invoke-command

Powershell – Debugging Techniques

This blog discuses some high level Debugging Techniques while working with Powershell. The contents of this blog are taken from the book – “Learn WINDOWS POWERSHELL, In a month of Lunches” By, Don Jones (All the credit goes to Don)

I am writing this blog, because we do not have enuf content on the web that discusses about the “Debugging” or “Troubleshooting” approaches while working with Powershell. And Don’s book helped me a lot in my personal endeavour. Now it is my turn to give it back to the Powershell Community.

First Computer Bug

There are two broad categories of bugs in the Powershell world. Firstly, the syntax errors and secondly, logic errors.

Syntax Errors

A Syntax Error means you typed something wrong. It might be a command name that you misspelled (Gte-Content instead of Get-Content) or it might be that you got the actual syntax of the command wrong (forgetting the hyphen between the verb and noun of a cmdlet name, or using a colon instead of a space to separate a parameter’s name and value). Whatever the cause, correcting your typing will solve the problem.

Powershell will usually tell you, in explicit detail, where the problem is. Powershell might not know what the problem is, but it will usually get pretty close to the location of the error. Example, here is a syntactically incorrect command and the resulting error message:

get-content

That is pretty clear: We used a parameter named -file, and Powershell could not find one. The error occurred on the line 1 of my  command and at character position 18.

Here is a checklist that will help you in overcoming the Syntax Errors:

  •  Make sure you typed the cmdlet name correctly. If you used an alias, make sure it is typed correctly, and that it points to the cmdlet you think it does.
  • Make sure parameter names are preceded by a dash and are followed by a space. Make sure you are using the correct parameter name (read the help!) and if you are abbreviating the parameter name, make sure you are providing enough characters to uniquely identify that parameter.
  • Most of the Powershell’s punctuation comes in pairs: single quotes, double quotes, square brackets, curly braces, and parenthesis are all good examples. Improper nesting, like ({this)}, means you are ending a pair before ending the pair it encloses.
  • Watch your spaces. In Powershell, spaces are special characters that indicate a separation between command elements. Powershell is not that case-sensitive, but it is very space-sensitive. There is a space in between parameter names and values. There is a space after a cmdlet name and before any parameter or values. There is a space after one parameter and before the next. Do not forget those.

Red Alert

The RED Text usually freaks us out. You can change the colour of the error message text to green.

(Get-Host).PrivateData.ErrorForegroundColor = 'green'

That only lasts for the duration of the shell session, so we can either put that in a profile script or we can do it before start working in the shell.

Logic Errors

Logic errors mean that your script or command is not doing what you want it to do, but it is not necessarily generating an error.

Some logic error will provide straightforward errors. You should know what to do with a  “file not found” error, or an “access denied” message, but sometimes errors are not always so clear, Get-WmiObject, for example, can produce an “RPC server not found” error if it is not able to locate a remote computer, etc.,

But the most vexing logic errors are the ones that do not produce any error at all – they just prevent your script or command from working properly. Check the below code snippet:

$name = Read-Host "Enter computer name"
if (Test-Connection $name) {
    Get-WmiObject Win32_BIOS -ComputerName $nmae
}

Logic errors, like syntax errors, can come from typos and one of such errors is listed in the above code snippet.

Debugging causes a lot of frustration, and we need to know the below things before we debug any script or command:

  • You cannot debug a script or command unless you have a clear expectation of what it is going to do.
  • You must execute your script an examine its reality (what  it actually does), and compare that reality to your expectations. When reality and your expectations differ, you have found the bug.
  • While executing the script and examining it, you need to read very, very carefully, so that you can spot typos. Sometimes using a different font can help.

Identifying your expectations

If you do not know what a command or a script should do, then you can never debug it. We are using the same code snippet as above to see the process involved.

$name = Read-Host "Enter computer name"

I expect that this will display “Enter Computer name:” on the screen, and allow me to type something. What ever I type will be stored in the variable $name.

if (Test-Connection $name) {

My expectation is that this will run the Test-Connection command, passing it the computer name in $name. I expect that this will ping the computer name I typed. I see that this is enclosed in an IF construct, so I expect that Test-Connection must return a True or False value. So, if the computer can be pinged, it will return True, and whatever is inside the IF construct will execute next.

Get-WmiObject Win32_BIOS -ComputerName $name

I expect that this will return Get-WmiObject and retrieve the Win32_BIOS class. We know that -class is the first parameter, so the Win32_BIOS will be fed to the -class parameter. I see that the -computername parameter is also specified, and it is being passed the computer name from the $name variable. Oh, wait – there is a typo. See, just a careful read-through of the script found a problem. I am going to leave the typo in there, though, and pretend that I was not being so careful with it (for now). I want to show you some other ways that you might have found it.

}

That closes the IF construct. With my expectations written down, it is time to start seeing where they differ from reality.

Adding Trace Code

The first trick is to add trace code to the script. It all starts with a helpful command called Write-Debug, which simply takes a message that you want it to display.

Write-Debug "Test Message

If you are following along, you will notice that Write-Debug will not produce any output.

Write-Debug actually sends your message to an alternate pipeline called Debug pipeline. Powershell has several of these alternate pipelines: Error, Warning, Debug and so forth. By default, the Debug pipeline’s switch  is set to SilentlyContinue, which is the same as OFF. The result is that all Write-Debug messages are suppressed.

Lets go ahead and change the setting in the script by adding this to the top:

$DebugPreference = "Continue"

Now we are free to add Write-Host statements to our script. Below is the revised script:

$DebugPreference = "Continue"

$name = Read-Host "Enter computer name"
Write-Debug "`$name contains $name"

if (Test-Connection $name) {
Write-Debug "Test-Connection was True"
    Get-WmiObject Win32_BIOS -ComputerName $nmae
} else {
    Write-Debug "Test-Connection was False"
}
You can see that I have added a Write-Debug to the inside of the IF construct, I even added an Else potion to the construct, containing a third Write-Debug message. That way, no matter which way the IF construct’s logic goes, I’ll see some output and know what is happening inside the script.
If you run the command, you will get the below error:
typo
That is where you will realize that there is a typo in that $name variable. Powershell is clearly telling us that there is a problem with the -computername parameter; if we look carefully at just that portion of the script, the $nmae typo is more obvious.
After fixing that and running  the script again:
notypo
That looks like what we want.
Now, we need to test the opposite situation, what happens when we provide a computer name that is not valid?
testfailed
That is certainly not what we are looking for. That is a logic error: the Test-Connection cmdlet clearly is not doing what I expected, which was to return a simple True or False value.
Stepping out of the script and just work with the Test-Connection from the command line:
tesst-connection
failagainOkay, that is definitely not what I expected. When I used the command with a valid computer name, I get back a table of results, not a True or False value. When I use it with an invalid computer name, I still get an error.
Reading the help, Help Test-Connection -full. I see that the command “returns the echo response replies”. The help also says that, “unlike the traditional ‘ping’ command, Test-Connection returns a Win32_PingStatus Object… bot you can use the -quiet parameter to force it to return only a Boolean value.” Yes, Please !
Looking at the -quiet parameter, I see it “supresses all errors and return $True if any pings succeed and $False if all failed.” That is what I want, so I will modify the script accordingly.
$DebugPreference = "Continue"

$name = Read-Host "Enter computer name"
Write-Debug "`$name contains $name"

if (Test-Connection $name -Quiet) {
Write-Debug "Test-Connection was True"
    Get-WmiObject Win32_BIOS -ComputerName $name
} else {
    Write-Debug "Test-Connection was False"
}
Running the script again, testing it with both a valid and invalid computer name. Below is the output:
works
In short:
  • Whenever I change the contents of a variable, I use Write-Debug to output the variable, just to I can check those contents.
  • Whenever I am going to read the value of a property or a variable, I use Write-Debug to output that property or variable, so that I can see what is going on inside the script.
  • Any time I have a loop or logic construct, I build it in such a way that I get a Write-Debug message no matter how the loop or logic works out.

Working with break points

Having to wade through a lot of debug messages is cumbersome in lengthy /complex scripts. So we fall to use Breakpoints, to make the debugging process a bit easy.

A breakpoint is a defined area where a script will pause its execution, allowing you to examine the environment that the script is running within. Powershell can be configured to break when:

  • Your script reaches a certain line
  • A variable is read and/or changed
  • A specific command is executed

In the first instance, you must specify the script that you are referring to. In the second and third situations, you can choose to specify a script, and the breaking point will only be active for that script. If you don’t, the breakpoint will occur globally through out the shell when that variable is read or written, or that command is executed.

Example, suppose I want to have the script stop immediately after Line 1 finishes executing, meaning that I want to break before the Line 2. Then I will run the command:

Set-PSBreakpoint -Script E:\Work\Powershell\scripts\demo.ps1 -Line 2

breakpoint

I also want to be notified whenever the $name variable is accessed, so I will run the below command:

Set-PSBreakpoint -Script E:\Work\Powershell\scripts\demo.ps1 -Variable name -mode Read

anotherbreakpoint

Notice that the variable name is just “name” and not “$name”. Variable names do not  include the dollar sign; $ is just a cue to the shell telling it that you wish to use the contents of a variable.

With those two breakpoints set, I will run the script. After entering the computer name, the script will break. The shell modifies the command-line prompt, indicating that I am in a suspended mode. Here I can examine the contents of variables, execute commands and so on. When I am done, I can run Exit to resume script execution. Here is how it all works:

breakpointfinal

This gives me the chance to test commands, see what is inside variables or properties, and so forth, without having to add a  lot of write-debug commands.

When I am done debugging, I can remove the breakpoints:

Get-PSBreakpoint | Remove-PSBreakpoint

Breakpoints are also supported in Powershell ISE. To set a line breakpoint, move your cursor to the desired line and press F9.The ISE will visually indicate where line breakpoints occur, using a red highlight. If you run the script within the ISE, breakpoints lines will be highlighted in yellow when the script reaches one of those lines. At that time, you can hover your cursor over any variable to see a tooltip with the contents of that variable.

 

Powershell – Writing a Filtering Function

We are discussing on how to write a Powershell Function. This is part 2 of the three part series. Please check my blog on Parameterized function, which is the part 1 of this series.

The second type of function is called a Filtering Function or also called as Pipeline Function.

  • You can accept one kind of information through the pipeline. This might be computer names, processes, or any other kind of single kind of information.
  • What ever you accept through the pipeline can come as a single object, or multiple objects can be piped in. You will write one (or many) commands that execute against each piped-in object, no matter how many there are.
  • You can designate additional parameters for other input elements. The values provided to these parameters will be used for each execution of your commands.
Below is an example of a Filtering function:

function Get-ServerInfo {

BEGIN{}

PROCESS{
 $computername = $_

$os = Get-WmiObject Win32_OperatingSystem -ComputerName $computername

$disk = Get-WmiObject Win32_LogicalDisk -Filter "DeviceID='C:'" -ComputerName $computername


 $obj = New-Object -TypeName PSObject

$obj | Add-Member -MemberType NoteProperty -Name ComputerName -Value $computername
 $obj | Add-Member -MemberType NoteProperty -Name BuildNumber -Value ($os.BuildNumber)
 $obj | Add-Member -MemberType NoteProperty -Name SPVersion -Value ($os.ServicePackMajorVersion)
 $obj | Add-Member -MemberType NoteProperty -Name SysDriveFree -Value ($disk.free / 1MB -as [int])

Write-Output $obj
 
 }

END{}

}

Get-Content names.txt | Get-ServerInfo | Format-Table -AutoSize

Here is what we did:

  • We have added a BEGIN, PROCESS and END block
  • What ever is in the BEGIN block will execute the first time this function is called in the pipeline; the END block will execute when the function is almost finished.
  • The PROCESS script block will execute one time for each object that is piped into the function. (If you do not pipe any input then PROCESS block will execute once)
  • This script expects computer names to be piped in, so if you pipe four names, the PROCESS block will execute four times. Each time, the “$_” placeholder will be automatically populated with a new object from the pipeline.
  • The last line of the script shows how you would execute this function: pipe a bunch of string objects to it.

 

Adding a parameter to a filtering function

What if we want the cmdlet to accept computer names either from the pipeline or from a parameter? In other words, you want both of theses to work:

Get-Content names.txt | Get-ServerInfo

Get-ServerInfo -computername (Get-Content names.txt)

Rite now the function does not accept parameters, so lets include a parameter.

function Get-ServerInfo {

param (
 [String] $computername
 )

BEGIN{}

PROCESS{
 $computername = $_

$os = Get-WmiObject Win32_OperatingSystem -ComputerName $computername

$disk = Get-WmiObject Win32_LogicalDisk -Filter "DeviceID='C:'" -ComputerName $computername


 $obj = New-Object -TypeName PSObject

$obj | Add-Member -MemberType NoteProperty -Name ComputerName -Value $computername
 $obj | Add-Member -MemberType NoteProperty -Name BuildNumber -Value ($os.BuildNumber)
 $obj | Add-Member -MemberType NoteProperty -Name SPVersion -Value ($os.ServicePackMajorVersion)
 $obj | Add-Member -MemberType NoteProperty -Name SysDriveFree -Value ($disk.free / 1MB -as [int])

Write-Output $obj
 
 }

END{}
}

Get-Content names.txt | Get-ServerInfo | Format-Table -AutoSize

However, we run into a problem. The original way of running the command – which is included at the bottom if the script listing – will continue to work. But the other way of running the function does not work.

Get-ServerInfo -computername vaio

Get-WmiObject : Cannot validate argument on parameter 'ComputerName'. The argument is null or 
empty. Supply an argument that is not null or empty and then try the command again.
At line:12 char:65
+ $os = Get-WmiObject Win32_OperatingSystem -ComputerName $computername
+ ~~~~~~~~~~~~~
 + CategoryInfo : InvalidData: (:) [Get-WmiObject], ParameterBindingValidationExcepti 
 on
 + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.PowerShell.Commands.GetW 
 miObjectCommand
 
Get-WmiObject : Cannot validate argument on parameter 'ComputerName'. The argument is null or 
empty. Supply an argument that is not null or empty and then try the command again.
At line:14 char:87
+ ... -ComputerName $computername
+ ~~~~~~~~~~~~~
 + CategoryInfo : InvalidData: (:) [Get-WmiObject], ParameterBindingValidationExcepti 
 on
 + FullyQualifiedErrorId : ParameterArgumentValidationError,Microsoft.PowerShell.Commands.GetW 
 miObjectCommand

This is because, all of the code in the function lives within the PROCESS block. We are taking the computer name from the $_ placeholder, which is populated with an object from the pipeline input. Except that we did not pipe anything in, so the PROCESS block only executes once, and $_ never contains anything, so $computername never contains anything, so nothing works.

 

Breaking the function into two parts

So, to fix the issue, we need to break the function into two pieces. The main working part can stand alone and can be used whether you are getting input from the pipeline or from a parameter, called as worker function. The second part will be the public function that you want people to actually use. It’s job is to determine, where the input is coming from and to pass the one computer name at a time to the worker function.

We have two things to keep in mind:

  • When input comes from the pipeline, the shell will enumerate through the objects automatically, allowing you to work with one at a time. You can pass those objects, as they are processed, to the worker function.
  • When the input comes form a parameter, you may gave either one object or many objects, but the PROCESS script block will only execute once regardless. So you will have to manually enumerate the parameter so that you can get to each object, one at a time.

We have a Powershell’s built in variable called $PSBoundParameters, and it contains each parameter that was manually specified. It has a ContainsKey() method that will let you test to see if a particular parameter was used or not.

function GetServerInfoWork {
 param (
 [String]$computername
 )

$os = Get-WmiObject Win32_OperatingSystem -ComputerName $computername

$disk = Get-WmiObject Win32_LogicalDisk -Filter "DeviceID='C:'" -ComputerName $computername


 $obj = New-Object -TypeName PSObject

$obj | Add-Member -MemberType NoteProperty -Name ComputerName -Value $computername
 $obj | Add-Member -MemberType NoteProperty -Name BuildNumber -Value ($os.BuildNumber)
 $obj | Add-Member -MemberType NoteProperty -Name SPVersion -Value ($os.ServicePackMajorVersion)
 $obj | Add-Member -MemberType NoteProperty -Name SysDriveFree -Value ($disk.free / 1MB -as [int])

Write-Output $obj

}

function Get-ServerInfo {
 param(
 [String[]]$computername
 )

BEGIN{
 $usedParameter = $false
 if($PSBoundParameters.ContainsKey('computername')){
 $usedParameter = $true
 }

}

PROCESS{
 if($usedParameter) {
 foreach($computer in $computername) {
 GetServerInfoWork -computername $computer
 }else {
 GetServerInfoWork -computername $_
 }
 }

}

END{}
}

Get-ServerInfo -verbose -computername (Get-Content E:\Work\Powershell\scripts\names.txt.txt)
Get-Content E:\Work\Powershell\scripts\names.txt.txt | Get-ServerInfo | Format-Table -AutoSize

Here is what we did :

  • We started by pulling most of the function code into a worker function.
  • In the public function, we declared the parameter to accept multiple stings
  • In the BEGIN block, which execute first, we want to see  if the input is coming from the pipeline or via the parameter. We start by assuming that the input came from the pipeline, setting $usedParameter to $false. Then test the $PSBoundParameters variable, and if it does indeed contain the comptername key, then we know that the  -computername parameter was used, so we set the $usedParameter to $true.
  • Even though the $usedParameter is created in the BEGIN block, it will be accessible in the PROCESS and END block.
  • In the PROCESS block, we check the variable to see what to do.
  • We use a foreach loop to enumerate the parameter input, passing each object to the worker function one at a time.
  • If the input came from the pipeline, the PROCESS block is already handling the enumeration, so we let it do its job and pass the $_ placeholder’s contents to the workers function.

Powershell – InvokeCommand versus ComputerName

As IT Pros, we will be fetching a lot of data from remote computers. But, when we are working in the production environment, we have to pay attention to the load that is exerted by the powershell cmdlets on the machine from which we run the cmdlets.

Choosing the correct cmdlet can make a huge difference in the performance.

To demonstrate this, I am browsing thru 4000 lines of System EventLogs and then selecting only those with the eventId as 1014.

I have two servers. DC and Server1. I am running the commands from Sever1. The reason why I am going thru 4000 entries of EventLog is to have a considerable amount of traffic for the Measure-Command, since the cmdlet has one local system (Server1) and one remote computer (DC).

Now, try running the below cmdlets. If you do not have a set up like this, you may use the Microsoft Virtual Labs

Cmdlet 1:

Measure-Command {Get-EventLog system -ComputerName server1, dc -Newest 4000 | where {$_.EventID -eq 1014}}

Cmdlet 2:

Measure-Command {Invoke-Command -ComputerName dc,server1 -ScriptBlock {Get-EventLog system -Newest 4000 | where {$_.EventID -eq 1014}}}

And below is the result:

bothResults

If you pay attention to the number of seconds it took for each command to run, it is pretty clear that the “Invoke-Command” has a better performance compared to the cmdlet with “-ComputerName“.

This is because, by using the Invoke-Command, the processing is done locally in the remote computers. But, while using cmdlets with “-ComputerName“, all the data is first fetched to the local system (from where the cmdlet was fired) and then the processing is done locally.

We can see a difference of 4 seconds just for two computers (local and remote). But in reality we will be working with hundreds and thousands of servers, so the difference will be exponential.