Skip to main content

Using Calculated Properties with Select-Object

By
From time to time, I take a good look at how I am teaching PowerShell just to make sure that I am providing the most effective delivery.  That may be why when someone audits the class, they say they learn new material.  Besides, if I did not make changes, I would get bored.

Over the last few months I’ve been teaching how to create calculated properties with Select-Object a little differently.  This is a key skill to have so I need to make sure that we get it right.  Here is an explanation of how to create a custom property.

A custom property simply adds an additional property to an object in the PowerShell pipeline.  It does not change the original object, just the copy of it in the pipeline. Take a look at the properties of Get-Date.
PS C:\> Get-date | Select *


DisplayHint : DateTime
DateTime    : Thursday, February 11, 2016 7:46:10 PM
Date        : 2/11/2016 12:00:00 AM
Day         : 11
DayOfWeek   : Thursday
DayOfYear   : 42
Hour        : 19
Kind        : Local
Millisecond : 564
Minute      : 46
Month       : 2
Second      : 10
Ticks       : 635908167705642214
TimeOfDay   : 19:46:10.5642214
Year        : 2016

We are going to add a new property called OurDate. This new property will have a value consisting of the current year, month, and day in this format: YYYY-MM-DD.  Here is the code to accomplish this:
Get-Date |
    Select-Object -Property *,
    @{
        Name="OurDate";
        Expression={"$($_.Year)-$($_.Month)-$($_.Day)"}
    }

We first place a DateTime object into the pipeline by calling Get-Date. Next we pipe it to Select-Object.  We use the –Property parameter to select which properties to keep from the original object.  In this case, we use ‘*’ to keep all of them.  Next we add a second property.  This one is a hash table.
A hash table is a key/value pair.  In our case, a Property Name / Property Value pair.  Hash tables in PowerShell are denoted by @{}.  Inside of the curly braces is where we declare the property name and the property value.

The Name value provides us with the property name.  You can see that we set this value equal to a string of what we want to call this new calculated property, “OurDate”.

The Expression is the calculated part.  The actual calculation is done inside of its own set of curly braces ‘{}’. While calculating this property, you can reference the current object in the PowerShell pipeline by using one of two generic variables.  $_ is usable in all versions of PowerShell.  $PSItem is for PowerShell 3 and higher.  They both represent the current object in the PowerShell pipeline.  Because this variable represents the object, you have access to all the properties and methods of the object. In our example, we are creating a string that contains the values of 3 properties from the object in the pipeline.  Since we are creating this value as a string, each time we reference the object, we need to place it inside of a subexpression.  That would be the $() surrounding each call to the object.  Here is the end result.

DisplayHint : DateTime
DateTime    : Thursday, February 11, 2016 8:56:23 PM
Date        : 2/11/2016 12:00:00 AM
Day         : 11
DayOfWeek   : Thursday
DayOfYear   : 42
Hour        : 20
Kind        : Local
Millisecond : 151
Minute      : 56
Month       : 2
Second      : 23
Ticks       : 635908209831513532
TimeOfDay   : 20:56:23.1513532
Year        : 2016
OurDate     : 2016-2-11

Take a look at the last item.  It is our calculated property.
You can shorten this code up a bit:
Get-Date |
    Select-Object -Property *,
    @{
        N="OurDate";
        E={"$($_.Year)-$($_.Month)-$($_.Day)"}
    }

Or
Get-Date |
    Select-Object -Property *,
    @{N="OurDate" ; E={"$($_.Year)-$($_.Month)-$($_.Day)"}}

You can also look at the examples in the help file for Select-Object. Take a look at example #4 for another example of creating a calculated property.



Labels:

Comments

Post a Comment

Popular posts from this blog

Sticky Key problem between Windows Server 2012 and LogMeIn

By
This week I instructed my first class using Windows Server 2012 accessed via LogMeIn and discovered a Sticky Key problem every time you press the Shift key. Here is my solution to resolve this.  First off, in the Preferences of LogMeIn for the connection to the Windows Server, click General . Change the Keyboard and mouse priority to Host side user and click Apply at the bottom. On the Windows 2012 server, open the Control Panel – Ease of Access – Change how your keyboard works . Uncheck Turn on Sticky Keys . Click Set up Sticky Keys . Uncheck Turn on Sticky Keys when SHIFT is pressed five times . Click OK twice. If you are using Windows Server 2012 as a Hyper-V host, you will need to redo the Easy of Use settings on each guest operating system in order to avoid the Sticky Key Problem. Updated Information: March 20, 2013 If you continue to have problems, Uncheck Turn on Filter Keys .
6 comments
Read more

Where did a User’s Account Get Locked Out?

By
Updated: May 15, 2015 When this article was originally published, two extra carriage returns were add causing the code to malfunction.  The code below is correct.   My client for this week’s PowerShell class had a really interesting question. They needed to know where an account is being locked out at. OK, interesting. Apparently users hop around clients and forget to log off, leading to eventual lock out of their accounts. The accounts can be unlocked, but are then relocked after Active Directory replication. This problem is solved in two parts. The first one is to modify the event auditing on the network. The second part is resolved with PowerShell. The first part involves creating a group policy that will encompass your Domain Controllers. In this GPO, make these changes. Expand Computer Configuration \ Policies \ Windows Settings \ Security Settings \ Advanced Audit Policy Configuration \ Audit Policies \ Account Management Double click User Account Management C...
18 comments
Read more

Backup and Restore AD LDS with DSDBUTIL.exe

By
Active Directory Lightweight Directory Services allow you to create a directory service that allows applications to have access to user accounts, groups, and authentication similar to Active Directory Domain Services.  The big advantage here is that the schema of the directory service will not be bound by the rules of an Active Directory database.  Exchange 2007/2010, for example, use an instance of AD LDS on the Edge Transport Server to provide for user authentication from the internet.  Because your Active Directory database is not exposed to the internet, this is more secure. Applications will handle most of the dirty work should they require AD LDS.  You may want to make sure the database is being backed up and also have a restore plan in place.  Should the database become corrupt, the application that uses that database will fail.  This document will walk you through backing up and restoring an instance of AD LDS using the dsdbutil.exe command. Fi...
6 comments
Read more