http://blog.softartisans.com/2011/01/24/managing-sharepoint-2010-farm-solutions-with-windows-powershell/

Adding solution to the solution store

Prerequisite: None

Because of the way PowerShell handles file path, the cmdlet’s -literalpath switch only accepts a full path to the WSP file. Using a relative path will result in a File Not Found error.

Deploying solution

Prerequisite: The solution has been added to the solution store.

This solution contains an assembly which must be registered in Web.config (as a SafeControl), so I use the -url switch with stsadm to specify the location of the desired Web application (to which Web.config belongs); alternatively, I could also use the -allcontenturls switch to deploy to all non-administrative web applications. With PowerShell, the -webapplication switch performs the same purpose.

With stsadm it’s also a good idea to run execadmsvcjobs to ensure that the operation is performed immediately. The cmdlet performs this step automatically.

Upgrading solution

Prerequisite: The solution is currently deployed.

Again, note that the cmdlet’s -literalpath switch requires a full path to the WSP file.

Retracting solution

Prerequisite: The solution is currently deployed.

As mentioned in the section Deploying solution, the solution requires modification to Web.config. With PowerShell, you must specify with the -webapplication switch the Web application from which the solution is to be retracted. Failure to do so will result in the following error:

Uninstall-SPSolution : This solution contains resources scoped for a Web application and must be retracted from one or more Web applications.

The cmdlet asks you to confirm the operation before proceeding. For a completely automated operation, you can turn off the confirmation with the -confirm switch.

Removing solution from solution store

Prerequisite: The solution has been retracted.