PowerShell cmdLet: Invoke-Expression

Executing a string as PowerShell command

invoke-expressions seems to be the equivalent of a statement that in other programming languages might be called eval: it executes the content of a string variable as PowerShell command.

The following example creates a string whose content is write-output (3*14) and then passes it to invoke-expression. Thus, when run, the example prints 42:

$num_1     =  3
$num_2     = 14
$operation = '*'

invoke-expression "write-output ($num_1 $operation $num_2)"

Github repository about-PowerShell, path: /cmdlets/expression/invoke/eval.ps1

Executing the commands in a file

invoke-expression can also be used to execute the commands that are stored in a file (that is: a script).

The following script uses split-path $myInvocation.myCommand.path to determine the directory where it is located and assigns the value to the variable $dir. It then uses $dir to execute a powershell that is located in $dir and is named commands.ps1. Note, that the file extension .ps1 is not required when using the -command parameter:

$dir = split-path $myInvocation.myCommand.path

invoke-expression -command "$dir/commands.ps1"

Github repository about-PowerShell, path: /cmdlets/expression/invoke/source.ps1

I am not sure if this construct allows to do something that is not possible just using the call operator (&).

Combining with get-content

Here's a file with a simple expression:

$THE_NUMBER = 7 * $someNumber

Github repository about-PowerShell, path: /cmdlets/expression/invoke/an-expression.txt

The content of this file is read using get-content and then piped into invoke-expression:

$someNumber = 6

get-content an-expression.txt | invoke-expression

write-output "The number is: $THE_NUMBER"

Github repository about-PowerShell, path: /cmdlets/expression/invoke/get-content.ps1

When executed, it prints

The number is: 42

Piping the contents of a file into invoke-expression

An expression that is stored in a file can be read with get-content and then piped into invoke-expression:

$dict = get-content -raw expression.txt | invoke-expression

"num = $($dict.num)"
"txt = $($dict.txt)"

Github repository about-PowerShell, path: /cmdlets/expression/invoke/pipe-into/pipeline.ps1

The file with the expression:

@{
  num = 42
  txt = 'hello world'
}

Github repository about-PowerShell, path: /cmdlets/expression/invoke/pipe-into/expression.txt

Passing a multi-line string

In order to evaluate an expression that is stored in a multi-line string, the ends of the continued lines need to be escaped twice:

set-strictMode -version latest

function func($p_1, $p_2, $p_3) {
   write-host "p_1 = $p_1"
   write-host "p_2 = $p_2"
   write-host "p_3 = $p_3"
}

invoke-expression "func ``
  -p_2   2              ``
  -p_1   one            ``
  -p_3   III
"

Github repository about-PowerShell, path: /cmdlets/expression/invoke/multiline.ps1

Applying a transformation on each element of an array

invoke-expression is handy to apply a transformation on each element of an array:

$hex_numbers = '0a', 'ff', '10'

$numbers     = $hex_numbers | foreach-object { invoke-expression "0x$_" }

write-host $numbers

Github repository about-PowerShell, path: /cmdlets/expression/invoke/apply-on-array-elements.ps1