formatting - Why is the output format changed when running two PowerShell commands in one line?

Question

I'm getting unexpected results when executing two PowerShell commands separated by a semicolon. The output of the second command changes. If I run them in reverse order, I don't see the second command output.

Here I'm simply trying to get a time stamp and a list of groups a user belongs to in AD, as a one-liner.

If I run this line, i get the following output:

Get-ADPrincipalGroupMembership username | Select-Object name

name
----
Domain Users
CL-Inventory-Group
...

However if I run the following, this behavior changes:

get-date; Get-ADPrincipalGroupMembership username | Select-Object name

Wednesday, April 3, 2019 2:31:35 PM

name : Domain Users


name : CL-Inventory-Group


...

Stranger yet, If i run the in reverse, meaning i say get-date after the first command, the date stamp never shows up after the groups are listed.

Am I improperly separating commands?

Answer 1

tl;dr:

Submitting multiple ;-separated commands at the prompt (interactively) still sends their output to a single pipeline (you can think of each command line submitted as an implicit script file).

In short: in your case, the first command output's automatic display formatting also determined the display formatting for the 2nd command, so which command comes first matters:

• get-date; Get-ADPrincipalGroupMembership username | Select-Object name

  • locked in implicit use of Format-List for all output following Get-Date, which explains the each-property-on-its-own line output from the Get-ADPrincipalGroupMembership ... command.

• If i run the reverse, meaning i say get-date after the first command, the date stamp never shows up after the groups are listed.

  • Select-Object output instances of type [pscustomobject], which, due to their having just 1 property in this case, locked in tabular display, i.e., implicit use of Format-Table, with the selected property as the only column, i.e., just Name here. Since the [datetime] type output by Get-Date doesn't have a Name property, Get-Date's output was effectively invisible.

Read on for background information and the complete rules.

---

PowerShell's default display formatting is optimized for objects of the same type, as that is the typical case.

If a pipeline contains a mix of types, the specific formatting that results by default depends on:

• the order of objects in the pipeline
• and their default formatting behavior

See the next section for details.

You can use explicit Format-* calls to control the formatting; in your case, you can use Format-Table on your 2nd command to force tabular output:

Get-Date; Get-ADPrincipalGroupMembership username | Select name | Format-Table

Caveat: The output from Format-* cmdlets are formatting instructions rather than the original data, which makes this output unsuitable for further programmatic processing.

---

How PowerShell formats objects of different types in the same pipeline for display:

In the absence of explicit formatting commands (Format-Table, Format-List, ...), PowerShell automatically chooses a suitable display format, based on a given object's type:

• If present for a given type, PowerShell uses predefined formatting instructions (see Get-Help about_Format.ps1xml)
• In their absence:
  • If the type is a primitive type (see below): the object's .ToString() representation is output.
  • Otherwise: the format style is chosen based on the following simple rules: 4 or fewer properties? -> Format-Table; 5 or more? -> Format-List.

Note: Primitive is used loosely here to refer to:

• all primitive CLR types - those for which .IsPrimitive returns $true, namely
  [Boolean], [Byte], [SByte], [Int16], [UInt16], [Int32], [UInt32], [Int64], [UInt64], [IntPtr], [UIntPtr], [Char], [Double], [Single]
• types [decimal], [bigint], [string] and [securestring]
• any other property-less types.

If all objects in a pipeline are of the same type, the above by definition applies to all of them.

By contrast, if there is a mix of types in the pipeline, the following logic applies:

• Any instances of primitive types always print the same, namely as a representation of the single value that they are (not an as an object with properties), obtained via a call to their .ToString() method; e.g., 12 or 3.0 or hi; primitive types have no bearing on the formatting of subsequent objects in the pipeline.

• The first non-primitive object in the pipeline:

  • is itself printed based on either its predefined formatting instructions or the default rules stated above (based on the number of properties).

  • locks in the format style - list vs. table - for all remaining non-primitive objects:

    • If the object itself implicitly uses Format-Table or Format-List, so will all remaining non-primitive objects.
    • If the object implicitly uses Format-Custom (e.g, in the case of Get-Date, via predefined formatting), it is Format-List that is locked in.

• All subsequent non-primitive objects then use the locked-in format style.

  • Caveat: If Format-Table is locked in, the first non-primitive object alone determines the set of properties displayed as table columns, which can cause subsequent objects to seemingly disappear if they don't have these properties - such objects are still in the output stream, however, they're just not displayed - see this answer for a demonstration.

    • On a side note: Since PSv5, implicit use of Format-Table results in asynchronous behavior that may be surprising; see this answer.

  • If it is Format-List that is locked in, no information is "lost", as each object's properties are then listed individually, on their own lines.