Performing Assertions with Should

⚠️ All information on this page are relevant to Pester v. 4.x. You can read older version of this page - relevant to Pester v. 3.x - here.

Should is a command that provides assertion convenience methods for comparing objects and throwing test failures when test expectations fail. Should is used inside It blocks of a Pester test script.

Negative Assertions

When reviewing the operators listed below, keep in mind that all of them can be negated by putting the word "Not" between "Should" and the operator. For example:

$true | Should -Be $true

$true | Should -Not -Be $false

Should Operators

Be

Compares one object with another for equality and throws if the two objects are not the same. This comparison is not case sensitive.

$actual="Actual value"

$actual | Should -Be "actual value" # Test will pass

$actual | Should -Be "not actual value" # Test will fail

Also compares an entire array for equality and throws if the array is not the same.

$array = @(1, 2, 3, 4, 'I am a string', (New-Object psobject -Property @{ IAm = 'An Object' }))

$array | Should -Be $array # Test will pass

$string = 'I am a string'

$array = @(1, 2, 3, 4, $string)

$arrayWithCaps = @(1, 2, 3, 4, $string.ToUpper())

$array | Should -Be $arrayWithCaps # Test will pass

Comparisons will fail if the arrays have the same values, but not the same order.

[int32[]]$array = (1..10)

$arrayoutoforder = (1,10,2,3,4,5,6,7,8,9)

$array | Should -Be $arrayOutOfOrder # Test will fail

BeExactly

Compares one object with another for equality and throws if the two objects are not the same. This comparison is case sensitive.

$actual="Actual value"

$actual | Should -BeExactly "Actual value" # Test will pass

$actual | Should -BeExactly "actual value" # Test will fail

BeGreaterThan

Asserts that a number is greater than an expected value. Uses PowerShell's -gt operator to compare the two values.

$Error.Count | Should -BeGreaterThan 0

[version]'2.3.4.5' | Should -BeGreaterThan ([version]'1.0.0.1')

(Get-Date).AddMinutes(5) | Should -BeGreaterThan (Get-Date)

BeGreaterOrEqual

Asserts that a number (or other comparable value) is greater than or equal to an expected value. Uses PowerShell's -ge operator to compare the two values.

2 | Should -BeGreaterOrEqual 0

2 | Should -BeGreaterOrEqual 2

BeIn

Asserts that the actual value is contained by the array/collection

'b' | Should -BeIn @('a','b','c')

27 | Should -BeIn (1..100)

BeLessThan

Asserts that a number is less than an expected value. Uses PowerShell's -lt operator to compare the two values.

$Error.Count | Should -BeLessThan 1

BeLessOrEqual

Asserts that a number (or other comparable value) is lower than, or equal to an expected value. Uses PowerShell's -le operator to compare the two values.

1 | Should -BeLessOrEqual 10

10 | Should -BeLessOrEqual 10

BeLike

Asserts that the actual value matches a wildcard pattern using PowerShell's -like operator. This comparison is not case-sensitive.

$actual="Actual value"

$actual | Should -BeLike "actual *" # Test will pass

$actual | Should -BeLike "not actual *" # Test will fail

BeLikeExactly

Asserts that the actual value matches a wildcard pattern using PowerShell's -clike operator. This comparison is case-sensitive.

$actual="Actual value"

$actual | Should -BeLikeExactly "Actual *" # Test will pass

$actual | Should -BeLikeExactly "actual *" # Test will fail

BeOfType

Asserts that the actual value should be an object of a specified type (or a subclass of the specified type) using PowerShell's -is operator:

$actual = Get-Item $env:SystemRoot

$actual | Should -BeOfType System.IO.DirectoryInfo # Test will pass; object is a DirectoryInfo

$actual | Should -BeOfType System.IO.FileSystemInfo # Test will pass; DirectoryInfo base class is FileSystemInfo

$actual | Should -BeOfType System.IO.FileInfo # Test will fail; FileInfo is not a base class of DirectoryInfo

BeTrue

Asserts that the value is true, or truthy.

$true | Should -BeTrue

1 | Should -BeTrue

1,2,3 | Should -BeTrue

BeFalse

Asserts that the value is false, or falsy.

$false | Should -BeFalse

0 | Should -BeFalse

$null | Should -BeFalse

HaveCount

Asserts that a collection has the expected amount of items.

1,2,3 | Should -HaveCount 3

Contain

Asserts that the collection contains value specified using PowerShell's -contains operator.

'a','b','c' | Should -Contain b

1..100 | Should -Contain 42

Exist

Does not perform any comparison but checks if the object calling Exist is present in a PS Provider. The object must have valid path syntax. It essentially must pass a Test-Path call.

$actual=(Dir . )[0].FullName

Remove-Item $actual

$actual | Should -Exist # Test will fail

To test path containing [ ] wildcards, escape each bracket with two back-ticks as such "TestDrive:\``[test``].txt" or use Test-Path -LiteralPath $something | Should -Be $true.

FileContentMatch

Checks to see if a file contains the specified text. This search is not case sensitive and uses regular expressions.

Set-Content -Path TestDrive:\file.txt -Value 'I am a file'

'TestDrive:\file.txt' | Should -FileContentMatch 'I Am' # Test will pass

'TestDrive:\file.txt' | Should -FileContentMatch '^I.*file$' # Test will pass

'TestDrive:\file.txt' | Should -FileContentMatch 'I Am Not' # Test will fail

Tip: Use [regex]::Escape("pattern") to match the exact text.

Set-Content -Path TestDrive:\file.txt -Value 'I am a file.'

'TestDrive:\file.txt' | Should -FileContentMatch 'I.am.a.file' # Test will pass

'TestDrive:\file.txt' | Should -FileContentMatch ([regex]::Escape('I.am.a.file')) # Test will fail

Warning: Make sure the input is either a quoted string or an Item object. Otherwise PowerShell will try to invoke the path, likely throwing an error Cannot run a document in the middle of a pipeline.

c:\file.txt | Should -FileContentMatch something # Will throw an error

'c:\file.txt' | Should -FileContentMatch something # Will evaluate correctly

FileContentMatchExactly

Checks to see if a file contains the specified text. This search is case sensitive and uses regular expressions to match the text.

Set-Content -Path TestDrive:\file.txt -Value 'I am a file.'

'TestDrive:\file.txt' | Should -FileContentMatchExactly 'I am' # Test will pass

'TestDrive:\file.txt' | Should -FileContentMatchExactly 'I Am' # Test will fail

FileContentMatchMultiline

As opposed to FileContentMatch and FileContentMatchExactly operators, FileContentMatchMultiline presents content of the file being tested as one string object, so that the expression you are comparing it to can consist of several lines.

$Content = "I am the first line.`nI am the second line."

Set-Content -Path TestDrive:\file.txt -Value $Content -NoNewline

'TestDrive:\file.txt' | Should -FileContentMatchMultiline 'first line\.\r?\nI am' # Test will pass

'TestDrive:\file.txt' | Should -FileContentMatchMultiline '^I am the first.*\n.*second line\.$' # Test will pass.

When using FileContentMatchMultiline operator, ^ and $ represent the beginning and end of the whole file, instead of the beginning and end of a line.

$Content = "I am the first line.`nI am the second line."

Set-Content -Path TestDrive:\file.txt -Value $Content -NoNewline

'TestDrive:\file.txt' | Should -FileContentMatchMultiline '^I am the first line\.$' # Test will fail.

Match

Uses a regular expression to compare two objects. This comparison is not case sensitive.

"I am a value" | Should -Match "I Am" # Test will pass

"I am a value" | Should -Match "I am a bad person" # Test will fail

Tip: Use [regex]::Escape("pattern") to match the exact text.

"Greg" | Should -Match ".reg" # Test will pass

"Greg" | Should -Match ([regex]::Escape(".reg")) # Test will fail

MatchExactly

Uses a regular expression to compare two objects. This comparison is case sensitive.

"I am a value" | Should -MatchExactly "I am" # Test will pass

"I am a value" | Should -MatchExactly "I Am" # Test will fail

Throw

Checks if an exception was thrown in the input ScriptBlock. Takes an optional argument to indicate the expected exception message.

{ foo } | Should -Throw # Test will pass

{ $foo = 1 } | Should -Throw # Test will fail

{ foo } | Should -Not -Throw # Test will fail

{ $foo = 1 } | Should -Not -Throw # Test will pass

{ throw "This is a test" } | Should -Throw "This is a test" # Test will pass

{ throw "bar" } | Should -Throw "This is a test" # Test will fail

Note: The exception message match is a substring match, so the following assertion will pass:

{throw "foo bar baz"} | Should -Throw "bar" # Test will pass

Warning: The input object must be a ScriptBlock, otherwise it is processed outside of the assertion.

Get-Process -Name "process" -ErrorAction Stop | Should -Throw # Should pass but fails the test

NOTE: Throw is used to validate terminating errors (i.e. exceptions that were thrown). If you want to perform validation against non-terminating errors (i.e. Write-Error messages), you can use the technique described here.

NOTE: If you are calling a cmdlet and want to force all errors to be terminating errors so that they can be caught by | -Should -Throw, then append -ErrorAction Stop to the cmdlet parameters as shown in the example above.

BeNullOrEmpty

Checks values for null or empty (strings). The static [String]::IsNullOrEmpty() method is used to do the comparison.

$null | Should -BeNullOrEmpty # Test will pass

$null | Should -Not -BeNullOrEmpty # Test will fail

@() | Should -BeNullOrEmpty # Test will pass

"" | Should -BeNullOrEmpty # Test will pass

Because

Adds message to test failure message.

$actual = "Actual value"

$actual | Should -Be "actual value" -Because 'Test must pass'

$actual | Should -Be "not actual value" -Because 'Test must fail'

# OUTPUT

# Expected strings to be the same, because Test must fail, but they were different.

# Expected length: 16

# Actual length: 12

# Strings differ at index 0.

# Expected: 'not actual value'

# But was: 'Actual value'

# -----------^

HaveParameter

An assertion operator -HaveParameter allows you to check function parameters, and their properties like this:

Get-Command "Invoke-WebRequest" | Should -HaveParameter Uri -Mandatory

function f ([String] $Value = 8) { }

Get-Command f | Should -HaveParameter Value -Type String

Get-Command f | Should -Not -HaveParameter Name

Get-Command f | Should -HaveParameter Value -DefaultValue 8

Get-Command f | Should -HaveParameter Value -Not -Mandatory

Using Should in a Test

function Add-Numbers($a, $b) {

return $a + $b

}

Describe "Add-Numbers" {

It "adds positive numbers" {

$sum = Add-Numbers 2 3

$sum | Should -Be 5

}

It "ensures that 2 + 2 does not equal 5" {

$sum = Add-Numbers 2 2

$sum | Should -Not -Be 5

}

}