Execute a command in Linux using Java and fetch the output

Question

I am using Groovy to execute commands on my Linux box and get back the output, but I am not able to use | pipes somehow (I think) or maybe it is not waiting for

Answer 1

You cannot do pipes or redirects using String.execute(). This doesn't work in Java, so it doesn't work in Groovy either...

You can use Process.pipeTo with Groovy to simplify things:

Process proca = 'uname -a'.execute()
Process procb = 'awk {print\$2}'.execute()

(proca | procb).text

A more generic version could be:

String process = 'uname -a | awk {print\$2}'

// Split the string into sections based on |
// And pipe the results together
Process result = process.tokenize( '|' ).inject( null ) { p, c ->
  if( p )
    p | c.execute()
  else
    c.execute()
}
// Print out the output and error streams
result.waitForProcessOutput( System.out, System.out )

Answer 2

The pipe | is a feature of a shell like bash. To use a pipe you need to run a shell, like

"/bin/bash", "-c", "uname -a | awk '{print $2}'"

To use ProcessBuilder with redirection you can do

public static void main(String... args) throws IOException, InterruptedException {
    final String cmd = "uname -a | awk '{print $1 \" \" $3}'";
    System.out.println(cmd + " => " + run(cmd));
}

private static String run(String cmd) throws IOException, InterruptedException {
    final ProcessBuilder pb = new ProcessBuilder("/bin/bash", "-c", cmd);
    pb.redirectErrorStream();
    final Process process = pb.start();
    final InputStream in = process.getInputStream();
    final byte[] bytes = new byte[1024];
    final ByteArrayOutputStream baos = new ByteArrayOutputStream();

    // you must read the output as the program runs or it can stall.
    for (int len; (len = in.read(bytes)) > 0;)
        baos.write(bytes, 0, len);
    process.waitFor();
    return baos.toString(); // assuming the default char encoding is ok.
}

prints

uname -a | awk '{print $1 " " $3}' => Linux 2.6.18-274.3.1.el5

Answer 3

Pipe is a feature of shell. So if you want pipes to be supported you have to run your command in shell context, i.e. your command line that you pass to exec in java should look like /bin/sh YOUR_COMMAND.