There is a difference between single and double quotes. (I am using PowerShell 4).

You can do this (as Benjamin said):

$name = 'Slim Shady'
Write-Host 'My name is'$name
-> My name is Slim Shady

Or you can do this:

$name = 'Slim Shady'
Write-Host "My name is $name"
-> My name is Slim Shady

The single quotes are for literal, output the string exactly like this, please. The double quotes are for when you want some pre-processing done (such as variables, special characters, etc.)

So:

$name = "Marshall Bruce Mathers III"
Write-Host "$name"
-> Marshall Bruce Mathers III

Whereas:

$name = "Marshall Bruce Mathers III"
Write-Host '$name'
-> $name

(I find How-to: Escape characters, Delimiters and Quotes good for reference).

When you read a string that contains a variable name out of a file and into a variable, you are going to need something other than double quotes to cause string expansion. There is a tool that's relevant here. It's called ExpandString. Take a look at this sample code:

$selhost = 'spr-it-minion'

selhost is offline!'
ExecutionContext.InvokeCommand.ExpandString(c

What's going on here is that first I have given $selhost and $b literal values, similar to the ones you would read out of the files you are using. Of course, $b isn't right, because the reference to $selhost isn't resolved, as outlined in the accepted answer. But $c gets the value produced by expanding $selhost, which you can use in your output.

I'll leave applying this to your case as a coding exercise.

So I find this post somewhat confusing and amusing. Steven's answer is a good one but seems to not have gotten through somehow so I will try to just throw some code out there and hope something sticks. If not, at least I tried right?

Copyfunction Append-BarToString {
    param(
        [string]$InputString
    )
    # explicit return keyword not needed.  
    # Any output inside function not assigned  
    # to a variable or to $null will be sent out
    $InputString + "Bar" 
}

# function can also be written without param block 
# more like C# like this, though it is unconventional
function Append-BarToString ([string]$InputString)
{
    # $InputString + "Bar"
    # or 
    "${InputString}Bar"
}

$someVar = 'foo'
$someOtherVar = Append-BarToString $someVar


$someOtherVar
# or
Write-Host $someOtherVar

# output
# fooBar
# fooBar

A FOR loop can provide the path to the file. If the default directory sorting order is not the result needed, specify additional command line switches on the DIR command.

FOR /F "delims=" %F IN ('DIR /B') DO (SET "THE_FILE=%~fF")
ECHO THE_FILE is "%THE_FILE%"

In a .bat file script, double the percent characters on FOR loop variables.

FOR /F "delims=" %%F IN ('DIR /B') DO (SET "THE_FILE=%%~fF")
ECHO THE_FILE is "%THE_FILE%"

The .bat file scripts can also run PowerShell scripts. It is best practice to not use aliases such as ls in scripts.

FOR /F "delims=" %%F IN ('powershell -NoLogo -NoProfile -Command ^
    "(Get-ChildItem -File | Select-Object -Last 1).FullName"') DO (SET "THE_FILE=%%~fF")
ECHO THE_FILE is "%THE_FILE%"