PowerShell Invoke-Expression

Introduction to PowerShell's Invoke-Expression

Invoke-Expression is a brilliant PowerShell cmdlet, which mimics us typing instructions into a 'DOS box'.  A typical scenario is where you are expert with cmd.exe, and now you want to execute the same commands, but using PowerShell.  This is a job for the Invoke family of cmdlets, specifically Invoke-Expression.

Topics for Invoke-Expression Cmdlet

I have two 'Vehicles' for you to test Invoke-Expression, firstly, WOL Wake-on-Lan, and secondly Test-Connection.

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WOL - A Scenario for Employing Invoke-Expression

I have chosen Wake-on-Lan (WOL) as a vehicle to show how to combine PowerShell and cmd.  Thanks to the use of variables, it is easy to modify the Invoke-Expression instructions to run your particular command-line program instead of wolcmd.exe.

Invoke-Expression Example

While PowerShell has aliases for dir (List-ChildItem) and cd (Set-Location), there is no direct equivalent for cmd.exe.  However, the Invoke-Expression cmdlet allows you to launch an executable and crucially, append command-line instructions.  The result is that Invoke-Expression executes the string as if you had typed it in the 'DOS box'.

How to Bring the Command-Line into PowerShell - WOL Example

PowerShell Invoke-Expression CMD Command-line

Take the scenario where you want to mimic cmd.exe, see screenshot above, my idea is to execute Wake-On-Lan with a PowerShell script containing two variables.   Firstly, control the path to the executable, secondly, append the command-line instructions that you want to pass to that program.

Summary of WOL Command
In DOS: Wolcmd 00248C1F9023 192.168.1.66 255.255.255.0 7

In PowerShell: Invoke-Expression Wolcmd.exe 00248C1F9023 192.168.1.66 255.255.255.0 7

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Example PowerShell Script Using Invoke-Expression to Wake-on-Lan

Vital preliminary step: download wolcmd.  I strongly recommend that you 'play' with wolcmd in the DOS box before you use it in a PowerShell script.

In the script below I have added two variables because I wanted to be sure that we control the path to the executable, and also I wanted to direct you to making your own version of my example.

# PowerShell Wake-on-Lan command-line instruction
$ExePath = "E:\Downloads\Applications\wolgui\wolcmd.exe"
$CLine = "00248C1F9023 192.168.1.66 255.255.255.0 7"
Invoke-Expression "$ExePath $CLine" | Out-Null

Note 1: You probably want to change the values of $ExePath and of $CLine

Note 2: Observe the double quotes surrounding both variables.  "$ExePath $CLine"

Note 3: Experiment with and without | Out-Null.  This command merely suppresses any output.

Troubleshooting

If you are following my actual PowerShell Wake-on-Lan example, then download wolcmd here.

If you have changed the values of $ExePath and $CLine, but your script still does not work try a manual walk-through in the cmd dos box.  See if you can get ipconfig to work with the command-line switch /all.

Invoke-Expression Using a String in a PS1 File

My idea in this PowerShell Invoke-Expression example is to store the string values in a text file, and then call that file and run those instructions.  The scenario is that you wish to measure the server's response time with PowerShell's Test-Connection, this is the equivalent of ping in cmd.exe.

Concept 1: My instructions ping a website using PowerShell's Test-Connection cmdlet
Concept 2: These instructions are saved in a text file with a .ps1 file extension.
Concept 3: Invoke-Expression then 'calls' this text file with its PowerShell instructions.

Save this into a file, let us call it C:\PingWeb.ps1

# PowerShell Test-Connection .ps1 file
Clear-host
$WebPing = Test-Connection www.computerperformance.co.uk -count 10
Write-host Average ($WebPing | measure-Object ResponseTime -average).Average

Now let us use Invoke-Expression to execute these instructions.

# PowerShell Invoke-Expression example from .ps1 file
$Path = "C:\PingWeb.ps1"
Invoke-Expression "$Path"

As with many of my scripts, this example contains extra code which aids learning, and directs you to making changes to suit your circumstances.  However, you could simplify the expression to:
Invoke-Expression "C:\PingWeb.ps1"

See more on PowerShell Measure-Command ยป

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Research Invoke-Expression

You can research Invoke-Expression with these two trusty PowerShell commands
Get-Help Invoke-Expression

You can also append Get-Member thus:

# PowerShell WOL - list Methods for Invoke-Expression
$ExePath = "E:\Downloads\Applications\wolgui\wolcmd.exe"
$CLine = "00248C1F9023 192.168.1.66 255.255.255.0 7"
Invoke-Expression "$ExePath $CLine" | Get-Member

More Members of the Invoke Family

# PowerShell script to list Invoke family of cmdlets
Get-Command -verb invoke

Invoke-Command
Invoke-Expression
Invoke-History
Invoke-Item
Invoke-WebRequest
Invoke-WmiMethod
Invoke-WSManAction

The two most promising cmdlets are Invoke-Command and Invoke-Item.

Problem with Invoke-Expression -ComputerName .... -command "xyz"

If it's any consolation commands like this did not work for me:
Invoke-Expression -ComputerName localhost, BigServer -command "Get-process powershell*"

What I did was switch to Invoke-Command thus:

Invoke-Command -ComputerName bigserver -scriptblock {Get-process powershell}

Summary of PowerShell's Invoke-Expression

Invoke-Expression is a wonderful way of executing a string of DOS instructions, but in PowerShell.  I chose Wake-on-lan (WOL) as a vehicle to give you a grounding in this cmdlet's syntax.  The ideas is that Invoke-Expression executes the string as if you typed it at the cmd.exe command-line.  I hope that you will be able to modify my examples to suit your task.

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See more Microsoft PowerShell tasks:

PowerShell Home   • Shell Application   • New-Object   • PowerShell Add Printer   • PowerShell -com

PowerShell Logon Script  • Map Network Drive  • PowerShell Create Shortcut  • Free CSV Import Tool

Invoke-Expression   • Invoke-Command   • Invoke-Item   • PowerShell Expression v Command Mode

Please email me if you have a better example script. Also please report any factual mistakes, grammatical errors or broken links, I will be happy to correct the fault.

 

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