Is there a way to determine if Java System.in is "interactive"?

Question

If the input is interactive, i.e. from the console, I want to print a command prompt e.g. ">"

But if it is redirected e.g. from a file, then I do not want to prompt.

System.in is an abstract InputStream which does not appear to have any method for this.

Maybe you could use instanceof if the concrete type is different?

As well, if System.out is redirected to a file I also do not want to prompt

For a linux-specific solution using JNI, see here. You'd just need to change fileno(stderr) to fileno(stdin). — Brad Mace
– Brad Mace, Commented Aug 22, 2013 at 3:29
possible duplicate of How can I check if a Java program's input/output streams are connected to a terminal? — Kristian Holdhus
– Kristian Holdhus, Commented May 2, 2014 at 1:49

Stephen C · Accepted Answer · 2021-10-31 02:32:13Z

5

AFAIK, there is no way to do this in pure Java, and even doing it in JNI / JNA would be complicated.

An alternative might be to use the new Console API introduced in JDK 1.6. This allows you to try to get a reader/writer for the console. If it succeeds, the result is guaranteed to be interactive ... in the sense that you mean.

A second alternative would be to do the "check for interactivity" in the wrapper script that you use to launch your application, and pass the information to Java via the system properties. For instance, on a GNU/Linux system the tty(1) command can be used to tell if stdin is a connected to a "tty" device.

Note that there are other ways to deal with a requirement to avoid unwanted prompts when running non-interactively:

If System.out is redirected to a file I also do not want to prompt.

^{(I think you might mean that System.in is redirected. That is the normal way to non-interactively run an application that normally takes interactive input from the user ...)}

The alternatives include:

You could modify the program to write the prompts to (say) System.err and redirect that to a different place.
You could modify the program to have options that mean "don't prompt" or "take input from a file".

edited Oct 31, 2021 at 2:32

answered Mar 10, 2010 at 0:08

Stephen C

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5 Comments

Andrew McKinlay Over a year ago

That should work. It sounds like System.console() will return null if there is no console attached. java.sun.com/javase/6/docs/api/java/io/Console.html

Stephen C Over a year ago

The Console API works in one sense, but if the user wants the app's input from a (say) a file, then opening a new console stream subverts. IMO, the Console API should only be used when it makes no sense (or is dangerous) for the application to read from a non-interactive stdin.

Andrew McKinlay Over a year ago

I wasn't thinking of opening a new console stream. I was just thinking you could check for a non-null return from System.console() to determine if you were attached to a console.

Stephen C Over a year ago

@Andrew - but how does that help? It does NOT tell you is System.in/out are attached to a console.

Andrew McKinlay Over a year ago

Yeah, you're right, I wasn't thinking. It only tells you if if java was started from a console, but obviously stdin/out could still be redirected.

tangens · Accepted Answer · 2022-11-22 12:26:19Z

0

Since Java 6 there is the method java.lang.System.console():

Whether a virtual machine has a console is dependent upon the underlying platform and also upon the manner in which the virtual machine is invoked. If the virtual machine is started from an interactive command line without redirecting the standard input and output streams then its console will exist and will typically be connected to the keyboard and display from which the virtual machine was launched. If the virtual machine is started automatically, for example by a background job scheduler, then it will typically not have a console.

If this virtual machine has a console then it is represented by a unique instance of this class which can be obtained by invoking the java.lang.System.console() method. If no console device is available then an invocation of that method will return null.

answered Nov 22, 2022 at 12:26

tangens

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1 Comment

Andrew McKinlay Over a year ago

See the previous discussion of System.console()

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