8 - intoduction to bash scripting.txt

// CONSENSUS IS IF IT'S LONGER THAN 50 LINES THEN USE PYTHON IF NOT THEN USE BASH 
// LESSON 1.1 - INTRO AND REFRESHER 
shell commands refresher 
    grep - filters input based on regex pattern matching 
    cat - concatenates file contents line by line 
    tail/head - give only the last -n lines 
    wc -  does a word or line country (with flags -w -l)
    sed - does pattern matched string replacement 

//LESSON 1.2 - BASIC BASH SCRIPT
bash script example - save this into group.sh then execute the script by using bash group.sh 
    #!/usr/bash 
    echo "Hello world"
bash and shell commands 
    - each line of your bash script can be a shell command 

// LESSON 1.3 - STANDARD STREAMS AND ARGUMENTS 
three streams 
    stdin: standard input 
    stdout: standard output 
    stderr: standard error 
argument 
    - each argument can be accessed via the $ notation. the first as $1, the second as $2 etc. 

    example script - args.sh 
        #!/usr/bashecho 
        $1echo # first argument 
        $2echo # 2nd argument 
        $@echo # all argument 
        "There are " $# "arguments" # number of arguments 

    calling the script args.sh
        bash args.sh one two three four five  

        doing this would produce 
            one 
            two
            one two three four five 
            There are 5 arguments 

// LESSON 2.1 - BASIC VARIABLES IN BASH 
assigning a variable 
    - must have no space 
    - can be accessed with $ dollar notation 
single vs double quote in calling variables 
    a='apple ' # assigning a string 

    # callign the variable 
    echo $a 
    echo "$a" - this would produce apple | double quotes grabs the value of variable when there is $
    echo '$a' - this would produce $a | single quotes take it literally 

    # assigning variable in weird way 
    now_var='NOW' # assigning variable must have no space 
    new_var="$now_var" # remember this is double quote meaning it would grab the value rather than literally assigning the word $now_var 
    echo $new_Var # this would produce NOW 
date program 
    date # this would produce the current time and date 
shell within a shell 
    right_now="The date is $(date)" # using the value of date by using $(), usign shell within a shell or in other words using shell commadns inside a variable initialization
    echo $right_now # this would produce: The date is Mon 2 Dec 2019 14:13:35 AEDT.

// LESSON 2.2 - NUMERIC VARIABLES IN BASH 
introducing expr 
    - to do arithmetic in shell 
    expr 1 + 4 # this would produce 5 

    double bracket notation - different sytnax for using expr 
    echo $((5 + 7))
introducing bc(basic calculator)
    - doing arithmetic calculation for multiple lines  
    bc  # type this 
    1 + 4 # would produce 5 
    2 + 4 # would produce 6 
    quit # quit bc, to do arithmetic calculations you now need to use expr or bc again 

    bc scale argument 
        eho "10/3" | bc # this owuld produce just 3 with no decimal, to add decimal use scale argument 
        echo "scale=3; 10/3" | bc # remmeber we're piping bc in here, semicolon is used to separate statemetns, this would produce 3.333
numbers in bash script 
    dog_age=6 # assign a number 6 in this variable 

// LESSON 2.3 - ARRAYS IN BASH 
array 
    - normal numerical indexed structure
        declare withotu adding elemetns 
            declare -a my_first_array 
        create and add elements at the same time 
            my_first_array=(1 2 3) # just like in python, but instead of comma we use space, weird  
    - called a list in python 
accessing array properties 
    my_array=(1 2 3 4)
    echo ${my_array[@]} # accessing array should be wrapped with ${}, [@] means grab all of the value of the array, use my_array[0] to access first element 
    my_array[0]=999 # changing element array is just like the same with python 
slicing thru array 
    my_array=(1 2 3 4 5)
    echo ${my_array[@]:3:2} # means select everything starting from 3rd index or 4th elemetn then only pick 2 elements 
appending to arrays 
    array+=(elements) # syntax for appending to array 
    my_array=(1 2 3)
    my_array+=(10) # my_array is now (1 2 3 10)
associate array (key value pair, kinda like dictionary in python or objects in javascript)
    # Create empty associative array
    declare -A model_metrics # weird way to create array 

    # Add the key-value pairs
    model_metrics[model_accuracy]=98
    model_metrics[model_name]="knn"
    model_metrics[model_f1]=0.82

    # another way to create associate array 
    # Declare associative array with key-value pairs on one line
    declare -A model_metrics=([model_accuracy]=98 [model_name]="knn" [model_f1]=0.82)

    # Print out the entire array
    echo ${model_metrics[@]}

// LESSON 3.1 - IF STATEMENTS 
if statement syntax 
    x="Queen"
    if [ $x == "King" ]; then # you can also use ((condition)) instead of [], you can also use commadns for conditional, look at youtube for more of this 
        echo "$x is a King!"
    else 
        echo "$x is not a King!"
    fi # closing if statement, weird huh 

// LESSON 3.2 - FOR LOOP AND WHILE STATEMENTS 
regular for loops 
    for x in 1 2 3 # pretty much just like in python, you loop thru array 
    do 
        echo $x 
    done 
for loop number ranges 
    for x in {1..5..2} # start with 1, end with 5, increment with 2 
    do 
        echo $x 
    done 
for loop three expression syntax # just like in c++
    for ((x=2;x<=4;x+=2))
    do 
        echo $x 
    done 
glob expansion 
    for book in books/* # iterating thru files 
    do 
        echo $book 
    done 
shell within a shell # command line inside a shell 
    for book in $(ls books/ | grep -i 'air')
    do 
        echo $book 
    done 

while statement 
    x=1
    while[ $x <= 3 ];
    do
        echo $x 
        ((x+=1)) # uses double parentheses for manipulation of variable or declaration of variable 
    done 

// LESSON 3.3 - CASE STATEMENTS 
- substitute to if else, useful for equality checking 

# Create a CASE statement matching the first ARGV element
case $1 in
    # Match on all weekdays
    Monday|Tuesday|Wednesday|Thursday|Friday)
    echo "It is a Weekday!";;
    # Match on all weekend days
    Saturday|Sunday)
    echo "It is a Weekend!";;
    # Create a default
    *) 
    echo "Not a day!";;
esac 

// LESSON 4.1 - BASIC FUNCTIONS IN BASH 
- just like in python 
function structure 
    function_name () {
        # function_code 
        echo #return is not similar to other programming languages 
    }
abnother function structure 
    function function_name { # can add parenthesis or not it doesnt matter since we do not pass arguments into parenthesis just like in normal programming language, super weird 
        # function code 
        return # something 
    }
    # calling the function 
    function_name 

// LESSON 4.2 - ARGUMETNS, RETURN VALUES, AND SCOPE 
arguments 
    - value we passed into the function 
return values 
    - only meant to determine if the function was a success or a failure (0 for fail, 1 for success)
local scope 
    - locally available, those within the curcly brackets 
global scope 
    - available everywhere, all variables in bash are globally scoped 
argument in a function 
    function function_name2 {
        echo "$1 $2 $@" # lmao these variables dont need no declarations in parenthesis beside function name 
    }
    # calling the function 
    function_name2 Hello there my dudes # passing argumetns into a function 
return values in a function 
    - bash has a return statement but the only use it has is to determine whether the functions execution is success(0) or not (values from 1-255)

    # returning values with global scope - not suggested 
    function myfunc()
    {
        myresult='some value'
    }

    myfunc
    echo $myresult

    # returning values while using local scope with command substitution
    function myfunc()
    {
        local  myresult='some value'
        echo "$myresult"
    }

    result=$(myfunc)   # or result=`myfunc`
    echo $result # calling the echo inside the myfunc 
restricting scope in bash functions 
    function print_filename { 
        local first_filename=$1 # by putting the local keyword, these variable is now only avaialble within the scope 
    }

// UNFINISHED - LESSON 4.3 - SCHEDULING YOUR SCRIPTS WITH CRON - JUST LIKE AIRFLOW