Executing a Windows Command

[Dennis Putnam]

I am trying to run the 'attrib' command to make a created file hidden. I get no errors but the file never gets the hidden attribute (works just fine from the command line).

try {
	Process p=Runtime.getRuntime().exec("cmd /c attrib +H "+file.getPath());
	p.waitFor();
} catch (IOException | InterruptedException e) {
	MessageScreen.showWindow("Unable to set inf file attributes: "+e.getMessage());
}

Can someone spot what I am doing wrong or suggest how to debug this? TIA.

[Campbell Ritchie]

How are you emptying the output and error streams from that process?

[Dennis Putnam]

Thanks for the reply but in this case there is no output stream to empty. This try/catch immediately follows a try/catch for 'file.createNewFile()'. Data will be written to the file a some other point.

[Steve Luke]

That's not the point. Every process has its own output and error streams. You need to consume those streams for a couple of reasons (to find out what feedback is provided if - for example - the processes didn't work, and because a process can freeze if the output or error streams fill without being consumed.)

You should read Deconta's great article on this: When Runtime.exec() won't.

[Paul Clapham]

Could I suggest just using the setAttribute method of the java.nio.file.Files class? I notice that the API documentation for that method includes

Usage Example: Suppose we want to set the DOS "hidden" attribute...

[Dennis Putnam]

Paul: Thanks. I'll take a look at that.

Steve: I thought I understood the article but it didn't seem to matter. Here is what I have now (this may become academic after I look at Paul's suggestion):

try {
	Process p=Runtime.getRuntime().exec("cmd /c attrib +H "+file.getPath());
	InputStream stderr=p.getErrorStream();
	InputStream stdout=p.getInputStream();
	InputStreamReader isre=new InputStreamReader(stderr);
	InputStreamReader isro=new InputStreamReader(stdout);
	BufferedReader bre=new BufferedReader(isre);
	BufferedReader bro=new BufferedReader(isro);
	String line=null;
	while ((line=bre.readLine())!=null) {
		MessageScreen.showWindow("Attribute command error: "+line);
	}
	while ((line=bro.readLine())!=null) {
	}
	p.waitFor();
	bre.close();
	bro.close();
} catch (IOException | InterruptedException e) {
	MessageScreen.showWindow("Unable to set inf file attributes: "+e.getMessage());
	return(null);
}

[Dennis Putnam]

Paul,

Thanks, that worked, sort of. When I did a search for this specific issue, all the solutions that came up were the convoluted 'exec' method. Why were so many people unaware of your method? Anyway, the problem I have now seems to be a Windows issue but it makes no sense and I am wondering if Java is interjecting itself. When I try to write the file I am getting a permission denied error. I can write to a hidden file from Windows so it is not clear why I get this. Since it is so easy, I will try turning that off and back on when I need to write the file to see what happens.

[Steve Luke]

Dennis Putnam wrote:Paul,

Thanks, that worked, sort of. When I did a search for this specific issue, all the solutions that came up were the convoluted 'exec' method. Why were so many people unaware of your method? Anyway, the problem I have now seems to be a Windows issue but it makes no sense and I am wondering if Java is interjecting itself. When I try to write the file I am getting a permission denied error. I can write to a hidden file from Windows so it is not clear why I get this. Since it is so easy, I will try turning that off and back on when I need to write the file to see what happens.

That is new - added in Java7.

[Paul Clapham]

Steve Luke wrote:That is new - added in Java7.

That's right. And it was not one of the high-profile features, so very few people know about it. But I did know that Dennis was using Java 7 because of this line of code:

} catch (IOException | InterruptedException e) { 

[Dennis Putnam]

Ah, that explains it. What it does not explain is why Java would imposes its own file restrictions contrary to the OS. By turning the hidden attribute off and on I can write to the file. Obviously that means it is Java that is prohibiting writes to a hidden file and returning a false permission denied error.

[Ivan Jozsef Balazs]

Dennis Putnam wrote:What it does not explain is why Java would imposes its own file restrictions contrary to the OS. By turning the hidden attribute off and on I can write to the file. Obviously that means it is Java that is prohibiting writes to a hidden file and returning a false permission denied error.

Do you mean a file remains writable on the OS level when the hidden attribute is set, whereas Java handles it as not writable?

[Campbell Ritchie]

I thought this discussion would become too difficult for “beginning”. It can be moved.