PowerShell pass a variable to background process [duplicate] - powershell

Question

within powershell I'd like to learn the best way to call a variable to a start job so I don't have to edit the script for each server as it will be specific based on the client I've placed my script on.
$Servername = 'Server1'
$pingblock = {
pathping $servername | Out-File C:\client\PS\ServerPing.TXT
}
start-job $pingblock
when I run my code above I just get a file with the help as if I forgot the specify the $servername.

Answer 1

Use the -ArgumentList parameter on Start-Job e.g.:
Start-Job -Scriptblock {param($p) "`$p is $p"} -Arg 'Server1'
In your case:
$pingblock = {param($servername) pathping $servername | Out-File C:\...\ServerPing.txt}
Start-Job $pingblock -Arg Server1

Answer 2

To complement Keith Hill's helpful answer with a PSv3+ alternative:
The $using: scope modifier can be used to reference the values of variables in the caller's scope inside the script block passed to Start-Job, as an alternative to passing arguments (by default, a script block executed as a background job does not see any of the caller's variables or other definitions):
$Servername = 'Server1'
Start-Job { "Target: " + $using:ServerName } | Receive-Job -Wait -AutoRemoveJob
The above yields:
Target: Server1
Note:
The same technique can be used with:
Invoke-Command for remote execution - see this question.
Start-ThreadJob, available by default in PowerShell (Core) v6+, installable on demand in Windows PowerShell.
ForEach-Object -Parallel, available in PowerShell (Core) v7+ only.
Note that, as with -ArgumentList (-Args), it is only variable values that are being passed, not the variables themselves; that is, you cannot modify the caller's variables.[1]
[1] However, the thread-based concurrency features - Start-ThreadJob and ForEach-Object Parallel - permit indirect modification, namely if the variable value at hand happens to be an instance of a (mutable) .NET reference type, such as a hashtable, in which case the object that that the variable "points to" can be modified (if it is mutable). Note that taking advantage of that requires additional, nontrivial effort to make the concurrent modifications thread-safe, such as by use of concurrent (synchronized) collections - see this answer - and/or explicit locking of individual objects - see this answer.

Answer 3

Some other ways, $args and $input. This goes for invoke-command too, which I think uses the same mechanism. The $input method works in an unexpected way with arrays.
start-job { $args[0] } -args hi | receive-job -wait -auto
hi
echo hi | start-job { $input } | receive-job -wait -auto
hi
echo hi there | start-job { $input.gettype() } | receive-job -wait -auto
IsPublic IsSerial Name BaseType
-------- -------- ---- --------
False False <GetReadEnumerator>d__20 System.Object
For arrays, it's probably better to use a foreach-object (%) loop instead, so it runs on each array item in parallel. See also start-threadjob or foreach-object -parallel in powershell 7. There's actually no -throttlelimit option to start-job, so use with care.
echo yahoo.com facebook.com |
% { $_ | start-job { test-netconnection $input } } |
receive-job -wait -auto | select * -exclude runspaceid,pssourcejob* | ft
ComputerName RemoteAddress ResolvedAddresses PingSucce
eded
------------ ------------- ----------------- ---------
yahoo.com 74.6.143.25 {74.6.143.25,...} True
facebook.com 31.13.71.36 {31.13.71.36} True