Using Java to call Linux terminal: How to flush the output?

Question

1) I'm using Java to call Linux terminal to run foo.exe and save the output in a file:

    String[] cmd = {"/bin/sh", "-c", "foo >haha.file"};
    Runtime.getRuntime().exec(cmd);

2) The problem is when I plan to read haha.file later in the code, it hasn't been written yet:

File f=new File("haha.file"); // return true
in = new BufferedReader(new FileReader("haha.file"));
reader=in.readLine();
System.out.println(reader);//return null

3) Only after the program is done will the haha.file be written. I only know how to flush "Writers" but don't know how to flush sth. like this. How can I force java to write the file in the terminal?

Thanks in advance E.E.

dbyrne · Accepted Answer · 2010-07-12 19:48:10Z

2

This problem is caused by the asynchronous nature of Runtime.exec. foo is being executed in a seperate process. You need to call Process.waitFor() to insure the file has been written.

String[] cmd = {"/bin/sh", "-c", "foo >haha.file"};
Process process = Runtime.getRuntime().exec(cmd);
// ....
if (process.waitFor() == 0) {
    File f=new File("haha.file");
    in = new BufferedReader(new FileReader("haha.file"));
    reader=in.readLine();
    System.out.println(reader);
} else {
    //process did not terminate normally
}

edited Jul 12, 2010 at 19:48

answered Jul 12, 2010 at 19:41

dbyrne

61.5k13 gold badges91 silver badges103 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Mike Q Over a year ago

Be careful with this approach. There are nasties lurking with the stdout/stderr stream when using exec(). You really need to ASYNCHRONOUSLY consume the output/error streams while waitFor() is blocking otherwise it may never return because the stdout/err buffers fill up and block the forked processes. Checkout apache commons-exec for a lib that solves this.

Maurice Perry · Accepted Answer · 2010-07-12 19:38:09Z

0

You can either wait for the completion of the process:

Process p = Runtime.getRuntime().exec(cmd);
int result = p.waitFor();

Or use the p.getInputStream() to read directly from the standard output of the process.

answered Jul 12, 2010 at 19:38

Maurice Perry

32.8k9 gold badges72 silver badges97 bronze badges

Collectives™ on Stack Overflow

Using Java to call Linux terminal: How to flush the output?

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related