Prof. Powershell

Express Yourself with the Invoke-Expression Cmdlet

The Invoke-Expression cmdlet: Turn it on and you can do almost anything.

• By Jeffery Hicks
• 09/21/2010

Windows PowerShell is not too difficult to operate. You type a command and something happens. Usually you are running PowerShell cmdlets, scripts, functions or even non-PowerShell commands like ipconfig.

But maybe you'd like to define a command, and run it later. Or perhaps construct a command on the fly. To run these commands you'll need to turn to the Invoke-Expression cmdlet. Here's a very simple example:

PS C:\> $c="arp -a"
PS C:\> invoke-expression $c

Interface: 192.168.56.1 --- 0x20
  Internet Address Physical Address  Type
  192.168.56.255   ff-ff-ff-ff-ff-ff static
  224.0.0.22       01-00-5e-00-00-16 static
  224.0.0.252      01-00-5e-00-00-fc static
  224.0.1.60       01-00-5e-00-01-3c static
  255.255.255.255  ff-ff-ff-ff-ff-ff static

I defined a variable, $c, that contains a command string. Whenever I want to run it, I call Invoke-Expression. You can also use the alias iex. But be careful. Suppose you want to take this expression and save it as a string so you can use Invoke-Expression:

get-service | where {$_.status -eq "Running"}

You'll need to enclose this is single quotes:

PS C:\> $c=' get-service | where {$_.status -eq "Running"}'
PS C:\> iex $c | measure-object

Count : 68
Average :
Sum :
Maximum :
Minimum :
Property :

You can also construct an expression, and include variables:

PS C:\> $c="get-wmiobject"
PS C:\>$c+=' -class $class'

These two statements combine to create the expression get-wmiobject -class $class. This is intentionally a very simple example to make my point. Let's continue:

PS C:\> $class="win32_bios"
PS C:\> iex $c

SMBIOSBIOSVersion : 2ACN21WW
Manufacturer : LENOVO
Name : Rev 1.0
SerialNumber :
Version : LENOVO - 6040000

I can reset $class, and re-invoke $c anytime I want.

PS C:\> $class="win32_computersystem"
PS C:\> iex $c

Domain : WORKGROUP
Manufacturer : LENOVO
Model : S10-3
Name : QUARK
PrimaryOwnerName : Jeff
TotalPhysicalMemory : 2136391680

Finally, you can also pipe one or more expressions to Invoke-Expression, and the cmdlet will execute each one:

PS C:\> "arp -a","nbtstat -n" | iex | out-file c:\work\stats.txt

With this expression I executed two commands and saved the results to a text file.

Most of the time, you know what command you want to run and simply do it. But when you might need to construct a command on the fly or execute something arbitrarily, take a look at Invoke-Expression.

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