-> ls :

    List all the file or folder present in the current directory.

        Option:

        * -l :  List all the file and folder with permission, owner,
                and group details in form of vertically alinged list.
        
        * -a :  Display hidden (.)dotfile as well.

-> pwd :

    Print the current directory, present on the terminal.

-> date :

    Print the current date and time.

-> chmod :

    Used to change the permission of file or folder

        Symbol Meaning :

        * a : For all (including owner, group and others)
        * u : For owner
        * g : For group
        * o : For others
        * r : read permission ( number 4 )
        * w : write permission ( number 2 )
        * x : execute permission ( number 1 )

        Option :

        * a+x : Add execute permission for all.
        * a-x : Remove execute permission for all.
        * 755 : rwx - user , r-x - group, r-x - others

    How to use?

        chmod a+x "file/folder name"
        chmod 755 "file/folder name"

-> echo :

    To Print argument on the terminal

        Option:

        * -n : Not to add newline.
        * ".." : String to print
        * $PATH : variable ( environment variable, etc. )

-> $PATH environment variable :

    Store the paths which help in invoking executable, script, etc,
    without specifying whole path, where its present.

    How to add path to $PATH?

    export PATH=$PATH:$(pwd)
                        or full path to intended directory.

-> SHABANG (#!/bin/bash)

    Tells to use this interpreter to run the script.
    Same as /bin/bash ./script

-> Variables :

    To stores data and can be changed.

    1. Explicit Defination :
        VAR=value
        COUNT=5
        PATH=/var/lib

    2. Read command :
        read VAR
        read COUNT
        read PATH

        Option :

        * -p :  to print the parameter before read.

            How to use?

                read -p "Your name: " NAME

        * -s :  to hide the input to be taken. Best use
                for password.

            How to use?
            
                read -sp "Your age: " AGE

    3. Command Substitution :

        To store the output of the command executed to the variable.

        How to use?

            VAR=$(pwd) or VAR=`pwd`


    How to use variables?

        Use $variableName for using variables.


-> For Math Calculation :

    1 . let

        let RESULT=NUMBER+5
        let NUMBER++
        let NUMBER--
        let NUMBER+=5
        let NUMBER-=5

    2. (( ))

        RESULT=$(( NUMBER + 5 ))
    
    3. []

        RESULT=$[ NUMBER + 5 ]
    
    4. expr 

        RESULT=$(expr $NUMBER + 5)
        RESULT=`expr $NUMBER + 5`
        RESULT=`expr 2 + 3`

    5. bc ( used for floating point number )

        RESULT=`echo "$NUMBER * 1.9" | bc`


-> Argument :


    $0 - script name
    $1 - first argument
    $2 - second argument
    $n - nth argument
    "$@" - all argument, expands as "$1" "$2" "$3"
            and so on.
    "$*" - all argument as string, "$1c$2c$3", 
            where c is IFS (Internal field
            separator), which is usally whitespace.
    $# - arguments count

-> Redirecting and piping:

    Redirecting

    * STDIN (0) - Standard input (data provided to program)
    * STDOUT (1) - Standard output (what program prints, defaultly to the terminal)
    * STDERR (2) - Standard error (what error message program prints, defaultly to terminal)

    examples:
    cat file1.txt > output_from_cat.txt
    cat file1.txt 1>output_from_cat.txt
    cat file1.txt 2>error.txt
    cat file1.txt 1>output_from_cat.txt 2>error.txt
    cat file1.txt &> output_from_cat.txt (from both 1 and 2)
    cat file1.txt 1> output_from_cat.txt 2>&1(redirecting error to output)

    cat file1.txt >> output_from_cat.txt(adding output instead of replacing it)

    Piping

    output of preceeding program is input to succeeding
    program.

    Program1 | Program2

    examples:
    cat file.txt | wc -l
    cat file.txt | head -5 | tail -3 | wc -l

-> Exit Status

    echo $?

    $? will give a return value of previous command.
    It's the return value, called EXIT STATUS.
    Successfully exiting give 0 value otherwise
    non-zero value.

    We can exit script using exit command.
    exit command can be used with number 0 - 255.