sshj: How to pipe input to automated host-side script

Question

So, here's the situation:

I would like to use the sshj library to connect to a host which automatically runs a script on connection. Let's say the script merely logs whatever json formated input it receives. In the terminal I can run something like:

echo '{ "name" : "Hubert", "status" : "alive" }' | ssh -i ~/.ssh/id_ed25519 [email protected]

and upon connection the host would log the info { "name" : "Hubert", "status" : "alive" }.

What would an (equivalent) implementation of the above command look like in sshj?

Answer 1

Okay, I am not 100% sure whether all I do in the following code is entirely necessary, but it works for me:

final SSHClient ssh = new SSHClient();
/* 
 * Connect to host and authenticate
 */
try (Session session = ssh.startSession()){
        // open a shell on host
        final Shell shl = session.startShell();
        
        // just a thread to stream stdout of host
        Thread t = new Thread() {
                @Override
                public void run() {
                        try {
                                InputStream in = shl.getInputStream();
                                BufferedReader br = new BufferedReader(new InputStreamReader(in));
                                String line;
                                while ((line = br.readLine()) != null) {
                                        System.out.println("## " + line);
                                }
                        } catch (Exception e) {
                                e.printStackTrace();
                        }
                }
        };
        t.start();

        // another thread to stream stderr of host
        Thread err_t = new Thread() {
                @Override
                public void run() {
                        try {
                                InputStream in = shl.getErrorStream();
                                BufferedReader br = new BufferedReader(new InputStreamReader(in));
                                String line;
                                while ((line = br.readLine()) != null) {
                                        System.out.println("## " + line);
                                }
                        } catch (Exception e) {
                                e.printStackTrace();
                        }
                }
        };
        err_t.start();
        
        // you might want to escape the input string
        String input = "Some String";
        byte[] data = input.getBytes();
        // getOutputStream() corresponds to the hosts stdin
        OutputStream out = shl.getOutputStream();
        out.write(data);
        // ensure all written bytes get flushed to host
        out.flush();
        out.close();
        shl.join(5, TimeUnit.SECONDS);
        shl.close();
    } finally {
        ssh.disconnect();
    }