Odd exec output in node

Question

I'm trying to determine if a java runtime environment is installed on the computer running my node app. Here's the promise I'm starting to build out to determine if it's installed:

FileSystemTools.prototype.checkToSeeIfJavaRuntimeEnvIsAvailable = function (){ return new Promise((resolve, reject) => { exec('java -version', (error, stdout, stderr) => { if(error) reject(error) if(stderr) reject(stderr) resolve(stdout) }) }) }

The generator function I'm building out to run this check looks like this so far:

co(function *(){ let jreVersion = yield fst.checkToSeeIfJavaRuntimeEnvIsAvailable() console.log(jreVersion) }).catch(error => { throw new Error(error) })

When I run this I get no output at all. However, if I resolve all of the arguments returned from the exec command:

 FileSystemTools.prototype.checkToSeeIfJavaRuntimeEnvIsAvailable = function (){ return new Promise((resolve, reject) => { exec('java -version', (error, stdout, stderr) => { // resolving early with everything to see what's going on resolve({error, stdout, stderr}) if(error) reject(error) if(stderr) reject(stderr) resolve(error, stdout, stderr) }) }) }

I'm seeing that my command is firing successful, but it's returning the result in the standard error, not standard output:

{ error: null, stdout: '', stderr: 'java version "1.8.0_101"\nJava(TM) SE Runtime Environment (build 1.8.0_101-b13)\nJava HotSpot(TM) 64-Bit Server VM (build 25.101-b13, mixed mode)\n' }

I double checked the docs on the exec callback thinking that I got the callback parameters wrong, but they're definitely in the right order:

This raises two questions for me:

Why is my successful output of the command returning in standard error?
If it is returning in standard error, why did my if statement to check for standard error and reject if it is truthy not fire?

@zack6849 I approved the edit. I def agree it's not a java specific question, though I'm assuming that this means there's nothing known to the java community about the version information returning on standard error instead of standard output. Is that correct? — Chris Schmitz
– Chris Schmitz, Commented Oct 19, 2016 at 14:29
Ah, I just saw @thefourtheye's comment about version returning on standard error so I guess that is a java specific thing ;P — Chris Schmitz
– Chris Schmitz, Commented Oct 19, 2016 at 14:30

thefourtheye · Accepted Answer · 2016-10-19 14:29:16Z

1

By default, java -version writes its result in standard error only, even though the command is executed successfully. You can confirm that by redirecting the stderr to a file, like this

$ sh -c "java -version 2>/tmp/test" $ cat /tmp/test java version "1.8.0_102" Java(TM) SE Runtime Environment (build 1.8.0_102-b14) Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

If there was an error, the error parameter in the callback would have been an object, with the error code and the signal.

answered Oct 19, 2016 at 14:29

thefourtheye

241k53 gold badges466 silver badges505 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

Chris Schmitz Over a year ago

Awesome. Thanks re: java version command returning the result on standard error. Regarding the error, the thing is the stderr param does have something in it, though it's not tripping the if check. Any idea why that is happening?

thefourtheye Over a year ago

@ChrisSchmitz You mean the if(stderr) reject(stderr)? Yes, the promise will be rejected. I am referring the first program in your question.

Chris Schmitz Over a year ago

Gotcha. I actually think I figured out the second part of my original question (javascript "truthiness" is an odd thing). Thanks for the help!

Chris Schmitz Over a year ago

Side note, I have 55 secs before I can actually give you the check ;)

Collectives™ on Stack Overflow

Odd exec output in node

1 Answer 1

4 Comments

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Related