very esoteric and unexpected. I’ll enumerate them in a future post. For now I want to concentrate on a source of confusion – especially to newcomers to PowerShell – namely the PowerShell foreach statements.

The confusion arises because there are effectively two foreach statements. One is a PowerShell keyword that initiates a loop and the other is an alias for  a cmdlet.

Lets start with the foreach loop.

$numbers = 1..10
foreach ($number in $numbers){
  [math]::Pow($number, 2)
}

foreach in this case is used to iterate over a collection. In the example above $numbers is an array of numbers 1 to 10. Foreach number in the array it is raised to the power 2 – squared.

Remember that PowerShell is unique among shells in that you can use pipelines in many places that other languages insist on variables so you could change the example to

foreach ($number in 1..10){
  [math]::Pow($number, 2)
}

The array is generated and then iterated over as earlier.

If you see foreach as the first command on a line you’re dealing with the foreach keyword and therefore a loop.

On the other hand if you see foreach in a pipeline

1..10 | foreach {
  [math]::Pow($_, 2)
}

or

1..10 | foreach {
  [math]::Pow($psitem, 2)
}

you’re dealing with the cmdlet. $_ or $psitem denote the object currently on the pipeline. foreach is an alias for the Foreach-Object cmdlet and you’re using –Process as a position parameter for the scriptblock. Written in full you’re doing this

1..10 | ForEach-Object -process {
  [math]::Pow($_, 2)
}

or

1..10 | ForEach-Object -process {
  [math]::Pow($psitem, 2)
}

Just to add to the confusion you also have the option to use the foreach method on the collection

(1..10).foreach({[math]::Pow($psitem, 2)})

This isn’t seen as much though it should be remembered as this is the fastest way to iterate over a collection.

In summary

foreach starting a line is the looping keyword. Faster than the pipeline but increases memory overheads as  the collection has to pre-generated

foreach on the pipeline is an alias for foreach-object. Lower memory requirements as the collection is passed down the pipeline but a bit slower

().foreach({}) is a method on the collection (we treat it as an operator in PowerShell in Action) and is fast but in terms of coding style may be more intuitive to developers than admins.