Like many other languages, PowerShell has commands for controlling the flow of execution within your scripts. One of those statement is the switch statement and in PowerShell, it offers features that are not found in other languages. Today we will take a deep dive into working with the PowerShell switch.

Index

If statement

One of the first statements that you will learn is the if statement. It lets you execute a script block if a statement is $true.

    if ( Test-Path $Path )
    {
        Remove-Item $Path
    }

You can have much more complicated logic by using elseif and else statements. Here is an example where I have a numeric value for day of the week and I want to get the name as a string.

    $day = 3

    if ( $day -eq 0 ) { $result = 'Sunday'        }
    elseif ( $day -eq 1 ) { $result = 'Monday'    }
    elseif ( $day -eq 2 ) { $result = 'Tuesday'   }
    elseif ( $day -eq 3 ) { $result = 'Wednesday' }
    elseif ( $day -eq 4 ) { $result = 'Thursday'  }
    elseif ( $day -eq 5 ) { $result = 'Friday'    }
    elseif ( $day -eq 6 ) { $result = 'Saturday'  }

    $result

    # Output
    Wednesday

It turns out that this is a very common pattern and there are a lot of ways to deal with this. One of them is with a switch.

Switch statement

The switch statement allows you to provide a variable and a list of possible values. If the value matches the variable, then it’s scriptblock will be executed.

    $day = 3

    switch ( $day )
    {
        0 { $result = 'Sunday'    }
        1 { $result = 'Monday'    }
        2 { $result = 'Tuesday'   }
        3 { $result = 'Wednesday' }
        4 { $result = 'Thursday'  }
        5 { $result = 'Friday'    }
        6 { $result = 'Saturday'  }
    }

    $result

    # Output
    'Wednesday'

For this example, the value of $day matches one of the numeric values, then the correct name will be assigned to $result. We are only doing a variable assignment in this example, but any PowerShell can be executed in those script blocks.

Assign to a variable

We can write that last example in another way.

    $result = switch ( $day )
    {
        0 { 'Sunday'    }
        1 { 'Monday'    }
        2 { 'Tuesday'   }
        3 { 'Wednesday' }
        4 { 'Thursday'  }
        5 { 'Friday'    }
        6 { 'Saturday'  }
    }

We are placing the value on the PowerShell pipeline and assigning it to the $result. You can do this same thing with the if and foreach statements.

Default

We can use the default keyword to identify the what should happen if there is no match.

    $result = switch ( $day )
    {
        0 { 'Sunday' }
        # ...
        6 { 'Saturday' }
        default { 'Unknown' }
    }

Here we return the value Unknown in the default case.

Strings

I was matching numbers in those last examples, but you can also match strings.

    $item = 'Role'

    switch ( $item )
    {
        Component
        {
            'is a component'
        }
        Role
        {
            'is a role'
        }
        Location
        {
            'is a location'
        }
    }

    # Output
    is a role

I decided not to wrap the Component,Role and Location matches in quotes here to hilight that they are optional. The switch treats those as a string in most cases.

Arrays

One of the cool features of the PowerShell switch is the way it handles arrays. If you give a switch an array, it will process each element in that collection.

    $roles = @('WEB','Database')

    switch ( $roles ) {
        'Database'   { 'Configure SQL' }
        'WEB'        { 'Configure IIS' }
        'FileServer' { 'Configure Share' }
    }

    # Output
    Configure IIS
    Configure SQL

If you have repeated items in your array, then they will be matched multiple times by the appropriate section.

PSItem

You can use the $PSItem or $_ to reference the current item that was processed. When we do a simple match, $PSItem will be the value that we are matching. I will be performing some advanced matches in the next section where this will be used.

Parameters

A unique feature of the PowerShell switch is that it has a number of switch parameters that change how it performs.

-CaseSensitive

The matches are not case sensitive by default. If you need to be case sensitive then you can use -CaseSensitive. This can be used in combination with the other switch parameters.

-Wildcard

We can enable wildcard support with the -wildcard switch. This uses the same wildcard logic as the -like operator to do each match.

    $Message = 'Warning, out of disk space'

    switch -Wildcard ( $message )
    {
        'Error*'
        {
            Write-Error -Message $Message
        }
        'Warning*'
        {
            Write-Warning -Message $Message
        }
        default
        {
            Write-Information $message
        }
    }

    # Output 
    WARNING: Warning, out of disk space

Here we are processing a message and then outputting it on different streams based on the contents.

-Regex

The switch statement supports regex matches just like it does wildcards.

    switch -Regex ( $message )
    {
        '^Error'
        {
            Write-Error -Message $Message
        }
        '^Warning'
        {
            Write-Warning -Message $Message
        }
        default
        {
            Write-Information $message
        }
    }

I have more examples of using regex in another article I wrote: The many ways to use regex.

-File

A little known feature of the switch statement is that it can process a file with the -File parameter. You use -file with a path to a file instead of giving it a variable expression.

    switch -Wildcard -File $path
    {
        'Error*'
        {
            Write-Error -Message $PSItem
        }
        'Warning*'
        {
            Write-Warning -Message $PSItem
        }
        default
        {
            Write-Output $PSItem
        }
    }

It works just like processing an array. In this example, I combine it with wildcard matching and make use of the $PSItem. This would process a log file and convert it to warning and error messages depending on the regex matches.

Advanced details

Now that you are aware of all these documented features, we can use them in the context of more advanced processing.

Expressions

The switch can be on an expression instead of a variable.

    switch ( ( Get-Service | Where status -eq 'running' ).name ) {...}

Whatever the expression evaluates to will be the value used for the match.

Multiple matches

You may have already picked up on this, but a switch can match to multiple conditions. This is especially true when using -wildcard or -regex matches. Be aware that you can add the same condition multiple times and all of them will trigger.

    switch ( 'Word' )
    {
        'word' { 'lower case word match' }
        'Word' { 'mixed case word match' }
        'WORD' { 'upper case word match' }
    }

    # Output
    lower case word match
    mixed case word match
    upper case word match

All three of these statements will fire. This shows that every condition is checked (in order). This holds true for processing arrays where each item will check each condition.

Continue

Normally, this is where I would introduce the break statement, but it is better that we learn how to use continue first. Just like with a foreach loop, continue will continue onto the next item in the collection or exit the switch if there are no more items. We can rewrite that last example with continue statements so that only one statement executes.

    switch ( 'Word' )
    {
        'word' 
        {
            'lower case word match'
            continue
        }
        'Word' 
        {
            'mixed case word match'
            continue
        }
        'WORD' 
        {
            'upper case word match'
            continue
        }
    }

    # Output
    lower case word match

Instead of matching all three items, the first one is matched and the switch continues to the next value. Because there are no values left to process, the switch exits. This next example is showing how a wildcard could match multiple items.

    switch -Wildcard -File $path
    {
        '*Error*'
        {
            Write-Error -Message $PSItem
            continue
        }
        '*Warning*'
        {
            Write-Warning -Message $PSItem
            continue
        }
        default
        {
            Write-Output $PSItem
        }
    }

Because a line in the input file could contain both the word Error and Warning, we only want the first one to execute and then continue processing the file.

Break

A break statement will exit the switch. This is the same behavior that continue will present for single values. The big difference is when processing an array. break will stop all processing in the switch and continue will move onto the next item.

    $Messages = @(
        'Downloading update'
        'Ran into errors downloading file'
        'Error: out of disk space'
        'Sending email'
        '...'
    )

    switch -Wildcard ($Messages)
    {
        'Error*'
        {
            Write-Error -Message $PSItem
            break
        }
        '*Error*'
        {
            Write-Warning -Message $PSItem
            continue
        }
        '*Warning*'
        {
            Write-Warning -Message $PSItem
            continue
        }
        default
        {
            Write-Output $PSItem
        }
    }

    # Output 
    Downloading update
    WARNING: Ran into errors downloading file
    write-error -message $PSItem : Error: out of disk space
    + CategoryInfo          : NotSpecified: (:) [Write-Error], WriteErrorException
    + FullyQualifiedErrorId : Microsoft.PowerShell.Commands.WriteErrorException

In this case, if we hit any lines that start with Error then we will get an error and the switch will stop. This is what that break statement is doing for us. If we find Error inside the string and not just at the beginning, we will write it as a warning. We will do the same thing for Warning. It is possible that a line could have both the word Error and Warning, but we only need one to process. This is what the continue statement is doing for us.

Break labels

The switch statement supports break/continue labels just like foreach.

    :filelist foreach($path in $logs)
    {
        :logFile switch -Wildcard -File $path
        {
            'Error*'
            {
                Write-Error -Message $PSItem
                break filelist
            }
            'Warning*'
            {
                Write-Error -Message $PSItem
                break logFile
            }
            default
            {
                Write-Output $PSItem
            }
        }
    }

I personally don’t like the use of break labels but I wanted to point them out because they are confusing if you have never seen them before. When you have multiple switch or foreach statements that are nested, you may want to break out of more than the inner most item. You can place a label on a switch that can be the target of your break.

Enum

PowerShell 5.0 gave us enums and we can use them in a switch.

    enum Context {
        Component
        Role
        Location
    }

    $item = [Context]::Role

    switch ( $item )
    {
        Component
        {
            'is a component'
        }
        Role
        {
            'is a role'
        }
        Location
        {
            'is a location'
        }
    }

    # Output
    is a role

If you want to keep everything as strongly typed enums, then you can place them in parentheses.

    switch ($item )
    {
        ([Context]::Component)
        {
            'is a component'
        }
        ([Context]::Role)
        {
            'is a role'
        }
        ([Context]::Location)
        {
            'is a location'
        }
    }

The parentheses are needed here so that the switch does not treat the value [Context]::Location as a literal string.

ScriptBlock

We can use a scriptblock to perform the evaluation for a match if needed.

    $age = 37

    switch ( $age )
    {
        {$PSItem -le 18} 
        {
            'child'
        }
        {$PSItem -gt 18} 
        {
            'adult'
        }
    }

    # Output
    'adult'

This adds a lot of complexity and can make your switch hard to read. In most cases where you would use something like this it would be better to use if and elseif statements. I would consider using this if I already had a large switch in place and I needed 2 items to hit the same evaluation block.

One thing that I think helps with legibility is to place the scriptblock in parentheses.

    switch ( $age )
    {
        ({$PSItem -le 18})
        {
            'child'
        }
        ({$PSItem -gt 18})
        {
            'adult'
        }
    }

It still executes the same way and give a better visual break when quickly looking at it.

Regex $matches

We need to revisit regex to touch on something that is not immediately obvious. The use of regex populates the $matches variable. I do go into the use of $matches more when I talk about The many ways to use regex. Here is a quick sample to show it in action with named matches.

    $message = 'my ssn is 123-23-3456 and credit card: 1234-5678-1234-5678'

    switch -regex ($message)
    {
        '(?<SSN>\d\d\d-\d\d-\d\d\d\d)'
        {
            Write-Warning "message contains a SSN: $($matches.SSN)"
        }
        '(?<CC>\d\d\d\d-\d\d\d\d-\d\d\d\d-\d\d\d\d)'
        {
            Write-Warning "message contains a credit card number: $($matches.CC)"
        }
        '(?<Phone>\d\d\d-\d\d\d-\d\d\d\d)'
        {
            Write-Warning "message contains a phone number: $($matches.Phone)"
        }
    }

    # Output
    WARNING: message may contain a SSN: 123-23-3456
    WARNING: message may contain a credit card number: 1234-5678-1234-5678

$null

You can match a $null value that does not have to be the default.

    $value = $null

    switch ( $value )
    {
        $null
        {
            'Value is null'
        }
        default
        {
            'value is not null'
        }
    }

    # Output
    Value is null

Same goes for an empty string.

    switch ( '' )
    {
        ''
        {
            'Value is empty'
        }
        default
        {
            'value is a empty string'
        }
    }

    # Output
    Value is empty

Constant expression

Lee Dailey pointed out that we can use a constant $true expression to evaluate [bool] items. Imagine if we have a lot of boolean checks that need to happen.

    $isVisible = $false
    $isEnabled = $true
    $isSecure = $true

    switch ( $true )
    {
        $isEnabled
        {
            'Do-Action'
        }
        $isVisible
        {
            'Show-Animation'
        }
        $isAdmin
        {
            'Enable-AdminMenu'
        }
    }

    # Output
    Do-Action
    Enabled-AdminMenu

This is a very clean way to evaluate and take action on the status of several boolean fields. The cool thing about this is that you can have one match flip the status of a value that has not been evaluated yet.

    $isVisible = $false
    $isEnabled = $true
    $isSecure = $false

    switch ( $true )
    {
        $isEnabled
        {
            'Do-Action'
            $isVisible = $true
        }
        $isVisible
        {
            'Show-Animation'
        }
        $isAdmin
        {
            'Enable-AdminMenu'
        }
    }

    # Output
    Do-Action
    Show-Animation

Setting $isEnabled to $true in this example will make sure the $isVisible is also set to $true. Then when the $isVisible gets evaluated, its scriptblock will be invoked. This is a bit counter-intuitive but is a very clever use of the mechanics.

Other patterns

Hashtables

One of my most popular posts is the one I did on everything you ever wanted to know about hashtables. One of the example use-cases for a hashtable is to be a lookup table. That is an alternate approach to a common pattern that a switch statement is often addressing.

    $day = 3

    $lookup = @{
        0 = 'Sunday'
        1 = 'Monday'
        2 = 'Tuesday'
        3 = 'Wednesday'
        4 = 'Thursday'
        5 = 'Friday'
        6 = 'Saturday'
    }

    $lookup[$day]

    # Output
    Wednesday

If I am only using a switch as a lookup, I will quite often use a hashtable instead.

Enum

PowerShell 5.0 introduced the Enum and it is also an option in this case.

    $day = 3

    enum DayOfTheWeek {
        Sunday
        Monday
        Tuesday
        Wednesday
        Thursday
        Friday
        Saturday
    }

    [DayOfTheWeek]$day

    # Output
    Wednesday

We could go all day looking at different ways to solve this problem. I just wanted to make sure you knew you had options.

Final words

The switch statement is simple on the surface but it offers some advanced features that most people don’t realize are available. Stringing those features together makes this into a really powerful feature when it is needed. I hope you learned something that you had not realized before.