SQL Server 2012 Express Service Pack 2 - installation error 0x84B40001
@Nissanna, As you said, you are getting the error during cmd installation of sql server express 2012.


  The syntax of argument "/HIDECONSOLE" is incorrect;Either the
  delimiter '=' is missing or there is one or more space characters
  before the '='" Error 0x84B40001


when you are running this syntax:


  SQLEXPR_x86_ENU.exe /ACTION=Install /HIDECONSOLE  /FEATURES=SQL
  /INSTANCENAME=SQLEXPRESS /SECURITYMODE=SQL /SAPWD="SOMEPWD"
  /SQLSVCACCOUNT="NT AUTHORITY\SYSTEM"
  /SQLSYSADMINACCOUNTS="Builtin\Administrators" /AGTSVCACCOUNT="NT
  AUTHORITY\Network Service"


I would like to say that definitely you are doing some arguments mistake or permission. As MSDN BOL https://msdn.microsoft.com/en-us/library/ms144259(v=sql.110).aspx the simple syntax of installation would be as mention below format:


  Setup.exe /q /ACTION=Install /FEATURES=SQL /INSTANCENAME=MSSQLSERVER
  /SQLSVCACCOUNT=""
  /SQLSVCPASSWORD=""
  /SQLSYSADMINACCOUNTS="" /AGTSVCACCOUNT="NT
  AUTHORITY\Network Service" /IACCEPTSQLSERVERLICENSETERMS


NB: For local installations, you must run Setup as an administrator.

AS local installations, you must run Setup as an administrator. If you install SQL Server from a remote share, you must use a domain account that has read and execute permissions on the remote share. For failover cluster installations, you must be a local administrator with permissions to login as a service, and to act as part of the operating system on all failover cluster nodes.

As your refence https://social.msdn.microsoft.com/forums/sqlserver/en-US/87bb5999-1126-4809-a353-0f5b06c9f81d/sql-express-2008-command-line-options and https://msdn.microsoft.com/en-us/library/ms144259(v=sql.110).aspx