Windows Environment Variables

Environment variables are mainly used within batch files, they can be created, modified and deleted using the SET command.

Variables can be displayed using either SET or ECHO.

Variables have a percent sign on both sides: %ThisIsAVariable%
The variable name can include spaces, punctuation and mixed case: %_Another Ex.ample%
(This is unlike Parameter variables which only have one % sign and are always one character long: %A )

In a batch file the default behaviour is to read and expand variables one line at a time, for example:

 SET /P _price=”Enter the price: ”
ECHO %_price%

The above will work, but the version below will fail because %_price% will remain undefined until processing reaches the next line, this behaviour can be changed using SETLOCAL EnableDelayedExpansion

 SET /P _price=”Enter the price: ” & ECHO %_price%
ECHO That didnt work!

Standard (built-in) Environment Variables

Variable	Default value: Windows XP	Default value: Windows 7/2008
%ALLUSERSPROFILE%	C:\Documents and Settings\All Users	C:\ProgramData
%APPDATA%	C:\Documents and Settings\{username}\Application Data	C:\Users\{username}\AppData\Roaming
%CommonProgramFiles%	C:\Program Files\Common Files	C:\Program Files\Common Files
%COMMONPROGRAMFILES(x86)%	C:\Program Files (x86)\Common Files	C:\Program Files (x86)\Common Files
%COMPUTERNAME%	{computername}	{computername}
%COMSPEC%	C:\Windows\System32\cmd.exe	C:\Windows\System32\cmd.exe
%HOMEDRIVE%	C:	C:
%HOMEPATH%	\Documents and Settings\{username}	\Users\{username}
%LOCALAPPDATA%	N/A (but can be manually addedLOCALAPPDATA=%USERPROFILE%\Local Settings\Application Data)	C:\Users\{username}\AppData\Local
%LOGONSERVER%	\\{domain_logon_server}	\\{domain_logon_server}
%PATH%	C:\Windows\System32\;C:\Windows\;C:\Windows\System32\Wbem;{plus program paths}	C:\Windows\System32\;C:\Windows\;C:\Windows\System32\Wbem;{plus program paths}
%PATHEXT%	.COM; .EXE; .BAT; .CMD; .VBS; .VBE; .JS ; .WSF; .WSH;	.COM; .EXE; .BAT; .CMD; .VBS; .VBE; .JS ; .WSF; .WSH; .MSC
%ProgramData%	N/A	C:\ProgramData
%ProgramFiles%	C:\Program Files	C:\Program Files
%ProgramFiles(x86)% 1	C:\Program Files (x86)	C:\Program Files (x86)
%PROMPT%	Code for current command prompt format,usually $P$G C :>	Code for current command prompt format,usually $P$G C :>
%PSModulePath%	N/A	%SystemRoot%\system32\WindowsPowerShell\v1.0\Modules\
%Public%	N/A	C:\Users\Public
%SYSTEMDRIVE%	C:	C:
%SYSTEMROOT%	C:\Windows	C:\Windows
%TEMP% and %TMP%	C:\Documents and Settings\{username}\Local Settings\Temp	C:\Users\{Username}\AppData\Local\Temp
%USERDOMAIN%	{userdomain}	{userdomain}
%USERNAME%	{username}	{username}
%USERPROFILE%	%SystemDrive%\Documents and Settings\{username}	%SystemDrive%\Users\{username}
%WINDIR%	C:\Windows	C:\Windows

¹ Only on 64 bit systems, is used to store 32 bit programs.

By default, files stored under Local Settings do not roam with a roaming profile.

%ERRORLEVEL% is a dynamic variable that is automatically set when a program exits.

Dynamic Variables

There are also 6 dynamic environment variables, these are computed each time the variable is expanded.
n.b. you should not attempt to directly SET a dynamic variable.

%CD% – The current directory (string).

%DATE% – The current date using same region specific format as DATE.

%TIME% – The current time using same format as TIME.

%RANDOM% – A random decimal number between 0 and 32767.

%CMDEXTVERSION% – The current Command Processor Extensions version number.

%CMDCMDLINE% – The original command line that invoked the Command Processor.

Pass a variable from one batch script to another

Where one batch script CALLs another it is recommended that you SETLOCAL in both scripts to prevent any possible naming conflicts, so each script should start with:
@ECHO OFF
SETLOCAL

Then to pass a value back to the original calling script, finish the script with a line like:

ENDLOCAL & SET _output=%_variable%

In the line above %_variable% is a local variable used and visible within just that one batch script
%_output% is an output variable that is passed back to the original calling script

“Men may be convinced, but they cannot be pleased against their will. But though taste is obstinate, it is very variable, and time often prevails when arguments have failed” – Samuel Johnson

Related:

CALL – Evaluate environment variables
SET – Display, set, or remove variables
Q100843 – The four types of environment variable
Q286705 – Set compatibility variables
Q242557 – Registry Settings for Folder Redirection
Managing Roaming User Data Deployment Guide – Microsoft.com (Word Doc)

ที่มา : http://ss64.com

http://ss64.com