Difference between System.out and System.err

In Java, System.out print messages to the standard out of the system you are using, where as System.err print messages to the standard error of the system. When you run your Java program from command prompt/terminal, by default all the print statements of System.out, System.err are print to system console (command prompt/terminal).

For Example,

public class Test {
 public static void main(String args[]) {
  System.out.println("Information message 1");
  System.err.println("error message 1");
  System.out.println("Information message 2");
  System.err.println("error message 2");
 }
}

Output

Information message 1
error message 1
Information message 2
error message 2

If you run the same application in any IDE like Eclipse, System.err statements are displayed in red color, where as System.out statements are displayed in black color.

Redirecting error messages from command prompt

Try to redirect the output of Test class to some file 'myLog'.

$ java Test > myLog
error message 1
error message 2
$
$
$ cat myLog
Information message 1
Information message 2

As you observe, error messages are redirected to console, where as System.out messages are redirected to myLog file. It is because, When you redirect console output using the ">" symbol, you are only redirecting STDOUT. In order to redirect STDERR you have to specify "2>" for the redirection symbol.

$ java Test > mylog.out 2>mylog.err
$
$ cat mylog.out
Information message 1
Information message 2
$
$ cat mylog.err
error message 1
error message 2

java Test > mylog.out 2>mylog.err

Above statement redirects all the System.out messages to mylog.out file and all error messages to mylog.err file.

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