Topic: Do-Until

Notice: This post is a part of the PowerShell Monday series — a group of quick and easy to read mini lessons that briefly cover beginning and intermediate PowerShell topics. As a PowerShell enthusiast, this seemed like a beneficial way to ensure those around me at work were consistently learning new things about Windows PowerShell. At some point, I decided I would share these posts here, as well. Here’s the PowerShell Monday Table of Contents.

I said I would say it this week too, so let’s get it out of the way: The Do-While and Do-Until loops ensure that the statement list is run at least once. Your code is going to execute at least one time using a Do construct. This is because the condition, that determines whether or not we loop again, isn’t evaluated until the statement list has run once. If you need to ensure your code executes at least one time, then the Do-While and Do-Until variations of the Do loop, may be the language construct you need.

Here’s the conceptual examples of the Do-Until.

Do {<statement list>} Until (<condition>)

Do {
    <statement list>

} Until (<condition>)

Our first below example should look somewhat familiar. We saw it first last week in the Do-While examples. We assign 1 to the $Number variable and then enter the Do loop. We output a string statement and then increment the $Number variable. Then, we check the condition. The condition indicates that we’ll continue to loop until $Number is less than 10. Well, 1 is always less than 10, so we exit the construct. This is a clear example that we always run the statement list at least once in a Do loop, before we check the condition.

$Number = 1

Do {
    Write-Output -InputObject "In the Do loop ($Number)."
    $Number++

} Until ($Number -lt 10)

In the Do loop (1).

Let’s modify this example. We’ll reinitialize the $Number variable back to 1 and start again. This time our condition states that we’ll continue to loop until $Number is greater than 10. As $Number is lower, you can likely guess that we’re going to run through this loop several times. Ten times, to be exact.

$Number = 1

Do {
    Write-Output -InputObject "In the Do loop ($Number)."
    $Number++

} Until ($Number -gt 10)

In the Do loop (1).
In the Do loop (2).
In the Do loop (3).
In the Do loop (4).
In the Do loop (5).
In the Do loop (6).
In the Do loop (7).
In the Do loop (8).
In the Do loop (9).
In the Do loop (10).

In closing out the Do variations, we’ll add a final example. Much like last week, we’ll transition from using numeric values to date times. In this example, we’ll add 10 seconds to the current time. Then, we’ll output a string value and sleep (pause) for a second. Eventually, the current time will be greater than the time when we began; therefore, the condition will be $true and we’ll exit the looping construct.

$Time = (Get-Date).AddSeconds(10)

Do {
    Write-Output -InputObject 'Waiting for time to pass...'
    Start-Sleep -Seconds 1

} Until ((Get-Date) -ge $Time)

Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...
Waiting for time to pass...

Next week is our final language construct — the While loop. By then, you’ll have a full and complete understanding of the various control structures we can use to respond to conditions in PowerShell. In case you hadn’t considered it, learning these concepts — even if it’s with PowerShell — is going to make learning other scripting, and even programming languages, easier to do.