I have a quite big project with many functions in there.
Just 2 questions:
What would be here the "best practice" regarding Error-Handling? To use a local handling per Function, or use only one Error-Logging in the Main-Section?
The tricky part (!), let's have a look at the strange Error behaviour in the function F_XXX: only $_ delivers an error message, $Error[0] is here empty! Strange enough, when I start the function F_XXX separately (cut out from the module), it behaves as expected, it means: $Error[0] gives an error back. The code:
Blockquote
$ErrorActionPreference = "Stop"
Function F1
{
try
{
# do something
}
catch
{
# 1. cascade Error to Main?
# throw $Error[0].InnerException
# or
# local Error-Logging?
write-MyErrorLogging -message $Error[0].InnerException
}
}
Function F2
{
try
{
# do something
}
catch
{
# 1. cascade Error to Main?
# throw $Error[0].InnerException
# or
# local Error-Logging?
write-MyErrorLogging -message $Error[0].InnerException
}
}
Function F_XXXXXX
{
try
{
cls
write-host "The install data is copied.."
$share = "\\my_wrong_path\sql_sources\"
Copy-Item $share -Destination $installDrive -Force -Recurse
}
catch
{
$Error[0] #here is nothing!
$null -eq $Error[0] # here true
$_.Exception # only here the error-message: wrong path!
}
}
Blockquote
# here Main
try
{
F1
F2
F_XXXXXX
}
catch
{
write-MyErrorLogging -message $Error[0].InnerException
}
Blockquote
Inside a catch block, it's best to avoid $Error[0], given that the error at hand is reliably reflected in the automatic $_ variable.
If you do need access to previous errors via the automatic $Error variable, use $global:Error inside modules - see the bottom section for details.
Unless you need to perform additional actions when an error occurs, you can let a script-terminating (fatal) error (which your $ErrorActionPreference = "Stop" statement turns all errors in your code into) bubble up the call stack until it is either caught by a try / catch statement or, in the absence of one, terminates the entire call stack (i.e., the scripts and its callers).
If you do need to perform additional actions, use try / catch, and place the actions inside the catch block (as well as potential cleanup actions in a finally block), followed by re-throwing the error simply by calling throw without an argument.
Thus, you can make do with a single try / catch statement in the top-level scope of your script:
# Turn all errors in this and descendant scopes into
# script-terminating (fatal) ones.
$ErrorActionPreference = 'Stop'
# ... other function definitions, without try / catch
# Top-level code that calls the other functions and catches
# any errors.
try
{
F1
F2
F_XXXXXX
}
catch
{
write-MyErrorLogging -message $_.InnerException
}
The automatic $Error variable in modules:
Strangely, up to at least PowerShell 7.2.3 (current as of this writing):
Errors occurring in modules - just like ones occurring outside modules - are recorded in the $Error variable that exists in the global scope.
However, a seemingly unused, module-local copy of $Error exists, which shadows the global variable.
The workaround is to use use $global:Error from inside modules.
The behavior suggests a bug, given that the module-local copy is seemingly never touched and serves no apparent purpose.
I can write the following code in javascript:
function sum(num1, num2) {
return num1 + num2;
}
and then get a value
var someNum = sum(2,5);
I would like to do the same thing in Powershell, but I read the following guide:
PowerShell also knows the return keyword; however, it follows a
different logic. In general, the purpose of return is to end the
execution of a code section and to give the control back to the parent
block.
If you add a parameter to the return statement, the value will indeed
be returned to the calling subroutine. However, this also applies for
all other statements with an output. This means that any output
produced in the function will be stored in the variable together with
the return parameter.
I want to do this for the sake of having pure functions. However, it seems doing
var someNum = sum(2,5);
is entirely redundant, when I can just call the function above, define someNum inside of it, and it will be available in the global scope.
Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?
A bit tangential, but here is my actual code:
function GetPreviousKeyMD5Hashes() {
$query = "SELECT Name, MD5, executed FROM [AMagicDb].[dbo].cr_Scripts";
$command = New-Object System.Data.SQLClient.SQLCommand;
$command.Connection = $connection;
$command.CommandText = $query;
try {
$reader = $command.ExecuteReader();
while ($reader.Read()) {
$key = $reader.GetString(1)
$previousScripts.Add($key) | Out-Null
}
$reader.Close();
Write-Output "$(Get-Date) Finished querying previous scripts"
}
catch {
$exceptionMessage = $_.Exception.Message;
Write-Output "$(Get-Date) Error running SQL at with exception $exceptionMessage"
}
}
and then:
$previousScripts = New-Object Collections.Generic.HashSet[string];
GetPreviousKeyMD5Hashes;
This code isn't clear to me at all - running GetPreviousKeyMD5Hashes does set $previousScripts, but this is entirely unclear to whoever modifies this after me. My only other alternative (afaik) is to have all this in line, which also isn't readable.
is entirely redundant, when I can just call the function above, define someNum inside of it, and it will be available in the global scope.
No: functions execute in a child scope (unless you dot-source them with .), so variables created or assigned to inside a function are local to it.
Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?
Yes: The implicit output behavior only applies to statements whose output is neither captured - $var = ... - nor redirected - ... > foo.txt
If there are statements that happen to produce output that you'd like to discard, use $null = ... or ... > $null
Note: ... | Out-Null works in principle too, but will generally perform worse, especially in earlier PowerShell versions - thanks, TheIncorrigible1.
If there are status messages that you'd like to write without their becoming part of the output, use Write-Host or, preferably Write-Verbose or, in PSv5+, Write-Information, though note that the latter two require opt-in for their output to be visible in the console.
Do NOT use Write-Output to write status messages, as it writes to the success output stream, whose purpose is to output data ("return values").
See this answer of mine for more information about PowerShell's output streams.
The equivalent of your JavaScript code is therefore:
function sum($num1, $num2) {
Write-Host "Adding $num1 and $num2..." # print status message to host (console)
$num1 + $num2 # perform the addition and implicitly output result
}
PS> $someNum = sum 1 2 # NOTE: arguments are whitespace-separated, without (...)
Adding 1 and 2... # Write-Host output was passed through to console
PS> $someNum # $someNum captured the success output stream of sum()
3
Am I missing something or is it possible to write pure functions in Powershell that don't return everything inside the function?
You can't have your cake and eat it too...
If you have no out put in your function, then it is "pure" like you desire. If you have output, that also becomes part of the return.
You can use [ref] params. See below for example.
function DoStuff([ref]$refObj)
{
Write-Output "DoStuff: Enter"
$refObj.Value += $(1 + 2)
$refObj.Value += "more strings"
Write-Output "DoStuff: Exit"
}
$refRet = #()
$allRet = DoStuff([ref]$refRet)
"allRet"
$allRet
"refRet"
$refRet
"`n`nagain"
$allRet = DoStuff([ref]$refRet)
"allRet"
$allRet
"refRet"
$refRet
Note: Powershell doesn't need semicolons at the end of each statement; only for separating multiple statements on the same line.
Whenever possible, it's a good idea to avoid changing global state within a function. Pass input as parameters, and return the output, so you aren't tied to using the function in only one way. Your sample could look like this:
function sum
{
param($num1,$num2)
return $num1+$num2
}
$somenum=sum 2 5
Now, with Powershell, the return statement isn't needed. The result of every statement that isn't otherwise assigned, captured, redirected, or otherwise used, is just thrown in with the return value. So we could replace the return statement above with simply
$num1+$num2
You're already making use of this in your code with:
$previousScripts.Add($key) | Out-Null
where you are discarding the result of .Add(). Otherwise it would be included in the return value.
Personally, I find using return to explicitly mark the return value makes it easier to read. Powershell's way of putting all if the output in the return caused a lot of trouble for me as I was learning.
So, the only fixes to your code I would make are:
Move $previousScripts = New-Object Collections.Generic.HashSet[string] to inside the function, making it local.
Add return $previousScripts to the end of the function.
I need an example that use try..catch..finally clause where the finally is NECESSARY vs try..catch clause where finally is optional. The example below only demonstrated that finally is optional because with or without it, it won't make any different to my output.
My Example (note: $ErrorActionPreference is Continue):
try {
$value = 5 / 0
} catch {
Write-Output "illegal operation"
}
$t = Get-Date
Write-Output ("operation is done at " + "$t")
The reason is I need to know where finally clause become necessary vs just put finally clause no matter what.
A finally clause is just a logical construct saying "this statement or group of statements should always be run at the end of the try block, regardless of whether there was an error or not". It tells people reading the code that there is a logical connection between the code in the try and finally blocks (e.g. opening and closing a database connection). However, beyond that there is no essential difference between
try {
5 / 0
} catch {
'illegal operation'
}
'continued'
and
try {
5 / 0
} catch {
'illegal operation'
} finally {
'continued'
}
You can find some discussion of the subject here.
I think the only way it would make a difference is if you return or exit in the try block:
try {
'foo' # <-- displayed
exit
} finally {
'bar' # <-- displayed
}
'baz' # <-- not displayed
but maybe something like that is just bad design.
The powershell ise sometimes prints out my source code, if I have:
function f
{
$a=2
}
$a
It prints:
C:\Users\vics> function f
{
$a=2
}
$a
Why so weired?
If you are not saving your files, the code is written down into the console window. If your file is saved, it is just executed. You will then see the execution path like
C:\Users\vics> C:\Users\vics\Documents\test.ps1
...
How can I retrieve the name of the function that is currently running in powershell? Here is an example of what I want:
Function write-FunctionName
{
write-host "The name of this function is: *SomethingGoesHereButWhat?*"
}
Then when I execute it, it will display this:
>write-FunctionName
The name of this function is: write-FunctioName
>
Can this be done? If so how?
The $MyInvocation variable contains information about whatever is currently executing:
Function write-FunctionName
{
write-host ("The name of this function is: {0} " -f $MyInvocation.MyCommand)
}
For more information, see get-help about_automatic_variables, or the technet site here.