shell-scripting-notes.txt

shell scripting
===============

#shell scripting123

What is shell?

-> shell refers to a user interface that allows users to interact with the operating system (OS) or execute commands. 

-> It acts as a command interpreter, taking commands entered by the user (via a command-line interface or graphical interface) and executing them by communicating with the OS kernel.

-> In most of Linux OS BASH[Bourne Again SHell] is the default shell type.

-> There are other shell programs are there,
   1. Korn shell
   2. Bash
   3. C Shell(csh)
   4. Z Shell(zsh),, etc

How to check How many shells are there in my Linux server?

cat /etc/shells

How to install csh?

sudo yum install csh -y
sudo yum install zsh -y
sudo yum install ksh -y


How to check which shell we are using now?

echo $SHELL
echo $0
ps -p $$

How switch to another shell?
step 1: check what are the shells are available
       cat /etc/shells
step 2: How to switch to another shell
      /bin/sh ---> run this
     ps -p $$
step 3: come back to bash ---> /bin/bash

  
===================================

what is shell scripting?
========================
--> It is a file , contains collections of commands, What ever the order we specified based on that order it will be executed.

what is the extension for shell scripting?
ANS:  .sh

Why we need to learn the shell script?
=======================================
Ans: To automating the manual tasks.

Is it only for DevOps Engineers?
Ans: NO

EX: 
1. serverresourseutilization.sh
2. dbbackup.sh


What are the prerequisites to learn shell scripting?
=====================================================
1. Linux commands
2. Basic programming knowledge[depends]
3. commitment 
4. Problem solving skills

How to write a shell script?
=============================
vi Demo.sh

#!/bin/bash  --> #! --> shebang line
echo "Welcome to shellscript KK FUNDA"
echo "Today date is"
date

How many ways you are going to run the script?
==============================================
1. sh Demo.sh
2. ./Demo.sh  --> chmod u+x Demo.sh
3. . Demo.sh
4. bash Demo.sh


How to run the shell script in debug mode?
===========================================
Ans: 
1.  sh -x Demo.sh  ---> It will gives the output as command and output

2.  To run few commands in debug mode.
set -x  ---> starting point
echo "Welcome to shellscript KK FUNDA"
echo "Today date is"
set +x -----> end point
date

3. sh -x Demo.sh  ---> From starting it will run debug mode until reaches +x

comments in shell scripting
===========================

--> comments are essential for improving code readability
--> documenting your script
--> explaining the purpose of specific commands or sections

Types of Comments:
=================
Single-line Comments:
---------------------

Single-line comments start with # and extend to the end of the line.
Example:
# This is a single-line comment

Inline Comments:
----------------
Inline comments are placed at the end of a command line to provide brief explanations.
Example:
command1  # This command does something

Multi-line Comments:
--------------------

Shell scripting does not support native multi-line comments like some other programming languages (e.g., /* */ in C/C++). However, you can achieve a similar effect by commenting out multiple lines using #.
Example:
# This is
# a multi-line
# comment

<<comment
comment

In java/terraform/groovy script
===============================
// single line comment
/*
  multi line comment
*/

XML
===
<!-- 

-->


Variables in shell scripting???????????
======================================
Def: variables are used to store data or values that can be referenced and manipulated throughout the script.

Variable Naming and Assignment:
-------------------------------
Variable Naming Rules:
----------------------

-->Variable names can consist of letters (a-z, A-Z), digits (0-9), and underscores (_).
-->They must start with a letter or an underscore (_).
-->Variable names are case-sensitive (myVar is different from MYVAR).

Variable Assignment:
--------------------
Assign values to variables using = (no spaces around =)

Example:
myVar="Hello, World!"
Quotes (" " or ' ') are used to enclose values to preserve spaces and special characters.


Accessing Variables:
---------------------

Use $ followed by the variable name to access its value.

Example:
echo "The value of myVar is: $myVar"

Variable Expansion:
-------------------
Variables are expanded when referenced with $.
Example:
name="Alice"
echo "Hello, $name!"

Concatenation:
--------------
Concatenate strings using variables.
Example:
greeting="Hello"
target="World"
echo "$greeting, $target!"


Types of variables
=================
Environment Variables[System defined variables]
-----------------------------------------------

Variables inherited from the shell environment or explicitly exported using export.

env or printenv ---> Used to get the system variables.

echo "Current user: $USER"

expose
------
It is a command to override the system defined values.
EX: echo $HISTSIZE
    export HISTSIZE=300
    echo $HISTSIZE

NOTE: It is only for particular session, How to change permanently?
     ~/.bash_profile
     export HISTSIZE=300




Local Variables or user defined variables
-----------------------------------------

Variables defined within a script are local by default and are only accessible within the script.

EX:
   a=10
   b=90
   name="kkfunda"

  echo $a
  echo $b
  echo $name




Naming conventions
==================
-> file name max limit is 255 characters.
-> The file name may contain alphabet, numbers, dots and underscore
-> System commands or Linux reserved words cant be used.
-> File System is case sensitive.
 EX:
     if
     fi
     for
     mkdir
     pwd






Command line arguments in shell scripting
==========================================

Command line arguments in shell scripting allow you to pass arguments or parameters to a script when it is executed from the command line.

Accessing Command Line Arguments:
----------------------------------
$0 ---> script name
$1 ---> First argument
$2 ---> second argument

Note: If it is more than one number keep it like this ${10}

Ex:
#!/bin/bash
echo "Script name: $0"
echo "First argument: $1"
echo "Second argument: $2"

sh Demo.sh Prasanth reddy


Special Variables:
------------------
$#: Number of arguments passed to the script.
$@: All arguments passed to the script as separate words ("$1" "$2" ...).
$*: All arguments passed to the script as a single string ("$1 $2 ..." ).
$$: It will display the process id[PID]

$? ---> Previous command execution status.
 0--> previous command execution status is success
 127--> Command not found






08/07/2024
==========

How to read a value at runtime in shell scripting?
===================================================

1. Read basic input
-----------------------

#!/bin/bash

# Prompt user to enter their name
echo "Enter your name:"
read name

echo "Hello, $name! Welcome to the script."



2. Using -p Option for Prompt
-----------------------------

#!/bin/bash

# Prompt user for input with a custom prompt
read -p "Enter your age: " age

echo "You entered: $age years old."


3. Read the input one after another
-----------------------------------
#!/bin/bash

# Reading multiple values
echo "Please enter your name:"
read name

echo "Please enter your age:"
read age

# Output a summary of the entered details
echo "Hello, $name! You are $age years old."



#!/bin/bash

# Reading multiple values
echo "Please enter your name and Age:"
read name age1

# Output a summary of the entered details
echo "Hello, $name! You are $age1 years old."

4. Reading input as an array
-----------------------------

we will discuss in arrays



5. Reading Input with Timeout
------------------------------

#!/bin/bash

# Prompt user for input with a timeout
read -t 10 -p "Enter your password within 10 seconds: " password

if [ -z "$password" ]; then
    echo "No password entered within 10 seconds."
else
    echo "Password entered: $password"
fi


6. Reading Input Silently (Password Input)
------------------------------------------

#!/bin/bash

# Prompt user for password input (silent)
read -s -p "Enter your password: " password
echo    # Move to a new line after password input

# Validate or process the password input
echo "Password entered."


Operators in shell scripting
=============================

In shell scripting, operators are used to perform various operations like arithmetic, comparison, logical operations, and string manipulations. Here's a detailed overview of different types of operators in shell scripting with examples:

1. Arithmetic Operators
--------------------------
#!/bin/bash

# Arithmetic Operators
a=20
b=5
echo "Arithmetic Operations:"
echo "Addition o two numbers $(( a + b ))"
echo "a - b = $(( a - b ))"
echo "a * b = $(( a * b ))"
echo "a / b = $(( a / b ))"
echo "a % b = $(( a % b ))"

sh open1.sh  20 5

$1, $2

2. Comparison Operators
------------------------

a=20
b=30

echo "Comparison Operations:"
if [ $a -eq $b ]; then  #==
    echo "a is equal to b"
fi
if [ $a -ne $b ]; then #!=
    echo "a is not equal to b"
fi
if [ $a -gt $b ]; then #>
    echo "a is greater than b"
fi
if [ $a -lt $b ]; then  #<
    echo "a is less than b"
fi

-ge(>=) ---> greater than equal to
-le(<=) ---> less than equal to

3. Logical Operators
--------------------
&&[AND]
===

A   B  A&&B
------------
T   T   T
T   F   F
F   T   F
F   F   F


||[OR]
---
A    B   A||B
--------------
T    T     T
T    F     T
F    T     T
F    F     F


a=30
b=5
echo "Logical Operations:"
if [ $a -eq 20 ] && [ $b -eq 5 ]; then
    echo "Both conditions are true"
fi
if [ $a -eq 10 ] || [ $b -eq 5 ]; then
    echo "At least one condition is true"
fi
if ! [ $a -eq 0 ]; then
    echo "a is not equal to 0"
fi

4. String Operators
-------------------

# String Operators
str1="Hello"
str2="World"
echo "String Operations:"
echo "=================="
if [ "$str1" = "$str2" ]; then
    echo "Strings are equal"
fi
if [ "$str1" != "$str2" ]; then
    echo "Strings are not equal"
fi
str3="$str1 $str2"
echo "Concatenated string: $str3"
length=${#str1}
echo "Length of str1 is $length"

control statements in shell scripting
======================================

Control statements in shell scripting allow you to control the flow of execution based on certain conditions or loops. They help in making decisions, repeating code blocks, and branching based on conditions. Here’s a detailed explanation of the main control statements in shell scripting:


1. if statement
----------------
if [ condition ]; then
    # Code block to execute if condition is true
fi

2. if-else Statement
--------------------

The if-else statement allows you to execute a block of code if a specified condition is true, otherwise, another block of code is executed.

Syntax:

if [ condition ] 
then
    # Code block to execute if condition is true
else
    # Code block to execute if condition is false
fi

Example 1:
-----------
#!/bin/bash

# Example of if-else statement
a=10

if [ $a -eq 10 ]; then
    echo "Value of a is 10"
else
    echo "Value of a is not 10"
fi

Example 2:
----------
#!/bin/bash

# Example of checking file existence using if and else statement
file="demo.sh"

if [ -f "$file" ]
then
    echo "File $file exists."
else
    echo "File $file does not exist."
fi

Example 3:  Checking Connectivity
----------  

#!/bin/bash

# Example of using if-else to check internet connectivity
ping_result=$(ping -c 1 google.com 2>&1)

if [[ $ping_result == *"icmp_seq"* ]]
then
    echo "Internet connectivity is up."
else
    echo "Internet connectivity is down."
fi

3. if-elif-else Statement
-------------------------

The if-elif-else statement allows you to test multiple conditions and execute a block of code as soon as one of the conditions evaluates to true.

Syntax:
-------


if [ condition1 ]; then
    # Code block to execute if condition1 is true
elif [ condition2 ]; then
    # Code block to execute if condition2 is true
else
    # Code block to execute if all conditions are false
fi


Example: 1
-----------

#!/bin/bash

# Example of if-elif-else statement

a=20
if [ $a -eq 10 ]; then
    echo "Value of a is 10"
elif [ $a -eq 20 ]; then
    echo "Value of a is 20"
else
    echo "Value of a is neither 10 nor 20"
fi

Example 2:  Checking Disk Space and Taking Action
-----------

#!/bin/bash

# Example of using if-elif-else to check disk space and take action
threshold_critical=15
threshold_warning=12
current_usage=$(df -h / | awk 'NR==2 {print $5}' | cut -d'%' -f1)

echo "Current disk usage: $current_usage%"

if [ $current_usage -ge $threshold_critical ]  # >=
then
    echo "Disk usage is critical ($current_usage%). Taking immediate action!"
    # Add commands to free up disk space or notify administrators
elif [ $current_usage -ge $threshold_warning ]
then
    echo "Disk usage is high ($current_usage%). Consider freeing up space."
    # Add commands to optimize disk usage
else
    echo "Disk usage is normal ($current_usage%)."
fi


Example 3:
----------

#!/bin/bash

# Example of using if-elif-else to handle different types of user input
read -p "Enter a number (1-3): " num

if [ "$num" -eq 1 ]
then
    echo "You entered the number one."
elif [ "$num" -eq 2 ]
then
    echo "You entered the number two."
elif [ "$num" -eq 3 ]
then
    echo "You entered the number three."
else
    echo "Invalid input: Please enter a number between 1 and 3."
fi


Example 4:  Checking Network Connectivity Status
----------

#!/bin/bash

# Example of using if-elif-else to check network connectivity status
ping_result=$(ping -c 1 google.com 2>&1)

if [[ $ping_result == *"icmp_seq"* ]]
then
    echo "Internet connectivity is up."
elif [[ $ping_result == *"unknown host"* ]]
then
    echo "Unable to resolve DNS. Check your network configuration."
else
    echo "Internet connectivity is down."
fi



4. case statement in shell scripting
------------------------------------

syntax:
-------
case variable in
    pattern1)
        # Commands to execute if variable matches pattern1
        ;;
    pattern2)
        # Commands to execute if variable matches pattern2
        ;;
    pattern3 | pattern4)
        # Commands to execute if variable matches either pattern3 or pattern4
        ;;
    *)
        # Default commands to execute if variable matches none of the above patterns
        ;;
esac





Example 1:
--------
#!/bin/bash

# Example of a basic case statement
fruit="apple"

case $fruit in
    apple)
        echo "It's a fruit: apple."
        ;;
    banana)
        echo "It's a fruit: banana."
        ;;
    orange | mandarin)
        echo "It's a citrus fruit."
        ;;
    *)
        echo "Unknown fruit."
        ;;
esac

Example 2: Checking Days of the Week
---------
#!/bin/bash

# Example of using case to check days of the week
day="Monday"

case $day in
    Monday | Tuesday | Wednesday | Thursday | Friday)
        echo "$day is a weekday."
        ;;
    Saturday | Sunday)
        echo "$day is a weekend day."
        ;;
    *)
        echo "Invalid day."
        ;;
esac


Example 3: Handling Menu Options
----------
#!/bin/bash

# Example of using case to handle menu options
echo "Menu:"
echo "====="
echo "1. Display date"
echo "2. Display calendar"
echo "3. Display current directory"
echo "4. Exit"

read -p "Enter your choice: " choice

case $choice in
    1)
        date
        ;;
    2)
        cal
        ;;
    3)
        pwd
        ;;
    4)
        echo "Exiting program."
        exit 0
        ;;
    *)
        echo "Invalid choice: $choice. Please enter a number between 1 and 4."
        ;;
esac



scp -i /c/Users/gpras/Downloads/testmc.pem /c/Users/gpras/Downloads/Shell_Scripting_New.txt ec2-user@13.232.216.223:/home/ec2-user/


scp -i /c/Users/gpras/Downloads/testmc.pem ec2-user@13.232.216.223:/home/ec2-user/R2.sh  /c/Users/gpras/Downloads/R2.sh 




9/07/2024
==========


what is a loop in SS?
----------------------


for Loop
========

syntax:
-------
for variable in list
do
    # Commands to execute for each item in the list
done


Example 1:
----------

#!/bin/bash

# Example of a for loop iterating over a list of numbers
for num in 1 2 3 4 5
do in.
    echo "Number: $num"
done

Example 2: Iterating over Files and directories in a Directory
---------
#!/bin/bash

# Example of a for loop iterating over files in a directory
echo "Files in current directory:"
for file in *
do
    echo "$file"
done

Example 3: Processing Arguments Passed to a Script
----------
#!/bin/bash

# Example of a for loop iterating over script arguments
echo "Arguments passed to the script:"
for arg in "$@"
do
    echo "$arg"
done

Example 4:  Generating a Sequence of Numbers
----------
#!/bin/bash

# Iterate 5 times
echo "for loop example"
for (( i=1; i<=5; i++ ))
do
    echo "Iteration: $i"
done

Example 5: Nested for loop
----------

#!/bin/bash

# Example of nested for loops to generate a multiplication table
echo "Multiplication Table (1 to 5):"

for (( i=1; i<=5; i++ ))
do
    echo "Multiplication table for $i:"
    for (( j=1; j<=5; j++ ))
    do
        echo "$i * $j = $((i * j))"
    done
    echo "----------------------"
done

j++
++j
j--
--j

6. while Loop
-------------

The while loop executes a block of code as long as a specified condition is true.

Syntax:
while [ condition ]; 
do
    # Code block to execute as long as condition is true
done

Example 1: Counting Down from 5 to 1
----------

#!/bin/bash

# Example of a while loop counting down from 5 to 1
counter=5

echo "Counting down:"
while [$counter -gt 0]; do
    echo "$counter"
    (( counter-- ))
done

echo "you are a good automation engineer"

Example 2: Reading Lines from a File
----------

#!/bin/bash

# Example of a while loop reading lines from a file
filename="Shell_Scripting_New.txt"

echo "Contents of file:"
while IFS= read -r line    #IFS: internal Field Separator
do
    echo "$line"
done < "$filename"
echo "you are a great"

name
====
1.
2.
3.
4.
5.
exit 


Example 3: Processing User Input Until Exit
----------
#!/bin/bash

# Example of a while loop processing user input until 'exit' is entered
echo "Enter names (type 'exit' to quit):"

while :
do
    read -p "Name: " name
    if [ "$name" = "exit" ]; then
        break
    fi

    echo "Hello --->$name!"

done


echo "you are exited"

Example 4: Calculating Factorial
----------
#!/bin/bash

# Example of a while loop calculating factorial of a number
echo "Enter a number to calculate its factorial:"
read num

if [ $num -gt 0 ]; then

factorial=1
counter=$num

while [ $counter -gt 0 ]
do
    factorial=$(( factorial * counter ))
    (( counter-- ))
done

echo "Factorial of $num is: $factorial"

else

echo "invalid input"

fi

7. until Loop
-------------

Example 1: Counting Up from 1 to 5
----------

#!/bin/bash

# Example of an until loop counting up from 1 to 5
counter=1

echo "Counting up:"
until [ $counter -gt 10 ]
do
    echo "$counter"
    (( counter++ ))
done

echo "Done counting!"

Arrays in shell scripting
=========================
Def: arrays provide a convenient way to store and manipulate multiple values under a single variable name. 

NOTE: Array values can be accessed by the index [0 to length-1]

Declaring Arrays
-----------------
1. Implicit Declaration: 
Arrays are implicitly declared when you assign values to indexed positions:

name_array=("kk" "sai" "raghu")

2. Explicit Declaration:
 You can explicitly declare an array by using the declare built-in command:

declare -a name_array
name_array=("kk" "sai" "raghu")


Accessing Array Elements
--------------------------
To access individual elements of an array, you use the index (starting from 0):


name_array=("kk" "sai" "raghu")
echo "${name_array[0]}"  # prints "value1"
echo "${name_array[1]}"  # prints "value2"
echo "${name_array[2]}"  # prints "value3"


Array Operations
----------------


1. Appending Elements

You can append elements to an array using the += operator:

name_array=("kk" "sai" "raghu")

name_array+=("jaswanth")


2. Length of Array

To find the length of an array, use ${#name_array[@]}

name_array=("kk" "sai" "raghu")

echo "Length of array: ${#name_array[@]}"  # prints the number of elements in the array


3. Iterating Over an Array
--------------------------

You can iterate over all elements in an array using a loop:

name_array=("kk" "sai" "raghu")
for element in "${name_array[@]}"
do
    echo "$element"
done

4. Removing Elements

You can unset individual elements by index:

unset name_array[1]  # removes the element at index 1

5. Clearing an Array

To clear all elements in an array:

name_array=()

Example 1
---------
#!/bin/bash

# Declare an array
declare -a fruits=("Apple" "Banana" "Orange")

# Print all elements
echo "All fruits:"
for fruit in "${fruits[@]}"
do
    echo "$fruit"
done

# Print length of array
echo "Number of fruits: ${#fruits[@]}"

# Append an element
fruits+=("Grapes")

# Print length of array
echo "Number of fruits: ${#fruits[@]}"

# Access individual element
echo "First fruit: ${fruits[0]}"

# Remove an element
unset fruits[1]

# Print updated array
echo "Remaining fruits:"
for fruit in "${fruits[@]}"
do
    echo "$fruit"
done


Example 2: Processing File Names
----------
#!/bin/bash

# Declare an array to store file names
declare -a files=()

# Populate the array with file names ending in .txt in current directory
files=(*.sh)

# Iterate over each file and perform an operation (e.g., count lines)
for file in "${files[@]}"
do
    line_count=$(wc -l < "$file")
    echo "File: $file has $line_count lines."
done

Example 3: Processing Command-Line Arguments
---------


You might want to process command-line arguments passed to a script and store them in an array.

#!/bin/bash

# Store command-line arguments in an array
args=("$@")

# Print all arguments
echo "All arguments:"
for arg in "${args[@]}"
do
    echo "$arg"
done

echo "Length of array: ${#args[@]}"  # prints the number of elements in the array

# Access individual argument
echo "First argument: ${args[0]}"

Example 4: Dynamic Menu with User Selection
----------

You might want to create a dynamic menu using an array of options and allow the user to select an option.

#!/bin/bash

# Array of menu options
declare -a menu=("Option 1: l1.sh" "Option 2: l2.sh" "Option 3: Exit")

# Display menu options
echo "Select an option:"
select option in "${menu[@]}"
do
    case $REPLY in
        1)
            sh l1.sh
           ;;
        2)
             sh l2.sh
            ;;
        3)
            echo "Exiting..."
            break
            ;;
        *)
            echo "Invalid option"
            ;;
    esac
done

10/07/2024
===========

File Descriptors
================

>
>>
<


0  ---> std i/p

1 ---> std o/p

2 ---> std error

2>&1



exit code
=========

0 to 127  ---->128



sleep 
-----

functions in shell scripting
=============================


Def:
Functions in shell scripting allow you to reuse the code.

---> you can reduse the number of lines of code

Syntax of Defining Functions
-----------------------------

# Syntax 1: 

function_name () 
{
    # Commands to be executed
    command1
    command2
    # ...
}

# Syntax 2: 

data() 
{
echo "I am from Nellore"
echo "My name Is KK FUNDA"
date
echo "KK FUNDA YOUTUBE CHANNEL"

}


Example:
--------

#!/bin/bash

# Define a function that prints a greeting
testing() 
{
    echo "Hello, World!"
}

# Call the function
testing
pwd
uptime
testing


Passing Parameters to Functions
-------------------------------

Functions in shell scripts can accept parameters similar to scripts. Parameters are accessed inside the function using positional parameters ($1, $2, ...) or the special variables ($@, $*). Here’s how you define and use a function with parameters:

#!/bin/bash

# Function with parameters
greet_user() {
    local name="$1"  # First parameter
    echo "Hello, $name!"
}

greet_user "kkfunda"

Example of Using Parameters
---------

#!/bin/bash

# Function to calculate the sum of two numbers
sum() {
    local num1="$1"
    local num2="$2"
    local total=$(( num1 + num2 ))
    echo "Sum of $num1 and $num2 is: $total"
}

# Call the function with arguments
sum 10 20

Return Values
-------------

Shell functions do not have a traditional return statement like other programming languages. Instead, the return status of the last executed command is used as the return value of the function. To explicitly return a value, you can use echo or modify a global variable.

Example
--------

#!/bin/bash

# Function to calculate the square of a number and return it
square() {
    local num="$1"
    local result=$(( num * num ))
    echo "$result"  # Return the result
}

# Call the function and capture the returned value
output=$(square 5)
echo "Square of 5 is: $output"

Local Variables and global variables and its Scope
--------------------------

--> Variables declared inside a function called as a local variable, and variables outside function called as global variables.
--> Local variable scope is with  in that block only, Global variable scope is entire script.

Example
-------
#!/bin/bash


global_var="Global"


demo_local_var() 
{
    local local_var="Local"
    echo "Inside function: local_var=$local_var, global_var=$global_var"
}


demo_local_var

echo "Outside function: local_var=$local_var"
echo "Outside function: local_var=$global_var"

Example
--------

#!/bin/bash

# Function to print numbers from 1 to n
print_numbers() {
    local n="$1"
    for (( i=1; i<=n; i++ ))
    do
        echo "$i"
       sleep 5
    done
}


print_numbers 5

Realtime examples using functions
=================================

Example 1: User Management Script
----------

Scenario: You want to create a script that manages user accounts by adding, deleting, and listing users using functions.

#!/bin/bash

# Function to add a user
add_user() {
    local username="$1"
    sudo useradd -m "$username"
    echo "User '$username' added successfully."
}

# Function to delete a user
delete_user() {
    local username="$1"
    sudo userdel -r "$username"
    if [ $? -eq 0 ] ; then
    echo "User '$username' deleted successfully."
    else
    echo "Invalid user"
     fi
}

# Function to list all users
list_users() {
    local users=$(cut -d: -f1 /etc/passwd)
    echo "List of users:"
    echo "$users"
}

# Main script logic
echo "Choose an option:"
echo "1. Add a user"
echo "2. Delete a user"
echo "3. List all users"
read "Enter your choice: " choice

case $choice in
    1) read -p "Enter username: " username
       add_user "$username"
       ;;
    2) read -p "Enter username to delete: " username
       read -p "Are you sure want to delete then press yes or else no:::" decision
       if [ $decision = "yes" ]
         then
         delete_user "$username"
       else
       echo "OK no problem leave it"
        fi
       ;;
    3) list_users ;;
    *) echo "Invalid choice. Exiting." ;;
esac

Example 2: Function with Conditional Logic
---------
#!/bin/bash

# Function to check if a number is even or odd
check_even_odd() {
    local number="$1"
    if (( number % 2 == 0 )); then
        echo "$number is even."
    else
        echo "$number is odd."
    fi
}


for ((i=1;i<=5;i++))
{
read -p "enter a number::" num
if [ $num -gt -1 ]
then 
  check_even_odd num
else
echo "pls enter valid input"
fi
}

11/07/2024
-----------
--> shell
--> shell scripting
---> command line arguments
---> debug mode
--> read command
---> variables
--> operators
--> control statements
--> arrays
--> functions

Example 3: Function with Array Handling
---------
#!/bin/bash

# Function to print elements of an array
print_array() {
    local arr=("$@")  # Copy all arguments into the array
    echo "Array elements:"
    for item in "${arr[@]}"
    do
        echo "$item"
    done
}

# Example usage with an array
my_array=("apple" "banana" "cherry" "date")
print_array "${my_array[@]}"

Example 4: Deleting a file using functions
---------

step 1: create a function   delete()
step 2: rm -rf <fn>
step 3: 


stpe 0: delete "fn"

#!/bin/bash

# Function to perform a risky operation
risky_operation() {
    local fname="$1"
    if [ -f "$fname" ]; then
        echo "Processing file: $fname"
        # Add risky operation here (e.g., deleting file)
        rm -rf "$fname" 
        echo "$file deleted successfully."
    else
        echo "Error: File $fname does not exist."
    fi
}

read -p "enter aa file name : " fname
risky_operation  "$fname"

Example 5: Function to Check Disk Usage
---------


#!/bin/bash

# Function to check disk usage
check_disk_usage() {
    local threshold=80  # Threshold percentage
    local disk_usage=$(df -h / | awk 'NR==2 {print $5}' | sed 's/%//')
    echo "Disk usage: $disk_usage%"

    if (( disk_usage >= threshold )); then
        echo "Warning: Disk usage exceeds $threshold%."
        # Add logic to send alert (e.g., email or notification)
    else
        echo "Disk usage within limits."
    fi
}

# Example usage
check_disk_usage



zip_files()
{
  zip backup.zip *.log
  mv backup.zip backup/ 
}

zip_files

Example 6: Function to Calculate Average
---------

#!/bin/bash

# Function to calculate average of numbers
calculate_average() {
    local total=0
    local count=0
    for num in "$@"
    do
        total=$(( total + num ))
        (( count++ ))
    done
    local average=$(( total / count ))
    echo "Average: $average"
}

# Example usage
calculate_average 10 20 30
calculate_average 5 15 25 35 45


Example 7:  install any package
---------
#!/bin/bash

# Function to check and restart service if not running
install() {
    local arg="$1"
   sudo yum install $arg -y
  
}

un_install()
{
    local arg="$1"
   sudo yum remove $arg -y
  
}
echo "enter package name"
read package
install "$package"


Example 8: Function for System Information Reporting
----------

#!/bin/bash

# Function to gather system information
gather_system_info() {
    local hostname=$(hostname)
    local kernel=$(uname -r)
    local cpu_cores=$(cat /proc/cpuinfo|grep -i "cpu cores" |awk '{print $4}')
    local memory=$(free -h |awk '/Mem/{print $7}')
    local disk_usage=$(df -h / | awk 'NR==2 {print $5}')

    echo "System Information:"
    echo "=================="
    echo "Hostname: $hostname"
    echo "kernel : $kernel"
    echo "CPU cores: $cpu_cores"
    echo "Total memory: ${memory}MB"
    echo "Disk usage: $disk_usage used"
}


gather_system_info

#!/bin/bash

# Directory to clean up
TARGET_DIR="/home/ec2-user/"

# Number of days to keep files
DAYS=2

# Find and delete files older than $DAYS
aaa=find "$TARGET_DIR" -type f -mtime +$DAYS
echo $aaa


0 2 * * * /path/to/delete.sh