Visual FoxPro

Read an SSH Channel with a Byte Threshold

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).

Background: The threshold makes it practical to stream large output in manageable chunks instead of buffering an entire multi-megabyte result. Treat it as a return threshold, not a hard cap: a complete SSH packet is buffered as a unit, so the amount returned can exceed what you asked for. Data already buffered before the call does not by itself trigger an immediate return.

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LOCAL lnSuccess
LOCAL loSsh
LOCAL lnSshPort
LOCAL lcPassword
LOCAL lnChannelNum
LOCAL lnPollTimeoutMs
LOCAL lnMaxNumBytes
LOCAL lnResult
LOCAL lcOutput

lnSuccess = 0

*  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.

loSsh = CreateObject('Chilkat.Ssh')

lnSshPort = 22
lnSuccess = loSsh.Connect("ssh.example.com",lnSshPort)
IF (lnSuccess = 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
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"

lnSuccess = loSsh.AuthenticatePw("mySshLogin",lcPassword)
IF (lnSuccess = 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
ENDIF

lnChannelNum = loSsh.OpenSessionChannel()
IF (lnChannelNum < 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
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.
lnSuccess = loSsh.SendReqShell(lnChannelNum)
IF (lnSuccess = 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
ENDIF

lnSuccess = loSsh.ChannelSendString(lnChannelNum,"cat /var/log/syslog" + CHR(10),"utf-8")
IF (lnSuccess = 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
ENDIF

*  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.
lnPollTimeoutMs = 500
lnMaxNumBytes = 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.
lnResult = loSsh.ChannelReadAndPoll2(lnChannelNum,lnPollTimeoutMs,lnMaxNumBytes)
IF (lnResult = -1) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
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 = 0) THEN
    ? loSsh.LastErrorText
    RELEASE loSsh
    CANCEL
ENDIF

? lcOutput

loSsh.Disconnect()

RELEASE loSsh