What does the special character "!" mean in PowerShell? - powershell

Question

What does the special character ! mean in PowerShell?
Or a site which lists all special characters and meaning.
Example:
$string = blah
!$String
(Returns $false)

Answer 1

PowerShell uses the ! character as an alias for the logical -not operator:
$true
!$true
$false
!$false
True
False
False
True

Answer 2

PowerShell interprets everything that is empty, $Null, or 0 to the Boolean $False. Bool can only have $True or $False.
By casting the value to a Boolean you can see what PowerShell interprets for each value:
[bool]0 # False
[bool]1 # True
[bool]"" # False
[bool]"test" # True
[bool]$null # False
The locical NOT operation turns each Boolean into its opposite:
!$True # Is $False
!$False # Is $True
![bool]0 # True
![bool]1 # False
![bool]"" # True
![bool]"test" # False
![bool]$null # True
You were assigning a string to a variable and then checking whether it is empty or not.
$string = blah
!$String # $String is not $Null or Empty so it is $True
# But the !(NOT) operation turns it to $False
Conditionals and loops in programming languages only work with Boolean values.
So when getting user input you can use this to check whether the user has input text, or not, and react on it:
$UserName = Read-Host -Prompt "Whats your Name Sir?"
If ($UserName) {
Write-Output "Happy Birthday $UserName"
}
Else {
Write-Output "I can't congratulate you as I don't know your name :("
}

Answer 3

The ! (exclamation mark) character in PowerShell is a shortcut to the -not operator ('not equal').
For example:
$a = $null;
if(!$a) {
Write-Host '$a is null'
}
Output.
$a is null

Answer 4

The symbol ! is an alias for -Not, which is a unary (one-argument) operator that casts its argument to a Boolean value and then returns the logical opposite of that value. (Most of the time, spelling out -Not is a better choice for readability.)
Boolean is a data type with only two possible values: true and false. In PowerShell the type is known as Bool and the two values are written $True and $False. The -Not/! operator just flips the value to its opposite: -Not $True is $False and ! $False is $True.
But in PowerShell, as in many other programming languages, you can apply Boolean operators (which also include -And and -Or) to non-Boolean values; those values just get automatically cast to Bool before being operated on, as if you had put a [Bool] in front of them.
Values that become $True when so cast are called "truthy"; values that become $False are called "falsy" (sometimes spelled "falsey"). Different programming languages have different conventions for what goes in each category; for instance, an empty array is falsy in Perl and Python, but truthy in Ruby.
In PowerShell, both the null value $Null and the empty string are falsy. So in your code example, !$String would be true if the string is either null or empty (since it would boolify to false and then get flipped to true by the !). Such an expression would likely show up in, for example, an If statement that sets a variable to a default value if it has not already been set to a different one by earlier code.
Besides the empty string and $Null, other falsy values in PowerShell include the number 0 and the empty array. However, an empty HashTable is truthy. You can see these correspondences by casting the values to Boolean, either with [Bool] or the shortcut !!. (! casts the value to Boolean but then flips it; the second ! flips it back. I recommend using !! at the prompt for quicker typing, but spelling out [Bool] for better self-documentation in production code.) Here are some examples:
PS /> [Bool]$Null
False
PS /> !!''
False
PS /> !!0
False
PS /> !!#()
False
PS /> !!#{}
True