How does pipeline affect Select-Object?

Question

I'm trying to make a table in PowerShell with custom headers, I am able to do this with

"var" | Select @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}

First Second Third
----- ------ -----
1     2      3

However, without the initial object, there is no output

Select @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}

I can't tell the difference between these other than one is after a pipeline while the other isn't. Why won't this work?

Why doesn't this work?

Nico Nekoru · Accepted Answer · 2020-07-07 22:19:24Z

3

The cmdlet Select-Object (alias Select) has one required parameter which is -inputObject Which also happens to be the object that gets passed through the pipeline.

Select-Object -InputObject "Example" -Property @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}

Will have the output


First Second Third
----- ------ -----
1     2      3

While without -InputObject, it will not have an output because there is no input (Thanks @mklement0).

Select-Object "Example" -Property @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}
# No Output

and with the pipeline, it will

"Example" | Select-Object "Example" -Property @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}

First Second Third
----- ------ -----
1     2      3

The -inputObject Parameter will usually contain the table you would like to select columns from or other things (if you are not using expressions).

edited Jul 7, 2020 at 22:19

answered Jul 7, 2020 at 19:37

Nico Nekoru

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3 Comments

Nationalism Over a year ago

What does the InputObject parameter do?

Nico Nekoru Over a year ago

If you are selecting a certain column instead of doing an expression, the table goes in the inputObject parameter

mklement0 Over a year ago

Nice, but worth adding to "without -InputObject, it will not have an output": because there is no input.

iRon · Accepted Answer · 2020-07-08 07:55:29Z

Although the answer from @Neko answers the exact question, I think it is important to mention that the Select-Object cmdlet is not mentioned to construct new custom objects:

Quote from the Get-Help Select-Object -Online:

The Select-Object cmdlet selects specified properties of an object or set of objects.

In other words; the object needs to exist in order to select its properties.

For what you are doing with

"var" | Select @{n="First";e={"1"}}, @{n="Second";e={"2"}},@{n="Third";e={"3"}}`

you're creating a new pscustomobject by removing all the default properties from the "Var" (string) object and adding new properties with a hard-coded (static) expression which is a long-winded syntax and quiet expensive for building a new object.

To construct a new pscustomobject ("table"), you can simply use the constructor syntax:

[pscustomobject]@{First = '1'; Second = '2'; Third = '3'}

For legacy (prior PSv3) POwerShell versions:

New-Object PSObject -Property @{First = '1'; Second = '2'; Third = '3'}

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