Java: Can't execute external exe with arguments

Question

I’m trying to run an external program with arguments. The program can take different types of arguments, for instance avl tip.avl or avl < test.ops

I can get avl tip.avl running through

try {
    String[] list = {"avl", "test_0.avl"};
    ProcessBuilder pb = new ProcessBuilder(list);
    pb.command(list);
    final Process p = pb.start();
    BufferedReader br = new BufferedReader( new InputStreamReader( p.getInputStream() ) );
    String line;
    while ((line = br.readLine()) != null) {
        System.out.println(line);
    }
} catch (Exception ex) {
    System.out.println(ex);
}

but when I try

String[] list = {"avl", "<", "test_0.ops"};
ProcessBuilder pb = new ProcessBuilder(list);
pb.command(list);
final Process p = pb.start();

the "<" does not get sent as an argument, but as an input after the program runs. avl < test.ops works ok when I try it from command line, but cant get it to work through ProcessBuilder.

I think avl tip.avl works because running avl tip.avl is the same as just running avl and then typing tip.avl. Which is what ProcessBuilder seems to be doing actually ...

I assumed the arguments would be passed all at one. What would be the right way of doing what the command prompt input does avl < test.ops + ENTER

what happens if you send it as one string rather than array of them? — dave
– dave, Commented Apr 6, 2018 at 19:19
That doesn't work because piping from a file is a shell feature, processbuilder doesn't use a shell, so the < is treated as an argument — jrtapsell
– jrtapsell, Commented Apr 6, 2018 at 19:21

Michael Piefel · Accepted Answer · 2018-04-06 19:26:45Z

4

You cannot redirect input like that from Java. Using < is a special shell pipe redirection command.

You will either have to use processBuilder.getOutputStream() to write data to the process, or else you can use redirectInput.

edited Apr 6, 2018 at 19:26

Michael Piefel

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answered Apr 6, 2018 at 19:23

Jamie

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Comments

Michael Piefel · Accepted Answer · 2018-04-06 19:35:49Z

the "<" does not get sent as an argument, but as an input after the program runs

No, that’s not right. It is passed as an argument, just as test_0.avl and test_0.ops are.

[…] the same as just running avl and then typing tip.avl

No, that is never what happens in the shell. The shell will pass tip.avl as the first argument.

That said, it is your shell that special-handles the < sign, as it would special-handle > and |. When you use ProcessBuilder, that special handling will not happen. Your second invocation is equivalent to this in the shell:

avl '<' test_0.ops

This will disable the sepcial meaning of the <. That’s not what you wanted here, of course.

Dersu Uzala · Accepted Answer · 2018-04-07 07:46:10Z

1

Thank you both, the below works as expected.

File pipe = new File("test_0.ops");
ProcessBuilder pb = new ProcessBuilder("avl");
pb.redirectInput(pipe);
final Process p = pb.start();

edited Apr 7, 2018 at 7:46

answered Apr 6, 2018 at 20:13

Dersu Uzala

331 silver badge4 bronze badges

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