PowerShell's Common Parameters

Take a look at thehelp for any PowerShell cmdlet and you'll notice <CommonParameters> at the end of each one's syntax list. What's that mean?

PowerShell has a set of parameters that the shell itself adds to every cmdlet. In other words, the developer of a cmdlet doesn't need to code these - they're just there, like magic. They're also added to the scripts and functions you write in the shell. You can learn about the complete set by running help about_common_parameters in the shell, but here's a quick rundown of the most frequently-used ones.

-Verbose

This switch sets the shell's $VerbosePreference variable to "Continue" for the cmdlet, script, or function, meaning that any embedded writes to the Verbose pipeline (a la Write-Verbose) will display output. There's no guarantee that a cmdlet will actually produce verbose output, but the -Verbose switch will enable whatever's in there.

-Debug

Works a lot like -Verbose, setting the $DebugPreference variable. 

-OutVariable

This parameter takes a variable name. Now, keep in mind that variable names do not include the $. The idea is that the cmdlets' output will be captured into whatever variable you specify. So, to capture a list of services into the variable $MyServices, and then display the contents of that variable, you'd do this:

Get-Service -outvariable myservices
$myservices

You need to use some caution here. Consider this wrong example:

$x = 'hello'
Get-Service -outvariable $x
$x

Here, the services will be stored in a variable $hello. That's because I put the dollar sign in front of x when I passed it to -OutVariable. The dollar sign makes PowerShell use the contents of the variable. This one's easy to mess up, and can be very confusing if you do.

-ErrorVariable

Works a lot like -OutVariable, accepting a variable name (no dollar sign!). Any errors that occur while running the command will be stored in this variable for you.

-ErrorAction

Accepts one of these values: Stop, Continue, SilentlyContinue, Inquire. This is a key trick to error handling, changing the command's default behavior when it encounters an error.


Again, you can read more about these and the other common parameters in PowerShell's help.