Why does the set command display more environment variables than the environment variables gui?

Question

When I run the set command in command prompt the first variable I see is

But when I open my Environment Variables GUI I don't see ALLUSERSPROFILE

Why is that? There are numerous other variables listed in set that don't show up in the GUI as well. Shouldn't they all show the same info?

Answer 1

After a bit of trial and error, I discovered that these environment variables are the additional ones reported by SET on my Windows 8.1 system:

ALLUSERSPROFILE
APPDATA
COMPUTERNAME
CommonProgramFiles
CommonProgramFiles(x86)
CommonProgramW6432
HOMEDRIVE
HOMEPATH
LOCALAPPDATA
LOGONSERVER
PROMPT
PUBLIC
ProgramData
ProgramFiles
ProgramFiles(x86)
ProgramW6432
SESSIONNAME
SystemDrive
SystemRoot
USERDOMAIN
USERDOMAIN_ROAMINGPROFILE

I suspect that they are all automatically generated and updated by Windows, which is probably why I couldn't find some of them listed anywhere in Windows Registry.

I know that HOMEDRIVE, HOMEPATH and HOMESHARE are updated automatically from this TechNet Forums post.

I tried permanently changing the PROMPT variable to something other than $P$G (which is the default) using SETX and noticed that it created a new PROMPT environment variable in the User variables for <username> section in the Environment Variables GUI.

On deleting that variable, my prompt was reset back to $P$G. I noticed the same behavior with COMPUTERNAME environment variable. The same goes for:

CommonProgramFiles
CommonProgramW6432
PROCESSOR_ARCHITECTURE
ProgramFiles
ProgramW6432

as explained in this MSDN article about WOW64 Implementation Details.

Answer 2

According to SS64.com (Autoexec.bat section), those variables may be boot-time variables, that are "not available to 32 bit gui programs". Since Windows XP, boot-time environment variables should be set with the registry in:
HKEY_CURRENT_USER\Environment (USER environment variables) or:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment(SYSTEM environment variables)
as shown in Microsoft's KB