To read Environment variables in T-SQL, you can use the xp_cmdshell extended stored procedure in SQL Server.

The following example shows how to read the %windir% environment variable, which gives the Windows directory path on SQL Server using xp_cmdshell:

DECLARE @windir nvarchar(255)
CREATE TABLE #Tmp
(
EnvVar nvarchar(255)
)
INSERT INTO #Tmp exec xp_cmdshell 'echo %windir%'
SET @windir = (SELECT TOP 1 EnvVar from #Tmp)
SELECT @windir as 'Windows Directory'

NOTE:To run this command, you need to be a member of the sysadmin fixed server. If you want others to be able to execute this command, you will have to explicitly grant them permission to execute the xp_cmdshell stored procedure.

Find more information about this stored procedure at MSDN.