Powershell: Force variable expansion

Question

How can I force string variable expansion?

I need to read a string with one or more variable names in it (a template) and then expand it after I read the file. The key is that I must read the contents of the file before I declare the variables that will be used in the expansion. I've tried several ways but I can't get it to work.

It's not an option to read the file after $environment is defined.

Contents of name.txt: $environment-RPT-INT

#example 1
$name = gc "c:\temp\name.txt"
$environment = "9065DEV" 
$expanded = $ExecutionContext.InvokeCommand.ExpandString($name)
$expanded

#example 2
$name = gc "c:\temp\name.txt"
$environment = "9065DEV" 
$expanded = $expanded = Invoke-Expression "`"$template`""
$expanded

#example 3
$name = gc "c:\temp\name.txt"
$environment = "9065DEV"
$name = $name.Clone()
$expanded = $ExecutionContext.InvokeCommand.ExpandString($name)
$expanded

Any help is appreciated.

Updated: Example 1 is now working for me.

Hello there! Are you saying $name contains the name of a variable that you want to get? If so, you can use Get-Variable -Name $Name -ValueOnly. If not, mind clarifying your desired goal with a little pseudo code? — Cookie Monster
– Cookie Monster, Commented Apr 11, 2014 at 20:49
I'm using Powershell v2 and your first example works for me, what version are you using and what is your output? — SpellingD
– SpellingD, Commented Apr 11, 2014 at 20:53
$name contains the contents of the file c:\temp\name.txt, which I specify is the string: $environment-RPT-INT — Matthew S
– Matthew S, Commented Apr 11, 2014 at 20:54

Jason Shirk · Accepted Answer · 2014-04-12 01:49:26Z

4

It looks like you've found some possible solutions, but I'll suggest another that is in my opinion a bit smarter and more robust.

Instead of requiring variable names in your text file, why not use format specifiers. For example, the contents of name.txt:

{0}-RPT-INT

And in your script:

$name = gc "c:\temp\name.txt"
$environment = "9065DEV" 
$expanded = $name -f $environment
$expanded

This way, you can rename the variable w/o changing any of your text files. As a bonus, if your text file comes from unknown sources, your script is vulnerable to code injection. For example, say you are given a text file with these contents:

$(rm -whatif -recurse -force c:\)-RPT-INT

Then ExpandString or Invoke-Expression will happily execute that code.

answered Apr 12, 2014 at 1:49

Jason Shirk

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user189198 · Accepted Answer · 2014-04-11 21:06:20Z

2

Your Invoke-Expression example is pretty close. Instead of using $template though, you need to use $name.

#example 2
$name = gc 'c:\temp\name.txt';
$environment = '9065DEV';
$expanded = Invoke-Expression -Command "`"$name`"";
$expanded;

answered Apr 11, 2014 at 21:06

user189198

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Walter Mitty · Accepted Answer · 2022-02-05 12:55:24Z

1

There is a function called ExpandString predefined in powershell. It's inside $ExecutionContext, as follows.

$mystring = @'
This is a here string with some embedded variables.

Here is variable foo -- $foo
Here is variable bar -- $bar
Here is variable bletch -- $bletch

'@

#This displays the here string as is.

$mystring

#now define foo, bar and bletch

$foo = 5
$bar = Get-Date
$bletch = "George Washington"

#now run the here string through Expandstring.

$ExecutionContext.InvokeCommand.ExpandString($mystring)

answered Feb 5, 2022 at 12:55

Walter Mitty

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Inventologist · Accepted Answer · 2019-09-02 15:30:09Z

0

If you are willing to store your Setting/Values in a CSV, I wrote a module to pull values from a CSV, put it into a HereString... any variables you put into the CSV become fully expanded inside the Here-String. That way, you can normally address the field names and values.

I might be able to change this to also work with traditional INI's if anyone is interested.

https://github.com/Inventologist?tab=repositories

Look for: HereStringFromCSV

answered Sep 2, 2019 at 15:30

Inventologist

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