I'm working on a function that accepts [Single]$DurationMS as an optional parameter. This is supposed to be float value. So in my function I have the following code to check if its been provided to the function. if it is provided I want to add the value to an object nested in another object.
if ($DurationMS -ne $null) {
$MyObject.attributes | Add-Member -MemberType NoteProperty -Name 'duration.ms' -Value $DurationMS
}
All looks fine except when I test it I get 0 in the duration and I can't figure out why.
duration.ms
-----------
0
so my condition is evaluating to true but I don't understand why.
[single] is a .NET value type, and instances of such types can never be $null.
[single].IsValueType returning $true tells you that it is a value type.
$null only applies to .NET reference types and tells you that is a reference to no object.
It is therefore pointless to test your [single]-typed $DurationMS parameter variable for being $null:
A [single] instance's default value is 0, so that your $DurationMS -ne $null conditional is effectively 0 -ne $null by default, which is $true.
The robust way to check if an argument was passed to a given (non-mandatory) parameter in a given invocation is to consult the automatic $PSBoundParameters variable, as Santiago Squarzon suggests.
This variable contains a dictionary that has entries for all explicitly passed arguments, keyed by their parameter names (sans prefix -); e.g., if your function is invoked with -DurationMS 1.2, $PSBoundParameters['DurationMS'] returns 1.2, and $PSBoundParameters.ContainsKey('DurationMS') indicates $true
Therefore:
# Was an argument passed to -DurationMS?
if ($PSBoundParameters.ContainsKey('DurationMS')) {
$MyObject.attributes |
Add-Member -MemberType NoteProperty -Name 'duration.ms' -Value $DurationMS
}
The following aspects are incidental:
if ($DurationMs) would only work if you also wanted to consider an explicit argument of 0 to signal "no value was provided", because with a [single]-typed $DurationMs, if ($DurationMs) is the same as if ($DurationMs -ne 0)
PowerShell allows you to use an expression of any type in a Boolean context; with numeric types, 0 maps to $false, and any nonzero value to $true.
While this implicit to-Boolean conversion behavior is generally convenient, it has its pitfalls - see the bottom section of this answer for a summary of the rules.
Given that many PowerShell operators can implicitly operate on arrays (collections) as the LHS - in which case they act as filters, returning the subarray of matching items - it is generally better to place a scalar comparison operand on the LHS (in the case at we know that the non-literal operand is by definition also a scalar - a [single] instance - so that doesn't matter).
Placing the scalar on the LHS avoids false positives / negatives, such as in the following example:
$arr = 0, $null
# !! -> 'null', because (0, $null) -ne $null filters the
# !! array to #(0), and [bool] #() - perhaps surprisingly - is $false
if ($arr -ne $null) { 'not null' } else { 'null' }
# OK, with $null on the LHS
# -> 'not null'
if ($null -ne $arr) { 'not null' } else { 'null' }
However, even on the LHS $null can exhibit unexpected behavior, namely with the -lt, -le, -gt, and -ge operators, as discussed in this answer; e.g.:
$null -lt 0 # !! -> $true - even though [int] $null yields 0
If PowerShell offered a dedicated test for $null, these pitfalls could be avoided; implementing such a test - in the form $var -is $null or $var -isnull - was the subject of GitHub PR #10704; unfortunately, that PR was abandoned by its creator, and no one has picked up the work since, which is why no such test exists as of PowerShell 7.2.2.
As Lee Dailey points out, a property name such as duration.ms can be problematic, given that it contains ., which normally suggests a nested property access, given that an (unquoted) . serves as the member-access operator.
Warning: Powershell novice, terms/code may be incorrect
Say I have a Powershell array with data from Invoke-Sqlcmd, say 10,000 "rows" with a half dozen (Col1-Col6) columns each. Say I want to find every row with an empty Col5. Right now I'm doing
foreach ($row in $sqlimport.col1)
{ $col5data = $sqlimport.Where({$_.col1 -eq $row}) | Select-Object -ExpandProperty Col5
if ( '' -eq $col5data ) {$col5data+1} else {} $col5data
Returns 1, which it should in my test. Code seems wrong and slow. It takes several minutes to run through. When something like
$sqlimport.Where({$_.col5 -eq 'somedatathatisthere'})
takes milliseconds
However,
$sqlimport.Where({$_.col5 -eq ''})
Returns blank
($sqlimport.Where({$_.col5 -eq ''})).Count
Returns 0
Right now, you're asking PowerShell to create an array consisting of all the values in column col1, and then for each iteration you search the entire array again to find the corresponding col5 value. That's entirely unnecessary.
Simply loop over the array itself:
foreach($row in $sqlimport){
if($row.Col5 -like ''){
Write-Host "Col5 is empty in row with id $($row.Col1)"
}
}
This only iterates over the entire array once.
Mathias' answer explains the problem with the inefficiency of your first attempt well and offers a solution that ultimately performs best, due to use of a (single) foreach statement (as opposed to the much slower ForEach-Object cmdlet).
However, a solution using a (single) .Where() array method call (or the related .ForEach() method):
is only slightly slower[1]
while being more concise and arguably conceptually more elegant.
It is what you tried with $sqlimport.Where({ $_.col5 -eq '' }), which, based on your feedback, requires only one small tweak to make it work:
Instead of -eq '' use -like '', which is a somewhat obscure[2] shortcut to testing for an empty string ('') and also for a $null value (as also used in Mathias' answer) - it is, in essence, the equivalent of .NET's [string]::IsNullOrEmpty() method.
# -like matches both '' and $null
$sqlimport.Where({ $_.col5 -like '' })
Note:
If you wanted to test for $null values only, use the following; note how $null is placed on the LHS, which is generally advisable for robust comparisons[3]:
$sqlimport.Where({ $null -eq $_.col5 })
As an aside: GitHub issue #10656 proposes introducing a simplified test for $null with something like -is $null; unfortunately, the associated PR has been abandonded.
As an aside: [string]-type-constrained PowerShell variables never store $null values: [string] $str = $null causes $str to contain '', not $null. However, [string]-typed property values not populated in PowerShell code can contain $null values, as in your case.
For passing a true $null value to a string-typed .NET API in PowerShell code, a special singleton must be used (passing $null directly would again result in ''): [NullString]::Value`
See this answer for background information.
[1] For a performance comparison of PowerShell's various enumeration (iteration) features, see the bottom section of this answer.
[2] The real purpose of the -like operator is to perform wildcard-expression matching. The shortcut relies on -like - which operates on (non-null) strings only - auto-converting non-string operands to strings, which in the case of $null causes conversion to '' (the empty string).
[3] To reliably test for $null, place it on the LHS of an -eq / -ne operation; e.g., $null -eq $var. If you place $null on the RHS - $var -eq $null - and $var happens to be a collection (such as an array), the return value is the array of matching elements, i.e. the array of those elements in $var whose value is $null, which is a different operation - see about_Comparison_Operators.
I'm trying to return TRUE from searching Get-ComplianceSearch's output for 'Completed'. My code below is a simple wait loop. But I don't think I'm returning the value correctly because the loop never finishes. I'm fairly new to PowerShell. Please assist or direct.
I'm using Powershell Core 7.1. There are no errors but the Search-String condition never returns TRUE.
try {
$timer = [Diagnostics.Stopwatch]::StartNew()
while (($timer.Elapsed.TotalSeconds -lt $Timeout) -and (-not (Get-ComplianceSearch -
Identity $searchName | Select-String 'Completed' -SimpleMatch -Quiet))) {
Start-Sleep -Seconds $RetryInterval
$totalSecs = [math]::Round($timer.Elapsed.TotalSeconds, 0)
Write-Verbose -Message "Still waiting for action to complete after [$totalSecs]
seconds..."
}
$timer.Stop()
if ($timer.Elapsed.TotalSeconds -gt $Timeout) {
throw 'Action did not complete before timeout period.'
} else {
Write-Verbose -Message 'Action completed before timeout period.'
}
} catch {
Write-Error -Message $_.Exception.Message
}
(This is the expected output of the command Get-ComplianceSearch)
Okay, you don't want to use Select-String here (although you can, see #mklement0's helpful answer, looking at object properties is usually preferred). That is returning an object and you want to check the Status property for "Completed". Make the following change to the -not subexpression:
(-not (Get-ComplianceSearch -Identity $searchName | Where-Object {
$_.Status -eq 'Completed'
}))
The above can be on one line but I broke it up for readability.
Basically, Select-String looks for content in strings. If you are looking for a particular value of an object property however, you can use Where-Object to test for a condition and return any objects matching that condition. In this case, we want to return any object that have a Status of 'Completed', so we can negate that in the if statement.
You (or others) might be wondering how this works since Where-Object returns matching objects, but not booleans. The answer is "truthiness". PowerShell objects are "truthy", which means anything can be evaluated as a [bool].
The following values evaluate to $false in most cases. I've included some gotchas to watch out for when relying on "truthy" values:
A numeric value of 0
A string value of 0 evaluates as $true
Empty arrays
Empty strings
A whitespace-only string or strings consisting only of non-printable characters evaluates as $true
$false
A string value of False evaluates as $true
Most everything else will evaluate to $true. This is also why comparison operators are syntactically optional when checking whether a variable is $null or not. Although there are times when an explicit value check is a good idea as comparison operators compare the actual values instead of only whether the variable "is" or "isn't".
How does this apply to the expression above then? Simple. if statements, always treat the condition expression as a [bool], no conversion required. In addition, logical operators and conditional operators also imply a boolean comparison. For example, $var = $obj assigns $obj to $var, but$var = $obj -eq $obj2 or $var = $obj -and $obj2 will assign $true or $false.
So knowing the above, if Where-Object returns nothing, it's $false. If it returns a tangible object, it's $true.
Bender the Greatest's helpful answer shows a better alternative to using Select-String, because OO-based filtering that queries specific properties is always more robust than searching string representations.
That said, for quick-and-dirty interactive searches, being able to search through a command's formatted display output can be handy, and, unfortunately, Select-String does not do that by default.
As for what you tried:
To make your Select-String work, you need to insert Out-String -Stream before the Select-String call, so as to ensure that the for-display representation is sent through the pipeline, line by line.
# `oss` can be used in lieu of `Out-String -Stream` in PSv5+.
# `sls` can be used in lieu of `Select-String`.
Get-ComplianceSearch | Out-String -Stream | Select-String 'Completed' -SimpleMatch -Quiet
Note:
If you want to search a for-display representation other than the default one, you can insert a Format-* cmdlet call before the Out-String -Stream segment; e.g.
Get-Item / | Format-List * | Out-String -Stream | Select-String ... would search through a list representation of all properties of the object output by Get-Item.
Perhaps surprisingly, Select-String does not search an input object's for-display representation, as you would see it in the console, using the rich formatting provided by PowerShell's display-formatting system.
Instead, it performs simple .ToString() stringification, whose results are often unhelpful and cannot be relied upon to include the values of properties. (E.g.,
#{ foo = 'bar' } | Select-String foo does not work as intended; it is equivalent to
#{ foo = 'bar' }.ToString() | Select-String foo and therefore to
'System.Collections.Hashtable' | Select-String foo
Arguably, Select-String should always have defaulted to searching through the input objects' formatted string representations:
That there is demand for this behavior is evidenced by the fact that PowerShell versions 5 and above (both editions) ship with the oss convenience function, which is a wrapper for Out-String -Stream.
GitHub issue #10726 asks that the current behavior of Select-String be changed to search the for-display string representations by default.
I just discovered that when you apply bool operators on a collection, it acts as a filter on that collection.
So the following will return all elements that are not null:
$objectArray = #('a','b','c','d')
$objectArray -ne $null
But how can I check if the collection's reference is null?
Trevor Sullivan's if () test forces the $objectArray to cast to a boolean.
[bool]$null #is $false
[bool]#(1,2,3) # is $true , so it looks good.
But empty arrays mislead it:
[bool]#() # is $false , so it's not an accurate test.
I suggest $null -eq $objectArray:
NB. It really opens the question of why you want to know if it's $null, specifically. Trevor's answer is typical and good enough for any common use.
NB. My answer includes an uncommon, but useful suggestion - when you have a literal value for one side of a comparison, put it on the left if you can.
0 -lt $counter
$null -eq $thing
"text" -eq $variable
4 -in $collection
It's less common, so looks less familiar, but it's more resilient against PowerShell implicit casting doing something you don't expect.
All you have to do is test the variable for $true or $false. If it's $false, then it's a null reference, otherwise the opposite is true.
if (!$objectArray) {
}
The following tells you if the reference is null:
[Object]::ReferenceEquals($objectArray, $null)
Testing if the variable is $true or $false does not always work because an empty collection will cast to false:
$objectArray = #()
if (!$objectArray) {
'The array is not actually null'
}
What Powershell pitfalls you have fall into? :-)
Mine are:
# -----------------------------------
function foo()
{
#("text")
}
# Expected 1, actually 4.
(foo).length
# -----------------------------------
if(#($null, $null))
{
Write-Host "Expected to be here, and I am here."
}
if(#($null))
{
Write-Host "Expected to be here, BUT NEVER EVER."
}
# -----------------------------------
function foo($a)
{
# I thought this is right.
#if($a -eq $null)
#{
# throw "You can't pass $null as argument."
#}
# But actually it should be:
if($null -eq $a)
{
throw "You can't pass $null as argument."
}
}
foo #($null, $null)
# -----------------------------------
# There is try/catch, but no callstack reported.
function foo()
{
bar
}
function bar()
{
throw "test"
}
# Expected:
# At bar() line:XX
# At foo() line:XX
#
# Actually some like this:
# At bar() line:XX
foo
Would like to know yours to walk them around :-)
My personal favorite is
function foo() {
param ( $param1, $param2 = $(throw "Need a second parameter"))
...
}
foo (1,2)
For those unfamiliar with powershell that line throws because instead of passing 2 parameters it actually creates an array and passes one parameter. You have to call it as follows
foo 1 2
Another fun one. Not handling an expression by default writes it to the pipeline. Really annoying when you don't realize a particular function returns a value.
function example() {
param ( $p1 ) {
if ( $p1 ) {
42
}
"done"
}
PS> example $true
42
"done"
$files = Get-ChildItem . -inc *.extdoesntexist
foreach ($file in $files) {
"$($file.Fullname.substring(2))"
}
Fails with:
You cannot call a method on a null-valued expression.
At line:3 char:25
+ $file.Fullname.substring <<<< (2)
Fix it like so:
$files = #(Get-ChildItem . -inc *.extdoesntexist)
foreach ($file in $files) {
"$($file.Fullname.substring(2))"
}
Bottom line is that the foreach statement will loop on a scalar value even if that scalar value is $null. When Get-ChildItem in the first example returns nothing, $files gets assinged $null. If you are expecting an array of items to be returned by a command but there is a chance it will only return 1 item or zero items, put #() around the command. Then you will always get an array - be it of 0, 1 or N items. Note: If the item is already an array putting #() has no effect - it will still be the very same array (i.e. there is no extra array wrapper).
# The pipeline doesn't enumerate hashtables.
$ht = #{"foo" = 1; "bar" = 2}
$ht | measure
# Workaround: call GetEnumerator
$ht.GetEnumerator() | measure
Here are my top 5 PowerShell gotchas
Here is something Ive stumble upon lately (PowerShell 2.0 CTP):
$items = "item0", "item1", "item2"
$part = ($items | select-string "item0")
$items = ($items | where {$part -notcontains $_})
what do you think that $items be at the end of the script?
I was expecting "item1", "item2" but instead the value of $items is: "item0", "item1", "item2".
Say you've got the following XML file:
<Root>
<Child />
<Child />
</Root>
Run this:
PS > $myDoc = [xml](Get-Content $pathToMyDoc)
PS > #($myDoc.SelectNodes("/Root/Child")).Count
2
PS > #($myDoc.Root.Child).Count
2
Now edit the XML file so it has no Child nodes, just the Root node, and run those statements again:
PS > $myDoc = [xml](Get-Content $pathToMyDoc)
PS > #($myDoc.SelectNodes("/Root/Child")).Count
0
PS > #($myDoc.Root.Child).Count
1
That 1 is annoying when you want to iterate over a collection of nodes using foreach if and only if there actually are any. This is how I learned that you cannot use the XML handler's property (dot) notation as a simple shortcut. I believe what's happening is that SelectNodes returns a collection of 0. When #'ed, it is transformed from an XPathNodeList to an Object[] (check GetType()), but the length is preserved. The dynamically generated $myDoc.Root.Child property (which essentially does not exist) returns $null. When $null is #'ed, it becomes an array of length 1.
On Functions...
The subtleties of processing pipeline input in a function with respect to using $_ or $input and with respect to the begin, process, and end blocks.
How to handle the six principal equivalence classes of input delivered to a function (no input, null, empty string, scalar, list, list with null and/or empty) -- for both direct input and pipeline input -- and get what you expect.
The correct calling syntax for sending multiple arguments to a function.
I discuss these points and more at length in my Simple-Talk.com article Down the Rabbit Hole- A Study in PowerShell Pipelines, Functions, and Parameters and also provide an accompanying wallchart--here is a glimpse showing the various calling syntax pitfalls for a function taking 3 arguments:
On Modules...
These points are expounded upon in my Simple-Talk.com article Further Down the Rabbit Hole: PowerShell Modules and Encapsulation.
Dot-sourcing a file inside a script using a relative path is relative to your current directory -- not the directory where the script resides!
To be relative to the script use this function to locate your script directory: [Update for PowerShell V3+: Just use the builtin $PSScriptRoot variable!]
function Get-ScriptDirectory
{ Split-Path $script:MyInvocation.MyCommand.Path }
Modules must be stored as ...Modules\name\name.psm1 or ...\Modules\any_subpath\name\name.psm1. That is, you cannot just use ...Modules\name.psm1 -- the name of the immediate parent of the module must match the base name of the module. This chart shows the various failure modes when this rule is violated:
2015.06.25 A Pitfall Reference Chart
Simple-Talk.com just published the last of my triumvirate of in-depth articles on PowerShell pitfalls. The first two parts are in the form of a quiz that helps you appreciate a select group of pitfalls; the last part is a wallchart (albeit it would need a rather high-ceilinged room) containing 36 of the most common pitfalls (some adapted from answers on this page), giving concrete examples and workarounds for most. Read more here.
There are some tricks to building command lines for utilities that were not built with Powershell in mind:
To run an executable who's name starts with a number, preface it with an Ampersand (&).
& 7zip.exe
To run an executable with a space anywhere in the path, preface it with an Ampersand (&) and wrap it in quotes, as you would any string. This means that strings in a variable can be executed as well.
# Executing a string with a space.
& 'c:\path with spaces\command with spaces.exe'
# Executing a string with a space, after first saving it in a variable.
$a = 'c:\path with spaces\command with spaces.exe'
& $a
Parameters and arguments are passed to legacy utilities positionally. So it is important to quote them the way the utility expects to see them. In general, one would quote when it contains spaces or does not start with a letter, number or dash (-).
C:\Path\utility.exe '/parameter1' 'Value #1' 1234567890
Variables can be used to pass string values containing spaces or special characters.
$b = 'string with spaces and special characters (-/&)'
utility.exe $b
Alternatively array expansion can be used to pass values as well.
$c = #('Value #1', $Value2)
utility.exe $c
If you want Powershell to wait for an application to complete, you have to consume the output, either by piping the output to something or using Start-Process.
# Saving output as a string to a variable.
$output = ping.exe example.com | Out-String
# Piping the output.
ping stackoverflow.com | where { $_ -match '^reply' }
# Using Start-Process affords the most control.
Start-Process -Wait SomeExecutable.com
Because of the way they display their output, some command line utilities will appear to hang when ran inside of Powershell_ISE.exe, particularly when awaiting input from the user. These utilities will usually work fine when ran within Powershell.exe console.
PowerShell Gotchas
There are a few pitfall that repeatedly reappear on StackOverflow. It is recommend to do some research if you are not familiar with these PowerShell gotchas before asking a new question. It might even be a good idea to investigate in these PowerShell gotchas before answering a PowerShell question to make sure that you teach the questioner the right thing.
TLDR: In PowerShell:
the comparison equality operator is: -eq
(Stackoverflow example: Powershell simple syntax if condition not working)
parentheses and commas are not used with arguments
(Stackoverflow example: How do I pass multiple parameters into a function in PowerShell?)
output properties are based on the first object in the pipeline
(Stackoverflow example: Not all properties displayed)
the pipeline unrolls
(Stackoverflow example: Pipe complete array-objects instead of array items one at a time?)
a. single item collections
(Stackoverflow example: Powershell ArrayList turns a single array item back into a string)
b. embedded arrays
(Stackoverflow example: Return Multidimensional Array From Function)
c. output collections
(Stackoverflow example: Why does PowerShell flatten arrays automatically?)
$Null should be on the left side of the equality comparison operator
(Stackoverflow example: Should $null be on the left side of the equality comparison)
parentheses and assignments choke the pipeline
(Stackoverflow example: Importing 16MB CSV Into Variable Creates >600MB's Memory Usage)
the increase assignment operator (+=) might become expensive
Stackoverflow example: Improve the efficiency of my PowerShell scrip
The Get-Content cmdlet returns separate lines
Stackoverflow example: Multiline regex to match config block
Examples and explanations
Some of the gotchas might really feel counter-intuitive but often can be explained by some very nice PowerShell features along with the pipeline, expression/argument mode and type casting.
1. The comparison equality operator is: -eq
Unlike the Microsoft scripting language VBScript and some other programming languages, the comparison equality operator differs from the assignment operator (=) and is: -eq.
Note: assigning a value to a variable might pass through the value if needed:
$a = $b = 3 # The value 3 is assigned to both variables $a and $b.
This implies that following statement might be unexpectedly truthy or falsy:
If ($a = $b) {
# (assigns $b to $a and) returns a truthy if $b is e.g. 3
} else {
# (assigns $b to $a and) returns a falsy if $b is e.g. 0
}
2. Parentheses and commas are not used with arguments
Unlike a lot of other programming languages and the way a primitive PowerShell function is defined, calling a function doesn't require parentheses or commas for their related arguments. Use spaces to separate the parameter arguments:
MyFunction($Param1, $Param2 $Param3) {
# ...
}
MyFunction 'one' 'two' 'three' # assigns 'one' to $Param1, 'two' to $Param2, 'three' to $Param3
Parentheses and commas are used for calling (.Net) methods.
Commas are used to define arrays. MyFunction 'one', 'two', 'three' (or MyFunction('one', 'two', 'three')) will load the array #('one', 'two', 'three') into the first parameter ($Param1).
Parentheses will intepret the containing contents as a single collection into memory (and choke the PowerShell pipeline) and should only be used as such, e.g. to call an embedded function, e.g.:
MyFunction (MyOtherFunction) # passes the results MyOtherFunction to the first positional parameter of MyFunction ($Param1)
MyFunction One $Two (getThree) # assigns 'One' to $Param1, $Two to $Param2, the results of getThree to $Param3
Note: that quoting text arguments (as the word one in the later example) is only required when it contains spaces or special characters.
3. Output properties are based on the first object in the pipeline
In a PowerShell pipeline each object is processed and passed on by a cmdlet (that is implemented for the middle of a pipeline) similar to how objects are processed and passed on by workstations in an assembly line. Meaning each cmdlet processes one item at the time while the prior cmdlet (workstation) simultaneously processes the upcoming one. This way, the objects aren't loaded into memory at once (less memory usage) and could already be processed before the next one is supplied (or even exists). The disadvantage of this feature is that there is no oversight of what (or how many) objects are expected to follow.
Therefore most PowerShell cmdlets assume that all the objects in the pipeline correspond to the first one and have the same properties which is usually the case, but not always...
$List =
[pscustomobject]#{ one = 'a1'; two = 'a2' },
[pscustomobject]#{ one = 'b1'; two = 'b2'; three = 'b3' }
$List |Select-Object *
one two
--- ---
a1 a2
b1 b2
As you see, the third column three is missing from the results as it didn't exists in the first object and the PowerShell was already outputting the results prior it was aware of the exists of the second object.
On way to workaround this behavior is to explicitly define the properties (of all the following objects) at forehand:
$List |Select-Object one, two, three
one two three
--- --- -----
a1 a2
b1 b2 b3
See also proposal: #13906 Add -UnifyProperties parameter to Select-Object
4. The pipeline unrolls
This feature might come in handy if it complies with the straightforward expectation:
$Array = 'one', 'two', 'three'
$Array.Length
3
a. single item collections
But it might get confusing:
$Selection = $Array |Select-Object -First 2
$Selection.Length
2
$Selection[0]
one
when the collection is down to a single item:
$Selection = $Array |Select-Object -First 1
$Selection.Length
3
$Selection[0]
o
Explanation
When the pipeline outputs a single item which is assigned to a variable, it is not assigned as a collection (with 1 item, like: #('one')) but as a scalar item (the item itself, like: 'one').
Which means that the property .Length (which is in fact an alias for the property .Count for an array) is no longer applied on the array but on the string: 'one'.length which equals 3. And in case of the index $Selection[0] , the first character of the string 'one'[0] (which equals the character o) is returned .
Workaround
To workaround this behavior, you might force the scalar item to an array using the Array subexpression operator #( ):
$Selection = $Array |Select-Object -First 1
#($Selection).Length
1
#($Selection)[0]
one
Knowing that in the case the $Selection is already an array, it will will not be further increased in depth (#(#('one', 'two')), see the next section 4b. Embedded collections are flattened).
b. embedded arrays
When an array (or a collection) includes embedded arrays, like:
$Array = #(#('a', 'b'), #('c', 'd'))
$Array.Count
2
All the embedded items will be processed in the pipeline and consequently returns a flat array when displayed or assigned to a new variable:
$Processed = $Array |ForEach-Object { $_ }
$Processed.Count
4
$Processed
a
b
c
d
To iterate the embedded arrays, you might use the foreach statement:
foreach ($Item in $Array) { $Item.Count }
2
2
Or a simply for loop:
for ($i = 0; $i -lt $Array.Count; $i++) { $Array[$i].Count }
2
2
c. output collections
Collections are usually unrolled when they are placed on the pipeline:
function GetList {
[Collections.Generic.List[String]]#('a', 'b')
}
(GetList).GetType().Name
Object[]
To output the collection as a single item, use the comma operator ,:
function GetList {
,[Collections.Generic.List[String]]#('a', 'b')
}
(GetList).GetType().Name
List`1
5. $Null should be on the left side of the equality comparison operator
This gotcha is related to this comparison operators feature:
When the input of an operator is a scalar value, the operator returns a Boolean value. When the input is a collection, the operator returns the elements of the collection that match the right-hand value of the expression. If there are no matches in the collection, comparison operators return an empty array.
This means for scalars:
'a' -eq 'a' # returns $True
'a' -eq 'b' # returns $False
'a' -eq $Null # returns $False
$Null -eq $Null # returns $True
and for collections, the matching elements are returned which evaluates to either a truthy or falsy condition:
'a', 'b', 'c' -eq 'a' # returns 'a' (truthy)
'a', 'b', 'c' -eq 'd' # returns an empty array (falsy)
'a', 'b', 'c' -eq $Null # returns an empty array (falsy)
'a', $Null, 'c' -eq $Null # returns $Null (falsy)
'a', $Null, $Null -eq $Null # returns #($Null, $Null) (truthy!!!)
$Null, $Null, $Null -eq $Null # returns #($Null, $Null, $Null) (truthy!!!)
In other words, to check whether a variable is $Null (and exclude a collection containing multiple $Nulls), put $Null at the LHS (left hand side) of the equality comparison operator:
if ($Null -eq $MyVariable) { ...
6. Parentheses and assignments choke the pipeline
The PowerShell Pipeline is not just a series of commands connected by pipeline operators (|) (ASCII 124). It is a concept to simultaneously stream individual objects through a sequence of cmdlets. If a cmdlet (or function) is written according to the Strongly Encouraged Development Guidelines and implemented for the middle of a pipeline, it takes each single object from the pipeline, processes it and passes the results to the next cmdlet just before it takes and processes the next object in the pipeline. Meaning that for a simple pipeline as:
Import-Csv .\Input.csv |Select-Object -Property Column1, Column2 |Export-Csv .\Output.csv
As the last cmdlet writes an object to the .\Output.csv file, the Select-Object cmdlet selects the properties of the next object and the Import-Csv reads the next object from the .\input.csv file (see also: Pipeline in Powershell). This will keep the memory usage low (especially where there are lots of object/records to process) and therefore might result in a faster throughput. To facilitate the pipeline, the PowerShell objects are quiet fat as each individual object contains all the property information (along with e.g. the property name).
Therefore it is not a good practice to choke the pipeline for no reason. There are two senarios that choke the pipeline:
Parentheses, e.g.:
(Import-Csv .\Input.csv) |Select-Object -Property Column1, Column2 |Export-Csv .\Output.csv
Where all the .\Input.csv records are loaded as an array of PowerShell objects into memory before passing it on to the Select-Object cmdlet.
Assignments, e.g.:
$Objects = Import-Csv .\Input.csv
$Objects |Select-Object -Property Column1, Column2 |Export-Csv .\Output.csv
Where all the .\Input.csv records are loaded as an array of PowerShell objects into $Objects (memory as well) before passing it on to the Select-Object cmdlet.
7. the increase assignment operator (+=) might become expensive
The increase assignment operator (+=) is syntactic sugar to increase and assign primitives as .e.g. $a += $b where $a is assigned $b + 1. The increase assignment operator can also be used for adding new items to a collection (or to String types and hash tables) but might get pretty expensive as the costs increases with each iteration (the size of the collection). The reason for this is that objects as array collections are immutable and the right variable in not just appended but *appended and reassigned to the left variable. For details see also: avoid using the increase assignment operator (+=) to create a collection
8. The Get-Content cmdlet returns separate lines
There are probably quite some more cmdlet gotchas, knowing that there exist a lot of (internal and external) cmdlets. In contrast to engine related gotchas, these gotchas are often easier to highlight (with e.g. a warning) as happend with ConvertTo-Json (see: Unexpected ConvertTo-Json results? Answer: it has a default -Depth of 2) or "fix". But there is very clasic gotcha in Get-Content which tight into the PowerShell general concept of streaming objects (in this case lines) rather than passing everything (the whole contents of the file) in once:
Get-Content .\Input.txt -Match '\r?\n.*Test.*\r?\n'
Will never work because, by default, Get-Contents returns a stream of objects where each object contains a single string (a line without any line breaks).
(Get-Content .\Input.txt).GetType().Name
Object[]
(Get-Content .\Input.txt)[0].GetType().Name
String
In fact:
Get-Content .\Input.txt -Match 'Test'
Returns all the lines with the word Test in it as Get-Contents puts every single line on the pipeline and when the input is a collection, the operator returns the elements of the collection that match the right-hand value of the expression.
Note: since PowerShell version 3, Get-Contents has a -Raw parameter that reads all the content of the concerned file at once, Meaning that this: Get-Content -Raw .\Input.txt -Match '\r?\n.*Test.*\r?\n' will work as it loads the whole file into memory.
alex2k8, I think this example of yours is good to talk about:
# -----------------------------------
function foo($a){
# I thought this is right.
#if($a -eq $null)
#{
# throw "You can't pass $null as argument."
#}
# But actually it should be:
if($null -eq $a)
{
throw "You can't pass $null as argument."
}
}
foo #($null, $null)
PowerShell can use some of the comparators against arrays like this:
$array -eq $value
## Returns all values in $array that equal $value
With that in mind, the original example returns two items (the two $null values in the array), which evalutates to $true because you end up with a collection of more than one item. Reversing the order of the arguments stops the array comparison.
This functionality is very handy in certain situations, but it is something you need to be aware of (just like array handling in PowerShell).
Functions 'foo' and 'bar' looks equivalent.
function foo() { $null }
function bar() { }
E.g.
(foo) -eq $null
# True
(bar) -eq $null
# True
But:
foo | %{ "foo" }
# Prints: foo
bar | %{ "bar" }
# PRINTS NOTHING
Returning $null and returning nothing is not equivalent dealing with pipes.
This one is inspired by Keith Hill example...
function bar() {}
$list = #(foo)
$list.length
# Prints: 0
# Now let's try the same but with a temporal variable.
$tmp = foo
$list = #($tmp)
$list.length
# Prints: 1
Another one:
$x = 2
$y = 3
$a,$b = $x,$y*5
because of operators precedence there is not 25 in $b; the command is the same as ($x,$y)*5
the correct version is
$a,$b = $x,($y*5)
The logical and bitwise operators don't follow standard precedence rules. The operator -and should have a higher priority than -or yet they're evaluated strictly left-to-right.
For example, compare logical operators between PowerShell and Python (or virtually any other modern language):
# PowerShell
PS> $true -or $false -and $false
False
# Python
>>> True or False and False
True
...and bitwise operators:
# PowerShell
PS> 1 -bor 0 -band 0
0
# Python
>>> 1 | 0 & 0
1
This works. But almost certainly not in the way you think it's working.
PS> $a = 42;
PS> [scriptblock]$b = { $a }
PS> & $b
42
This one has tripped me up before, using $o.SomeProperty where it should be $($o.SomeProperty).
# $x is not defined
[70]: $x -lt 0
True
[71]: [int]$x -eq 0
True
So, what's $x..?
Another one I ran into recently: [string] parameters that accept pipeline input are not strongly typed in practice. You can pipe anything at all and PS will coerce it via ToString().
function Foo
{
[CmdletBinding()]
param (
[parameter(Mandatory=$True, ValueFromPipeline=$True)]
[string] $param
)
process { $param }
}
get-process svchost | Foo
Unfortunately there is no way to turn this off. Best workaround I could think of:
function Bar
{
[CmdletBinding()]
param (
[parameter(Mandatory=$True, ValueFromPipeline=$True)]
[object] $param
)
process
{
if ($param -isnot [string]) {
throw "Pass a string you fool!"
}
# rest of function goes here
}
}
edit - a better workaround I've started using...
Add this to your custom type XML -
<?xml version="1.0" encoding="utf-8" ?>
<Types>
<Type>
<Name>System.String</Name>
<Members>
<ScriptProperty>
<Name>StringValue</Name>
<GetScriptBlock>
$this
</GetScriptBlock>
</ScriptProperty>
</Members>
</Type>
</Types>
Then write functions like this:
function Bar
{
[CmdletBinding()]
param (
[parameter(Mandatory=$True, ValueFromPipelineByPropertyName=$True)]
[Alias("StringValue")]
[string] $param
)
process
{
# rest of function goes here
}
}
Forgetting that $_ gets overwritten in blocks made me scratch my head in confusion a couple times, and similarly for multiple reg-ex matches and the $matches array. >.<
Remembering to explicitly type pscustom objects from imported data tables as numeric so they can be sorted correctly:
$CVAP_WA=foreach ($i in $C){[PSCustomObject]#{ `
County=$i.county; `
TotalVote=[INT]$i.TotalBallots; `
RegVoters=[INT]$i.regvoters; `
Turnout_PCT=($i.TotalBallots/$i.regvoters)*100; `
CVAP=[INT]($B | ? {$_.GeoName -match $i.county}).CVAP_EST }}
PS C:\Politics> $CVAP_WA | sort -desc TotalVote |ft -auto -wrap
County TotalVote RegVoters Turnout_PCT CVAP CVAP_TV_PCT CVAP_RV_PCT
------ --------- --------- ----------- ---- ----------- -----------
King 973088 1170638 83.189 1299290 74.893 90.099
Pierce 349377 442985 78.86 554975 62.959 79.837
Snohomish 334354 415504 80.461 478440 69.832 86.81
Spokane 227007 282442 80.346 342060 66.398 82.555
Clark 193102 243155 79.453 284190 67.911 85.52
Mine are both related to file copying...
Square Brackets in File Names
I once had to move a very large/complicated folder structure using Move-Item -Path C:\Source -Destination C:\Dest. At the end of the process there were still a number of files in source directory. I noticed that every remaining file had square brackets in the name.
The problem was that the -Path parameter treats square brackets as wildcards.
EG. If you wanted to copy Log001 to Log200, you could use square brackets as follows:
Move-Item -Path C:\Source\Log[001-200].log.
In my case, to avoid square brackets being interpreted as wildcards, I should have used the -LiteralPath parameter.
ErrorActionPreference
The $ErrorActionPreference variable is ignored when using Move-Item and Copy-Item with the -Verbose parameter.
Treating the ExitCode of a Process as a Boolean.
eg, with this code:
$p = Start-Process foo.exe -NoNewWindow -Wait -PassThru
if ($p.ExitCode) {
# handle error
}
things are good, unless say foo.exe doesn't exist or otherwise fails to launch.
in that case $p will be $null, and [bool]($null.ExitCode) is False.
a simple fix is to replace the logic with if ($p.ExitCode -ne 0) {},
however for clarity of code imo the following is better: if (($p -eq $null) -or ($p.ExitCode -ne 0)) {}