How can I check if a Java program's input/output streams are connected to a terminal?

Question

I would like a Java program to have different default settings (verbosity, possibly colored output where supported) depending on its use. In C, there is an isatty() function which will return 1 if a file descriptor is connected to a terminal, and 0 otherwise. Is there an equivalent for this in Java? I haven't seen anything in the JavaDoc for InputStream or PrintStream.

Answer 1

System.console() vs isatty()

System.console(), as already mentioned by @Bombe, works for simple use cases of checking console-connectedness. The problem with System.console() however, is that it doesn't let you determine whether it's STDIN or STDOUT (or both or neither) that is connected to a console.

The difference between Java's System.console() and C's isatty() can be illustrated in the following case-breakdown (where we pipe data to/from a hypothetical Foo.class):

1) STDIN and STDOUT are tty

%> java Foo System.console() => <Console instance> isatty(STDIN_FILENO) => 1 isatty(STDOUT_FILENO) => 1 

2) STDOUT is tty

%> echo foo | java Foo System.console() => null isatty(STDIN_FILENO) => 0 isatty(STDOUT_FILENO) => 1 

3) STDIN is tty

%> java Foo | cat System.console() => null isatty(STDIN_FILENO) => 1 isatty(STDOUT_FILENO) => 0 

4) Neither STDIN nor STDOUT are tty

%> echo foo | java Foo | cat System.console() => null isatty(STDIN_FILENO) => 0 isatty(STDOUT_FILENO) => 0 

I can't tell you why Java doesn't support better tty-checking. I wonder if some of Java's target OS's don't support it.

Using JNI to call isatty()

It technically is possible to do this in Java (as stephen-c@ pointed out) with some fairly simple JNI, but it will make your application dependent on C-code that may not be portable to other systems. I can understand that some people may not want to go there.

A quick example of what the JNI would look like (glossing over a lot of details):

Java: tty/TtyUtils.java

public class TtyUtils {     static {         System.loadLibrary("ttyutils");     }     // FileDescriptor 0 for STDIN, 1 for STDOUT     public native static boolean isTty(int fileDescriptor); } 

C: ttyutils.c (assumes matching ttyutils.h), compiled to libttyutils.so

#include <jni.h> #include <unistd.h>  JNIEXPORT jboolean JNICALL Java_tty_TtyUtils_isTty           (JNIEnv *env, jclass cls, jint fileDescriptor) {     return isatty(fileDescriptor)? JNI_TRUE: JNI_FALSE; } 

Other languages:

If you have the option of using another language, most other languages I can think of support tty-checking. But, since you asked the question, you probably already know that. The first that come to mind for me (aside from C/C++) are Ruby, Python, Golang and Perl.

Answer 2

System.console() will return the console your application is connected to if it is connected, otherwise it returns null. (Note that it’s only available from JDK 6 on.)