Net::SSH Manual :: Chapter 5: User Shells

Net::SSH— Secure Shell for Ruby | Net::SSH Version: 1.1.2 Manual Last Updated: 2007-06-18 18:12 UTC

| Previous (4. Executing Commands) | Up | Next (6. Port Forwarding) # 5. User Shells ## 5.1. Introduction A user’s shell is the environment in which commands are executed on a remote machine. There are a variety of different shells, including bash, csh, tcsh, ksh, and many others. A shell may be executed like any other program, and when run, it (more-or-less) enters a read-execute-prompt cycle in which the user may interactively run commands. SSH has special support for shells, allowing you to start a shell on a channel and have all subsequent data sent to that channel interpreted as the user’s input to the shell. Also, SSH supports pty’s (pseudo-tty’s, or terminals), which allow the shell to act as if it were running locally on the users own machine. Because a shell is just a program, you can always start a shell simply by executing it (as described in the previous chapter). However, you can also take advantage of SSH’s builtin shell support to execute the user’s preferred shell. This chapter will discuss this approach. ## 5.2. Using Channels At the lowest level, starting a shell is a matter of sending a “shell” request over a channel. Sending a shell request [ruby]
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    Net::SSH.start( host ) do |session|

      session.open_channel do |channel|

        channel.on_success do
          puts "shell was started successfully!"
          channel.send_data "exit\n" # tell the shell to exit
        end
        channel.on_failure do
          puts "shell could not be started!"
        end
        channel.on_data do |ch,data|
          puts "recieved #{data} from shell"
        end
        channel.on_close do
          puts "shell terminated"
        end

        channel.send_request "shell", nil, true

      end

      session.loop

    end
The `#send_request` method accepts three parameters—the name of the request (in this case, “shell”), any additional data to send with the request (none, in this case), and whether or not you want the server to reply with the success or failure of the request. In general, it is a good idea to ask for the server to reply, so that you know when you can start sending data to the shell. If you want to open a pty before starting the shell, you can use the #request_pty method of the channel: Opening a pty on a channel [ruby]
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    Net::SSH.start( host ) do |session|

      session.open_channel do |channel|

        channel.on_success do
          puts "pty was requested successfully!"

          channel.on_success do
            puts "shell was started successfully!"
            channel.send_data "exit\n" # tell the shell to exit
          end

          channel.send_request "shell", nil, true
        end

        channel.on_failure do
          puts "shell could not be started!"
        end
        channel.on_data do |ch,data|
          puts "recieved #{data} from shell"
        end
        channel.on_close do
          puts "shell terminated"
        end

        channel.request_pty :want_reply => true

      end

      session.loop

    end
First, the pty is requested (with an indicator to the server that it should return a “success” or “failure” notification). When the pty is successfully opened, the “on_success” callback is changed, and the shell is then requested. It’s a lot of hoops to jump through, but it gives you the finest-grained control over opening a shell. For most things, though, you can live with less control. For those tasks, there are the shell and sync services. ## 5.3. Shell Service To make interacting with shells, simpler, version 0.9 of Net::SSH introduced the shell service. This allows you to open a shell (with or without a pty), send a series of commands to the shell, and then wait for the output from the shell. Using the shell service [ruby]
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    Net::SSH.start( 'localhost' ) do |session|

      shell = session.shell.open

      # script what we want to do
      shell.pwd
      shell.cd "/"
      shell.pwd
      shell.test "-e foo"
      shell.cd "/really/bogus/directory"
      shell.send_data "/sbin/ifconfig\n"
      shell.pwd
      shell.ruby "-v"
      shell.cd "/usr/lib"
      shell.pwd
      shell.exit

      # give the above commands sufficient time to terminate
      sleep 0.5

      # display the output
      $stdout.print shell.stdout while shell.stdout?
      $stderr.puts "-- stderr: --"
      $stderr.print shell.stderr while shell.stderr?

    end
Any unrecognized method that is sent to the shell object is interpreted as a command to send to the server. To explicitly send a command, use the `#send_data` method (but remember to add a newline!). The `#send_data` method may also be used to send data to any process running in the shell as that process’ `stdin` stream. You can also specify that a pty should be allocated for this shell: Allocating a pty for the shell [ruby]
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    Net::SSH.start( 'localhost' ) do |session|

      shell = session.shell.open( :pty => true )

      # or

      shell = session.shell.open( :pty => { ... } )

    end
If you give a hash for the `:pty` option, it must be a map of the options that describe the new pty. See the API documentation for the `Channel#request_pty` method for more information. Note that this is still an asynchronous approach. You send all the commands through the pipe and then wait for the output. (And be sure to give sufficient time or the processes to terminate!) This is fine for scripts that you just want to throw at the server, but many times you want a more interactive interface, for executing a command and receiving its output before moving on. ## 5.4. SyncShell Service The SyncShell service allows you to execute commands on the shell and block until they finish. It is not fool-proof—it has to use some tricks to accomplish this task, and some commands may foul it up. But for most tasks, it works admirably. Using the SyncShell service [ruby]
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    Net::SSH.start( 'localhost' ) do |session|

      shell = session.shell.sync

      out = shell.pwd
      p out.stdout

      out = shell.test "-e foo"
      p out.status

      out = shell.cd "/really/bogus/directory"
      p out.stderr
      p out.status

      out = shell.ruby "-v"
      p out.stdout

      out = shell.cd "/usr/lib"

     out = shell.ls "-l"
      p out.stdout

      out = shell.send_command( "bc", <<CMD )
    5+5
    10*2
    scale=5
    3/4
    quit
    CMD
      p out.stdout

      p shell.exit

    end
The result of executing each command is an object that encapsulates the `stdout` and `stderr` streams, and the exit status of the command. To explicitly execute a command, use the `#send_command` instead of `#send_data`—otherwise, the command will be executed asynchronously, which is not what you want. Also, if you pass a second parameter to the `#send_command` method, it is interpreted as the `stdin` data to send to the new process. ## 5.5. Terminal Clients Using the shell service and pty’s, you can now create a simple SSH terminal client. (You’ll also want to download and install the ruby-termios library so that your input is not interpreted in a linewise fashion.) A simple SSH terminal client in Ruby [ruby]
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    begin
      require 'termios'
    rescue LoadError
    end

    def stdin_buffer( enable )
      return unless defined?( Termios )
      attr = Termios::getattr( $stdin )
      if enable
        attr.c_lflag |= Termios::ICANON | Termios::ECHO
      else
        attr.c_lflag &= ~(Termios::ICANON|Termios::ECHO)
      end
      Termios::setattr( $stdin, Termios::TCSANOW, attr )
    end

    host = ARGV.shift or abort "You must specify the [user@]host to connect to"
    if host =~ /@/
      user, host = host.match( /(.*?)@(.*)/ )[1,2]
    else
      user = ENV['USER'] || ENV['USER_NAME']
    end

    Net::SSH.start( host, user ) do |session|

     begin
        stdin_buffer false

        shell = session.shell.open( :pty => true )

        loop do
          break unless shell.open?
          if IO.select([$stdin],nil,nil,0.01)
            data = $stdin.sysread(1)
            shell.send_data data
          end

          $stdout.print shell.stdout while shell.stdout?
          $stdout.flush
        end
      ensure
        stdin_buffer true
      end

    end
The above code is also available as an example script in the Net::SSH distribution (`examples/ssh-client.rb`). Previous (4. Executing Commands) | Up | Next (6. Port Forwarding)