Prof. Powershell

Label Me Perfect

The power of properties allow you to use it as a, well, a PowerShell labelmaker.

• By Jeffery Hicks
• 04/14/2009

Last time I showed you how to extend PowerShell's reach by adding custom properties to an object with Select-Object. I demonstrated how to use an associative array, sometimes known as a hash table, to create new properties.

The reason to do this with Select-Object is that the new property becomes part of the pipeline:

PS C:\files> dir -recurse | select FullName,LastWriteTime,`
@{name="Size";Expression={$_.Length}},`
@{name="Owner";Expression={(Get-ACL $_).Owner}} | Sort Owner,Size -desc |
ConvertTo-HTML | out-file FileOwners.html

In this example I've used hash tables to rename the "Length" property to the more meaningful "Size" and created a new property called "Owner" that returns the file owner using the Get-ACL cmdlet. The resulting expression is then sorted by "Owner" and "Size" in descending order, converted to HTML and saved to a file.

If you only need to produce nicely formatted reports for the console or perhaps to save to a file, you could pipe the Select-Object expression to Format-Table. However, you can save a step and use hash tables with Format-Table to define the custom properties. The only change is that instead of defining a name, we are defining a label. Here's an example:

PS C:\> get-wmiobject Win32_logicaldisk -filter "drivetype=3" `
-computer "mycompany-dc01" | Format-table DeviceID,VolumeName,`
@{label="Size(MB)";Expression={$_.Size/1mb -as [int]}},`
@{label="Free(MB)";Expression={$_.FreeSpace/1MB -as [int]}},`
@{label="PercentFree";Expression={
"{0:P2}" -f (($_.FreeSpace/1MB)/($_.Size/1MB))}} -autosize

This example takes output from a Get-WMIObject expression querying logical drives on computer MYCOMPANY-DC01 and creates a more user friendly report. The values for "Size" and "FreeSpace" are in bytes. Using a hash table, they have been reformatted to return values in MB. The label has also been modified to reflect the change:

@{label="Size(MB)";Expression={$_.Size/1mb -as [int]}}

The expression also creates a new value called "PercentFree" that is calculated from dividing the "FreeSpace" by "Size":

@{label="PercentFree";Expression={"{0:P2}" -f (($_.FreeSpace/1MB)/($_.Size/1MB))}}

I had to add a step to convert these values to MB to get around a limitation in PowerShell 1.0. When you run this expression, you should get output like this:

DeviceID VolumeName Size(MB) Free(MB) PercentFree
-------- ---------- -------- -------- -----------
C:       OS_C           8182     1410     17.24 %
E:       pagetemp       2046     1215     59.39 %
F:       Backups        4094     3141     76.72 %

You can also use custom labels with the Format-List cmdlet. There's practically no limit to what you can do with custom properties and labels and it's all possible because of PowerShell objects.