Friday Fun Reverse PowerShell

These Friday Fun posts are not supposed to be practical, yet hopefully still entertaining and educational. Today's article is no exception but I 'll be you'll try it at least once. And you'll maybe even learn something about PowerShell that perhaps you didn't know before. It all starts with a string. We use strings all the time for things like passwords and names. Let me start by explaining some basic concepts and then we'll get to the fun stuff.

$t = "abcdefg"

The string object has a number of methods, but I'm not going to use any of them today. Instead, I want to start by showing that the string is an array, or collection, of characters. This means you can access any element by its index number.

PS C:\> $t[0]
a
PS C:\> $t[3]
d
PS C:\> $t[-1]
g

You can start at the end of the array with -1. You can also get several elements with the Range ( .. ) operator.

PS C:\> $t[0..2]
a
b
c

So, what about counting in reverse? We know we can start at -1. -2 would give us the 2nd to last element, -3 the third to last and so on. Therefore all we need to do is count backwards to the beginning.

PS C:\> $t[-1..-($t.length)]
g
f
e
d
c
b
a

I used the string length to know how far back to count. But wait, it gets better. Let's bring in the -Join operator. This operator is designed to join elements of an array into a single string.

PS C:\> $a="adam","beth","charlie","darlene"
PS C:\> $a
adam
beth
charlie
darlene
PS C:\> $a -join "||"
adam||beth||charlie||darlene

If you don't want to specify a delimiter, you can use -Join like this:

PS C:\> -join $a
adambethcharliedarlene

See where I'm going with this? Let's join the reversed array of string characters.

PS C:\> -join $t[-1..-($t.length)]
gfedcba

So if I can take any string and reverse it, why not reverse PowerShell? I created a function called Out-Reverse.

#requires -version 3.0

Function Out-Reverse {

<#
.Synopsis
Reverse command output to text.
.Description
This command will take any input and write it to the console as reverse text.
.Example
PS C:\> get-service m* | Out-Reverse

                           emaNyalpsiD               emaN   sutatS
                           -----------               ----   ------
                         rvSpotkseDTCM      rvSpotkseDTCM  gninnuR
            reludehcS ssalC aidemitluM              SSCMM  deppotS
           ecivreS ecnanetniaM allizoM ecnanetniaMallizoM  deppotS
                      llaweriF swodniW             cvSspM  gninnuR
   rotanidrooC noitcasnarT detubirtsiD              CTDSM  deppotS
     ecivreS rotaitinI ISCSi tfosorciM            ISCSiSM  deppotS
                     rellatsnI swodniW          revresism  deppotS
             retliF draobyeK tfosorciM   retliFdraobyeKsM  deppotS
              )REVRESLQSSM( revreS LQS        REVRESLQSSM  gninnuR
              revreS PCHD NAP sseleriW      SNDPCHDiFiWyM  deppotS
.Example
PS C:\> "PowerShell" | Out-Reverse
llehSrewoP

.Example
PS C:\> get-eventlog -list | format-list | out-reverse

noitacilppA :                  goL
eslaF :  stnevEgnisiaRelbanE
08402 :     setyboliKmumixaM
0 : syaDnoitneteRmuminiM
dedeeNsAetirwrevO :       noitcAwolfrevO

stnevEerawdraH :                  goL
eslaF :  stnevEgnisiaRelbanE
08402 :     setyboliKmumixaM
0 : syaDnoitneteRmuminiM
dedeeNsAetirwrevO :       noitcAwolfrevO

rerolpxE tenretnI :                  goL
eslaF :  stnevEgnisiaRelbanE
215 :     setyboliKmumixaM
7 : syaDnoitneteRmuminiM
redlOetirwrevO :       noitcAwolfrevO
...

.Notes
Last Updated: July 16, 2014
Version     : 0.9

.Inputs
Object

.Outputs
String

#>

[cmdletbinding()]
Param(
[Parameter(Position=0,ValueFromPipeline)]
[Object]$InputObject)

Begin {
    Write-Verbose -Message "Starting $($MyInvocation.Mycommand)"  
    $data = @()
} #begin

Process {
 $data += $InputObject
}
End {
    $Text = ($Data | Out-String).Split("`n")
    foreach ($line in $text) {
     -join $line[-1..-$($line.length)]
    }
    Write-Verbose -Message "Ending $($MyInvocation.Mycommand)"
} #end
}

#define an alias
Set-Alias -Name orv -Value Out-Reverse

The idea is that you can run any PowerShell expression, pipe it to Out-Reverse, and you'll get just that: reverse output.

The function takes any pipeline input and stores it in a temporary array. Once everything has been processed, the data is converted to a string with Out-String. Because I need to process each line separately, I split what would otherwise be a very long single string, into multiple strings on the end of line marker (`n).

$Text = ($Data | Out-String).Split("`n")

$Text is now an array of strings and I can reverse each one.

foreach ($line in $text) {
     -join $line[-1..-$($line.length)]
    }

It is kind of fun looking at reversed output. For you language geeks, this also makes a great palindrome tester.

PS C:\>  "able was I saw elba" | orv
able was I saw elba
PS C:\>  "madam i'm adam" | orv
mada m'i madam

Or maybe you need some simple obfuscation.

PS C:\> get-content C:\work\ComputerData.xml | orv
>sretupmoc<
>'tne-18niw-hj'=eman retupmoc<    
>/ lairessoib<        
>/ noisrevso<        
>retupmoc/<    
>'10cd-ihc'=eman retupmoc<    
>/ lairessoib<        
>/ noisrevso<        
>retupmoc/<    
>sretupmoc/<

Actually, if you find a production-worthy use of this function I hope you'll share it with me and the community. But as always I hope you picked up a PowerShell tidbit along the way that you can use in your "real" PowerShell work.

Enjoy your weekend. I'm sure you earned it.

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