Powershell – Variables

I’ve just started to play around with variables in Powershell. I used them a little bit before this, but am in the process of figuring out basic commands around variables. First thing to note is that variables are not case-sensitive.  That means that a variable called “$MyVariable” is the same as “$MYVARIABLE” or “$mYvARIABLE”.  You can declare a variable and assign a value easily using something like:

$variable = value

That pretty much covers the majority of basic assignments. You can also use the Set-Variable command to not only set a variable, but change some options such as making a variable read-only or making it into a constant. Once a constant has been declared/set, you cannot delete it. It will be cleaned up when the Powershell session ends.  The only somewhat tricky point around variables is using special characters.  All variables should be declared within {}’s if you need to use special characters.
It’s easy to list all of the variables and their values using:

dir variable:
dir variable: | Format-Table Name, Value, Description -autosize

The first command lists all variables and their values. The second lists the name, value, and a Description property as well and then auto-sizes everything accordingly. Useful if you want to set other properties of your variables besides name and value. You can also use that latter command with a “-wrap” parameter to show the definitions of the system-level variables.

If you want to use Windows Environment variables, you’ll want to reference the “env:” virtual drive within Powershell. These are the variables such as your Path or Windows folder that are used within Windows. Any changes you make to these variables are only set within the scope of your Powershell session.

By default, variables stay within the scope of the function or session, but you can override this with $private to limit a variable to a scope, $local to use default scoping and enable variables to be read by sessions the current session creates, $script to allow the variable to work anywhere in the current script, and $global to allow the variable to be used anywhere, even external functions and scripts.

Variable Types and Attributes

Working with variables is easy with the defaults because they are weakly or loosely typed. If I assign a string to a variable, that variable contains a string. If I assign a bit value, the variable contains a Byte. That means that variables can change types easily as the script runs. However, that also means that if I’m expecting a variable to contain a date value and it somehow gets a string or floating point value, I could be very surprised.  Powershell allows for strong data typing of variables as well. If you define a variable in this manner:

[datetime]$myDate = "2009-12-25"

You will explicitly declare a variable of type DateTime. If you then attempt to set the variable to a string, you’ll get an error message. The variable types are standard .NET types with a handful of specific Powershell types.

You can set other properties using the Attributes for a variable and can even clear out the strong Typing using an Attributes.Clear() command against the variable. Attributes can be used to set constraints around a variable such as whether or not the variable can store $Null values, check constraints, or even RegEx patterns for the data stored in the variable.

This is by no means a definitely explanation of everything around variables in Powershell, but it’s a good start for most people. You can always run get-help About_Variables within Powershell for more details or get-help Set-Variable to read more about setting variables. I may update or post a follow-up if something else pertinent about variables comes up. I’d love to know a way to list out variable datatypes from within Powershell, but haven’t found a way yet.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s