B4X

Poll an SSH Channel for Incoming Data

Demonstrates the Chilkat Ssh.ChannelPoll method, which polls a channel for incoming messages, waiting at most the given number of milliseconds. The first argument is the channel number and the second is the poll timeout; pass 0 for a nonblocking poll.

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).

Background: ChannelPoll gives you an explicit, per-call wait, which is what an event loop or responsive UI needs — poll briefly, do other work, poll again. Importantly, the timeout you pass governs the wait: shorter ReadTimeoutMs or IdleTimeoutMs values do not cut it short. A -2 return simply means nothing new arrived, which is normal when polling.

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Dim success As Boolean = False

'  Demonstrates the Ssh.ChannelPoll method, which polls a channel for incoming messages, waiting
'  at most the given number of milliseconds.  The 1st argument is the channel number and the 2nd
'  is the poll timeout.  Pass 0 for a nonblocking poll.

Dim ssh As ChilkatSsh
ssh.Initialize("ssh")

Dim sshPort As Int = 22
success = ssh.Connect("ssh.example.com", sshPort)
If success = False Then
    Log(ssh.LastErrorText)
    Return
End If


'  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.
Dim password As String = "mySshPassword"

success = ssh.AuthenticatePw("mySshLogin", password)
If success = False Then
    Log(ssh.LastErrorText)
    Return
End If


Dim channelNum As Int = ssh.OpenSessionChannel
If channelNum < 0 Then
    Log(ssh.LastErrorText)
    Return
End If


'  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.
success = ssh.SendReqShell(channelNum)
If success = False Then
    Log(ssh.LastErrorText)
    Return
End If


success = ssh.ChannelSendString(channelNum, "date" & Chr(10), "utf-8")
If success = False Then
    Log(ssh.LastErrorText)
    Return
End If


'  Wait up to 3 seconds for channel data.
Dim pollTimeoutMs As Int = 3000
'  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.
Dim result As Int = ssh.ChannelPoll(channelNum, pollTimeoutMs)
If result = -1 Then
    Log(ssh.LastErrorText)
    Return
End If


If result = -2 Then
    Log("No channel data arrived before the timeout.")
End If


If result = 0 Then
    Log("EOF or channel close was processed.  Buffered data may still remain.")
End If


If result > 0 Then
    Log("Total bytes buffered: " & result)
End If


Dim output As String = ssh.GetReceivedText(channelNum, "utf-8")
If ssh.LastMethodSuccess = False Then
    Log(ssh.LastErrorText)
    Return
End If

Log(output)

ssh.Disconnect