Check if an environment variable is defined without command extensions and without using a batch file?

39
votes

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?

5
windowsbatch-filecmd
At the command line you can just use the SET command to check if a variable is defined. – RBarryYoung
How would I use that in a conditional check? I tried: IF (SET UNDEFINED) (echo yes) and got UNDEFINED) was unexpected at this time. – Johann
Why would you need to use the IF at the command line? Just look at it and take the appropiate action. – RBarryYoung
The reason is that this is being used within a gyp build file, and the gyp reference says: "In a command expansion, the entire string contained within the parentheses is passed to the system’s shell." (code.google.com/p/gyp/wiki/…). However, when I use the IF "%VARIABLE%" == "" syntax, it never evaluates to true, even when VARIABLE is not defined. I'm assuming it's because it's running through the same command line parser. IF DEFINED does work as expected, but I cannot guarantee that command extensions will be enabled. – Johann
I think @RBarryYoung provided the answer. Use set with no args to print the environment, and use find to find the variable. I was going to post an answer with the same. But because of Barry's answer, it simply would have been a "me too" answer. – jww

5 Answers

36
votes

Errr... just:

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

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

9
votes

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.

8
votes

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.

5
votes

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
1
votes
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.