linux - Java exec() does not return expected result of pipes' connected commands

Question

I'm calling command line programs connected by pipes. All this works on Linux for sure.

My method:

protected String execCommand(String command) throws IOException {
    String line = null;
    if (command.length() > 0) {
        Process child = Runtime.getRuntime().exec(command);
        InputStream lsOut = child.getInputStream();
        InputStreamReader r = new InputStreamReader(lsOut);
        BufferedReader in = new BufferedReader(r);

        String readline = null;
        while ((readline = in.readLine()) != null) {
            line = line + readline;
        }
    }

    return line;
}

If I'm calling some cat file | grep asd, I'm getting the expected result. But not all commands works correctly. For example with this:

cat /proc/cpuinfo | wc -l

or this:

cat /proc/cpuinfo | grep "model name" | head -n 1 | awk -F":" '{print substr($2, 2, length($2))}

the method will return null. I'm guessing this problem depends on output formatting commands like head, tail, wc, etc. How I can work around this problem and get the final result of the output?

Answer 1

The pipe (like redirection, or >) is a function of the shell, and so execing directly from Java won't work. You need to do something like:

/bin/sh -c "your | piped | commands | here"

which executes a shell process with the command line (including pipes) specified after the -c (in quotes).

Note also that you have to consume stdout and stderr concurrently, otherwise your spawned process will block waiting for your process to consume the output (or errors). More info here.