contents

user interfaces

  • CLI - Command Line Interface
  • GUI - Graphical User Interface

terminal

  • the terminal originally meant a physical piece of interface hardware needed to interact with the computer
    • it was a replacement for punch cards
    • live input and output with computer program was possible with the terminal
    • punch cards were fed into the computer before the terminal
  • terminal is a popular CLI interface application in UNIX/LINUX based OSes
  • different programming languages have different CLIs
    • python has a default shell that is entered when python is called from the terminal app
    • IPython is an interactive shell
    • web browsers have a JavaScript shell in the console tab

GUI vs. terminal

  • terminal uses text instead of graphics
  • most things on the computer don’t need a graphical interface
    • somethings are easier to do in the terminal
  • terminal can be used to
    • edit and run a program
    • find files on the computer that have particular names
    • download a file from the internet if the URL is known
    • start a web-server on the computer
    • start a web server on the cloud
  • terminal cannot be used to
    • edit a large motion picture
    • mixing and mastering a music file
    • perform graphical 3D modelling
  • these operations need a GUI

advantages of CLI

  • CLI interface is programmable
  • efficient in terms of network bandwidth
  • files on a remote system can be edited without having to be copied locally
  • counter-intuitively, it is possible to be much more expressive on a CLI than over GUI
  • remote servers usually do not have a GUI
    • they are connected to and worked with through the CLI exclusively

virtual linux server

  • vagrant along with VirtualBox offers Ubuntu virtualization
    • VirutalBox is an application for running OSes on a virtual machine within a machine’s native OS
    • Vagrant helps quickly and consistently configure such a virtual machine
    • easy to manage multiple virtual machines in a virtual network within a single machine
  • see VirtualBox’s website
  • see Vagrant’s website
  • get a virtual machine up and running using vagrant and an Ubuntu virtualbox to follow along
    • login to the vm using vagrant ssh from the native terminal app

linux CLI

  • in the native terminal app, if successfully logged in to the vagrant machine
    • lands in the command prompt of the ubuntu terminal
    • the terminal points to the login user’s home directory by default when it starts
    • a login message is displayed by default
      • usually with login time and IP address
      • can be configured differently
  • the rest of these notes are in that command prompt

terminal interface

  • there is some variety in the way that terminal programs look
    • but the contents don’t change much
  • they can be configured differently on different systems, but usually displayed are:
    • the login user name
    • the host name/computer name
    • the current time
    • the current folder
  • typically display:
    • informational messages
    • output of previously run commands
    • prompt for entering new commands
  • new commands are typed in the prompt and enter is pressed
    • this is followed by actually running the command
    • the result of running command is displayed as the output after completion of command processing
    • after this, a fresh prompt is added to the bottom, ready for the next command
  • each line in the terminal is either an input or an output
    • input lines start with a command prompt - ends with a $
    • the output lines can vary, depending on the output produced by the program

escape characters

  • use a backslash before printing special characters like ' and ! as they usually have reserved meanings in a shell’s context

terminal vs. shell

  • terminal is an app that draws text on the screen and accepts input from the keyboard
    • technically, it is an emulator of the old-school terminal hardware
  • the terminal itself doesn’t know what to do with the input
    • it needs an another app to handle that
  • when things are typed into the terminal, it is sent to the shell
  • when enter is pressed, the shell interprets the command it receives from the terminal
    • then figures out what program to run,
    • runs it and
    • returns the output to the terminal
  • the user can see the shell output displayed in the terminal
  • the terminal can be used without the shell
    • the default app in the terminal is the shell, other programs can be run in the terminal as well
    • the python interpreter, for example, runs in the terminal when the shell calls it
    • when python interpreter is exited, the linux shell is started in the terminal again (default behavior)
  • there are multiple shells as well
    • linux and macOS default to bash (GNU Bash)
    • others are:
      • ksh: Korn shell
      • csh: C shell
      • zsh: Z shell
      • sh: Bourne shell (default for BSD Unix)
      • TCSH: TENEX C Shell
    • they all work slightly differently and have various different features
  • the shell can be used without the terminal as well
    • the shell commands can be arranged in a file
    • it can then be fed directly to the shell
    • this is called shell scripting

linux commands

  • syntax:
    • <command-name> <agruments>
    • the arguments follow the command name
    • this is entered at the terminal command prompt
  • ls
    • lists the files and folders in the current directory
    • stands for long listing
    • shows no output if there are no files in the current directory
      • dynamically shaped output like this common
  • curl
    • downloads files from the internet
      • needs an address
      • needs a file name to save the file as locally 
        curl http://udacity.github.io/ud595-shell/stuff.zip -o things.zip
  • date
    • prints date and time in terminal
  • expr 2 + 2
    • very simple calculator
    • simple prints the expression “2 + 2” adds to 4
  • echo 'hello world'
    • simply prints ‘hello world’
    • like print statement in python
  • uname
    • prints operating system name (“Linux”)
  • hostname
    • prints name of the computer
    • the virtual machine name if VM
  • host startpage.com
    • prints info about webpage startpage.com
    • includes IP address and mail handlers
  • bash --version
    • prints version of linux shell
    • some copyright info
  • python --version
    • print version on python 2 installed
  • history
    • prints the history of commands executed previously
  • uptime
    • current time, time the computer’s uptime D-M-H, users logged in, load average
  • top
    • displays processes running currently on the machine with the highest load at the top
  • typing random things into the CLI
    • usually throws an error if no command or program is recognized
      • command not found
    • the shell prompt is returned after the error message
      • ready to accept the next command

comparison between a shell command and a python function

  • similarities:
    • they are both units of code
    • both have a name and çan take arguments
    • rm weirdfile.txt
      • deletes the file ‘weirdfile.txt’ in current directory
    • similarly in the python shell,
      • os.remove("weirdfile.txt")
      • deletes the file ‘weirdfile.txt’ in python current directory
  • differences:
    • they exist in different contexts
    • most commands executed in the shell are whole programs
      • they are separate OS threads/processes
    • functions are used to organize scripts in languages rather than starting new processes

references