PowerShell: Use undeclared variable in string-CodePudding

I'm trying to figure out how I can use undeclared variables in a string. This is a simplified example to demonstrate my issue

[string]$sentence="My dog is $age years old"
[int]$age = 6 
$sentence | write-output

$age = 3
$sentence | write-output

Looking for this output:

   My dog is 6 years old

   My dog is 3 years old

Getting this output:

   My dog is  years old

   My dog is  years old

I've tried the following:

  "My dog is `$age years old" #My dog is $age years old
  $("My dog is `$age years old") #My dog is $age years old
  $("My dog is $age years old") #My dog is 3 years old

Is it possible in any way to make this more compact? or is this the only way?

CodePudding user response：

I don't know how to do this with interpolation, but I can do it with the older .Net String.Format() method like this:

$sentence = "My dog is {0} years old."

$age = 6
[string]::Format($sentence, $age) | Write-Output
# => My dog is 6 years old

$age = 3
[string]::Format($sentence, $age) | Write-Output
# => My dog is 3 years old

[string]::Format($sentence, 12)   | Write-Output
# => My dog is 12 years old

Which we can shorten like this (thank you commenters):

$sentence = "My dog is {0} years old."

$age = 6
Write-Output ($sentence -f $age)
# => My dog is 6 years old

$age = 3
Write-Output ($sentence -f $age)
# => My dog is 3 years old

Write-Output ($sentence -f 12)
# => My dog is 12 years old

Also note the different placeholder in the template string.

CodePudding user response：

This answer shows an overcomplicated alternative to the nice answer from Joel Coehoorn. You can use a Script Block, to store an expression (including your $age variable) and then execute it.

$sentence = { "My dog is $age years old" }
$age = 6 
& $sentence | write-output # => My dog is 6 years old

$age = 3
& $sentence | write-output # => My dog is 3 years old

If you want your script block to "remember" the value of a variable at the moment it was created you can use it's .GetNewClosure() method, for example:

$expressions = foreach($i in 0..5) {
    { "Value of `$i was $i in this iteration" }.GetNewClosure()
}
$expressions.foreach{ & $_ }