Truncated output when running Powershell command in a loop

Question

I wrote the following function as a rudimentary way of implementing basic functionality of the *nix watch command within Powershell:

function watch {
    Param(
        [Parameter(Mandatory=$true)][string]
        $command,
        [Parameter(Mandatory=$false)][int]
        $n = 2
    )

    while($true) {
        clear
        Write-Output (iex $command)
        sleep $n
    }
}

When working with cmdlets that return Powershell objects, I get strange behavior. For example if I run `watch 'get-command ls', on the first iteration I get the following formatted output of the object:

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Alias           ls -> Get-ChildItem

But on the second and subsequent iterations, it truncates the object property headers (and any description above it in certain other commands):

Alias           ls -> Get-ChildItem

I'm curious as to why this behavior happens, and how I can make the output identical to the first iteration for all subsequent iterations. I am running this in Powershell 5.1 on Windows 10.

Because Format-Table to which you implicitly pipe output does not know that you clear screen, and it just continue to print table, not printing new table. — user4003407
– user4003407, Commented Oct 21, 2016 at 5:06

TessellatingHeckler · Accepted Answer · 2016-10-21 05:29:32Z

1

I think it's because you're mashing up Write-Output which writes to the pipeline, and clear which is properly named Clear-Host and clears the local terminal, and knows nothing of the pipeline. It goes better with Write-Host.

Since your function loops eternally, the pipeline output never finishes, so what you would get get is this endless list with one set of headings:

CommandType     Name                                               Version    Source
-----------     ----                                               -------    ------
Alias           ls -> Get-ChildItem
Alias           ls -> Get-ChildItem
Alias           ls -> Get-ChildItem
Alias           ls -> Get-ChildItem
Alias           ls -> Get-ChildItem
Alias           ls -> Get-ChildItem

but you clear the /display/ in the middle of the list, so it carries on printing the subsequent items without the headings.

If you explicitly write to Format-Table in your command you can get the headings repeated:

watch { Get-Alias ls | Format-Table }

answered Oct 21, 2016 at 5:29

TessellatingHeckler

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2 Comments

codewario Over a year ago

I actually got the same behavior without combining write-output and iex, I combined write-output thinking it might fix the issue, but still got the same behavior. I'll give adding format-table a shot

codewario Over a year ago

Good news, Format-Table was exactly what I needed. Thanks!

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