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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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llSuccess = .F.

//  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.

loSsh = createobject("CkSsh")

lnSshPort = 22
llSuccess = loSsh.Connect("ssh.example.com",lnSshPort)
if (llSuccess = .F.) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

//  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.
lcPassword = "mySshPassword"

llSuccess = loSsh.AuthenticatePw("mySshLogin",lcPassword)
if (llSuccess = .F.) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

lnChannelNum = loSsh.OpenSessionChannel()
if (lnChannelNum < 0) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

//  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.
llSuccess = loSsh.SendReqShell(lnChannelNum)
if (llSuccess = .F.) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

llSuccess = loSsh.ChannelSendString(lnChannelNum,"date" + Chr(10),"utf-8")
if (llSuccess = .F.) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

//  Wait up to 3 seconds for channel data.
lnPollTimeoutMs = 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.
lnResult = loSsh.ChannelPoll(lnChannelNum,lnPollTimeoutMs)
if (lnResult = -1) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

if (lnResult = -2) then
    ? "No channel data arrived before the timeout."
endif

if (lnResult = 0) then
    ? "EOF or channel close was processed.  Buffered data may still remain."
endif

if (lnResult > 0) then
    ? "Total bytes buffered: " + str(lnResult)
endif

lcOutput = loSsh.GetReceivedText(lnChannelNum,"utf-8")
if (loSsh.LastMethodSuccess = .F.) then
    ? loSsh.LastErrorText
    release loSsh
    return
endif

? lcOutput

loSsh.Disconnect()


release loSsh