Select

Select-Object serves for multiple purposes for object transformation and picking from collection.

The following is a overview:

• Picking composed object by one or more properties with -Property.
  • select properties reversely by -ExcludeProperty.
• Picking single property value with -ExpandProperty.
• Take a count from start or end with -First or -Last.
• Skip a count from start or end -Skip or -SkipLast.
• Cherry-Pick one or more items by zero-based index.
• Distinct items selected from pipline.

TIP

Use select alias for Select-Object.

Select as an Object

Select-Object wraps selected properties as an PSCustomObject

gps | Select-Object -Property Name # gets an object[] instead of string[]
(gps | select Name -First).GetType().Name # PSCustomObject

You can select multiple properties:

$ gps | select Name, Id -First 1 | gm

   TypeName: Selected.System.Diagnostics.Process

Name        MemberType   Definition
----        ----------   ----------
Equals      Method       bool Equals(System.Object obj)
GetHashCode Method       int GetHashCode()
GetType     Method       type GetType()
ToString    Method       string ToString()
Id          NoteProperty int Id=12588
Name        NoteProperty string Name=ABDownloadManager

Nested Property and Custom Property

-Property accepts a Script Block to calculate the nested property you'd like to select.

$ gps | select Name, { $_.StartTime.DayOfWeek } -First 1

Name               $_.StartTime.DayOfWeek
----              ------------------------
ABDownloadManager                 Thursday

You can even define a custom calculation with a HashTable shaped as:

• lable: string: name of the property
• expression: ScriptBlock: an procedure returns the calculated property value.

$ gci -File | select Name, @{ label = 'Size(KB)'; expression = { $_.Length / 1KB } }

Name         Size(KB)
----         --------
.gitignore       0.01
dotfiles.ps1     1.69
flake.lock       1.64
flake.nix        1.02
home.nix         0.63
install.ps1      1.49
make_vs.ps1      2.06
README.md        0.85

Expand a Property

Select Value Only

To select single value of a property instead of being wrapped as an object, use -ExpandProperty. The return type is still an object[]. But each memeber should be string indeed in the following snippet.

gps | select -ExpandProperty Name
(gps | select -ExpandProperty Name).GetType().Name # object[]
(gps | select -ExpandProperty Name -First 1) -is [string] # True

NOTE

ForEach-Object can achieve the same thing

gps | foreach Name
# or use another alias of ForEach-Object %
gps | % Name

Append Extra NoteProperty

-ExpandProperty can be used together with -Property, selected properties by -Property will be added as NoteProperty to the selected -ExpandProperty.

WARNING

This approach mutates the selected property instead of generating a new object.

$player = @{
    Id = 123
    Level = 15
    Status = 'Empowered'
    Partner = @{
        Id = 234
        Level = 12
        Status = 'Poisoned'
    }
}

# Adds a NoteProperty `Status` with the same value from $player to $player.Partner
$partner = $john | select Status -ExpandProperty Partner

[object]::ReferenceEquals($partner, $john.Child) # True

$child | gm -MemberType NoteProperty # Status

$partner.Status # Empowered # . accessor prefers ETS property
$partner['Status'] # Poisoned

NOTE

-ExpandProperty can only take one property.

Calculated Property

Select-Object can generate new NoteProperty to the PSCustomObject returned by specifying a HashTable with following shape:

• Name(or N, Lable, L): the name of the new property.
• Expression(E): the calculation logic for the new property represented as a script block.

$person = [pscustomobject]@{
    FirstName = 'John'
    LastName = 'Smith'
}

# return a new PSCustomObject that has FirstName, LastName and FullName.
$person | select *, @{ Name = 'FullName'; Expression = { "$($_.FirstName) $($_.LastName)" } }

NOTE

All selected properties from a HashTable will be regenerated as NoteProperty. Those selected properties are essentially Keys of the HashTable, not real properties.

Take a Count

Select-Object can also take specific count of items from a collection, from start or end.

gps | select -First 5
gps | select -Last 5

Skip a Count

gps | select -Skip 5
gps | select -SkipLast 5

Cherry Pick

$dir = gci -Directory
$dir | select -Index 1, ($dir.Length - 1) # Pick first and last item

Distinction

# might have duplicated entries since file extensions should ignore casing.
ls | select Extension -Unique
# list all extensions appeared in current directory
ls | select Extension -Unique -CaseInsensitive