'Session' Parameter is null or empty in PowerShell script - session

Question

I'm very new to PowerShell (forgive my ignorance) and am trying to install a program remotely on multiple computers on my domain. Currently, I'm just trying to get it to work on one computer. The script below was found online and adapted for my needs. Yesterday it worked, but today it's complaining about the session parameter.
I don't fully understand "sessions", but I have ensured on the client machine that the winrm service is running and I have invoked Enable-PSRemoting -force.
Here's the script:
$computers = Get-Content "c:\tmpPS\computers.txt"
$rs = Get-PSSession
Get-PSSession | Get-Member
######
## Functions
################
foreach ($comp in $computers)
{
Write-Host "should work with $comp"
}
PushMSI
RemoteConnect
InstallMSI
Function PushMSI {
Write-Host "------------------------------------------------"
Write-Host "This will copy the MSI file from localhost c:\tmpPS\"
write-Host "------------------------------------------------"
Write-Host ""
Write-Host ""
foreach ($comp in $computers)
{
Copy-Item -path "c:\tmpPS\clientInstall.msi" -Destination \\$comp\c$\tmpPS
}
}
Function RemoteConnect
{
Write-Host "------------------------------------------------"
Write-Host "This will establish a PSSession with all computers in c:\temp\computers.txt"
write-Host "------------------------------------------------"
Write-Host ""
Write-Host ""
Get-Content C:\tmpPS\computers.txt | New-PSSession -ThrottleLimit 50
}
Function InstallMSI
{
Write-Host "------------------------------------------------"
Write-Host "This will Install UPS Update on all computers with an Established PSSession"
write-Host "------------------------------------------------"
Write-Host "After the Install PSSessions will be removed"
Write-Host ""
Invoke-Command -Session $rs -ScriptBlock {invoke-item "c:\tmpPS\ClientInstall.msi"}
}
Get-PSSession | Remove-PSSession
And here's the output:
PS C:\Users\Me> C:\tmpPS\remoteInstall.ps1
Get-Member : No object has been specified to the get-member cmdlet.
At C:\tmpPS\remoteInstall.ps1:3 char:17
+ Get-PSSession | Get-Member
+ ~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Get-Member], InvalidOperationException
+ FullyQualifiedErrorId : NoObjectInGetMember,Microsoft.PowerShell.Commands.GetMemberCommand
should work with eSignWin81.informa.local
------------------------------------------------
This will copy the MSI file from localhost c:\tmpPS\
------------------------------------------------
------------------------------------------------
This will establish a PSSession with all computers in c:\temp\computers.txt
------------------------------------------------
Id Name ComputerName State ConfigurationName Availability
-- ---- ------------ ----- ----------------- ------------
6 Session6 eSignWin81.i... Opened Microsoft.PowerShell Available
------------------------------------------------
This will Install UPS Update on all computers with an Established PSSession
------------------------------------------------
After the Install PSSessions will be removed
Invoke-Command : Cannot validate argument on parameter 'Session'. The argument is null or empty. Supply an argument that is not null or empty and then try
the command again.
At C:\tmpPS\remoteInstall.ps1:49 char:25
+ Invoke-Command -Session $rs -ScriptBlock {invoke-item "c:\tmpPS\ClientInstall.ms ...
+ ~~~
+ CategoryInfo : InvalidData: (:) [Invoke-Command], ParameterBindingValidationException
+ FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.PowerShell.Commands.InvokeCommandCommand

Answer 1

Try not to use Write-Host as it brakes the pipeline and kills puppy's.
I've tried to improve your script a little bit, so you understand the logic behind the structure better. It hasn't been tested, but it should do the trick.
# First parameters
[CmdletBinding()]
Param(
[ValidateScript({Test-Path $_ -PathType leaf})]
$ComputerList = "c:\tmpPS\computers.txt",
[ValidateScript({Test-Path $_ -PathType leaf})]
$MSI = "c:\tmpPS\clientInstall.msi"
)
# Then functions
Begin {
Function Copy-MSI {
foreach ($Com in $Computers) {
Copy-Item -path $MSI -Destination "\\$Com\c$\tmpPS"
}
}
Function Install-MSI {
foreach ($Com in $Computers) {
Enter-PSSession -ComputerName $Com
invoke-item "c:\tmpPS\ClientInstall.msi"
Exit-PSSession
}
}
}
# Then the actions
Process {
$Computers = Get-Content $ComputerList
Copy-MSI
Install-MSI
}
If you run this, you'll find the information you're looking for:
Get-Help Enter-PSSession