Why the following code doesn't work?

Question

i'm trying to run shell command in linux via java. most of the commands work, but when i run the following command i get an execption, although it works in the shell:

    String command = "cat b.jpg f1.zip > pic2.jpg";

    String s = null;
    try {
        Process p = Runtime.getRuntime().exec(command);

        BufferedReader stdInput = new BufferedReader(new
             InputStreamReader(p.getInputStream()));

        BufferedReader stdError = new BufferedReader(new
             InputStreamReader(p.getErrorStream()));

        System.out.println("Here is the standard output of the command:\n");

        while ((s = stdInput.readLine()) != null) {
            System.out.println(s);
        }
        System.out.println("Here is the standard error of the command (if any):\n");
        while ((s = stdError.readLine()) != null) {
            System.out.println(s);
        }
        System.exit(0);
    }
    catch (IOException e) {
        System.out.println("exception happened - here's what I know: ");
        e.printStackTrace();
        System.exit(-1);
    }

i am getting the error in the console:

cat: >: No such file or directory

cat: pic2.jpg: No such file or directory

I think this might be related - http://stackoverflow.com/questions/16238714/runtimes-exec-method-is-not-redirecting-the-output — Mirec Miskuf, Dec 15 '15 at 16:10

score 2 · Answer 1 · answered Dec 15 '15 at 16:21

The problem is the redirection.

cat: >: No such file or directory

The way to interpret this error message:

the program cat is trying to tell you about a problem
the problem is that there is no file named >

Indeed, > is not a file. It's not intended as a file, at all. It's a shell operator to redirect output.

You need to use ProcessBuilder to redirect:

ProcessBuilder builder = new ProcessBuilder("cat", "b.jpg", "f1.zip");
builder.redirectOutput(new File("pic2.jpg"));
Process p = builder.start();

Adam Siemion · Answer 2 · 2015-12-15T16:20:22.197

1

Because you need to start a shell (e.g. /bin/bash) which will execute your shell command, replace:

String command = "cat b.jpg f1.zip > pic2.jpg";

with

String command = "bash -c 'cat b.jpg f1.zip > pic2.jpg'";

edited Dec 15 '15 at 16:20

answered Dec 15 '15 at 16:10

Adam Siemion

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score 1 · Answer 3 · answered Dec 15 '15 at 16:19

When you run a command it doesn't start a shell like bash unless you do this explicitly. This means you are running cat with four arguments b.jpg f1.zip > pic2.jpg The last two files names don't exist so you get an error.

What you are likely to have intended was the following.

String command = "sh -c 'cat b.jpg f1.zip > pic2.jpg'";

This will run sh which in sees > as a special character which redirects the output.

Why the following code doesn't work?

3 Answers3