JUNIOR

ACADEMY

8/13/2016

note

• When typing commands into the terminal, make sure you know exactly what it's doing
  • Check the man if you don't
• Same goes for installing packages from apt
  • Don't just immediately apt install ...
  • First, apt search ... and read the description of the package
  • Also, before pressing "y" to confirm the installation, read the dependent packages that will be installed with it
    • If anything looks crazy (file size is huge, too many dependencies), look for alternatives! You don't want to clutter your filesystem

paths

• When writing out paths for anything (such as arguments), you can write them out as absolute or relative paths.
• Absolute Paths
  • Points to the file/directory regardless of the current working directory
  • Includes the root
  • Ex. python3 /home/john/Documents/JrDLA/blackhat.py
• Relative Paths
  • Points to the file/directory starting from the current working directory
  • Written without the root /
  • Ex. pwd == /home/john/
    • python3 Documents/JrDLA/whitehat.py
• Remember: ~ == /home/yourUserName/

bash review

• Create a directory ~/Documents/BashExercises and two folders within BashExercises in one command
  • Hint: use the -p option for mkdir
    • ​If you don't know what that does, check the manual!!!
• Pipeline to grep through your installed packages for feh
  • Hint: use the apt list command
  • If you don't have feh, find and install it through apt
• Download and install the Sublime Text package from its website on Chromium using dpkg. Then remove it (we'll be using a CLI text editor in this class)

alias

• You can map complex commands to whatever you would like to simplify them
  • Ex. alias ai='sudo apt install'
  • If I typed in "ai firefox-esr", the shell would interpret it as "sudo apt install firefox-esr"
• This way, you only have to type what you want to do things
• You don't have to type out options for things you would always want to use, like alias mv='mv -vi'
• Type "alias" into the terminal to see your current aliases
• Try making an alias now
  • alias mv='mv -vi'
  • Move a file with just mv to test it out

alias

• Close the terminal and reopen it
• Type alias and you'll see that your new alias disappeared!
  • They only work during that shell session :(
• If you want to keep your aliases permanently, you have to edit ~/.bashrc
  • ​.bashrc is a shell script that is run every time Bash starts
  • You can add your aliases directly into it, but it's better to make a separate file for management
• Look at .bashrc and you'll see:

if [ -f ~/.bash_aliases ]; then

    . ~/.bash_aliases

fi

alias

• In ~, make a file .bash_aliases
  • ​type your aliases in that file
• Use the type command (with a command as an arg) to see if a command already exists, and if it does, what it is
  • Try it out: type ls chromium-browser hello
• Aliases I use:
  • alias acs='sudo apt search'
  • alias ai='sudo apt install'
  • alias grep='grep --color=auto'
  • alias cp='cp -vi'
  • alias mv='mv -vi'
  • alias rm='rm -vI'
  • alias mkdir='mkdir -vp'
  • alias df='df -h'
• Come up with your own!

permissions

• GNU/Linux is a multiuser OS
• Files and commands can only be used and accessed/changed by specific users, whether it's the owner or root
  • Only the owner of a file or a group that is allowed can access a file
    • Ex. Only users in the "sudo" group can use sudo and only root/sudoers can access files outside of /home/*
• id shows the groups the user is in

permissions

• Look at the file's current permissions with ls -l
  • On the far left of each file's entry, there are 9 chars
  • drwxrwxrwx (or each character has a dash instead)
  • the first character is the file type
    • - == file
    • d == directory
    • l == link
  • The next 3 are the owner permissions
    • r == readable by file's owner
    • w == writable by file's owner
    • x == executable by file's owner

permissions

• The next 3 (bits 5-7) are the owner's permissions
  • rwx == readable, writable, and executable by members of the file's owner group
• The last 3 are the world permissions
  • rwx == readable, writable, and executable by everyone
• Some examples:
  • -rw-r--r-- is a regular file that is readable and writable by the file's owner, and readable by the owner's group and world (everyone)
  • drwxr-x--- is a directory that the owner may enter, and create, rename, and delete files within it. Only the group can enter, but do nothing else

permissions

• chmod changes file mode bits
  • chmod -OPTIONS MODE FILE
  • The file's owner or superuser can change the permissions/mode of a file/directory
  • It uses either octal or symbolic representation to do so in the mode part of the command syntax
• Use octal numbers to represent binary digits (bits) which maps to the file mode
  • 3 digits (ex. 777)
  • Each digit of the octal representation equates to each group of the file mode (first digit is owner, second digit is group owner, third digit is world)
• Use symbols to change the file mode as well
  • Three parts: who the change affects, which operation will be performed, and which permission will be set

octal

Ex. "chmod 600 foo.txt" sets the mode -rw------- to the file

The most common octals are: 7, 6, 5, 4, and 0

symbols

• Who it'll affect
  • u is short for user, but means the file owner
  • g is group owner
  • o is short for others but means world
  • a is short for all; u, g, & o combined. Default
• What will be performed
  • + adds the permission
  • - removes the permission
  • = means only that permission is applied, the rest are removed
• Permissions
  • rwx
• Ex. u+x == Add execute permission for owner
  • chmod +w file.txt

ownership

• chown changes the owner and group owner of a file/directory
  • Use the user or :group to apply that ownership to the file
  • chown bob file.txt

file compression

• Removes redundancy from data to (in the case of lossless algorithms) preserve the data in a file but make it take up less space
• gzip - compress & exapnd files
• bzip2 - block sorting compresser (higher compression at the cost of speed)
• Download the zip and unzip packages from apt to compress/extract zip archives
  • zip is both a compression tool and an archiver

archiving

• Archiving is gathering multiple files and bundling them into a single large file
  • Usually used in conjunction with compression and for backups
• tar - Tape Archiving Utility
  • Files with the .tar* or .tgz extensions are tape archives
  • Syntax: tar mode[options] pathname
    • Modes: c - create an archive, x - extract an archive, t - list the contents of an archive; see the man page for more
  • Options don't need a leading dash, they're appended directly to the mode

tar practice

• cd into your Documents and make a tar of your BashExercises
  • tar cvf BashExercises.tar BashExercises
  • The mode (c) and the options f (to specify the name) and v (verbose) are joined
  • The mode MUST be specified first
• List the contents of your tar
  • tar tvf BashExercises.tar
• Extract the tar into a new location
  • mkdir BE2
  • cd BE2
  • tar xvf ../BashExercises.tar
  • ls

putting tar to use

• A lot of binary packages that can't be found in the apt repo or don't have a deb file online are usually packaged in a tar archive.
• If you find software you want to "install", but can't, extract the archive and run the executable bin file!
• Downloading Firefox Nightly (pre-alpha build)
  • On Chromium, go to: https://nightly.mozilla.org
  • Download the tar.bz2 file Linux (Intel, 64-bit)
  • Navigate to your Downloads and mv the file to your Desktop
  • Extract the tar.bz2 file you just downloaded
  • cd into the resulting directory

  • ./firefox

next week

• For the GNU/Linux portion of next week, we'll be going over Vim and i3wm
• Vim is an advanced and feature-heavy text editor
  • Vi IMproved
  • Vi is a text editor that is in every installation of any UNIX-like OS
  • Plugins allow the potential to surpass IDEs
• i3 is a window manager
  • Instead of a DE, you can use a WM
    • 100x better
  • Maximizes/optimizes space
  • More efficient
  • Can be mouse-free
• Remind your parents to renew your enrollment!

python

• Conditional statements
• Loops
• Functions

review exercises

• Create a list of your favorite colors, and then create another list of your favorite fruits.
  • Create a third list, called "favorite_things" with three items in it.
    • The first and third elements of this list will be from your favorite colors, referencing the index of those colors
    •  The second item of this list will be one of the fruits, referencing the index of the fruit.
    • Print out the three lists

if-statements

• if' is followed by a 'condition' and a colon (:), and if the condition is met, the following block of code executes 
  • ex if age > 17:

print('you are too old!')

• Conditional operators
  • ==, !=, >, <, >=, <=
• What is the difference between = and ==?

white space

• White space is the white space between statements
• The white space before statements inside of something is important!
• A block of code is a group of statements, and the white space (tab) separate blocks of code
  • Consistent spacing
• The 3 dots(. . . instead of >>>) in the interpreter means you're typing in a block of code
• Blocks placed inside each other require the extra spacing as shown to the right

this is block 1
this is block 1
      this is block 2
      this is block 2
      this is block 2
            this is block 3
            this is block 3
      this is block 2
this is block 1
this is block 1

what else is there?

• If-statements can be extended with an if-then-else-statement
• So, if the condition isn't met, it would execute a different block of code (if something is true, then do this, otherwise do that); ex:

>>> age = 18

>>> if age == 12:

. . .      print('Hello')

. . .  else:

. . .      print('Goodbye')

else-if-statements

• If-statements can be further extended using elif (else-if)
• If the previous if-statement's condition isn't met (so it evaluates to False), then it check the else-if's condition, and so forth.

elif example

>>> pets = 2

>>> if pets == 1:

. . .      print("You have 1 pet")

. . .  elif pets == 2:

. . .      print("You have 2 pets")

. . .  elif age < 13:

. . .      print("You have less than 3 pets")

. . .  else:

. . .      print("You have more than 2 pets")

What will be printed?

combining conditions

• You can have multiple conditions in any conditional statements, by using and and or

>>> if age ==10 or age == 11 or age ==12:

. . .       print("You are %s" % age)

• If any of the conditions are met, then the code runs, hence the or

>>> if age >= 10 and age <= 12:

. . .       print("You are %s" % age)

• In this case, all of the conditions have to be true for the code to run, hence the and

none

• Another value that can be assigned to a variable is None 
• This is an empty value, with nothing in it (also known as null or NIL in other languages)
• None can be used to reset variables and make them unused
• An example is if we don't want to print a message when a variable is empty
  • >>> money = None
  • >>> if money == None:
  • . . .      print("You have no money!")

exercises

• Imagine an alien was just shot down in a game. Create a variable called alien_color and assign it a value of 'green', 'yellow', or 'red'
  • Write an if statement to test whether the alien's color is green. If it is, print a message that the player just earned 5 points. If it isn't green, print a statement saying the player earned 10 points.

• Write a sequence of conditional statements (or just one) that prints a number if it is equivalent to 20, 21, 22, 25, 26, 27, or 30)

loops

• Repeatedly executing code written in the loop until/while a condition is met
• for loop & while loop
• How can this be useful?

for loops

• For loops iterate a limited amount of times through a list with a variable taking on the values in the given list

• for variable in list:

  • The variable iterates through the list and is assigned the value of the current iteration

• Like if statements, the for loop is defined with a colon and followed by a block of code that is executed

for x in range(5):
    y = x + 1
    print(x+y)

for s in myList:
    print(s)

for loops

• The variable that iterates through the list only exists in the for loop, and is purely used for the loop
  • It takes on every value in the list before finishing the loop
• Example of a for loop: Assuming I earn $35 a week, and I have $0 in my savings account, I can calculate my money earned throughout the weeks for a year!

  • >>> savings = 0

  • >>> for week in range(1, 53):

  • ...     savings = savings + 35

  • ...     print("Week %s = $%s" % (week, savings))

nested loops

• Try typing this in your interpreter:

  • >>> myList = ['a', 'b', 'c']

  • >>> for u in myList:

  • ...     print(u)

  • ...     for v in myList:

  • ...         print(v)

loop exercise

If you have $100 saved in your bank account, and you obtain 3% interest every year, how much money will you have for each year, up to 10 years?

while we're on loops

• While loops iterate though the block of code while a condition is met
  • Different from for loops, because it's while condition == true, not iterating through a list

while condition:
    # block of code

while loop example

x = 1
while x < 200:
    x = 2*x
    print(x)

What will this print?

break

• You can break out of a loop before the condition is met
  • >>> while True:
  • . . .        # tons of code
  • . . .        if someCondition == True:
  • . . .                break
• Games use while True to keep the game running and refreshing the screen!

another loop exercise

Answer the previous question using a while loop

If you don't remember the task:

If you have $100 saved in your bank account, and you obtain 3% interest every year, how much money will you have for each year, up to 10 years?

input()

• You can take in user-inputted text in the code, and store it in a variable
• input("text")
  • The user types something in and presses Enter
  • Whatever they typed in is returned by input() as a string
• >>> name = input("Please enter your name: ")
• >>> print("Hello, %s!" % name)
• If they input a number, it is still returned as a string
  • use the int() function to change it to an int
  • >>> age = input("What is your age? ")
  • >>> age = int(age)

functions

• Functions allow you to write code just once, then reuse that code in your programs multiple times
• Functions take in parameters, which is a variable you give it that is only available in the body of the function (the block of code inside of it)

def function(parameter):
    # block of code

example of a function

You define (make) the function like this:

def printName(name):
    print("Hello, %s!" % name)

And call (use) it like this:

printName("John")

Which will print:

Hello, John!

example of a function

Another example, except with return instead of print():

def savings(chores, job, spending):
    return chores + job - spending
    # or return(chores+job-spending)

And call it like this:

print(savings(10, 10, 5))
# OR
money = savings(10, 10, 5)

scope

• Not all variables are accessible from all parts of our program.
• Where a variable is accessible depends on how it is defined.
• We call the part of a program where a variable is accessible its scope.
• ​A variable which is defined in the main body of a file is called a global variable.
  • It will be visible throughout the file, and also inside any file which imports that file.
• A variable which is defined inside a function is local to that function.
  • It is accessible from the point at which it is defined until the end of the function

# This is a global variable
a = 0

if a == 0:
    # This is still a global variable
    b = 1

def my_function(c):
    # this is a local variable
    d = 3
    print(c)
    print(d)

# Now we call the function, passing the value 7
# as the first and only parameter
my_function(7)

# a and b still exist
print(a)
print(b)

# c and d don't exist anymore -- these statements
# will give us name errors!
print(c)
print(d)

function exercise

• Write a function called practice_function() that takes in a number as a parameter. If the number is less than 50, print the value. If the number is greater than 50 but less than 1000, print a message. If the number is any other value, return the number, then print it. Call the function for three different values: 5, 50, and 200.

• Write a function called describeCity() that accepts the name of a city and its country as parameters. The function should return a simple sentence (for example: "Honolulu is in the USA"). Call your function for three different cities, store them in a list, and print the items in the list separately.