Determining whether STDERR is going to terminal

Question

I have a suite of Java programs which are used as command-line tools on our Linux servers. Most of them use a class that prints a progress bar on STDERR, similar to Perl's Term::ProgressBar.

I'd like to have the progress bar shown whenever STDERR is going to the terminal and automatically disable itself when STDERR is redirected so that there aren't all sorts of progress bar pieces in the redirected data.

Checking System.console() == null was my first thought, but redirecting STDOUT is enough to make this true, even if STDERR is still going to the terminal. Is there anything I can check that is specific to STDERR? A solution that is Linux-specific or that uses native APIs would be ok for my needs.

Answer 1

I think what you're looking for is isatty(3), in unistd.h. There's no way to tell whether a file handle has been redirected, period, but that'll tell you whether it's still interactive. See the source for the tty command in GNU coreutils.

Answer 2

After combining @chrylis's pointer with this answer and doing a little tweaking, what I finally ended up with is:

1. create and compile Java class with native method signature
2. use javah to generate C header file
3. create .cpp file, implementing function with isatty
4. compile C++ code into shared library
5. run Java program, using -Djava.library.path=... to tell it where your custom library is

Java class:

package com.example.cli;

class LinuxTerminalSupport {

    public native boolean isStderrVisible();

    static {
        System.loadLibrary("term");
    }
}

ant target to generate .h:

<target name="generate-native-headers">
    <javah destdir="native/" verbose="yes">
        <classpath refid="compile.class.path"/>
        <class name="com.example.cli.LinuxTerminalSupport" />
    </javah>
</target>

.cpp file:

#include "com_example_cli_LinuxTerminalSupport.h"
#include "unistd.h"

using namespace std;

JNIEXPORT jboolean JNICALL Java_com_example_cli_LinuxTerminalSupport_isStderrVisible(JNIEnv * env, jobject obj) {
    return isatty(fileno(stderr)) == 1;
}

Makefile (change java includes to reflect your $JAVA_HOME):

linux: LinuxTerminalSupport.o
    g++ -I/usr/java/jdk1.6.0_13/include -I/usr/java/jdk1.6.0_13/include/linux \
            -o libterm.so -shared -Wl,-soname,term.so LinuxTerminalSupport.o -lc

LinuxTerminalSupport.o: LinuxTerminalSupport.cpp
    g++ -c -I/usr/java/jdk1.6.0_13/include -I/usr/java/jdk1.6.0_13/include/linux LinuxTerminalSupport.cpp