Execute bash-command in Java won't give a return

Question

I am trying to execute some bash commands within some java code. The goal is to get the rx and tx data. I got a function for the command execution. I have tested the function with other commands and it works fine.

But in this example the variable interfaceName is always empty:

public static void main(String args[]) {
    String interfaceCommand = "route | grep default | tr -s ' ' | cut -d ' ' -f 8";
    String interfaceName = executeCommand(interfaceCommand);

    String getRXCommand = "cat /sys/class/net/" + interfaceName + "/statistics/rx_bytes";
    String rxData = executeCommand(getRXCommand);

    String getTXCommand = "cat /sys/class/net/" + interfaceName + "/statistics/tx_bytes";
    String txData = executeCommand(getTXCommand);

    System.out.println(interfaceName);
    System.out.println(getTXCommand);
    System.out.println(getRXCommand);
}

private static String executeCommand(String command) {
    StringBuffer output = new StringBuffer();
    Process p;

    try {
        p = Runtime.getRuntime().exec(command);
        p.waitFor();
        BufferedReader reader = new BufferedReader(new InputStreamReader(p.getInputStream()));
        String line = "";           
        while ((line = reader.readLine())!= null) {
            output.append(line + "\n");
        }
    } catch (Exception e) {
        e.printStackTrace();
    }

    return output.toString();
}

If I set the variable interfaceName manually to enp0s31f6 the commands to get the rx and tx data are working fine (just on my test-PC). But i like to use the program on several PCs and the interface name is not always the same. So i need to get the interface name first.

It seems to be a problem if the command uses the | because i tried some other command variations.

With ip -o link show | grep ' UP ' | awk '{print $2}' the variable is empty.
With ls /sys/class/net/ | grep enp the variable is

/sys/class/net/: enp0s31f6 lo vmnet1 vmnet8

That is not the proper return value for the command. And as I said other commands, without |, are working fine...

I am not quite sure where my error is neither how to start searching for a proper solution. Do i have somehow to escape the |? Or is there another solution to get the interface name?

score 0 · Answer 1 · answered Jun 22 '18 at 16:06

Damn it! I am so f*cking stupid. It isn't necessary to trim the output in the bash command. I can get the whole output and trim it in java. Or i try to get the interface name directly in java with NetworkInterface.getNetworkInterfaces() or something...

But for today it seems i have programmed enough and my brain needs a break O_o

score 0 · Answer 2 · answered Jun 22 '18 at 16:13

Your problem is that the shell don't recognize the command in the right way... Try the following it should work for linux systems:

String [] command = {"/bin/sh", "-c", "route | grep default | tr -s ' ' | cut -d ' ' -f 8"};
String response = executeCommand(command);

private static String executeCommand(String [] command) // Important: Change the prototype
{
    ...
    p = Runtime.getRuntime().exec(command);
    ...
}

Execute bash-command in Java won't give a return

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