How to read JSch command output?

Question

I have the following code:

JSch jsch = new JSch();
jsch.setKnownHosts(dotSshDir + "/known_hosts");
jsch.addIdentity(dotSshDir + "/id_rsa");

Session session = jsch.getSession(userName, hostname, 22);
session.connect();

ChannelExec channel = (ChannelExec) session.openChannel("exec");
channel.setCommand(command);
channel.setInputStream(null);
channel.setErrStream(System.err);
Reader reader = new InputStreamReader(channel.getInputStream());

char[] buf = new char[1024];
int numRead;
while ((numRead = reader.read(buf)) != -1) {
    String readData = String.valueOf(buf, 0, numRead);
    result.append(readData);
    buf = new char[1024];
}

It's hanging trying to read from the reader. How do I fix this? How do I go about hunting down what's happening?

Answer 1

The hanging was actually due to some unbalanced quotes in the command.

For posterity, the final code (after dealing with some other issues) is:

public String call(String hostname, String[] argv) throws SubprocessException {
    StringBuffer result = new StringBuffer();

    Session session = null;
    ChannelExec channel = null;

    try {
        // TODO: Emit friendly error if ~/.ssh doesn't exist.
        String dotSshDir = System.getProperty("user.home") + "/.ssh";
        String userName = System.getProperty("user.name");

        // TODO: Emit friendly error if ~/.ssh/known_hosts doesn't exist.
        jSch.setKnownHosts(dotSshDir + "/known_hosts");
        // TODO: Emit friendly error if ~/.ssh/id_rsa doesn't exist.
        jSch.addIdentity(dotSshDir + "/id_rsa");

        session = jSch.getSession(userName, hostname, 22);
        session.connect();

        channel = (ChannelExec) session.openChannel("exec");
        channel.setCommand(Joiner.on(" ").join(argv));
        channel.setInputStream(null);
        InputStream stdout = channel.getInputStream();
        InputStream stderr = channel.getErrStream();
        channel.connect();

        waitForChannelClosed(channel);

        if (channel.getExitStatus() != 0) {
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(stderr));
            readAll(bufferedReader, result);

            throw new Exception(result.toString());
        } else {
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(stdout));
            readAll(bufferedReader, result);
        }
    } catch (Exception e) {
        throw new SubprocessException(e);
    } finally {
        if (channel != null) {
            channel.disconnect();
        }

        if (session != null) {
            session.disconnect();
        }
    }

    return result.toString();
}

Answer 2

The accepted answer by @NoelYap is wrong.

One has to read the output continuously, while waiting for the command to finish. Otherwise, if the command produces enough output to fill in an output buffer, the command will hang, waiting for the buffer to be consumed, what never happens. So you get a deadlock.

The following example reads both stdout and stderr continuously, while monitoring a command status. It is based on the official JSch exec.java example (just adds a reading of stderr).

ChannelExec channel = (ChannelExec)session.openChannel("exec");

channel.setCommand(
    "for((i=1;i<=10000;i+=2)); do echo \"Long output - $i\"; done ; " +
    "echo error output >&2");
InputStream commandOutput = channel.getExtInputStream();

StringBuilder outputBuffer = new StringBuilder();
StringBuilder errorBuffer = new StringBuilder();

InputStream in = channel.getInputStream();
InputStream err = channel.getExtInputStream();

channel.connect();

byte[] tmp = new byte[1024];
while (true) {
    while (in.available() > 0) {
        int i = in.read(tmp, 0, 1024);
        if (i < 0) break;
        outputBuffer.append(new String(tmp, 0, i));
    }
    while (err.available() > 0) {
        int i = err.read(tmp, 0, 1024);
        if (i < 0) break;
        errorBuffer.append(new String(tmp, 0, i));
    }
    if (channel.isClosed()) {
        if ((in.available() > 0) || (err.available() > 0)) continue; 
        System.out.println("exit-status: " + channel.getExitStatus());
        break;
    }
    try { 
      Thread.sleep(1000);
    } catch (Exception ee) {
    }
}

System.out.println("output: " + outputBuffer.toString());
System.out.println("error: " + errorBuffer.toString());

channel.disconnect();

---

If you add while (!channel.isClosed()) {} after the channel.connect();, you will see that with a sufficiently large i in the shell for loop (10000 is enough with in my environment), the loop never finishes.