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).

local chilkat = require("chilkat")

-- 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.

local ssh = chilkat.Ssh.new()

local sshPort = 22
local success = ssh:connect("ssh.example.com", sshPort)
if not success then
    print(ssh.last_error_text)
    return
end

-- 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.
local password = "mySshPassword"

success = ssh:authenticate_pw("mySshLogin", password)
if not success then
    print(ssh.last_error_text)
    return
end

local channelNum = ssh:open_session_channel()
if channelNum < 0 then
    print(ssh.last_error_text)
    return
end

-- 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:send_req_shell(channelNum)
if not success then
    print(ssh.last_error_text)
    return
end

success = ssh:channel_send_string(channelNum, "cat /var/log/syslog\n", "utf-8")
if not success then
    print(ssh.last_error_text)
    return
end

-- 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.
local pollTimeoutMs = 500
local 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.
local result = ssh:channel_read_and_poll2(channelNum, pollTimeoutMs, maxNumBytes)
if result == (-1) then
    print(ssh.last_error_text)
    return
end

if result == (-2) then
    print("No channel data arrived before the timeout.")
end

if result == 0 then
    print("EOF or channel close was processed.  Buffered data may still remain.")
end

if result > 0 then
    print("Total bytes buffered: " .. result)
end

local output = ssh:get_received_text(channelNum, "utf-8")
if not ssh.last_method_success then
    print(ssh.last_error_text)
    return
end

print(output)

ssh:disconnect()