Class: Net::SSH::Connection::Channel

Inherits:
Object
  • Object
show all
Includes:
Constants, Loggable
Defined in:
lib/net/ssh/connection/channel.rb

Overview

The channel abstraction. Multiple “channels” can be multiplexed onto a single SSH channel, each operating independently and seemingly in parallel. This class represents a single such channel. Most operations performed with the Net::SSH library will involve using one or more channels.

Channels are intended to be used asynchronously. You request that one be opened (via Connection::Session#open_channel), and when it is opened, your callback is invoked. Then, you set various other callbacks on the newly opened channel, which are called in response to the corresponding events. Programming with Net::SSH works best if you think of your programs as state machines. Complex programs are best implemented as objects that wrap a channel. See Net::SCP and Net::SFTP for examples of how complex state machines can be built on top of the SSH protocol.

ssh.open_channel do |channel|
  channel.exec("/invoke/some/command") do |ch, success|
    abort "could not execute command" unless success

    channel.on_data do |ch, data|
      puts "got stdout: #{data}"
      channel.send_data "something for stdin\n"
    end

    channel.on_extended_data do |ch, type, data|
      puts "got stderr: #{data}"
    end

    channel.on_close do |ch|
      puts "channel is closing!"
    end
  end
end

ssh.loop

Channels also have a basic hash-like interface, that allows programs to store arbitrary state information on a channel object. This helps simplify the writing of state machines, especially when you may be juggling multiple open channels at the same time.

Note that data sent across SSH channels are governed by maximum packet sizes and maximum window sizes. These details are managed internally by Net::SSH::Connection::Channel, so you may remain blissfully ignorant if you so desire, but you can always inspect the current maximums, as well as the remaining window size, using the reader attributes for those values.

Constant Summary collapse

VALID_PTY_OPTIONS =

A hash of the valid PTY options (see #request_pty).

{ :term        => "xterm",
:chars_wide  => 80,
:chars_high  => 24,
:pixels_wide => 640,
:pixels_high => 480,
:modes       => {} }

Constants included from Constants

Net::SSH::Connection::Constants::CHANNEL_CLOSE, Net::SSH::Connection::Constants::CHANNEL_DATA, Net::SSH::Connection::Constants::CHANNEL_EOF, Net::SSH::Connection::Constants::CHANNEL_EXTENDED_DATA, Net::SSH::Connection::Constants::CHANNEL_FAILURE, Net::SSH::Connection::Constants::CHANNEL_OPEN, Net::SSH::Connection::Constants::CHANNEL_OPEN_CONFIRMATION, Net::SSH::Connection::Constants::CHANNEL_OPEN_FAILURE, Net::SSH::Connection::Constants::CHANNEL_REQUEST, Net::SSH::Connection::Constants::CHANNEL_SUCCESS, Net::SSH::Connection::Constants::CHANNEL_WINDOW_ADJUST, Net::SSH::Connection::Constants::GLOBAL_REQUEST, Net::SSH::Connection::Constants::REQUEST_FAILURE, Net::SSH::Connection::Constants::REQUEST_SUCCESS

Instance Attribute Summary collapse

Attributes included from Loggable

#logger

Instance Method Summary collapse

Methods included from Loggable

#debug, #error, #fatal, #info, #lwarn

Constructor Details

#initialize(connection, type, local_id, max_pkt_size = 0x8000, max_win_size = 0x20000, &on_confirm_open) ⇒ Channel

Instantiates a new channel on the given connection, of the given type, and with the given id. If a block is given, it will be remembered until the channel is confirmed open by the server, and will be invoked at that time (see #do_open_confirmation).

This also sets the default maximum packet size and maximum window size.


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# File 'lib/net/ssh/connection/channel.rb', line 110

def initialize(connection, type, local_id, max_pkt_size = 0x8000, max_win_size = 0x20000, &on_confirm_open)
  self.logger = connection.logger

  @connection = connection
  @type       = type
  @local_id   = local_id

  @local_maximum_packet_size = max_pkt_size
  @local_window_size = @local_maximum_window_size = max_win_size

  @on_confirm_open = on_confirm_open

  @output = Buffer.new

  @properties = {}

  @pending_requests = []
  @on_open_failed = @on_data = @on_extended_data = @on_process = @on_close = @on_eof = nil
  @on_request = {}
  @closing = @eof = @sent_eof = false
end

Instance Attribute Details

#connectionObject (readonly)

The underlying Net::SSH::Connection::Session instance that supports this channel.


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# File 'lib/net/ssh/connection/channel.rb', line 66

def connection
  @connection
end

#local_idObject (readonly)

The local id for this channel, assigned by the Net::SSH::Connection::Session instance.


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# File 'lib/net/ssh/connection/channel.rb', line 57

def local_id
  @local_id
end

#local_maximum_packet_sizeObject (readonly)

The maximum packet size that the local host can receive.


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# File 'lib/net/ssh/connection/channel.rb', line 69

def local_maximum_packet_size
  @local_maximum_packet_size
end

#local_maximum_window_sizeObject (readonly)

The maximum amount of data that the local end of this channel can receive. This is a total, not per-packet.


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# File 'lib/net/ssh/connection/channel.rb', line 73

def local_maximum_window_size
  @local_maximum_window_size
end

#local_window_sizeObject (readonly)

This is the remaining window size on the local end of this channel. When this reaches zero, no more data can be received.


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# File 'lib/net/ssh/connection/channel.rb', line 84

def local_window_size
  @local_window_size
end

#outputObject (readonly)

The output buffer for this channel. Data written to the channel is enqueued here, to be written as CHANNEL_DATA packets during each pass of the event loop. See Connection::Session#process and #enqueue_pending_output.


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# File 'lib/net/ssh/connection/channel.rb', line 97

def output
  @output
end

#pending_requestsObject (readonly)

The list of pending requests. Each time a request is sent which requires a reply, the corresponding callback is pushed onto this queue. As responses arrive, they are shifted off the front and handled.


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# File 'lib/net/ssh/connection/channel.rb', line 102

def pending_requests
  @pending_requests
end

#propertiesObject (readonly)

A hash of properties for this channel. These can be used to store state information about this channel. See also #[] and #[]=.


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# File 'lib/net/ssh/connection/channel.rb', line 92

def properties
  @properties
end

#remote_idObject (readonly)

The remote id for this channel, assigned by the remote host.


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# File 'lib/net/ssh/connection/channel.rb', line 60

def remote_id
  @remote_id
end

#remote_maximum_packet_sizeObject (readonly)

The maximum packet size that the remote host can receive.


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# File 'lib/net/ssh/connection/channel.rb', line 76

def remote_maximum_packet_size
  @remote_maximum_packet_size
end

#remote_maximum_window_sizeObject (readonly)

The maximum amount of data that the remote end of this channel can receive. This is a total, not per-packet.


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# File 'lib/net/ssh/connection/channel.rb', line 80

def remote_maximum_window_size
  @remote_maximum_window_size
end

#remote_window_sizeObject (readonly)

This is the remaining window size on the remote end of this channel. When this reaches zero, no more data can be sent.


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# File 'lib/net/ssh/connection/channel.rb', line 88

def remote_window_size
  @remote_window_size
end

#typeObject (readonly)

The type of this channel, usually “session”.


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# File 'lib/net/ssh/connection/channel.rb', line 63

def type
  @type
end

Instance Method Details

#[](name) ⇒ Object

A shortcut for accessing properties of the channel (see #properties).


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# File 'lib/net/ssh/connection/channel.rb', line 133

def [](name)
  @properties[name]
end

#[]=(name, value) ⇒ Object

A shortcut for setting properties of the channel (see #properties).


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# File 'lib/net/ssh/connection/channel.rb', line 138

def []=(name, value)
  @properties[name] = value
end

#active?Boolean

Returns true if the channel exists in the channel list of the session, and false otherwise. This can be used to determine whether a channel has been closed or not.

ssh.loop { channel.active? }

Returns:

  • (Boolean)

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# File 'lib/net/ssh/connection/channel.rb', line 259

def active?
  connection.channels.key?(local_id)
end

#closeObject

Requests that the channel be closed. If the channel is already closing, this does nothing, nor does it do anything if the channel has not yet been confirmed open (see #do_open_confirmation). Otherwise, it sends a CHANNEL_CLOSE message and marks the channel as closing.


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# File 'lib/net/ssh/connection/channel.rb', line 284

def close
  return if @closing
  if remote_id
    @closing = true
    connection.send_message(Buffer.from(:byte, CHANNEL_CLOSE, :long, remote_id))
  end
end

#closing?Boolean

Returns true if the channel is currently closing, but not actually closed. A channel is closing when, for instance, #close has been invoked, but the server has not yet responded with a CHANNEL_CLOSE packet of its own.

Returns:

  • (Boolean)

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# File 'lib/net/ssh/connection/channel.rb', line 276

def closing?
  @closing
end

#do_closeObject

Invokes the #on_close callback when the server closes a channel. The channel is the only argument.


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# File 'lib/net/ssh/connection/channel.rb', line 590

def do_close
  @on_close.call(self) if @on_close
end

#do_data(data) ⇒ Object

Invokes the #on_data callback when the server sends data to the channel. This will reduce the available window size on the local end, but does not actually throttle requests that come in illegally when the window size is too small. The callback is invoked with the channel as the first argument, and the data as the second.


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# File 'lib/net/ssh/connection/channel.rb', line 567

def do_data(data) #:nodoc:
  update_local_window_size(data.length)
  @on_data.call(self, data) if @on_data
end

#do_eofObject

Invokes the #on_eof callback when the server indicates that no further data is forthcoming. The callback is invoked with the channel as the argument.


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# File 'lib/net/ssh/connection/channel.rb', line 584

def do_eof
  @on_eof.call(self) if @on_eof
end

#do_extended_data(type, data) ⇒ Object

Invokes the #on_extended_data callback when the server sends extended data to the channel. This will reduce the available window size on the local end. The callback is invoked with the channel, type, and data.


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# File 'lib/net/ssh/connection/channel.rb', line 576

def do_extended_data(type, data)
  update_local_window_size(data.length)
  @on_extended_data.call(self, type, data) if @on_extended_data
end

#do_failureObject

Invokes the next pending request callback with false as the second argument.


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# File 'lib/net/ssh/connection/channel.rb', line 596

def do_failure
  if callback = pending_requests.shift
    callback.call(self, false)
  else
    error { "channel failure recieved with no pending request to handle it (bug?)" }
  end
end

#do_open_confirmation(remote_id, max_window, max_packet) ⇒ Object

Invoked when the server confirms that a channel has been opened. The remote_id is the id of the channel as assigned by the remote host, and max_window and max_packet are the maximum window and maximum packet sizes, respectively. If an open-confirmation callback was given when the channel was created, it is invoked at this time with the channel itself as the sole argument.


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# File 'lib/net/ssh/connection/channel.rb', line 508

def do_open_confirmation(remote_id, max_window, max_packet) #:nodoc:
  @remote_id = remote_id
  @remote_window_size = @remote_maximum_window_size = max_window
  @remote_maximum_packet_size = max_packet
  connection.forward.agent(self) if connection.options[:forward_agent] && type == "session"
  forward_local_env(connection.options[:send_env]) if connection.options[:send_env]
  @on_confirm_open.call(self) if @on_confirm_open
end

#do_open_failed(reason_code, description) ⇒ Object

Invoked when the server failed to open the channel. If an #on_open_failed callback was specified, it will be invoked with the channel, reason code, and description as arguments. Otherwise, a ChannelOpenFailed exception will be raised.


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# File 'lib/net/ssh/connection/channel.rb', line 521

def do_open_failed(reason_code, description)
  if @on_open_failed
    @on_open_failed.call(self, reason_code, description)
  else
    raise ChannelOpenFailed.new(reason_code, description)
  end
end

#do_request(request, want_reply, data) ⇒ Object

Invoked when the server sends a channel request. If any #on_request callback has been registered for the specific type of this request, it is invoked. If want_reply is true, a packet will be sent of either CHANNEL_SUCCESS or CHANNEL_FAILURE type. If there was no callback to handle the request, CHANNEL_FAILURE will be sent. Otherwise, CHANNEL_SUCCESS, unless the callback raised ChannelRequestFailed. The callback should accept the channel as the first argument, and the request-specific data as the second.


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# File 'lib/net/ssh/connection/channel.rb', line 546

def do_request(request, want_reply, data) #:nodoc:
  result = true

  begin
    callback = @on_request[request] or raise ChannelRequestFailed
    callback.call(self, data)
  rescue ChannelRequestFailed
    result = false
  end

  if want_reply
    msg = Buffer.from(:byte, result ? CHANNEL_SUCCESS : CHANNEL_FAILURE, :long, remote_id)
    connection.send_message(msg)
  end
end

#do_successObject

Invokes the next pending request callback with true as the second argument.


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# File 'lib/net/ssh/connection/channel.rb', line 606

def do_success
  if callback = pending_requests.shift
    callback.call(self, true)
  else
    error { "channel success recieved with no pending request to handle it (bug?)" }
  end
end

#do_window_adjust(bytes) ⇒ Object

Invoked when the server sends a CHANNEL_WINDOW_ADJUST packet, and causes the remote window size to be adjusted upwards by the given number of bytes. This has the effect of allowing more data to be sent from the local end to the remote end of the channel.


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# File 'lib/net/ssh/connection/channel.rb', line 533

def do_window_adjust(bytes) #:nodoc:
  @remote_maximum_window_size += bytes
  @remote_window_size += bytes
end

#enqueue_pending_outputObject

Enqueues pending output at the connection as CHANNEL_DATA packets. This does nothing if the channel has not yet been confirmed open (see #do_open_confirmation). This is called automatically by #process, which is called from the event loop (Connection::Session#process). You will generally not need to invoke it directly.


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# File 'lib/net/ssh/connection/channel.rb', line 484

def enqueue_pending_output #:nodoc:
  return unless remote_id

  while output.length > 0
    length = output.length
    length = remote_window_size if length > remote_window_size
    length = remote_maximum_packet_size if length > remote_maximum_packet_size

    if length > 0
      connection.send_message(Buffer.from(:byte, CHANNEL_DATA, :long, remote_id, :string, output.read(length)))
      output.consume!
      @remote_window_size -= length
    else
      break
    end
  end
end

#env(variable_name, variable_value, &block) ⇒ Object

Syntactic sugar for setting an environment variable in the remote process' environment. Note that for security reasons, the server may refuse to set certain environment variables, or all, at the server's discretion. If you are connecting to an OpenSSH server, you will need to update the AcceptEnv setting in the sshd_config to include the environment variables you want to send.

channel.env "PATH", "/usr/local/bin"

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# File 'lib/net/ssh/connection/channel.rb', line 187

def env(variable_name, variable_value, &block)
  send_channel_request("env", :string, variable_name, :string, variable_value, &block)
end

#eof!Object

Tells the remote end of the channel that no more data is forthcoming from this end of the channel. The remote end may still send data. The CHANNEL_EOF packet will be sent once the output buffer is empty.


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# File 'lib/net/ssh/connection/channel.rb', line 302

def eof!
  return if eof?
  @eof = true
end

#eof?Boolean

Returns true if the local end of the channel has declared that no more data is forthcoming (see #eof!). Trying to send data via #send_data when this is true will result in an exception being raised.

Returns:

  • (Boolean)

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# File 'lib/net/ssh/connection/channel.rb', line 295

def eof?
  @eof
end

#exec(command, &block) ⇒ Object

Syntactic sugar for executing a command. Sends a channel request asking that the given command be invoked. If the block is given, it will be called when the server responds. The first parameter will be the channel, and the second will be true or false, indicating whether the request succeeded or not. In this case, success means that the command is being executed, not that it has completed, and failure means that the command altogether failed to be executed.

channel.exec "ls -l /home" do |ch, success|
  if success
    puts "command has begun executing..."    # this is a good place to hang callbacks like #on_data...

  else
    puts "alas! the command could not be invoked!"
  end
end

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# File 'lib/net/ssh/connection/channel.rb', line 158

def exec(command, &block)
  send_channel_request("exec", :string, command, &block)
end

#on_close(&block) ⇒ Object

Registers a callback to be invoked when the server acknowledges that a channel is closed. This is invoked with the channel as the sole argument.

channel.on_close do |ch|
  puts "remote end is closing!"
end

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# File 'lib/net/ssh/connection/channel.rb', line 383

def on_close(&block)
  old, @on_close = @on_close, block
  old
end

#on_data(&block) ⇒ Object

Registers a callback to be invoked when data packets are received by the channel. The callback is called with the channel as the first argument, and the data as the second.

channel.on_data do |ch, data|
  puts "got data: #{data.inspect}"
end

Data received this way is typically the data written by the remote process to its stdout stream.


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# File 'lib/net/ssh/connection/channel.rb', line 330

def on_data(&block)
  old, @on_data = @on_data, block
  old
end

#on_eof(&block) ⇒ Object

Registers a callback to be invoked when the server indicates that no more data will be sent to the channel (although the channel can still send data to the server). The channel is the sole argument to the callback.

channel.on_eof do |ch|
  puts "remote end is done sending data"
end

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# File 'lib/net/ssh/connection/channel.rb', line 395

def on_eof(&block)
  old, @on_eof = @on_eof, block
  old
end

#on_extended_data(&block) ⇒ Object

Registers a callback to be invoked when extended data packets are received by the channel. The callback is called with the channel as the first argument, the data type (as an integer) as the second, and the data as the third. Extended data is almost exclusively used to send stderr data (type == 1). Other extended data types are not defined by the SSH protocol.

channel.on_extended_data do |ch, type, data|
  puts "got stderr: #{data.inspect}"
end

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# File 'lib/net/ssh/connection/channel.rb', line 345

def on_extended_data(&block)
  old, @on_extended_data = @on_extended_data, block
  old
end

#on_open_failed(&block) ⇒ Object

Registers a callback to be invoked when the server was unable to open the requested channel. The channel itself will be passed to the block, along with the integer “reason code” for the failure, and a textual description of the failure from the server.

channel = session.open_channel do |ch|
  # ..
end

channel.on_open_failed { |ch, code, desc| ... }

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# File 'lib/net/ssh/connection/channel.rb', line 410

def on_open_failed(&block)
  old, @on_open_failed = @on_open_failed, block
  old
end

#on_process(&block) ⇒ Object

Registers a callback to be invoked for each pass of the event loop for this channel. There are no guarantees on timeliness in the event loop, but it will be called roughly once for each packet received by the connection (not the channel). This callback is invoked with the channel as the sole argument.

Here's an example that accumulates the channel data into a variable on the channel itself, and displays individual lines in the input one at a time when the channel is processed:

channel[:data] = ""

channel.on_data do |ch, data|
  channel[:data] << data
end

channel.on_process do |ch|
  if channel[:data] =~ /^.*?\n/
    puts $&
    channel[:data] = $'
  end
end

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# File 'lib/net/ssh/connection/channel.rb', line 372

def on_process(&block)
  old, @on_process = @on_process, block
  old
end

#on_request(type, &block) ⇒ Object

Registers a callback to be invoked when a channel request of the given type is received. The callback will receive the channel as the first argument, and the associated (unparsed) data as the second. The data will be a Net::SSH::Buffer that you will need to parse, yourself, according to the kind of request you are watching.

By default, if the request wants a reply, Net::SSH will send a CHANNEL_SUCCESS response for any request that was handled by a registered callback, and CHANNEL_FAILURE for any that wasn't, but if you want your registered callback to result in a CHANNEL_FAILURE response, just raise Net::SSH::ChannelRequestFailed.

Some common channel requests that your programs might want to listen for are:

  • “exit-status” : the exit status of the remote process will be reported as a long integer in the data buffer, which you can grab via data.read_long.

  • “exit-signal” : if the remote process died as a result of a signal being sent to it, the signal will be reported as a string in the data, via data.read_string. (Not all SSH servers support this channel request type.)

    channel.on_request "exit-status" do |ch, data|
      puts "process terminated with exit status: #{data.read_long}"
    end
    

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# File 'lib/net/ssh/connection/channel.rb', line 441

def on_request(type, &block)
  old, @on_request[type] = @on_request[type], block
  old
end

#processObject

If an #on_process handler has been set up, this will cause it to be invoked (passing the channel itself as an argument). It also causes all pending output to be enqueued as CHANNEL_DATA packets (see #enqueue_pending_output).


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# File 'lib/net/ssh/connection/channel.rb', line 310

def process
  @on_process.call(self) if @on_process
  enqueue_pending_output

  if @eof and not @sent_eof and output.empty? and remote_id
    connection.send_message(Buffer.from(:byte, CHANNEL_EOF, :long, remote_id))
    @sent_eof = true
  end
end

#request_pty(opts = {}, &block) ⇒ Object

Requests that a pseudo-tty (or “pty”) be made available for this channel. This is useful when you want to invoke and interact with some kind of screen-based program (e.g., vim, or some menuing system).

Note, that without a pty some programs (e.g. sudo, or subversion) on some systems, will not be able to run interactively, and will error instead of prompt if they ever need some user interaction.

Note, too, that when a pty is requested, user's shell configuration scripts (.bashrc and such) are not run by default, whereas they are run when a pty is not present.

channel.request_pty do |ch, success|
  if success
    puts "pty successfully obtained"
  else
    puts "could not obtain pty"
  end
end

Raises:

  • (ArgumentError)

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# File 'lib/net/ssh/connection/channel.rb', line 218

def request_pty(opts={}, &block)
  extra = opts.keys - VALID_PTY_OPTIONS.keys
  raise ArgumentError, "invalid option(s) to request_pty: #{extra.inspect}" if extra.any?

  opts = VALID_PTY_OPTIONS.merge(opts)

  modes = opts[:modes].inject(Buffer.new) do |memo, (mode, data)|
    memo.write_byte(mode).write_long(data)
  end  # mark the end of the mode opcode list with a 0 byte

  modes.write_byte(0)

  send_channel_request("pty-req", :string, opts[:term],
    :long, opts[:chars_wide], :long, opts[:chars_high],
    :long, opts[:pixels_wide], :long, opts[:pixels_high],
    :string, modes.to_s, &block)
end

#send_channel_request(request_name, *data, &callback) ⇒ Object

Sends a new channel request with the given name. The extra data parameter must either be empty, or consist of an even number of arguments. See Net::SSH::Buffer.from for a description of their format. If a block is given, it is registered as a callback for a pending request, and the packet will be flagged so that the server knows a reply is required. If no block is given, the server will send no response to this request. Responses, where required, will cause the callback to be invoked with the channel as the first argument, and either true or false as the second, depending on whether the request succeeded or not. The meaning of “success” and “failure” in this context is dependent on the specific request that was sent.

channel.send_channel_request "shell" do |ch, success|
  if success
    puts "user shell started successfully"
  else
    puts "could not start user shell"
  end
end

Most channel requests you'll want to send are already wrapped in more convenient helper methods (see #exec and #subsystem).


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# File 'lib/net/ssh/connection/channel.rb', line 468

def send_channel_request(request_name, *data, &callback)
  info { "sending channel request #{request_name.inspect}" }
  msg = Buffer.from(:byte, CHANNEL_REQUEST,
    :long, remote_id, :string, request_name,
    :bool, !callback.nil?, *data)
  connection.send_message(msg)
  pending_requests << callback if callback
end

#send_data(data) ⇒ Object

Sends data to the channel's remote endpoint. This usually has the effect of sending the given string to the remote process' stdin stream. Note that it does not immediately send the data across the channel, but instead merely appends the given data to the channel's output buffer, preparatory to being packaged up and sent out the next time the connection is accepting data. (A connection might not be accepting data if, for instance, it has filled its data window and has not yet been resized by the remote end-point.)

This will raise an exception if the channel has previously declared that no more data will be sent (see #eof!).

channel.send_data("the password\n")

Raises:

  • (EOFError)

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# File 'lib/net/ssh/connection/channel.rb', line 249

def send_data(data)
  raise EOFError, "cannot send data if channel has declared eof" if eof?
  output.append(data.to_s)
end

#subsystem(subsystem, &block) ⇒ Object

Syntactic sugar for requesting that a subsystem be started. Subsystems are a way for other protocols (like SFTP) to be run, using SSH as the transport. Generally, you'll never need to call this directly unless you are the implementor of something that consumes an SSH subsystem, like SFTP.

channel.subsystem("sftp") do |ch, success|
  if success
    puts "subsystem successfully started"
  else
    puts "subsystem could not be started"
  end
end

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# File 'lib/net/ssh/connection/channel.rb', line 175

def subsystem(subsystem, &block)
  send_channel_request("subsystem", :string, subsystem, &block)
end

#waitObject

Runs the SSH event loop until the channel is no longer active. This is handy for blocking while you wait for some channel to finish.

channel.exec("grep ...") { ... }
channel.wait

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# File 'lib/net/ssh/connection/channel.rb', line 268

def wait
  connection.loop { active? }
end