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How does the PowerShell Pipeline Concept work?
(3 answers)
Closed 5 years ago.
I'm writing a PowerShell script that can take 2 parameters - Computer name or Serial number - and output some information about the corresponding computer. However, when I pass a value in from the pipeline, it always goes into the wrong parameter.
Here's my code:
Param
(
[Parameter(ValueFromPipeline=$true,ValueFromPipelineByPropertyName=$True)]
[String]$ComputerName,
[Parameter(ValueFromPipeline=$true,ValueFromPipelineByPropertyName=$True)]
[String]$SerialNumber
)
Then it uses those values to pull information from SCCM.
However, if I use the following line to call the script:
"<ComputerName>" | .\Get-SerialNumber.ps1 -ComputerName $_
The computer name gets put into the SerialNumber parameter instead of the ComputerName parameter.
If I put in
"<SerialNumber>" | .\Get-SerialNumber.ps1 -SerialNumber $_
The value gets passed into the ComputerName parameter. What is going on here? Why are they being passed backwards?
If I type in
.\Get-SerialNumber.ps1 -SerialNumber "<SerialNumber>"
or
.\Get-SerialNumber.ps1 -ComptuerName "<ComputerName>"
it works fine. It only screws up when values are passed through the pipeline.
For the record, I've tried to restart my PowerShell session to make sure something weird wasn't happening in my session, and even tried on another computer entirely. It still happens the same way.
The way you're calling these doesn't make sense. If you pass in a ComputerName via the pipeline, and then also specify -ComputerName as a named parameter, of course it binds the pipeline to the only other parameter that accepts pipeline input. How did you expect to accept two different values for the same parameter? How would you use it?
Related
I am trying to call a PS script via batch file, like so
Powershell.exe -file "C:\Scripts\Blah\Blah\Blah.ps1" -webUID "usernameValue" -webPWD "passwordValue" -Param "param value" -Param2 "param 2 value"
The issue seems to be the batch file is confusing Param and Param2. It thinks I am setting Param2 twice however Param and Param2 are separate parameters altogether. Has anyone experienced this? Is there perhaps a way to explicitly state the param names? Thanks
Param block
# Parameters
Param
(
[string]$WebUID,
[string]$WebPWD,
[string]$Param,
[string]$Param2
)
In an effort to support concise command-line use, PowerShell's "elastic syntax" allows specifying unambiguous prefix substrings of parameter names so that you only need to type as much of a parameter name as is necessary to identify it without ambiguity;
e.g., typing -p to refer to -Path is enough, if no other parameters start with p.
However, an exact match is always recognized, so that specifying -Param in your case should unambiguously match the -Param parameter, even though its full name happens to be a prefix substring of different parameter -Param2.
If the problem were an issue of ambiguity (it isn't), you'd see a different error message. For instance, were you to use the ambiguous -Para, you'd see:
Parameter cannot be processed because the parameter name 'para' is ambiguous. Possible matches include: -Param -Param2.
Instead, the wording of your error message suggests that the exact same parameter name - -Param2 - was indeed specified more than once - even though your sample code doesn't show that.
I've tested the behavior in PSv2 and PSv5.1 / 6.0 alpha 10 - it's conceivable, however, that other versions act differently due to a bug. Do let us know.
Consider an alternative approach:
If you invoked your script from within PowerShell, you could use a single, array-valued parameter - e.g. [string[]] $Params - and then simply pass as many parameters as needed, comma-separated, without needing to specify a distinct parameter name for each value.
Sadly, when invoking a script from outside of PowerShell, this approach won't work, because passing arrays isn't supported from the outside.
There is a workaround, however:
Declare the array-valued parameter decorated with [parameter(ValueFromRemainingArguments=$true)]
Invoke the script with the parameters as a space-separated list at the end of the command.
Applied to your scenario:
If your script defined its parameters as follows:
Param
(
[string]$WebUID,
[string]$WebPWD,
[parameter(ValueFromRemainingArguments=$true)]
[string[]] $Params
)
You could then invoke your script as follows:
Powershell.exe -file "C:\Scripts\Blah\Blah\Blah.ps1" `
-webUID "usernameValue" `
-webPWD "passwordValue" `
"param value" "param 2 value"
and $Params would receive an array of values: $Params[0] would receive param value, and $Params[1] would receive param 2 value.
Note that when calling from outside of PowerShell:
you must not use parameter name -Params in the invocation - just specify the values at the end.
you must not use , to separate the values - use spaces.
I'm no guru, but this looks like it's related to "Partial Parameters" and "Parameter Completion". See this article for more information.
Simply changing Param to Param1 should fix the issue.
Let's take the PowerShell statement below as an example:
powershell.exe c:\temp\windowsbroker.ps1 IIS
Is it possible to have it scripted within windowsbroker.ps1 to check for that IIS string, and if it's present to do a specific install script? The broker script would be intended to install different applications depending on what string followed it when it was called.
This may seem like an odd question, but I've been using CloudFormation to spin up application environments and I'm specifying an "ApplicationStack" parameter that will be referenced at the time when the powershell script is run so it knows which script to run to install the correct application during bootup.
What you're trying to do is called argument or parameter handling. In its simplest form PowerShell provides all arguments to a script in the automatic variable $args. That would allow you to check for an argument IIS like this:
if ($args -contains 'iis') {
# do something
}
or like this if you want the check to be case-sensitive (which I wouldn't recommend, since Windows and PowerShell usually aren't):
if ($args -ccontains 'IIS') {
# do something
}
However, since apparently you want to use the argument as a switch to trigger specific behavior of your script, there are better, more sophisticated ways of doing this. You could add a Param() section at the top of your script and check if the parameter was present in the arguments like this (for a list of things to install):
Param(
[Parameter()]
[string[]]$Install
)
$Install | ForEach-Object {
switch ($_) {
'IIS' {
# do something
}
...
}
}
or like this (for a single option):
Param(
[switch]$IIS
)
if ($IIS.IsPresent) {
# do something
}
You'd run the script like this:
powershell "c:\temp\windowsbroker.ps1" -Install "IIS",...
or like this respectively:
powershell "c:\temp\windowsbroker.ps1" -IIS
Usually I'd prefer switches over parameters with array arguments (unless you have a rather extensive list of options), because with the latter you have to worry about spelling of the array elements, whereas with switches you got a built-in spell check.
Using a Param() section will also automatically add a short usage description to your script:
PS C:\temp> Get-Help windowsbroker.ps1
windowsbroker.ps1 [-IIS]
You can further enhance this online help to your script via comment-based help.
Using parameters has a lot of other advantages on top of that (even though they probably aren't of that much use in your scenario). You can do parameter validation, make parameters mandatory, define default values, read values from the pipeline, make parameters depend on other parameters via parameter sets, and so on. See here and here for more information.
Yes, they are called positional parameters. You provide the parameters at the beginning of your script:
Param(
[string]$appToInstall
)
You could then write your script as follows:
switch ($appToInstall){
"IIS" {"Install IIS here"}
}
I am creating my own set of cmdlets. They all need the same state data (like location of DB and credentials for connecting to DB). I assume this must be a common need and wonder what the common idiom for doing this is.
the obvious one is something like
$db = my-make-dbcreds db=xxx creds=yyyy ...
my-verb1 $db | my-verb2 $db -foo 42...
my-verb8 $db bar wiz
.....
but i was wondering about other ways. Can I silently pipe the state from one to another. I know I can do this if state is the only thing I pipe but these cmdlets return data
Can I set up global variables that I use if the user doesnt specify state in the command
Passing the information state through the pipe is a little lost on me. You could update your cmdlets to return objects that the next cmdlet will accept via ValueFromPipeline. When you mentioned
like location of DB and credentials for connecting to DB
the best this I could think that you want is....
SPLATTING!
Splatting is a method of passing a collection of parameter
values to a command as unit. Windows PowerShell associates
each value in the collection with a command parameter.
In its simplest form
$params = #{
Filter = "*.txt"
Path = "C:\temp"
}
Get-ChildItem #params
Create a hashtable of parameters and values and splat them to the command. The you can edit the table as the unique call to the cmdlet would allow.
$params.Path = "C:\eventemperor"
Get-ChildItem #params
I changed the path but left the filter the same. You also dont have to have everything in $params you splat and use other parameters in the same call.
It is just a matter of populating the variables as you see fit and changing them as the case requires.
Spewing on the pipeline
Pretty that is what it is actually called. If you use advanced function parameters you can chain properties from one cmdlet to the next if you really wanted to. FWIW I think splatting is better in your case but have a look at the following.
function One{
param(
[parameter(Mandatory=$true,
ValueFromPipeline=$True,
ValueFromPipelineByPropertyName=$true)]
[String[]]
$Pastry
)
write-host "You Must Like $Pastry"
Write-Output (New-Object -TypeName PSCustomObject -Property #{Pastry= $pastry})
# If you have at least PowerShell 3.0
# [pscustomobject]#{Pastry= $pastry}
}
Simple function that writes the variable $pastry to the console but also outputs an object for the next pipe. So running the command
"Eclairs" | One | One | Out-Null
We get the following output
You Must Like Eclairs
You Must Like Eclairs
We need to pipe to Out-Null at the end else you would get this.
Pastry
------
{Eclairs}
Perhaps not the best example but you should get the idea. If you wanted to extract information between the pipe calls you could use Tee-Object.
"Eclair" | One | Tee-Object -Variable oneresults | One | Out-Null
$oneresults
Consider Parameter Default Values
Revisiting this concept after trying to find a better way to pass SQL connection information between many function working against the same database. I am not sure if this is the best thing to do but it certainly simplifies thing for me.
The basic idea is to add a rule for your cmdlet or wildcard rule if your cmdlets share a naming convention. For instance I have a series of functions that interact with our ticketing system. They all start with Get-Task.... and all configured with SQL connection information.
$invokeSQLParameters = #{
ServerInstance = "serverName"
Username = $Credentials.UserName
Password = $Credentials.GetNetworkCredential().Password
}
$PSDefaultParameterValues.Add("New-Task*:Connection",$invokeSQLParameters)
$PSDefaultParameterValues.Add("Get-Task*:Connection",$invokeSQLParameters)
So now in my functions I have a parameter called Connection that will always be populated with $invokeSQLParameters as long as the above is done before the call. I still use splatting as well
Invoke-Sqlcmd -Query $getCommentsQuery #Connection
You can read up more about this at about_parameters_default_values
I've got a wrapper powershell script that I'm hoping to use to automate a few things. It's pretty basic, and accepts a parameter that I want the script to run as if it were a line in the script. I absolutely cannot get it to work.
example:
param( [string[]] $p)
echo $p
# Adds the base cmdlets
Add-PSSnapin VMware.VimAutomation.Core
# Add the following if you want to do things with Update Manager
Add-PSSnapin VMware.VumAutomation
# This script adds some helper functions and sets the appearance. You can pick and choose parts of this file for a fully custom appearance.
. "C:\Program Files (x86)\VMware\Infrastructure\vSphere PowerCLI\Scripts\Initialize-VIToolkitEnvironment.ps1"
$p
In the example above, I want $p to execute as if it were a line in the script. I know this isn't secure, and that's probably where the problem lies.
Here is how I try running the script and passing in a parameter for $p:
D:\ps\test>powershell -command "D:\ps\test\powershell_wrapper.ps1" 'Suspend-VM servername -Verbose -Confirm:$False'
How can I get my parameter of 'Suspend-VM servername -Verbose -Confirm:$False' to run inside my script? If I just include the value in the script instead of pass it in as a parameter it runs without any issues...
You can basically approach this two ways, depending on what your needs really are and how you want to structure your code.
Approach #1 - Invoke-Expression
Invoke-Expression basically allows you to treat a string like an expression and evaluate it. Consider the following trivial example:
Invoke-Expression '{"Hello World"}'
That will evaluate the string as if it were an expression typed in directly, and place the string "Hello World" on the pipeline. You could use that to take your string parameter and run it on-the-fly in your script.
Approach #2 - Using a ScriptBlock
PowerShell has a special data type called a ScriptBlock, where you can bind a script to a variable, and then invoke that script as part of your code. Again, here is a trivial example:
function Test-SB([ScriptBlock]$sb) {
$sb.Invoke()
}
Test-SB -sb {"Hello World"}
This example creates a function with a single parameter $sb that is of type ScriptBlock. Notice the parameter is bound to the actual chunk of code {"Hello World"}? That code is assigned to the $sb parameter, and then a call to the .Invoke method actually executes the code. You could adapt your code to take in a ScriptBlock and invoke it as part of your script.
Approach #3 - Updating your profile
OK, so I said there were two ways to approach it. There is actually a third... sort of... You could add the VMWare cmdlets to your $profile so they are always present and you don't need your wrapper to load in those libraries. Granted, this is a pretty big hammer - but it might make sense if this is the environment you are constantly working in. You could also just create a shortcut to PowerShell that runs a .ps1 on startup that includes those libraries and hangs around (this is what MS did with the SharePoint admin shell and several others). Take a look at this TechNet page to get more info on the $profile and if it can help you out:
http://msdn.microsoft.com/en-us/library/windows/desktop/bb613488.aspx
I have a PowerShell script with a number of 'params' at the start:
param(
[switch] $whatif,
[string] $importPath = $(Read-Host "Full path to import tool"),
[string] $siteUrl = $(Read-Host "Enter URL to create or update"),
[int] $importCount = $(Read-Host "Import number")
)
Is there any way I can run this against an answer file to avoid entering the parameter values every time?
I am not getting the reason for the question. All you have to do to call your script is something like:
.\script.ps1 -whatif -importPath import_path -siteUrl google.com -importCount 1
The Read-Host are there as defaults, to be executed ( and then read and assign the values to the parameters ) only if you don't specify the values. As long you have the above comand ( saved in a file so that you can copy and paste into console or run from another script or whatever ), you don't have to enter the values again and again.
Start by setting the function or script up to accept pipeline input.
[CmdletBinding(SupportsShouldProcess=$True,ConfirmImpact='Low')]
param(
[Parameter(Mandatory=$True,ValueFromPipelineByPropertyName=$True)]
[string] $importPath,
[Parameter(Mandatory=$True,ValueFromPipelineByPropertyName=$True)]
[string] $siteUrl,
[Parameter(Mandatory=$True,ValueFromPipelineByPropertyName=$True)]
[int] $importCount
)
Notice that I removed your manually-created -whatif. No need for it - I'll get to it in a second. Also note that Mandatory=$True will make PowerShell prompt for a value if it isn't provided, so I removed your Read-Host.
Given the above, you could create an "answer file" that is a CSV file. Make an importPath column, a siteURL column, and an importCount column in the CSV file:
importPath,siteURL,importCount
"data","data",1
"x","y",2
Then do this:
Import-CSV my-csv-file.csv | ./My-Script
Assuming your script is My-Script.ps1, of course.
Now, to -whatif. Within the body of your script, do this:
if ($pscmdlet.shouldprocess($target)) {
# do whatever your action is here
}
This assumes you're doing something to $target, which might be a path, a computer name, a URL, or whatever. It's the thing you're modifying in your script. Put your modification actions/commands inside that if construct. Doing this, along with the SupportsShouldProcess() declaration at the top of the script, will enable -whatif and -confirm support. You don't need to code those parameters yourself.
What you're building is called an "Advanced Function," or if it's just a script than I guess it'd be an "Advanced Script." Utilizing pipeline input parameters in this fashion is the "PowerShell way of doing things."
To my knowledge, Powershell doesn't have a built-in understanding of answer files. You'll have to pass them in somehow or read them yourself from the answer file.
Wrapper. You could write another script that calls this script with the same parameters you want to use every time. You could also make a wrapper script that reads the values from the answer file, then pass them in.
Optional Parameters. Or you could change the parameters to use defaults that indicate no parameters were passed, then check for a file of a specific name to read values from. If the file isn't found, then prompt for the values.
If the format of the answer file is flexible, (i.e., you're only going to be using it with this Powershell script), you could get much closer to the behavior of an actual answer file by writing it as a Powershell script itself and dot-sourcing it.
if (test-path 'myAnswerfile'){
. 'myAnswerFile'
#process whatever was sourced from the answer file, if necessary
} else {
#prompt for values
}
It still requires removing the Read-Host calls from the parameters of the script.
Following on from Joel you could set up a different parameter set, based around the switch -answerfile.
If that's set the function will look for an answer file and parse though it - as he said you'll need to do that yourself. If it's not set and the others are then the functionis used with the parameters given. Minor benefit I see is that you can still have the parameters mandatory when used that way.
Matt