I include an external .ps1 into antother .ps1:
foo.ps1:
.("C:\test\bar.ps1");
$obj = [bar]::new();
$obj.out();
bar.ps1:
class bar{
$output;
bar(){
$this.output = 1;
}
[void] out(){
write-host $this.output;
}
}
The first time I execute foo.ps1 in the Windows PowerShell ISE the output is "1", as expected.
Then I go to bar.ps1 and change $this.output = 1; to $this.output = 2;. After executing foo.ps1 again the output is still "1". When I change something in foo.ps1, like simply appending a new line, and execute it once again, the output becomes "2". Changing back, like removing the new line, will make an output of "1" again.
For me it looks like an caching issue. Is it possible to clear or disable the caching?
Thanks in advance!
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.
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
}
$a
Why so weired?
If you are not saving your files, the code is written down into the console window. If your file is saved, it is just executed. You will then see the execution path like
C:\Users\vics> C:\Users\vics\Documents\test.ps1
...
I realize the title is a little confusing but I couldn't figure out a better way to phrase it.
I have a Powershell script with a couple dozen functions. Currently, I have the exact same code in every function to format the output. Here's a snippet:
function function1 () {
do something...
output code here
}
function function2 () {
do something...
output code here
}
The output code is exactly the same. Being a fan of code deduplication, this is driving me crazy because every time I add a new function I have this template code that I have to apply. I've tried putting the entire script in a try/catch block and throwing the object that is output but I couldn't get it to work and this still requires coding in the same throw statement in every function.
Does anyone know of something I can do to have all of these functions in this script to automatically send their output to another function or am I just going to have to live with this?
If the functions have no parameters, you can use this simple solution:
function addOutputCode {
param($name)
$oldBody = (get-item function:$name).ScriptBlock
$newBody = {
param($computer)
$funcOutput = . $oldBody $computer
# some formatting
$funcOutput | % { 'FORMATTED: ' + $_ }
}.GetNewClosure()
Set-Item function:$name -value $newBody
}
As you can see the functions gets the body of the function and assignes new body with formatting code. You can try it, just copy & paste the code below.
# this is your file with defined functions
function f1 { param($c) 'this is test of ' + $c }
function f2 { $c.Length; 'this was length of $c' }
# now f1 and f2 would return unformatted data
# f1
# f2
# add formatting code
addOutputCode f1
addOutputCode f2
# now if you call f1 or f2, they return formatted data
# f1 comp1
# f2 comp2
I use trap to write errors to file, and want write line number where error ocured.
$_.Exception.StackTrace is not answer.
Where I can get line number of error? Maybe some predefined variable?
You can retrieve the line number from the InvocationInfo object on $_. For example, the script...
"Hello, World!"
function foo() {
trap [Exception] {
$_.InvocationInfo.ScriptLineNumber
$_.InvocationInfo.OffsetInLine
continue;
}
[reflection.assembly]::loadfrom("C:\")
}
foo
... generates the output:
Hello, World!
10
34
You should use $_.InvocationInfo properties, for example: ScriptName, ScriptLineNumber, OffsetInLine, Line.
For example to format position info in Visual Studio style:
trap {
Write-Host "$($_.InvocationInfo.ScriptName)($($_.InvocationInfo.ScriptLineNumber)): $($_.InvocationInfo.Line)"
}
It will write something like:
C:\TEMP\test2.ps1(8): Get-Item missing
Also, you can just use $_.InvocationInfo.PositionMessage, see this post:
How can I get powershell exception descriptions into a string?
If you just want to find an error line after the script has been executed, you can view $Error array. $Error[0] corresponds to the last error.
More details here.