In case you missed the first three parts, you can check them out here:
Part 1 - simple string arrays
Part 2 - object arrays
Part 3 - building custom object arrays
Custom Parameter Names
From Part 2 - Selecting, we covered how to return only a subset of columns (parameters) from our object arrays and we can even order the parameters so they appear the way we want them in a table or CSV file, etc. Custom parameter names takes this one step further and instead of just selecting which parameters we want to output, we also choose new names for them.
To do this, we use hashtables, one for each parameter. Within the hashtable, we specify two name-value pairs. The first one is "name" (or "n" is also acceptable) with the value equal to whatever name we want to give the new parameter. The second pair is "expression" (or "exp", or even just "e") with the value of the old parameter's name in the form of a powershell expression.
Below you see an example where we replace the default "dir" output with our own names. Don't be afraid of the dollar-underscore part of the statement, we'll cover this later; for now, just understand that it's a special variable that represents the current object (record) in the table. The select statement transforms the incoming data object by object and the $_ variable just means the current one it's processing.
PS C:\> dir
Directory: C:\
Mode LastWriteTime Length Name
---- ------------- ------ ----
d-r--- 7/19/2015 3:39 AM Program Files
d-r--- 7/21/2015 12:49 PM Program Files (x86)
d-r--- 7/19/2015 3:39 AM Users
d----- 7/19/2015 3:40 AM Windows
-a---- 7/2/2015 7:39 PM 2544 myserverstatus.csv
-a---- 7/2/2015 7:39 PM 2544 myserverstatus.xml
PS C:\> dir |select Name
Name
----
Program Files
Program Files (x86)
Users
Windows
myserverstatus.csv
myserverstatus.xml
PS C:\> dir | select @{n="Nombre"; e={$_.name}}
Nombre
------
Program Files
Program Files (x86)
Users
Windows
myserverstatus.csv
myserverstatus.xml
PS C:\> dir | select @{n="Nombre"; e={$_.name}}, @{n="Type"; e={$_.mode}}
Nombre Type
------ ----
Program Files d-r---
Program Files (x86) d-r---
Users d-r---
Windows d-----
myserverstatus.csv -a----
myserverstatus.xml -a----
PS C:\> dir | select @{n="Nombre"; e={$_.name}}, @{n="Type"; e={$_.mode}}, @{n="File Write Time"; e={$_.LastWriteTime}}
Nombre Type File Write Time
------ ---- ---------------
AMD d----- 7/19/2015 3:38:16 AM
EFI d----- 5/23/2015 4:52:28 AM
Games d----- 6/20/2015 8:13:11 PM
HyperV d----- 7/7/2015 3:58:26 PM
Intel d----- 6/24/2015 6:59:47 PM
MSI d----- 6/24/2015 6:50:19 PM
PerfLogs d----- 7/10/2015 4:04:22 AM
Program Files d-r--- 7/19/2015 3:39:38 AM
Program Files (x86) d-r--- 7/21/2015 12:49:26 PM
Users d-r--- 7/19/2015 3:39:41 AM
Windows d----- 7/19/2015 3:40:35 AM
Windows.old d----- 7/19/2015 4:36:26 AM
myserverstatus.csv -a---- 7/2/2015 7:39:18 PM
myserverstatus.xml -a---- 7/2/2015 7:39:27 PM
PS C:\>
As you can see the expression part of the hashtable has it's own curly brackets. You can use this template to help you create your own custom parameter names:
$datatable | select @{n="New Parameter Name 1"; e={$_.OldName1}}, @{n="New Parameter Name 2"; e={$_.OldName2}}, @{n="New Parameter Name X"; e={$_.OldNameX}}
Rounding and changing units
Another very common use for using custom selects is to change data units and rounding fractions. Using another "dir" example, say we want to convert the units to GB or TB, as working with bytes can be annoying.
PS C:\> dir C:\HyperV\VHD\ | select Name, Length
Name Length
---- ------
BVadm1.vhdx 11446255616
BVDC1.vhdx 10305404928
BVServer10.vhdx 9030336512
BVSMA1.vhdx 9936306176
BVSQL1.vhdx 10741612544
BVSQL1_DATA.vhdx 1077936128
DBadm2.vhdx 18291359744
Dev10.vhdx 18996002816
PS C:\> dir C:\HyperV\VHD\ | select Name, @{n="Size (GB)"; e={$_.length / 1GB}}
Name Size (GB)
---- ---------
BVadm1.vhdx 10.66015625
BVDC1.vhdx 9.59765625
BVServer10.vhdx 8.41015625
BVSMA1.vhdx 9.25390625
BVSQL1.vhdx 10.00390625
BVSQL1_DATA.vhdx 1.00390625
DBadm2.vhdx 17.03515625
Dev10.vhdx 17.69140625
PS C:\> dir C:\HyperV\VHD\ | select Name, @{n="Size (GB)"; e={[math]::round($_.length / 1GB,2)}}
Name Size (GB)
---- ---------
BVadm1.vhdx 10.66
BVDC1.vhdx 9.6
BVServer10.vhdx 8.41
BVSMA1.vhdx 9.25
BVSQL1.vhdx 10
BVSQL1_DATA.vhdx 1
DBadm2.vhdx 17.04
Dev10.vhdx 17.69
PS C:\>
Notice how I can mix and match normal parameters with custom parameters, and that they appear in the order I list them in the select statement.
Computing new parameters
Perhaps the most powerful use of custom selects is to compute data from other parameters. When possible, using custom selects is the fastest way to transform and compute data from lists. It's much better than writing a whole script to create a new object array and build new objects with different parameters.
PS C:\> dir | select Name,
>>> @{n="Directory"; e={$_.mode -like "d*"}},
>>> @{n="Name Length"; e={$_.name.length}},
>>> @{n="Days Old"; e={((get-date) - $_.creationTime).days}}
Name Directory Name Length Days Old
---- --------- ----------- --------
AMD True 3 31
HyperV True 6 31
Intel True 5 27
Program Files True 13 11
Program Files (x86) True 19 11
Users True 5 11
Windows True 7 11
Windows.old True 11 2
myserverstatus.csv False 18 19
myserverstatus.xml False 18 19
PS C:\> gwmi win32_logicaldisk | select @{n="Letter"; e={$_.deviceid[0]}},
>>> @{n="Used Space (TB)"; e={[math]::round(($_.size - $_.freespace)/1TB,3)}},
>>> @{n="Percent Free"; e={[math]::round(100 * $_.freespace / $_.size,0)}}
Letter Used Space (TB) Percent Free
------ --------------- ------------
C 0.022 82
D 0.017 86
I 2.202 27
M 3.917 22
PS C:\>
As you can see, using custom selects with object arrays is a great way to extract exactly what you need from source data without doing too much work. Also note that you can put each parameter definition on a new line as long as you end the current line with a comma as I did here.
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