Java Runtime.getRuntime.exec() returns null but returns valid String in shell

Question

I've been writing a class to find the real userID, using the linux binary /usr/bin/logname that returns the current user associated with my TTY. The command works fine in shell.

/usr/bin/logname
scott

But I cannot get the same as a String in java with the following code that I wrote.

private String currentUser;
public void getRealUser() throws Exception{
        String cmd = "/usr/bin/logname";
        Process p = Runtime.getRuntime().exec(cmd);
        p.waitFor();
        BufferedReader stdInput = new BufferedReader(new InputStreamReader(p.getInputStream()));
        this.currentUser = stdInput.readLine();
        System.out.println(currentUser);
}

When I create the object, I am seeing null for the value currentUser, which means stdInput.readLine(); is not outputting anything.

Please let me know what am I doing wrong.

I have a similar app, but I read the input before p.waitFor(); Have you tried putting p.waitFor(); before the System.out.println ? — Clark Kent
– Clark Kent, Commented Nov 24, 2015 at 19:18
Yes. It is still null with p.waitFor() after System.out.println() — scott
– scott, Commented Nov 24, 2015 at 19:21
I meant after the this.currentUser = statement, but before the System.out. statement. On another note, there's the potential that the first line you get is a null line... I use a while loop to make sure I read all lines from the InputStream. — Clark Kent
– Clark Kent, Commented Nov 24, 2015 at 19:24
@Jan that was the one from the ideone.com link 4 comments above. - scott that's not a java problem, I get the same error in the terminal in some cases. The error means something like: your shell command runs in a login shell while the java one doesn't (the subprocess spawned from the java one). Or aplawrence.com/Forum/TonyLawrence8.html - in other words, linux sometimes doesn't know how the user is that's responsible for a process $LOGNAME or $USERNAME orso may have a value but they aren't reliable either — zapl
– zapl, Commented Nov 25, 2015 at 10:59

Amir Afghani · Accepted Answer · 2015-11-24 19:30:04Z

1

This is not answering your question directly, but did you try :

System.getProperty("user.name")

which is at least platform agnostic? Why bother writing code thats unix specific in a language like Java?

answered Nov 24, 2015 at 19:30

Amir Afghani

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1 Comment

scott Over a year ago

Yes, that will, but that will defeat the purpose of getting the real username, but returns the effective username. If I run the program as sudo, it will return root, but if I run /usr/bin/logname as sudo, it will return the real user, not root.

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