common ways to pass state from cmdlet to cmdlet - powershell

Question

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

Answer 1

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