Powershell pass variable to start-job

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.

Prolix

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

Judaism

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.}

Habitue

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

Carchemish