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.
- WOL – A Scenario for Employing Invoke-Expression
- Example PowerShell Script Using Invoke-Expression
- Invoke-Expression Using a String in a PS1 File
- More Members of the Invoke Family
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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
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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Encouraging computers to sleep when they’re not in use is a great idea – until you are away from your desk and need a file on that remote sleeping machine!
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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.
