Powershell – How to pass variables with the Invoke-Command cmdlet

invoke-commandpowershellvariables

I have to get Event-Logs from some servers and I don't want to read in the credentials for each server which is found.

I've tried to pass my variables by using the ArgumentList parameter but I doesn't work.

This is my code:

$User = Read-Host -Prompt "Enter Username"
$Password = Read-Host -Prompt "Enter Password" -AsSecureString
$BSTR = [System.Runtime.InteropServices.Marshal]::SecureStringToBSTR($Password)
$UnsecurePassword = [System.Runtime.InteropServices.Marshal]::PtrToStringAuto($BSTR)

Get-ADComputer -Filter "OperatingSystem -Like '*Server*'" | Sort-Object Name |
ForEach-Object{
    if($_.Name -like '*2008*'){
        Invoke-Command -ComputerName $_.Name -ArgumentList $User, $UnsecurePassword -ScriptBlock {  
            net use P: \\Server\dir1\dir2 /persistent:no /user:$User $UnsecurePassword
            Get-EventLog -LogName System -After (Get-Date).AddHours(-12) -EntryType Error, Warning | format-list | 
            out-file P:\EventLog_$env:COMPUTERNAME.log
            net use P: /delete /yes
        }
    }
}

How can I use the variables in the Invoke-Command ScriptBlock?

Best Answer

Alternatively you can use the $Using:scope. See Example 5 under this link.

Example:

$servicesToSearchFor = "*"
Invoke-Command -ComputerName $computer -Credential (Get-Credential) -ScriptBlock { Get-Service $Using:servicesToSearchFor }

With $Using: you don't need the -ArgumentList parameter and the param block in the scriptblock.

TLDR

JavaScript has lexical (also called static) scoping and closures. This means you can tell the scope of an identifier by looking at the source code.

The four scopes are:

Global - visible by everything
Function - visible within a function (and its sub-functions and blocks)
Block - visible within a block (and its sub-blocks)
Module - visible within a module

Outside of the special cases of global and module scope, variables are declared using var (function scope), let (block scope), and const (block scope). Most other forms of identifier declaration have block scope in strict mode.

Overview

Scope is the region of the codebase over which an identifier is valid.

A lexical environment is a mapping between identifier names and the values associated with them.

Scope is formed of a linked nesting of lexical environments, with each level in the nesting corresponding to a lexical environment of an ancestor execution context.

These linked lexical environments form a scope "chain". Identifier resolution is the process of searching along this chain for a matching identifier.

Identifier resolution only occurs in one direction: outwards. In this way, outer lexical environments cannot "see" into inner lexical environments.

There are three pertinent factors in deciding the scope of an identifier in JavaScript:

How an identifier was declared
Where an identifier was declared
Whether you are in strict mode or non-strict mode

Some of the ways identifiers can be declared:

var, let and const
Function parameters
Catch block parameter
Function declarations
Named function expressions
Implicitly defined properties on the global object (i.e., missing out var in non-strict mode)
import statements
eval

Some of the locations identifiers can be declared:

Global context
Function body
Ordinary block
The top of a control structure (e.g., loop, if, while, etc.)
Control structure body
Modules

Declaration Styles

var

Identifiers declared using var have function scope, apart from when they are declared directly in the global context, in which case they are added as properties on the global object and have global scope. There are separate rules for their use in eval functions.

let and const

Identifiers declared using let and const have block scope, apart from when they are declared directly in the global context, in which case they have global scope.

Note: let, const and var are all hoisted. This means that their logical position of definition is the top of their enclosing scope (block or function). However, variables declared using let and const cannot be read or assigned to until control has passed the point of declaration in the source code. The interim period is known as the temporal dead zone.

function f() {
    function g() {
        console.log(x)
    }
    let x = 1
    g()
}
f() // 1 because x is hoisted even though declared with `let`!

Function parameter names

Function parameter names are scoped to the function body. Note that there is a slight complexity to this. Functions declared as default arguments close over the parameter list, and not the body of the function.

Function declarations

Function declarations have block scope in strict mode and function scope in non-strict mode. Note: non-strict mode is a complicated set of emergent rules based on the quirky historical implementations of different browsers.

Named function expressions

Named function expressions are scoped to themselves (e.g., for the purpose of recursion).

Implicitly defined properties on the global object

In non-strict mode, implicitly defined properties on the global object have global scope, because the global object sits at the top of the scope chain. In strict mode, these are not permitted.

eval

In eval strings, variables declared using var will be placed in the current scope, or, if eval is used indirectly, as properties on the global object.

Examples

The following will throw a ReferenceError because the namesx, y, and z have no meaning outside of the function f.

function f() {
    var x = 1
    let y = 1
    const z = 1
}
console.log(typeof x) // undefined (because var has function scope!)
console.log(typeof y) // undefined (because the body of the function is a block)
console.log(typeof z) // undefined (because the body of the function is a block)

The following will throw a ReferenceError for y and z, but not for x, because the visibility of x is not constrained by the block. Blocks that define the bodies of control structures like if, for, and while, behave similarly.

{
    var x = 1
    let y = 1
    const z = 1
}
console.log(x) // 1
console.log(typeof y) // undefined because `y` has block scope
console.log(typeof z) // undefined because `z` has block scope

In the following, x is visible outside of the loop because var has function scope:

for(var x = 0; x < 5; ++x) {}
console.log(x) // 5 (note this is outside the loop!)

...because of this behavior, you need to be careful about closing over variables declared using var in loops. There is only one instance of variable x declared here, and it sits logically outside of the loop.

The following prints 5, five times, and then prints 5 a sixth time for the console.log outside the loop:

for(var x = 0; x < 5; ++x) {
    setTimeout(() => console.log(x)) // closes over the `x` which is logically positioned at the top of the enclosing scope, above the loop
}
console.log(x) // note: visible outside the loop

The following prints undefined because x is block-scoped. The callbacks are run one by one asynchronously. New behavior for let variables means that each anonymous function closed over a different variable named x (unlike it would have done with var), and so integers 0 through 4 are printed.:

for(let x = 0; x < 5; ++x) {
    setTimeout(() => console.log(x)) // `let` declarations are re-declared on a per-iteration basis, so the closures capture different variables
}
console.log(typeof x) // undefined

The following will NOT throw a ReferenceError because the visibility of x is not constrained by the block; it will, however, print undefined because the variable has not been initialised (because of the if statement).

if(false) {
    var x = 1
}
console.log(x) // here, `x` has been declared, but not initialised

A variable declared at the top of a for loop using let is scoped to the body of the loop:

for(let x = 0; x < 10; ++x) {} 
console.log(typeof x) // undefined, because `x` is block-scoped

The following will throw a ReferenceError because the visibility of x is constrained by the block:

if(false) {
    let x = 1
}
console.log(typeof x) // undefined, because `x` is block-scoped

Variables declared using var, let or const are all scoped to modules:

// module1.js

var x = 0
export function f() {}

//module2.js

import f from 'module1.js'

console.log(x) // throws ReferenceError

The following will declare a property on the global object because variables declared using var within the global context are added as properties to the global object:

var x = 1
console.log(window.hasOwnProperty('x')) // true

let and const in the global context do not add properties to the global object, but still have global scope:

let x = 1
console.log(window.hasOwnProperty('x')) // false

Function parameters can be considered to be declared in the function body:

function f(x) {}
console.log(typeof x) // undefined, because `x` is scoped to the function

Catch block parameters are scoped to the catch-block body:

try {} catch(e) {}
console.log(typeof e) // undefined, because `e` is scoped to the catch block

Named function expressions are scoped only to the expression itself:

(function foo() { console.log(foo) })()
console.log(typeof foo) // undefined, because `foo` is scoped to its own expression

In non-strict mode, implicitly defined properties on the global object are globally scoped. In strict mode, you get an error.

x = 1 // implicitly defined property on the global object (no "var"!)

console.log(x) // 1
console.log(window.hasOwnProperty('x')) // true

In non-strict mode, function declarations have function scope. In strict mode, they have block scope.

'use strict'
{
    function foo() {}
}
console.log(typeof foo) // undefined, because `foo` is block-scoped

How it works under the hood

Scope is defined as the lexical region of code over which an identifier is valid.

In JavaScript, every function-object has a hidden [[Environment]] reference that is a reference to the lexical environment of the execution context (stack frame) within which it was created.

When you invoke a function, the hidden [[Call]] method is called. This method creates a new execution context and establishes a link between the new execution context and the lexical environment of the function-object. It does this by copying the [[Environment]] value on the function-object, into an outer reference field on the lexical environment of the new execution context.

Note that this link between the new execution context and the lexical environment of the function object is called a closure.

Thus, in JavaScript, scope is implemented via lexical environments linked together in a "chain" by outer references. This chain of lexical environments is called the scope chain, and identifier resolution occurs by searching up the chain for a matching identifier.

Find out more.

Powershell – How to handle command-line arguments in PowerShell

You are reinventing the wheel. Normal PowerShell scripts have parameters starting with -, like script.ps1 -server http://devserver

Then you handle them in param section in the beginning of the file.

You can also assign default values to your params, read them from console if not available or stop script execution:

 param (
    [string]$server = "http://defaultserver",
    [Parameter(Mandatory=$true)][string]$username,
    [string]$password = $( Read-Host "Input password, please" )
 )

Inside the script you can simply

write-output $server

since all parameters become variables available in script scope.

In this example, the $server gets a default value if the script is called without it, script stops if you omit the -username parameter and asks for terminal input if -password is omitted.

Update: You might also want to pass a "flag" (a boolean true/false parameter) to a PowerShell script. For instance, your script may accept a "force" where the script runs in a more careful mode when force is not used.

The keyword for that is [switch] parameter type:

 param (
    [string]$server = "http://defaultserver",
    [string]$password = $( Read-Host "Input password, please" ),
    [switch]$force = $false
 )

Inside the script then you would work with it like this:

if ($force) {
  //deletes a file or does something "bad"
}

Now, when calling the script you'd set the switch/flag parameter like this:

.\yourscript.ps1 -server "http://otherserver" -force

If you explicitly want to state that the flag is not set, there is a special syntax for that

.\yourscript.ps1 -server "http://otherserver" -force:$false

Links to relevant Microsoft documentation (for PowerShell 5.0; tho versions 3.0 and 4.0 are also available at the links):