Start-Process with Process Priority in Loop doesn't recognize variable - powershell

Question

I'm using FFmpeg with PowerShell.
I have a loop that goes through a folder of mpg files and grabs the names to a variable $inputName.
FFmpeg then converts each one to an mp4.
Works
Batch Processing
$files = Get-ChildItem "C:\Path\" -Filter *.mpg;
foreach ($f in $files) {
$inputName = $f.Name; #name + extension
$outputName = (Get-Item $inputName).Basename; #name only
ffmpeg -y -i "C:\Users\Matt\Videos\$inputName" -c:v libx264 -crf 25 "C:\Users\Matt\Videos\$outputName.mp4"
}
Not Working
Batch Processing with Process Priority
$files = Get-ChildItem "C:\Path\" -Filter *.mpg;
foreach ($f in $files) {
$inputName = $f.Name; #name + extension
$outputName = (Get-Item $inputName).Basename; #name only
($Process = Start-Process ffmpeg -NoNewWindow -ArgumentList '-y -i "C:\Users\Matt\Videos\$inputName" -c:v libx264 -crf 25 "C:\Users\Matt\Videos\$outputName.mp4"' -PassThru).PriorityClass = [System.Diagnostics.ProcessPriorityClass]::AboveNormal;
Wait-Process -Id $Process.id
}
If I set the Process Priority using Start-Process PriorityClass, the $inputName variable is no longer recognized.
Error:
C:\Users\Matt\Videos\$inputName: No such file or directory

Answer 1

Lets go over a few basic things.
In powershell we love piping |, It allows use to pass the information from one command to another command.
A good example of this is the ForEach you have.
Instead of Foreach($F in $Files) you can pipe | into a foreach-object
Get-ChildItem "C:\Path\" -Filter *.mpg | Foreach-Object{
$_
}
When Piping | a command powershell automatically creates the variable $_ which is the object that is passed in the pipe |
The next thing is there are 2 types of quotes " and '.
If you use ' then everthing is taken literally. Example
$FirstName = "TestName"
'Hey There $FirstName'
Will return
Hey There $FirstName
While " allows you to use Variables in it. Example
$FirstName = "TestName"
'Hey There $FirstName'
Will return
Hey There TestName
Now one last thing before we fix this. In powershell we have a escape char ` aka a tick. Its located beside the number 1 on the keyboard with the tilde. You use it to allow the use of char that would otherwise break out of the qoutes. Example
"`"Hey There`""
Would return
"Hey There"
OK so now that we covered the basics lets fix up the script
Get-ChildItem "C:\Users\Matt\Videos\" -Filter *.mpg -File | Foreach-Object{
($Process = Start-Process ffmpeg -NoNewWindow -ArgumentList "-y -i `"$($_.FullName)`" -c:v libx264 -crf 25 `"C:\Users\Matt\Videos\$($_.Name)`"" -PassThru).PriorityClass = [System.Diagnostics.ProcessPriorityClass]::AboveNormal;
Try{
Wait-Process -Id $Process.id
}catch{
}
}
In the case above I changed
Add -File to the Get-ChildItem to designate that you only want Files returned not folders
Pipe | into a Foreach-Object
Changed the Outside Brackets in the -ArgumentList to be double quotes " instead of literal quotes '
Removed the $InputName and $OutputName in favor of the Foreach-Object variable $_