Check if an environment variable is defined withou

Question

I need to use a cmd.exe command line (cmd.exe is being called from the gyp build tool) to determine whether an environment variable is defined or not. How can I do this? I am okay assuming that the variable value does not contain single or double quotes, but cannot assume that command extensions are enabled.

I've tried the following, which works great in a .bat file, but fails when typed directly on the command line:

IF "%UNDEFINED%" == "" (echo yes)

When that exact line is in a .bat file and executed, I see yes as the output. When I type it on the command line, the output is empty. I am testing this on Windows XP SP3, though my coworker sees the same results on Windows 7. This is the method suggested by http://support.microsoft.com/kb/121170 and http://www.robvanderwoude.com/battech_defined.php. I do not want to use IF DEFINED UNDEFINED (echo yes) because that won't work if command extensions are disabled.

The top-voted answer in the following post has led me to believe that this issue is related to how percent-expansion is handled differently in the "CmdLineParser" vs. the "BatchLineParser," but still has not led me to a solution: How does the Windows Command Interpreter (CMD.EXE) parse scripts?

Answer 1

OK, this took a bit, but I think I've figured it out. Try this:

SET UNDEFINED 2>Nul | Findstr/I "."
IF ERRORLEVEL 1  ECHO Not Defined.

This works for all cases AFAIK, and does not rely on any command extension features.

Answer 2

IF NOT %CODE%==? do stuff.

This works on a W98 command line and in a batch file, so it ought to work anywhere from early MS-DOS onwards with no extensions needed. It assumes that CODE is usefully set or not set at all.

It results in a syntax error if CODE does not exist, or does nothing if CODE is a question mark (chosen because it could never exist in a path). Either way, nothing is done. The NOT makes sure action is only taken if CODE appears to be set to something useful.

I use it in compiler batch files to determine whether to use relative paths or use a fixed base directory if one is set in the CODE variable. It allows me to make a coding tree portable without having to modify all the batch files for each program if I move everything.

Answer 3

If the extensions are really disabled (I can't believe this),
then you can try different ways.

IF %UNDEFINED% == %^UNDEFINED% (echo yes)

This works as if undefined doesn't exists then it isn't replaced, also ^undefined but the caret will be removed in the next parser phase, so %undefined% is compared against %undefined%. The disadvantage are the missing quotes, as they also make the expression stable against special characters.

A better way is to use IF defined, but when extensions are disabled you need to enable them first.

cmd /E:on /c "if not defined undefined echo It's undefined"

The best way is to simply use a batch file, that should also work with gyp build system.

Answer 4

I tried this and it worked:

@echo off

setlocal disableextensions

set x=%path%
if "%x%"=="" (echo "PATH" does not exist) else (echo "PATH" exists)

set x=%pathx%
if "%x%"=="" (echo "PATHX" does not exist) else (echo "PATHX" exists)

endlocal

It returned:

"PATH" exists
"PATHX" does not exist

Answer 5

Errr... just:

if defined your-var-name ( 
    echo yarp
) else (
    echo narp
)

I should add, I do not believe this needs command extensions...