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?
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.
Errr... just:
if defined your-var-name (
echo yarp
) else (
echo narp
)
I should add, I do not believe this needs command extensions...
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.
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
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.
