Quick Learn – Clear-Host, Without Clearing the Host

After you read this, read part 2 (and download the TMModule)

I use the Clear-Host cmdlet alias, cls, throughout the day to clear out whatever typing I have inside my Windows PowerShell console. It does its job well, but recently I’ve wanted it to work differently. I wanted it to appear that the console host has been cleared, but still allow me to scroll back up to see what was on the screen before it was cleared. I started playing around with the console class, [Console]. While not necessary, this can also be written using the namespace, System, such as [System.Console]. I like the idea of being as complete as possible and so you’ll see me use the namespace even though it’s not necessary.

Before I could write something reusable, such as a function, I had to figure out if what I wanted to accomplish, was even possible. I knew I was working with [System.Console] and so I piped that to Get-Member, but it returned the methods and properties of System.RuntimeType, seen below.

PS C:\> [System.Console] | Get-Member

    TypeName: System.RuntimeType

I struggled for a moment until I remembered an article I had read on using static classes. I found that page again, http://technet.microsoft.com/en-us/library/dd347632.aspx, and was quickly reminded that using the -Static parameter of the Get-Member cmdlet would get me the correct results.

PS C:\> [System.Console] | Get-Member -Static

    TypeName: System.Console

Running the command above produces the TypeName as shown, but it also produces all the methods and properties. I started looking over the properties and a couple about the cursor quickly caught my eye, especially the CursorTop property. After the Get-Member command from above, and based on the results of returning the CursorTop property, my cursor was positioned at line 59 inside my console, as can been seen in the example below on line 2. I cleared the screen, and beginning on line 4 below, I reran the command three more times. Each time, it gave me the location where the cursor was last positioned.

PS C:\> [System.Console]::CursorTop
59
PS C:\> cls
PS C:\> [System.Console]::CursorTop
1
PS C:\> [System.Console]::CursorTop
3
PS C:\> [System.Console]::CursorTop
5
PS C:\>

I decided I would assign the value, 0, to the CursorTop property and suddenly I was writing over the text on the top line. Take a close look at line 1 below.

PS C:\> blahblahConsole]::CursorTop
59
PS C:\> cls
PS C:\> [System.Console]::CursorTop
1
PS C:\> [System.Console]::CursorTop
3
PS C:\> [System.Console]::CursorTop
5
PS C:\> [System.Console]::CursorTop = 0

I could move my cursor, great, but this wasn’t exactly what I wanted. What I wanted was to push that scroll bar down so that anything that was already on the screen was pushed off the top of my console, and all that was left was my PowerShell prompt. I still believed there was a way to do this and so I spent a little more time looking over the properties. I found four that began with Window – WindowHeight, WindowLeft, WindowTop, and WindowWidth – and began to experiment with them. I didn’t suspect I’d be doing anything with the height and width but I thought I would check out their values anyway – 50 and 120, respectively.

PS C:\> [System.Console]::WindowHeight
50
PS C:\> [System.Console]::WindowWidth
120
PS C:\>

WindowLeft didn’t seem to be that important, because no matter how much was typed before I entered [System.Console]::WindowLeft, the property value was still set to 0. Then I entered in [System.Console]::WindowTop and it was also 0 every time. Then it dawned on me, what if I changed its value like I did with CursorTop. I tried it and my scroll bar started jumping all over. I’m getting close!

PS C:\> [System.Console]::WindowTop = 10
PS C:\> [System.Console]::WindowTop = 200
PS C:\> [System.Console]::WindowTop = 2000
PS C:\> [System.Console]::WindowTop = 0

We know the CursorTop value changes, so what would happen if we set the value of WindowTop to CursorTop? I tried it, and it worked!

PS C:\> [System.Console]::WindowTop = [System.Console]::CursorTop
PS C:\>

I thought I was done when I took another moment and scanned over the methods. I found one called SetWindowPosition. Instead of simply assigning a new value to the property WindowTop, I decided I would use the method to do the work for me. I eventually ran both of these options through the Measure-Command cmdlet and determined that there was no gain in speed by using one option over the other.

So, once I knew what to do, I opened my profile ($PROFILE) and created an empty function. For whatever reason, I called it clx thinking that this would be a good option for me. Turns out that while clx has little meaning, I was able to quickly remember it and start using it right away. Now, every time I want it to appear that I’ve cleared my host, but didn’t really, I type clx and press Enter.

Function clx {
    [System.Console]::SetWindowPosition(0,[System.Console]::CursorTop)
}

I added one additional feature to this function as is seen in the example below. This option allowed me to run the clx function and leave the last n number of rows on the screen. Try it out by ensuring your have some output in your console and then entering clx 2. This will “clear” the console screen but still allow you to view the last two rows without scrolling back up. Try it and it may make more sense.

Function clx($SaveRows) {
    If ($SaveRows) {
        [System.Console]::SetWindowPosition(0,[System.Console]::CursorTop-($SaveRows+1))
    } Else {
        [System.Console]::SetWindowPosition(0,[System.Console]::CursorTop)
   }
}

Here’s a video of the function in action. The first thing we do is return 5 processes and then 5 services. Then we use cls and notice that we cannot scroll back up to see what was cleared. This is the typical behavior. When we add the processes and services back, and then use the clx function, we can see that we have the option to scroll back up and see what was on the screen, before we “cleared” it.

3 thoughts on “Quick Learn – Clear-Host, Without Clearing the Host

  1. DexterPOSH

    Well written !
    Based on your post I tweaked my PSReadline Keyhandler for Ctrl+l .
    Used you code
    #Placed this on my profile to overwrite the default keyhandler.
    Set-PSReadlineKeyHandler -Chord Ctrl+l -ScriptBlock {[System.Console]::SetWindowPosition(0,[System.Console]::CursorTop)

    Reply

Leave a Reply

Your email address will not be published.