Read an SSH Channel with a Byte Threshold

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Demonstrates the Chilkat Ssh.ChannelReadAndPoll2 method, which works like ChannelReadAndPoll but also returns once newly processed data brings the total buffered data to or above a byte threshold. The arguments are the channel number, the poll timeout, and the threshold.

The integer return value is -1 on error, -2 if nothing arrived before the timeout, 0 if EOF or channel close was processed, or — when greater than 0 — the total bytes currently buffered for the channel (not the bytes newly received by this call).

import { Ssh } from '@chilkat/react-native'

async function chilkatExample() {
  // Demonstrates the Ssh.ChannelReadAndPoll2 method, which works like ChannelReadAndPoll but also
  // returns once the total buffered data reaches or exceeds a byte threshold.  The 1st argument is
  // the channel number, the 2nd is the poll timeout, and the 3rd is the byte threshold.

  const ssh = new Ssh();

  const sshPort = 22;
  try {
    await ssh.connectAsync('ssh.example.com', sshPort);
  } catch {
    console.log(ssh.lastErrorText);
    return;
  }

  // Normally you would not hard-code the password in source.  You should instead obtain it
  // from an interactive prompt, environment variable, or a secrets vault.
  const password = 'mySshPassword';

  try {
    await ssh.authenticatePwAsync('mySshLogin', password);
  } catch {
    console.log(ssh.lastErrorText);
    return;
  }

  const channelNum = await ssh.openSessionChannelAsync();
  if (channelNum < 0) {
    console.log(ssh.lastErrorText);
    return;
  }

  // Start a shell WITHOUT requesting a PTY.  Without a PTY the shell sees it is connected to a
  // raw data pipe and enters non-interactive mode: it suppresses the command prompt, does not
  // echo typed commands back, and skips interactive login scripts.  The output is therefore
  // clean and easily parseable -- but because no prompt is ever sent, code must not wait for
  // one.  Instead, have each command print a predictable marker, or wait for EOF / channel close.
  try {
    await ssh.sendReqShellAsync(channelNum);
  } catch {
    console.log(ssh.lastErrorText);
    return;
  }

  try {
    await ssh.channelSendStringAsync(channelNum, 'cat /var/log/syslog\n', 'utf-8');
  } catch {
    console.log(ssh.lastErrorText);
    return;
  }

  // Return when output goes quiet for 500ms, or once about 64KB has been buffered.  The
  // threshold is not a hard buffer limit -- a whole SSH packet may push the total above it.
  const pollTimeoutMs = 500;
  const maxNumBytes = 65536;
  // The return value is: -1 on error, -2 if nothing arrived before the timeout, 0 if EOF or
  // channel close was processed, or the total number of buffered bytes when greater than 0.
  const result = await ssh.channelReadAndPoll2Async(channelNum, pollTimeoutMs, maxNumBytes);
  if (result === -1) {
    console.log(ssh.lastErrorText);
    return;
  }

  if (result === -2) {
    console.log('No channel data arrived before the timeout.');
  }

  if (result === 0) {
    console.log('EOF or channel close was processed.  Buffered data may still remain.');
  }

  if (result > 0) {
    console.log(`Total bytes buffered: ${result}`);
  }

  let output: string;
  try {
    output = ssh.getReceivedText(channelNum, 'utf-8');
  } catch {
    console.log(ssh.lastErrorText);
    return;
  }

  console.log(output);

  ssh.disconnect();
}