6. Conditional logic

Note

The below information is extensively based in information taken from the PowerShell® Notes for Professionals book. I plan to extend this information based on my day to day usage of the language.

6.1: if, else and else if

Powershell supports standard conditional logic operators, much like many programming languages. These allow certain functions or commands to be run under particular circumstances.

With an if the commands inside the brackets ({}) are only executed if the conditions inside the if(()) are met

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$test = "test"
if ($test -eq "test"){
Write-Host "if condition met"
}

You can also do an else. Here the else commands are executed if the if conditions are not met:

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$test = "test"
if ($test -eq "test2"){
  Write-Host "if condition met"
}
else{
  Write-Host "if condition not met"
}

or an elseif. An else if runs the commands if the if conditions are not met and the elseif conditions are met:

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$test = "test"
if ($test -eq "test2"){
  Write-Host "if condition met"
}
elseif ($test -eq "test"){
  Write-Host "ifelse condition met"
}

Note the above use -eq(equality) CmdLet and not = or == as many other languages do for equality.

6.2: Negation

You may want to negate a boolean value, i.e. enter an if statement when a condition is false rather than true. This can be done by using the -Not CmdLet

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$test = "test"
if (-Not $test -eq "test2"){
  Write-Host "if condition not met"
}

You can also use !:

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$test = "test"
if (!($test -eq "test2")){
  Write-Host "if condition not met"
}

there is also the -ne (not equal) operator:

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$test = "test"
if ($test -ne "test2"){
  Write-Host "variable test is not equal to 'test2'"
}

6.3: If conditional shorthand

If you want to use the shorthand you can make use of conditional logic with the following shorthand. Only the string 'false' will evaluate to true (2.0).

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#Done in Powershell 2.0
$boolean = $false;
$string = "false";
$emptyString = "";

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If($boolean){
  # this does not run because $boolean is false
  Write-Host "Shorthand If conditions can be nice, just make sure they are always boolean."
}

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If($string){
  # This does run because the string is non-zero length
  Write-Host "If the variable is not strictly null or Boolean false, it will evaluate to true as it is an object or string with length greater than 0."
}

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If($emptyString){
  # This does not run because the string is zero-length
  Write-Host "Checking empty strings can be useful as well."
}

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If($null){
  # This does not run because the condition is null
  Write-Host "Checking Nulls will not print this statement."
}