Powershell – Comparison Operators

I’m writing this one so I remember these operators. Powershell doesn’t use standard operators such as =, <, <>, !=, etc. These operators are used in the following manner.

Operator	Conventional Operator
-eq	=
-ne	<> or !=
-gt	>
-ge	>=
-lt	<
-le	<=
-contains	e.g., 1,2,3 –contains 1 is true
-notcontains	e.g., 1,2,3 –notcontains 1 is false
-not !	e.g., –not ($a –eq $b) Logical Not operator
-and	Logical AND e.g., ($age –ge 21) –and ($gender –eq “M”)
-or	Logical OR e.g., ($age –le 21) –or ($InSchool –eq $true)

 

I think this is one of the areas that is going to trip me up quite a bit. In Powershell, “=” is always used to assign.  “<” and “>” are used for redirection. The names make sense to me in a way, but I can tell I’ll be spending some time with these before I’m completely comfortable with this concept.

 

The above are the base comparisons.  If you prefix them with “i” the operator becomes case-insensitive.  If you prefix an operator with “c”, the operator will be case-sensitive.  Thus we would get the following:

  "johnson" -eq "Johnson" #True

  "johnson" -ieq "Johnson" #True

  "johnson" -ceq "Johnson" #False

Where-Object

This is a useful Cmdlet to filter out results from the pipeline, but this should also be used with care. The initial object will still contain everything with which it started. The results will then be filtered. If you have a way to filter these results prior to passing them through the pipeline, you’ll often have better performance. The following will find all instances of Firefox running on the machine:

  Get-Process | where-object {$_.name -eq 'firefox'}

We could then pass this through the pipeline to operate on that specific object. Where-Object can be abbreviated with a “?”.

 

If, ElseIf, Else

This is a pretty basic statement for Programming and seems to make sense in the way it was implemented in Powershell.

If (condition) {#do something}

ElseIf (next condition) {#do something else}


Else (final condition) {#final action}

As in most uses of If, the process will work through until it finds a condition that evaluates to $true. Once it hits that, it will execute the command(s) inside the corresponding curly braces and exit out of the If.

 

Switch

Sometimes it doesn’t make sense to write out a bunch of IfElse lines to keep checking conditions. Switch can help shorten that. A simple example would be something like this.

$Value = 15
Switch($Value)
{
  1 {"Value is 1"}
  2 {"Value is 2"}
  15 {"Value is 15"}
  default {"Value is $_"}
}

Powershell interprets these in order and will evaluate each one in turn. The default line tells Powershell what to do if no match is found.  If you want Powershell to stop evaluating when it finds a match, add a ;break inside the {} following the matching condition. Otherwise, it could hit the same matching condition and evaluate each one it finds. This may be desirable at some times when you want to act on all matching statements. Other times you may want to short-circuit the process.

Another advantage of the Switch statement is that you can evaluate expressions instead of just the values.  In the example above, the behavior behind the scenes is performing {$_ –eq 1} and {$_ –eq 2}, etc for the lines in the switch. You can substitute your own expressions in here easily to change the evaluations.

Switch supports character comparisons (defaulting to case-insensitive equals), but can do other types by default with a parameter on the Switch statement (e.g., –regex, –case, –wildcard, etc.)

The biggest drawback for Switch is that it would be considered a “blocking” component in the pipeline. It needs to wait for all results before it operates. If you just want to do some filtering on the results, you would be better off using Where-Object or filtering up front.