Today's Topics

New Material

• We Will Be Using Unix in This Class
• Working with Unix
• Connecting to a Unix Machine
• Your Home Directory
• The Hierarchical Filesystem
• Moving To A Different Directory
• Always Know Where You Are
• Creating A New Directory
• Types of Programming Languages
• Python
• Working with Python
• Running Python in Interactive Mode
• Experimenting with Python
• Python Scripts
• Scripts versus Interactive Mode
• Today's Class Exercise

Reading Assignment

You should read Chapter 1, Introduction to Computers and Programming, from our textbook, Starting Out with Python, before next week's class.

Homework 1

Remember, homework 1 is due this Sunday by 11:59 PM.

You will find it here.

Unix Lab Assistant

An IT student has been stationed in the Unix Lab to you with your Unix questions.

They can also provide basic help with your assignments.

You can find details here.

This link also appears on the class web page.

Completing the Apply Process

A few of you have not completed the Apply process.

You must do this to get a class directory.

Without a class directory you cannot submit assignments.

New Material

We Will Be Using Unix in This Class

• All work in this course will be done on the Unix machine users3.cs.umb.edu
• You will need to connect to this machine from your computer
• This will mean that you will have to learn a few simple Unix commands to do your work
• But it will help you in your professional career
• A great deal of IT work can only be done at the command line

Working with Unix

• All work you create for this course must be done on users3.cs.umb.edu
• You must also run your scripts on this Unix machine to make sure they work
• This means you must deal with the command line
• There are no menus on the command line
• You must type the name of the command and any arguments it needs
• Then hit Enter (PC) or Return (Mac)
• If you make a mistake you will get an error message which is usually not very helpful
• Fortunately, you only need to know a few Unix commands
• It will be frustrating at first
• But you will soon learn how to do what you need
• Be patient and ask for help in the class discussion area when you run into trouble
• You can also get help from other students
• And people in the Unix Lab on the 3rd floor of McCormack

Connecting to a Unix Machine

• You can connect to Unix machine on which you have an account if you have software that implements the SSH (Secure Shell) protocol
• This software is called an ssh client
• If you have a PC, you need to download an SSH client
• A good free one is PuTTY which you can download here
• You only need the file putty.exe
• You will find instructions on using Windows machine to connect to a Unix machine here
• On a Mac the ssh client is already installed
• To use it, you must go to the Macintosh command line
• You do this by running the Terminal program
• Which is located in the Utilities directory inside the Applications directory
• You run the ssh client like this

  ssh YOUR_UNIX_USERNAME@users3.cs.umb.edu

  where you substitute your Unix username for YOUR_UNIX_USERNAME
• You find instructions on connecting to a Unix machine from a Mac here
• You will then have to enter your password

Your Home Directory

• When you first connect to the Unix machine you will be in your home directory
• A directory holds files and other directories
• It is called a folder on Windows and the Mac
• Every Unix account on our system has a home directory
• This directory belongs to you and you alone
• You have complete control over this directory
• If you enter the Unix command ls you will see what is inside this directory
• ls is short for "list"
• If you have completed the Apply process you should see an it116 directory

The Hierarchical Filesystem

• Before we go any further, we needed to talk a little about the way Unix arranges files
• Unix uses a hierarchical filesystem
• Which means that all files are kept in directories (folders)
• And directories live inside other directories
• There is one special directory at the top called root
• But it is written as  / 

Moving To A Different Directory

• To move to a different directory you use the cd command
• cd stands for "change directory"
• The command has the following format

  cd  NAME_OF_DIRECTORY

• NAME_OF_DIRECTORY is a placeholder
• When you use cd you replace NAME_OF_DIRECTORY with the name of a directory
• To move to your it116 directory, type

  $ cd  it116

• The hierarchical filesystem can be a little confusing
• If you are ever confused by where you are when connected to Unix, there is a simple fix
• Use the cd command without an argument to go back to your home directory

Always Know Where You Are

• The hierarchical filesystem is like pyramid
• The root directory is at the top
• All other directories are inside this directory
• Or in sub-directories
• Or in sub-sub-Directories
• And so on
• The pyramid gets wider the deeper you go
• Many commands will not work properly if you are in the wrong directory
• Always know where you are in the hierarchical filesystem
• One way to learn your location is to use the pwd command

  $ pwd
  /home/ghoffman

• pwd stands for "print working directory"

Creating A New Directory

• You will have to create new directory for each homework assignment
• And for each Class Exercise
• You create a directory using the mkdir command
• mkdir stands for "make directory"
• It has the following format

  mkdir Directory Name

• You will have to create two new directories for today's Class Exercise
  • hw
  • ex
• These directories mustbe inside your it116 directory
• Otherwise the scripts I use to score your assignments will not work
• So it is always a good idea to run pwd before using mkdir
• After you create the directories us ls

  $ pwd
  /home/ghoffman/it116
  
  $ mkdir hw
  
  $ mkdir ex
  
  $ ls
  ex hw
  

Types of Programming Languages

• There are two broad classes of programming languages:
  • Compiled languages
  • Scripting languages
• Compiled languages, like C, C++ and Java are used in large programming projects
• In a compiled language, you create a text file consisting of instructions in the language
• This file is called the source code
• A source code file is written by programmers in a computer language
• But it cannot, itself, be run on a computer
• In order to run the program you must create an executable file
• For example, here is the source code for a simple program written in C:

  #include <stdio.h>
  
  main()
  {
      printf("Hello, world!\n");
  }

• In order to create an executable fileI must run the source code through a program called a C compiler

  $ gcc -o hello hello.c

• This program creates the executable file hello which I can run on the Unix machine

  $ ./hello 
  Hello, world!

• Compiled languages are designed for big programming projects
• They are overkill for simple system administration tasks
• Let's say we wanted to remove all files in a directory and all sub-directories that have the .tmp extension
• And were created before a certain date
• You would not want to write a C program to do this
• C is very finicky about syntax
• And would probably give you a lot of compile errors before you got a working program
• Scripting languages were created to write programs like this
• Scripting languages are not compiled
• But they are still written in a programming language and cannot be run directly on the machine
• Instead they are run using an interpreter
• An interpreter is a program that understands the scripting language
• The interpreter reads the script and performs the actions specified by the code
• There are a number of scripting languages commonly in use today:
  • Perl
  • PHP
  • Python
  • Ruby
• We will be learning Python

Python

• Python was conceived in the late 1980s
• It was first implemented in December 1989 by Guido van Rossum in the Netherlands
• There are two versions of Python currently in use
  • Python 2
  • Python 3
• Python 2 was released in October 2000
• Python 3 is the latest version of Python
• Unfortunately, scripts written in Python 3 will not always work in Python 2
• And vice versa
• We will be using Python 3 in this course
• The Python design philosophy emphasizes code readability
• It tries to express concepts in fewer lines of code than languages such as C
• The core philosophy of Python is spelled out in the document "PEP 20 (The Zen of Python)"
• It includes the following statements
  • Beautiful is better than ugly
  • Explicit is better than implicit
  • Simple is better than complex
  • Complex is better than complicated
  • Readability counts

Working with Python

• We will be working with Python on the Linux machine users3.cs.umb.edu
• All your work for this course must be done on this machine
• If you wish, you may install Python on your own machine
• But this is not necessary for this course
• The Resources page has a download link for Python
• If you own a Mac, Python is already installed, but it is the wrong version
• In this class we will be using Python 3.5.2

Running Python in Interactive Mode

• There are two ways to use Python
  • Interactively
  • Writing Python scripts
• If you enter python3 on the Unix command line and hit Enter you will be running Python in interactive mode

  $ python3
  Python 3.5.1 (v3.5.1:37a07cee5969, Dec  5 2015, 21:12:44) 
  [GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
  Type "help", "copyright", "credits" or "license" for more information.
  >>>

• Notice that the command I used was python3, not python
• If you run python you will be using the wrong version of Python
• In interactive mode you are talking directly to the Python interpreter
• In this mode you can type Python statements directly at the command line
• And get the results immediately
• Whenever you see  >>> , Python is ready for your input
• Typing help() will get you started

  >>> help()
  
  Welcome to Python 3.5's help utility!
  
  If this is your first time using Python, you should definitely check out
  the tutorial on the Internet at http://docs.python.org/3.5/tutorial/.
  
  Enter the name of any module, keyword, or topic to get help on writing
  Python programs and using Python modules.  To quit this help utility and
  return to the interpreter, just type "quit".
  
  To get a list of available modules, keywords, symbols, or topics, type
  "modules", "keywords", "symbols", or "topics".  Each module also comes
  with a one-line summary of what it does; to list the modules whose name
  or summary contain a given string such as "spam", type "modules spam".
  
  help>

• Typing help() with no arguments, puts you in help mode
• as you can see by the help> prompt
• When you are in help mode, you can just type a topic name to get information on that topic
• To see a list of topics type "topics"

  help> topics
  
  Here is a list of available topics.  Enter any topic name to get more help.
  
  ASSERTION           DELETION            LOOPING             SHIFTING
  ASSIGNMENT          DICTIONARIES        MAPPINGMETHODS      SLICINGS
  ATTRIBUTEMETHODS    DICTIONARYLITERALS  MAPPINGS            SPECIALATTRIBUTES
  ATTRIBUTES          DYNAMICFEATURES     METHODS             SPECIALIDENTIFIERS
  AUGMENTEDASSIGNMENT ELLIPSIS            MODULES             SPECIALMETHODS
  BASICMETHODS        EXCEPTIONS          NAMESPACES          STRINGMETHODS
  BINARY              EXECUTION           NONE                STRINGS
  BITWISE             EXPRESSIONS         NUMBERMETHODS       SUBSCRIPTS
  BOOLEAN             FLOAT               NUMBERS             TRACEBACKS
  CALLABLEMETHODS     FORMATTING          OBJECTS             TRUTHVALUE
  CALLS               FRAMEOBJECTS        OPERATORS           TUPLELITERALS
  CLASSES             FRAMES              PACKAGES            TUPLES
  CODEOBJECTS         FUNCTIONS           POWER               TYPEOBJECTS
  COMPARISON          IDENTIFIERS         PRECEDENCE          TYPES
  COMPLEX             IMPORTING           PRIVATENAMES        UNARY
  CONDITIONAL         INTEGER             RETURNING           UNICODE
  CONTEXTMANAGERS     LISTLITERALS        SCOPING             
  CONVERSIONS         LISTS               SEQUENCEMETHODS     
  DEBUGGING           LITERALS            SEQUENCES      
  
  help>

• To exit the help mode type "q"

  help> q
  
  You are now leaving help and returning to the Python interpreter.
  If you want to ask for help on a particular object directly from the
  interpreter, you can type "help(object)".  Executing "help('string')"
  has the same effect as typing a particular string at the help> prompt.
  >>>

• In interactive mode, you can use the Python interpreter as a calculator

  >>> 5 + 6
  11
  >>> 6 - 5
  1
  >>> 3 * 5
  15
  >>> 4.1 / 2
  2.05

• But the best way to use interactive mode is to experiment with Python
• The simplest thing a computer program can do is to print something

  >>> print("Hello world!")
  Hello world!

• When writing code we often need to store a value that will be needed later
• All computer languages have variables
• A variable is a place in memory with a name, that holds a single value
• Giving a value to a variable is called assignment
• In Python, the assignment operator is the equal sign,  = 

  >>> a = 5
  >>> b = 6
  >>> a * b
  30
  >>> a - b
  -1

• If you type something that is not a legal Python statement you will get an error

  >>> a $ b
    File "<stdin>", line 1
    a $ b
      ^
  SyntaxError: invalid syntax

• To leave the interactive mode enter the following

  exit()

• If you using a Mac or a Linux machine you can quit interactive mode using Control D

Experimenting with Python

• The interactive mode is a great way to experiment with Python
• Those of us who work with computers are lucky
• Whenever we have a question, we can ask the machine
• If you want to know if something is legal in Python, just type it in and see what happens
• Chemists are not so lucky
• To find out if a chemical reaction will work they need a laboratory stocked with chemicals and equipment
• But we carry our laboratories around in our laptops
• We can experiment at any time in any place

Python Scripts

• The interactive mode is a great way to learn Python
• But you would not want to use it to automate routine tasks
• All the variables you define in the interactive mode disappear when you leave
• If you define a number of variables and then quit Python all your work disappears
• You need something more permanent when you use Python to accomplish some task
• You need to create Python scripts
• A Python script is just a text file that contains a series of Python statements
• Here is a Python script, hello.py, that prints a simple message

  print("Hello world!")

• To run this file I need to use it as an argument to the Python interpreter

  $ python3 hello.py 
  Hello world!

• All Python script files must have the extension .py

Scripts versus Interactive Mode

• Interactive mode is OK for quick and dirty work
• And for experimenting with Python
• But any time you want to do something more than once you should write a script
• We will use both, at least in the beginning
• In general, anything you can do in interactive mode you can also do in a script
• And vice-versa
• But there are two important differences between them
• If you have defined a variable in an interactive session it will disappear the moment you leave
• The second difference involves printing values
• In interactive mode, once you have assigned a value to a variable you can print its value, just by typing the variable name

  >>> a = 5
  >>> a
  5

• This does not work with scripts

  $ cat var_test.py 
  #! /usr/bin/python3
  
  a = 5
  a
  
  $ ./var_test.py 
  
  $

Today's Class Exercise

• The Class Exercises give you hands on experience with Python
• All Class Exercises must be done on the Unix machine users3.cs.umb.edu
• In today's exercise you will practice some basic skills needed for this course
• You will find it here
• Let me show you how to connect to the machine and do the Exercise

Attendance