Sharepoint Powershell - To set master page

To find out which master page is currently applied

$web = Get-SPWeb http://siteurl

$web.CustomMasterUrl

$web.MasterUrl

To set a custom master page to the site

$web = Get-SPWeb http://siteurl

$web.CustomMasterUrl = "/_catalogs/masterpage/custom.master"

$web.MasterUrl = "/_catalogs/masterpage/custom.master"

$web.Update()