Please explain, function to add string to first line of a file - powershell

Question

Would you like to explain what is happing in the PowerShell code at the bottom of this post?
I got my first, lets say "hello world" in PowerShell, and it needed these lines of code. It works like a charm, but I am not sure what it does, exactly.
The questions starts at
$( ,$_; Get-Content $Path -ea SilentlyContinue) | Out-File $Path
So this is what I understand so far.
We create a function called Insert-Content. With the params (input that will be interpeted as a string and will be added to $path).
function Insert-Content {
param ( [String]$Path )
This is what the function does/processes:
process {
$( ,$_;
I am not sure what this does, but I guess it gets "the input" (the "Hello World" before the | in "Hello World" | Insert-Content test.txt).
And then we got -ea SilentylyContinue, but what does it do?
process {
$( ,$_; Get-Content $Path -ea SilentlyContinue) | Out-File $Path
It would be greatly appreciated if you could explain these two parts
$( ,$_;
-ea SilentylyContinue
Code needed/used: Add a string to the first line of a doc.
function Insert-Content {
param ( [String]$Path )
process {
$( ,$_;Get-Content $Path -ea SilentlyContinue) | Out-File $Path
}
}
"Hello World" | Insert-Content test.txt

Answer 1

process {...} is used for applying the code inside the scriptblock (the {...}) to each parameter argument that the function reads from a pipeline.
$_ is an automatic variable containing the current object. The comma operator , preceding the $_ converts the string value to a string array with a single element. It's not required, though. The code would work just as well with just $_ instead of ,$_.
Get-Content $Path reads the content of the file $Path and echoes it as to the success output stream as an array of strings (each line as a separate string).
The ; separates the two statements ,$_ and Get-Content $Path from each other.
| Out-File $Path writes the output back to the file $Path.
The subexpression operator $() is required to decouple reading the file from writing to it. You can't write to a file when a process is already reading from it, so the subexpression ensures that reading is completed before writing starts.
Basically this whole construct
$( ,$_;Get-Content $Path -ea SilentlyContinue) | Out-File $Path
echoes the input from the pipeline (i.e. the "Hello World") followed by the current content of the file $Path (effectively prepending the input string to the file content) and writes everything back to the file.
The -ea SilentlyContinue (or -ErrorAction SilentlyContinue) suppresses the error that would be thrown when $Path doesn't already exist.

Answer 2

The relevant section of code must be handled as a whole:
$( ,$_;Get-Content $Path -ea SilentlyContinue) | Out-File $Path
First, as others have said, -ea is the shortened version of -ErrorAction. -ErrorAction SilentlyContinue tells the cmdlet "Suppress any error messages and continue executing." See Get-Help about_Common_Parameters -ShowWindow.
Next, the $() is the sub-expression operator. It means "Evaluate what is between the parentheses as its own command and return the result(s)." See Get-Help about_Operators -ShowWindow.
This subexpression here is:
,$_;Get-Content $Path -ea SilentlyContinue
It contains two statements: ,$_ and Get-Content $Path -ea SilentlyContinue. The semicolon is just the end of statement identifier to separate the two.
,$_; is two kind of complex parts.
$_ is the special pipeline variable. It always contains whatever object is in the current pipeline. See Get-Help about_Automatic_Variables -ShowWindow for more about $_ (it's mostly used with ForEach-Object and Where-Object cmdlets, so check those out, too), and Get-Help about_pipelines -ShowWindow for more help with pipelines.
The comma here is the comma operator (see Get-Help about_Operators -ShowWindow again). It creates an array from the objects on either side. For example, 1,2,3 creates an array with three elements 1, 2, and 3. If you want a two item array, you can say 1,2.
What if you want a one item array? Well, you can't say 1, because Powershell will think you forgot something. Instead, you can say ,1.
You can test it with the -is operator:
PS C:\> 1,2,3 -is [Array]
True
PS C:\> 1 -is [Array]
False
PS C:\> ,1 -is [Array]
True
Why might you want to create a one item array? Well, if later on your code is assuming the item is an array, it can be useful. In early editions of Powershell, properties like .Count would be missing for single items.
For completeness, yes, I believe you could write:
$( #($_);Get-Content $Path -ea SilentlyContinue)
And I think you could rewrite this function:
function Insert-Content {
param ( [String]$Path )
process {
#Read from pipeline
$strings = #($_);
#Add content of specified file to the same array
$strings += Get-Content $Path -ea SilentlyContinue;
#Write the whole array to the file specified at $Path
$strings | Out-File $Path;
}
}
So this adds content from the pipeline to the start of a file specified by -Path.
It's also somewhat poor practice not to create a parameter for the pipeline object itself and define it. See... well, see all the topics under Get-Help "about_Functions*", but mostly the Advanced ones. This is an advanced topic.