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.
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