PowerShell Practice and Style
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Output and Formatting

Formatting Output

TODO: This whole document is STILL ROUGH DRAFT

Don't use Write-Host unless you really mean it

Previous to PowerShell 5, Write-Host has no functionality at all in non-interactive scripts. It cannot be captured or redirected, and therefore should only be used in functions which are "Show"ing or "Format"ing output, or to display something as part of an interactive prompt to the user.
That is: you should not use Write-Host to create script output unless your script (or function, or whatever) uses the Show verb (as in, Show-Performance) or the Format verb (as in, Format-Hex), or has a -Formatted switch parameter. You may also use it to build a interactions with the user in other cases (e.g. to write extra information to the screen before prompting the user for a choice or input).
Generally, you should consider the other Write-* commands first when trying to give information to the user.

Use Write-Progress to give progress information to someone running your script

When you're letting the user know how far through the script they are, or just making sure they know that something is happening, Write-Progress is the right command to use. In the case of graphical hosts or remote jobs, this output can be shown to the user in real time, even when verbose and other streams are being collected and logged.
Progress output is ephemeral, however, in that it doesn't stick around. You should not put anything exclusively in the progress stream that the user needs to see, or might want to review after the script finishes.

Use Write-Verbose to give details to someone running your script

You should use verbose output for information that contains details about the value of computation, or the reason why a certain execution path was chosen. It should be information that is useful but not necessary for anyone running the script, providing status information such as "Server1 processed", or logic information such as "Server2 up-to-date, skipped", but shouldn't be used for actual results or important information.

Use Write-Debug to give information to someone maintaining your script

You should use the debug output stream for output that is useful for script debugging (ie: "Now entering main loop" or "Result was null, skipping to end of loop"), or to display the value of a variable before a conditional statement, so the maintainer can break into the debugger if necessary.
TIP: When debugging you should be aware that you can set $DebugPreference = "Continue" to see this information on screen without entering a breakpoint prompt.

Use CmdletBinding if you are using output streams

As we've already written elsewhere, you should probably always use CmdletBinding.
Using CmdletBinding is particularly important, however, when you're using Write-Verbose and Write-Debug, as the Verbose and Debug output streams are off by default, and the [CmdletBinding()] attribute enables the common -Verbose and -Debug switches which turn those streams on.
CmdletBinding also enables the switches for the Warning and Error streams, as well as ways of collecting those streams into variables. You should read the always use CmdletBinding topic for more information.

Use Format Files for your custom objects

You should not use format commands inside functions. Instead you should include a modulename.format.ps1xml file in the FormatsToProcess field of your module's PSD1 manifest, and define a PSTypeName on your objects, so that PowerShell will format your output the way you want, automatically.

Only output one "kind" of thing at a time

You should avoid mixing different types of objects in the output of a single command, because you may get empty rows in your output or cause table output to break into list output, etc.
For the sake of tools and command-search, you should indicate with the [OutputType()] attribute the output type(s) of your scripts, functions or cmdlets (see about_Functions_OutputTypeAttribute for more information).
When you combine the output of multiple types objects, they should generally be derived from a common basetype (like FileInfo and DirectoryInfo come from System.IO.FileSystemInfo), or should have format or type files which cause them to output the same columns. In particular, you must avoid outputting strings interspersed in your output.

Two important exceptions to the single-type rule

For internal functions it's ok to return multiple different types because they won't be "output" to the user/host, and can offer significant savings (e.g. one database call with three table joins, instead of three database calls with two or three joins each). You can then call these functions and assign the output to multiple variables, like so:
$user, $group, $org = Get-UserGroupOrg
When you call Out-Default. If you must return multiple object types from an external command, you should name your function in such a way that it's obvious to users that you're returning multiple things, and you must call Out-Default separately for each type of object to ensure that the outputs don't ever get mixed up by the formatter.
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Outline
Formatting Output
Don't use Write-Host unless you really mean it
Use Write-Progress to give progress information to someone running your script
Use Write-Verbose to give details to someone running your script
Use Write-Debug to give information to someone maintaining your script
Use CmdletBinding if you are using output streams
Use Format Files for your custom objects
Only output one "kind" of thing at a time
Two important exceptions to the single-type rule