Bash Tutorials
====================================================================================================

   Explore Bash shell scripting tutorials and programming examples..

   This post provides tutorials on Bash commands and programming with examples.

   The course covers Bash and shell programming features, including variables, loops, conditional expressions, and operators.

   Shell is a command-line interpreters, It is an application to provide commands to the different operating systems such as Linux, Unix, and Mac.

   Bash is an enhanced version of shell, an layer between user and operating system function calls.

   User types the commands via command line or batch of commands also called scripts.

Requirement

   To follow this course, you should have a basic understanding of computer language, and the following are required:
     * Linux-flavored operating system
     * Basic knowledge of any programming language

What is Bash

   Bash, short for Bourne Again Shell, is an open-source command-line shell interpreter and scripting language. It interprets user-entered commands, either interactively or
   from a script file.

   It serves as an interface to call commands, enabling system function calls.

   Threre are two types of bash modes
     * Interactive mode: Also called a command interpreter, it allows the execution of commands in the terminal. It executes commands sequentially if there are multiple
       commands.
     * Non-interactive mode: This refers to scripts, allowing you to write Bash syntax that contains a sequence of multiple commands for script execution.

What is the difference between Bash and shell

   Shell, an alias for Bourne Shell, is a command-line interpreter for Unix and Linux OS. Bash, an alias for Bourne Again Shell, is an enhanced version.

What is bash scripting used for?

   Bash scripting has multiple use cases, including:
     * Writing scripts to automate programming tasks
     * Synchronizing tasks to copy files
     * Executing cron jobs for scheduling

How do I write code in bash?

   To write code in a Bash script, follow these steps:
    1. In the terminal, create a file using vi test.sh.
    2. Add #!/bin/bash at the top of the file.
    3. Add multiple shell code snippets.
    4. Save the shell file with a .sh extension.
    5. Execute the shell script using the ./test.sh command in the terminal.

Is bash coding language?

   Bash executes commands from the terminal or a file. It is a programming language that operates in Unix/Linux kernel operating systems, containing all the features to write
   complete code.

   Bash is a special type of shell that takes input from commands, runs the code, and processes the input, returning results.

Shell Types

   There are different types of shells in a Unix OS.
   Shell Type       Alias        First Line
   sh         Bourne Shell       #!/bin/sh
   bash       Bourne Again shell #!/bin/bash
   cshell     C shel             #!/bin/csh


Difference between Command Line and Script in bash

   Let’s see the difference between command line and scripts

   Commands line options
     * Command line has a prompt which takes input from a user
     * Commands are not saved to a file.
     * It supports only one command at a time.

   Script files
     * Support multiple commands in a single file
     * Prompt still can be written in the script file
     * Only one line in a file executes sequentially


Bash - Variables
====================================================================================================

   Learn how to declare and use variables in bash scripts. Types of global, local, and readonly variables, and understand proper syntax for accessing variables in shell
   scripts..

   This post is about how to bash commands and programming tutorials with examples.

Bash Shell Variable

   Variables form the fundamental building blocks of any programming language. Shell and bash script programming offer variables, much like other languages.

   A variable serves as a container used for storing data in programming. It includes a pointer to the memory location of the data.

   Declare variable: To create a variable, you must assign a value to it.
variableName=VariableValue

   variableName:

   It is the name of the variable, which can contain any combination of alphabets (a to z, A to Z), numbers (0 to 9), and underscores (_).

   VariableValue is the value stored in a variable, and it can be a string of numbers or a boolean. The equal symbol (=) is used to assign a value to a variable.

   For example
AGE=25

   A variable named AGE is created and assigned the value 25.

How to Access Variables in Bash

   After declaring and assigning values to variables, you can access them using a dollar symbol ($) followed by the variable name.
AGE=25
echo $AGE

   This above code declares a variable named AGE with a value of 25 and then uses echo to display the value of the AGE variable. The dollar symbol before the variable name is
   crucial for accessing its value.

Bash Shell Readonly Variables

   Once variables are assigned values, you can change them to new values using the assignment operator =.
AGE=25
echo $AGE
AGE=35
echo $AGE

   Output:
25
35

   How do you make variables not updatable?

   The readonly keyword prevents a variable from being updated, effectively turning it into a constant.
AGE=25
echo $AGE
readonly AGE
AGE=35
echo $AGE

   AGE is an constaints, assigning new value throws an error , and the error message is AGE: is read-only.

   Output:
25
 line 6: AGE: is read-only

Bash unset variable

   The unset keyword assists in removing the value from the specified variable. The variable remains accessible but prints an empty value.
AGE=25
echo $AGE
unset AGE
echo "empty":$AGE

   Output:
25
empty:

   Above code,
     * first sets the variable AGE to 25, prints its value,
     * then unsets it using unset AGE.
     * Subsequently, it prints “empty” followed by the value of AGE, which now appears as empty space.

Variable Scopes

   Every declared variable must have a scope, defining where in the program the variable can be used.

   For instance, if a variable is declared within a function, it is available only within that function and is not accessible outside of it.

   Variable scopes in Bash can be defined in two ways
     * Global variable
     * Local variable:

Bash Global Variables

   Variables declared in a shell script are referred to as global variables.

   Global variables can be accessed within a function or any nested blocks of a shell script file.

   The default variable declared in a script file is called a global variable.
setAge() {
    echo "Inside Function Age: $AGE"
}
AGE=40
setAge
echo "Script Age: $tmp"

   Output
Inside Function Age: 40
Script Age: 40

Bash Local variables

   Local variables are declared inside a block of code or a function. The scope of these variables is visible only within the block where they are declared.

   Syntax:
local variablename=variablevalue

   In this syntax, the variable is declared and assigned with the local keyword.
setAge() {
    local AGE=25
    echo "Local Variable Age: $AGE"
}
AGE=40
setAge
echo "Global Age: $tmp"

   Output
Local Variable Age: 25
Global Age: 40

   A local variable is declared inside a function and is only visible within that function. Variables declared outside functions are called global variables and are available
   for all functions.

Variables typing

   Bash scripting is not typed language, however you can declare an variable with a type of the data using declare command Based on the type of variable, it allows the type
   of the data .
declare options variablename=value

   variable is declared and assigned with a value.

   Options contains option to create a type of a varialbe

   Array: To create an array variable

   declare -a variable=
    Variable Type         Syntax                            Description
   Array            declare -a variable declare an indexedarray variable that stores strings
   Associated Array declare -A variable Associated Array
   Integer          declare -i variable numeric value to store in variable
   Readonly         declare -r variable readonly variable, cannot be changed or unset
   Export           declare -x variable export the variable and used by all child process

Display Environment variables in Bash

   In Bash, there are two types of commands for printing environment variables.
     * The printenv command
     * The env command

   Both of these commands list all the environment variables of the terminal.

Variable nameing convention

     * Variables are read by prefixing with the $ symbol.
     * Variable names are composed of alphabets, numbers, or underscores.
     * Variables are case-sensitive; for example, test and Test are considered two different variables in scripting.
     * While variable names are conventionally written in UPPERCASE, you can create them using UPPER or LOWER case if needed. And Environment and Shell variables are both in
       UPPERCASE.
     * Variable names can not contain spaces
     * Names conventinaly should be camelCase. Example firstName

Shell variables

   Shell variables are variables are set by the shell, not by the user. These are required by shell to work smoothly
     Variable                                      Description
   PWD          Current working directory
   Set-Location Change the working directory to new directory
   Rename-Item  Rename a file
   IFS          Internal FIeld Separator by default is space, set by Shell, Used for string split
   PATH         Contains semicolon separated path of commands, Configured to lookup for commands
   UID          Prints the User Identifcation number
   Home         Home directory of a current user


Bash - Loop File
====================================================================================================

   How to loop contents of a file in Bash Shell scripting tutorials and programming examples.

   Sometimes, You want to read the file content with bash programming.

   There are multiple ways we can do

How to read a file by line in bash shell?

     * using while loop

#!/bin/bash
while IFS= read -r line; do
   echo "$line"
done <filename.txt


Bash - Comments
====================================================================================================

   In this tutorial, Learn the syntax and examples of inserting single and multi-line comments in shell and bash script files..

   This post explains how to write comments in bash shell scripts, with accompanying examples.

   Comments are code statements containing user-readable text that the shell skips during execution. Every programming language includes a comments feature, providing a
   description for lines of code or statements.

   Inline comments within the code assist developers in editing and better understanding the code.

   The bash scripting language allows you to use the following comment types.
     * Single comments
     * Multi-line comments

   The comments useful for humans, code is written for scripting

Single line Comment in bash shell

   Single-line comments in shell scripts are denoted by the # symbol at the beginning of each line.

   These comments include strings providing information about the corresponding line of code in the shell script.

   It is essential to place single-line comments on a separate line for clarity.

   For inline comments, use the # symbol at the beginning of the comment. Single-line comments always start with the # symbol.

   Syntax:
# Single-line comments

   Blank space after the # symbol is not required. Following is a single-line comment example in a shell script.
# Single-line comments.
AGE=45
echo "Age:$AGE" ## printing age inline comment

Multi-Line comments in a shell script

   Multi-line comments involve using more than one line for comments.

   First way, to creating multi-line comments is by utilizing single-line comments with each line starting with the single-line comment symbol.

   Syntax:
# Line1 comments
# Line1 comments
# Line1 comments

   The second Way to create a multi-line comment is by enclosing multiple lines within (:) and (').

   This syntax involves:
     * Comments starting with a colon (:) followed by '.
     * It followed by multiple lines of comments.
     * Comments concluding with '. Here is the syntax:

: '
multiline comments example1
multiline comments example2
multiline comments example3
'

   Here is a Multi-Line Comment example
: '
multiline comments example1
multiline comments example2
multiline comments example3
'
echo "testing multi-line comments"

   It is useful for incorporating more text spanning multiple lines, serving documentation purposes as well.


Bash - Arrays
====================================================================================================

   How to create indexed and associated arrays add, remove get iterate loop first and last elements of array s syntax and array cheat sheet shell examples.

   Arrays in shells are variable to hold more than one value.

   Suppose, You have a list of numbers 1 2 3.. 10 and want to store these numbers in Shell Script

   Without arrays, You have to declare as follows
let number1=1
let number1=1
...
...
let number10=10

   Iteration is difficult and if we want to store 100 numbers, It is very difficult.

   So, You can use an array referring to a single variable and store it.

How to declare and create an array?

   There are two types of arrays we can create
     * indexed arrays: array elements are stored with the index starting from zero
     * associated arrays: array is stored with key-value pairs
     * Declare an array

   To create an array, We need to declare an array.
declares -a array; # indexed array
declare -A array; # associative array

   an array is declared with the keyword declare with option -a or A

   indexed array example In this, Array values are stored with index=0 onwards. these are created with declare and -a option
declare -a array
array=(one two three)

   This array is a store with index=0, incremented by 1 as follows.
array[0]=one
array[1]=two
array[2]=three

   associative array example In this, Array values are stored with keys. these are created with declare and -A option
declare -A array
array=(one two three)

   In this array is a store with index=0, incremented by 1 as follows
array[key1]=one
array[key2]=two
array[key3]=three

   Let’s assign the values.
array=(1,2,3,4)

     * Assign the values without declaring an array
arrayvariable[index]=value

       This means, that arrayvariable is declared and assigned an array index with value.

   Arrays are zero-indexed based on zero to the length of an array -1 index=0 - returns the first element index=-1 returns the last element

   Arrays can contain numbers, strings, and a mix of it Let’s create an array of examples.

Access the array values

   An array contains an index to get elements. Array elements can be accessed using the below syntax.
${array_name[index]}

Declare an Array of numbers and loop through

   Arrays can contain numbers This example contains an array of numbers and for loop to print
nums=(1 3 12)
for i in "${nums[@]}"
do
  echo "$i"
done

   Output:
1
3
12

Declare an Array of strings and loop through

   Arrays can contain numbers This example contains an array of numbers and for loop to print
numbers=("element1" "element2" "element3")
for i in "${numbers[@]}"
do
  echo "$i"
done

   Output:
element1
element2
element3

Access the first elements of an array

   In Array elements, the First element index is zero, and array[0] returns the first element
numbers=("element1" "element2" "element3")

echo ${numbers[0]}
echo ${numbers}

   Output:
element1
element1

Get the last element of an array

   In a bash script, you can use index=-1 to get the last array element.
numbers=("element1" "element2" "element3")
echo ${numbers[-1]}

   With the recent bash 4.0 version, you can use the below syntax to read the last element.
echo ${numbers[${#numbers[@]}-1]}

Iterate or loop array elements

   For loop is used to iterate elements.

   Here is an example loop array example to print all elements
numbers=("element1" "element2" "element3")

for i in "${numbers[@]}"
do
  echo "$i"
done

   Output:
element1
element2
element3

   Another way to print the index and elements of an array using for loop.
numbers=("element1" "element2" "element3")

for i in "${!numbers[@]}"
do
  echo "$i" "${numbers[$i]}"
done

   Output:
0 element1
1 element2
2 element3

Print all array elements

   Use [@] or [*] to print all elements of an array.
arr=("element1" "element2" "element3") //
echo ${arr[@]} #element1 element2 element3
echo ${arr[*]}  #element1 element2 element3

Remove an element from an array

   You can remove an element from an array using unset for a given index.
numbers=("element1" "element2" "element3")
echo ${numbers[*]}
unset numbers[-1]
echo ${numbers[*]}

Adding an element to an array

   You can add an element at any index position using the below syntax.
array[index]=value

   An example of adding elements starting and end as well as middle
numbers=("element1" "element2" "element3")
echo ${numbers[*]}

 numbers[0]="element0"
echo ${numbers[*]}
 numbers[5]="element5"
echo ${numbers[*]}
 numbers[6]="element6"
echo ${numbers[*]}

   Output:
element1 element2 element3
element0 element2 element3
element0 element2 element3 element5
element0 element2 element3 element5 element6

Length of an array

   In this, Find the count of all elements in an array.

   Shell script provides #
arr=("element1" "element2" "element3")
echo ${#arr[@]} # returns 3
echo ${#arr[*]} # returns 3

Array cheat sheet examples

                            Example                                             Description
   declare -a array                                          Declare an Indexed array
   declare -A array                                          Declare an Associative array
   declare -a array=()                                       Declare an indexed array with empty array
   array=()                                                  create an empty array with declaring is valid
   array=(1 6 3)                                             Initialize array with numbers
   array=(one two three)                                     Initialize the array with string
   array=(one two 1)                                         Initialize the array with mixed data
   ${array[0]}                                               Get first element
   ${array[1]}                                               Get Second element
   ${array[-1]}                                              Get Last element
   ${array[@]}                                               Get All elements
   ${array[*]}                                               Get All elements
   ${!array[!]}                                              Get All indexes
   ${#array[!]}                                              Array length
   array[0]=12                                               Add element to array at first position.i.e index=0
   array[-1]=22                                              Add element to array at last position.
   array+=(11)                                               Append value to an array
   ${array[@]:k:i} Get index=1 element starting from index=k


Bash - expansion
====================================================================================================

   Bash scripting tutorial on use parameter expansion in.

   This tutorial explains how to write a batch scripts in shell script and executes it.

   commands are input to OS to make a system calls and does actions. user input commands in the terminal to do operation such as ls, cd, mkdir etc.

   Another way, Multiple commands can be placed in a file, bash interpreter read the commands and run

   How to write a shell script in bash
     * Choose Editor or text editor
     * Create a file with extension .sh or .bash
     * Write commands in a file
     * Save the file as hello.sh

#!/bin/bash
echo "Hello World"

   Change the permission to execute the file
chmod +x hello.sh


Bash - Condional Expression
====================================================================================================

   Bash scripting tutorial on programming examples for concatenating string variables using simple append and shorthand arithmetic operator.

   Conditional expressions are evaluated at script execution time, based on results, It executes speci fic command blocks.

   thes

   There are different types of conitional expressions in Bash
     * String Comparision Operators
     * Numric Comparision Operator
     * File Operators
     * Logical Operators

File Operators

   Bash provides logical operators on FIle and directories to test condtional expressions. It allows you to check different operations such as existence, and permissions,
   size. It is used conditional expressions in conditional statements such as if else and case.

   Syntax:
if [[ conditiona_expressions]]; then
# code to handle
fi

   conditiona_expressions contains options, and filepath, that always returns true or false.

   following are options provided
      Operator                                       Description
   -e file         Returns true if given file exists, file can be normal file or directory
   -f file         Returns true if given file exists and a file(not directory)
   -d file         Returns true if file is an directory
   -r file         Returns true if file exists and has readable permission
   -w file         Returns true if file exists and has writable permission
   -x file         Returns true if file exists and has executable permission
   -s file         Returns true if file exists and size is not empty
   -G file         Returns true if file exists and isowned by a Group ID that matches
   -O file         Returns true if file exists and owned by a user ID that matches
   -N file         Returns true if file exists and modified by last read date
   -L file         Returns true if file exists and and is an symbolic Link
   file1 -ot file2 Returns true if file1 is older than file2 or file2 exists, file1 does not exist
   file1 -ne file2 Returns true if file1 is newer than file2,file1 exists, file2 does not exists
   file1 -ef file2 Returns true if file1 and file2 pointed to same device and inode


Bash - Case Statements
====================================================================================================

   Learn Case patterns in Bash Script Programming and tutorials..

   case statement is similar to switch case in other programming langauges.

   It is used to compaer the given input with a multiple patterns, and commands inside a matching pattern are executed.

   Syntax
case expression in

pattern1)
  ## Commands
  ;;
pattern1)
  ## Commands
  ;;
*)
  ## Default case to execute if none of the pattern is matched
  ;;

     * expression is an variable or valid expression to evaluate
     * It contains patterns defiend inside case which is evaluated with comparing expressions, matching case fuound, it executes commands inside it.
     * default case(*)) to execute if none of the pattern is matched
     * Each block of a pattern ends with a ;;
     * case is a beginning word and esac is a word terminate the case statement

   Here is an example
name="john1"

case $name in
    "john")
        echo "John."
        ;;
    "others" | "others2")
        echo "Other names."
        ;;

    *)
        echo "Default Name."
        ;;
esac


Bash - Special Characters
====================================================================================================

   Learn Special Characters in Bash Script Programming and tutorials..

   Special characters in bash evaluated with special meaning in interpretion of a command. These characters has special instructions, on using this characters has different
   meanings on different contexts

   Let’s expore supported special characters in bash scripting

Blankspace(" “):

   These also called whitespaces, contains tab,spaces,return, new line. These tells bash interepreter to separate the commands and content. It is an delimeter to separate a
   commands as well as strings.
echo "Hello World"

   The above example, echo is an command followed by a space, and strign contains spaces for words

Expansion($)

   Dollar sign symbol used for different types of expansions parameter expansion, ($variable, ${variable}) Substitution ($(expression)) arthematic expressions
   ($((expression)))

Ambersand (&)

   Adding & to an end of a command allows you to execute the command in the background.
command $

   For example, To run redis server in the background use, below command
redis-server &

Pipe (|)

   It is used to pass the output of one command to an input to another command from left to right. It allows to fomr a chain of commands

   Syntax is command1 | command2

   Example : echo "hello" | wc returns the character count.

Semicolon(;)

   It is used to separate a multiple commands using ; in a single line.; is an command separator to define multiple commands in a single line Syntax: command1;
   command2;command3

   Example: cd /app/;ls;

Single quotes

   Single quotes (') are used to define a string without a special meaning. It means all the variables and expansion are not interepreted and print the same literal string
name="Eric"

echo "Hi, $name"  # Hi, Eric
echo 'Hello, $name'  # Hi $name

   Example, First echo, name variable is expanded and interpreted as string and printed. Second echo, uses single quotes, and name variable is not expanded and printed as
   literal string.

   If single quote contains an nested single quoe, You need to escape it using ```.
echo 'test string with nested single quote: '\''Hello'\'''

   Examle contains ``` is an single quote character inside a single quote

Double quotes

   Double quotes (') are used to define a string literals with a special meaning.

   if string contains a variables and expansion syntax, These are intereprested and expanded, with a evaluated value at runtime.

   if string don’t want to expand their variables, then you can escape \ before $ dollar symbol
name="Eric"

echo "Hi, $name"  # Hi, Eric
echo "Hi, \$name"  # Hi, $name


   Example, First echo, name variable is expanded and interpreted as string and printed. Second echo, $ prefixed escape character \, printed as literal string.

Backslash Character(\)

   Backslash character used to escape the characters in strings. it is used in double quoted strings.

   For example
echo escape $$ example # escape 3225 example
echo escape \$$ example # escape $$ example

   In the first example, echo contains $$, which displays process id. In the second example, echo contains \$$, which displays $$ as literal string. escape character
   prefixed.

Comment ( #)

   Comments symbole used to comment out a line of code. Line of comment always starts with #.

   It is ignored by bash interpreter.
# Line comment
echo "comment example" # Inline comment

Question Mark(?)

   question mark has a different meanings in context.
     * In regular expression context
     * In

   check the exit status of a last command execution.

Dot (.)


Bash - if elif else
====================================================================================================

   Learn the essentials of Bash scripting with a tutorial on implementing decision-based condition execution using if, elif, and else statements..

   Bash scripting provides conditional expressions to execute different code based on specified conditions.

Bash Shell Conditional Statements

   At times, you may need to execute diverse code blocks depending on various decisions based on a given condition.

   Bash scripting facilitates this through conditional statements
  if condition; then
     # true code
  elif another_condition; then
     # condition is false, and another_condition is true
  else
     # none of the above conditions are true
  fi

     * The if statement is utilized to execute a code block if a condition is true, with the syntax if then fi.
     * The else statement is employed to execute code if a condition is false, following the syntax if then else fi.
     * The if..elif..else statement comes in handy when you need to execute code if none of the preceding conditions are true. The syntax is as follows:

   Notes:
     * A condition is an expression that evaluates to true or false in shell scripting.
     * A space is required before and after [ and ].
     * A semicolon before then is required.
     * if, else, then, elif, fi are reserved words in Bash.
     * A condition is an expression with a command.
          + A command containing single brackets syntax, the syntax example [expression] and is used for file string operations.
          + Double bracket syntax, example is [[expression]], which is employed for combining multiple conditions and handling regex patterns.
          + Double parentheses, syntax example is ((expression)), used for arithmetic operations.

If Conditional Statements

   The if statement in Bash is used to execute a code block when a specified condition is true.
if [ condition ]; then
   # Execute code block if the condition is true
fi

   In the above syntax:
     * Replace [ condition ] with the conditional expression.
     * The code block within the if statement is executed only if the specified condition evaluates to true.
     * Every if statement must conclude with fi.

   Example
age=10
if [ $age -lt 50 ]; then
   echo "$age is less than 50"
fi


   Output
10 is less than 50

If-Else Conditional Statements

   The if-else conditional statements in Bash allow you to execute different code blocks depending on whether a condition is true or false.
if [ condition ]; then
   # Execute code block if the condition is true
else
   # Execute code block if the condition is false
fi

   In the above syntax:
     * Replace [ condition ] with the expression to test.
     * The code block within the if statement is executed if the specified condition is true.
     * The code block within the else statement is executed if the condition is false.
     * Every if-else statement must conclude with fi.

#! /bin/sh
age=25
if [[ $age -gt 60 ]]; then
     echo "Senior Citizen"
else
     echo "Not Senior Citizen"
fi

   In this example, if the age is greater than 60, it outputs “Senior Citizen”; otherwise, it outputs “Not Senior Citizen.”

If..Elif..Else Statements

   Use if..elif..else conditional statements in Bash to execute different code blocks based on multiple conditions.
if [ condition1 ]; then
        # Execute code if condition1 is true
elif [ condition2 ]; then
        # Execute code if condition1 is false and condition2 is true
else
        # Execute code if both condition1 and condition2 are false
fi

     * The code block within the first if statement is executed if condition1 is true.
     * The code block within the first elif statement is executed if condition1 is false and condition2 is true``.
     * The else block is executed if both condition1 and condition2 are false.
     * Every if..elif..else statement must conclude with fi.

   Example
age=25
if [[ $age -gt 60 ]]; then
    echo "Senior Citizen"
elif [[ $age -lt 14 ]]; then
    echo "Child"
else
    echo "Adult"
fi

   In this example, the script checks if the age is greater than 60, less than 14, or falls into neither category, and it outputs the corresponding message.


Bash - Loops

   Bash scripting tutorial on supported loops with examples for, for index until, while loop.

   Loops are used to execute code block for a number of times.

   Suppose you want to execute commands repeately or prins an array.for loop is used in Bash

   Different types of loops

   Bash script provides multiple types of loops
     * for loop
     * for index loop
     * while loop
     * until loop

for loop

   for loop used to execute code multipe times based on
for element in [list]
do
 ## Code block
done

   The example iterates the list and prints to console.
for element in 1 2 3 4 5
do
  echo $element
done

for index loop

   for index loop is similar to C language for index loop It executes code multiple times based on condition is true, It starts with initial value and iteration contails the
   value to be incremented by 1.
for (( assignment; condition; iteration )); do
  # code block
done

   example
for ((i=0;i<5;i++));do
 echo $i
done

   It prints the numbers from 0 to 5

while loop in bash

   The while loop in Bash allows for the repeated execution of code as long as a specified condition is true. If the condition becomes false, the loop exits.

   The basic structure of a while loop is as follows:
while [ condition ]; do
  # code block
done

   Example:
i=0
while [[ i -lt 100 ]]; do
  echo "$i"
  i=$((i+1))
done

   while loop executes code as long as the specified condition([[ i -lt 100 ]]) is true.

   Code block increments the value by 1 and prints the value.

   if condition is false, loop exits.

Until loop in bash

   The until keyword in Bash is employed to repeatedly execute code until a specified condition becomes true, at which point the loop exits.

   The basic structure of an until loop is as follows.
until [ condition ]; do
  # code block
done

   until keyword is used in Bash and ends with done.
i=0
until [[ i -eq 100 ]];
do
 echo "$i"
 i=$((i+1))
done

   In the example, The code block executes as long as [[ i -eq 100 ]] is false. It increments the value by 1 and prints the value.

   output prints the numbers from 0 to 99 numbers


Bash - Append String
====================================================================================================

   Bash scripting tutorial on programming examples for concatenating string variables using simple append and shorthand arithmetic operator.

   This tutorial explains about arthematic expersssions and expansion.

Bash Athematic expressions

   Arthematic expressions are used to perform mathematical operations

   expressions is a term used in math to denote an operation. It contains operands and operatorto perform mathematical operatoions. a<b is an expression. It may contains
   binary or unary operators

   In bash, Experssions are created using (()) brackets with the operands and operators as arguments. ((a)) is an bash expression.

   Syntax is
((expression))

   expression are mathematical operations, It can contains sub expressions separated by comma.
result=$((12 + 11))
echo "$result"

   Arthematic expressions are constructed using below operators
     * Arthmatic Operators
     * Comparision Operator

   Comparision operators used to check one with another by comparing values The operators (<, <=, >, >=, ==, !=)
a=10
b=2
if ((a > b)); then
    echo "a is greater than b"
fi

     * Logical Operators
     * Conditional Statements

Bash Athematic Expansion

   Expansion are same as expressions, It calculates the value of an expression and result is replaced with an value. It always prefixed by dollar sign.
$((expression))

   For example, calculate the average of two numbers, print the result. Here used expansion syntax, It evaluates expression and result is replaced with an output of
   expression.
first=12
second=2
echo "The average is $(((first+second)/2))".

When to Use arthematic expressions and expansions

   [[]] is used to test an expression, returns boolean value(1 or 0). This will be used in the following cases
     *
     * Use artheatic expression
          + To perform mathematicl operations and comparision
     * Use artheatic expression [[]
          + Comparision of strings and numbers
          + Check file or directory exists


Bash - Functions

   Bash scripting tutorial on programming examples for concatenating string variables using simple append and shorthand arithmetic operator.

   Functions are reusable code that can be grouped under a single name.

   Declare a function Calling a Function Function with arguments Variable scope in Functions

How to declare a Function and call it

   Function defination contains the multiple lines of code to executes.

   Functions contains a name of a function enclosed in {}.

   It Can be declare in two ways
function function_name {
# Commands or valid bash code
# multiple lines
}

function function_name() {
  # Commands or valid bash code
  # multiple lines
}

How to pass a parameters to an function

function_name "parameter1" "parameter2" "parameter3".. "parametern"

   Parameters can be access using $1 $2 $3.. $n
function function_name() {
# $1 represents first paramter
# $2 represents second paramter

# $n represents nth paramter
}

Variable scope inside a function


Bash - Append String
====================================================================================================

   Learn Bash scripting with practical examples of how to concatenate string variables using both simple append and shorthand arithmetic operators..

   In this tutorial, you will learn various methods for appending string variables in Bash.

   There are multiple ways to append one string to another.

Simple variable append

   Begin by declaring two string variables in the Bash script, which can be printed to the console using echo by enclosing the variables in double-quotes.
string1="Hello, "
string2='Welcome to w3schools.'
echo "$string1 $string2 "

   You can also append without double quotes
echo $string1 $string2

   Output:
Hello, Welcome to w3schools.

   Another example involves concatenating a string to the same variable and printing it to the console:
result="My site is"
result="${result} w3schools"
echo "${result}"

   Output:
My site is w3schools

   This approach has pros and cons
     * It is simple and easy to append strings.
     * If you are appending multiple variables, it is less in terms of readability.
     * Understanding the syntax may be initially difficult for new Bash users.

Use Shorthand Arithmetic Operator

   The shorthand arithmetic operator (+=) is commonly used in arithmetic to add a value to a variable. It can also be used for strings to append a string to a variable.

   For example.
     * a+=1 is equivalent to a=a+1 in the case of numbers.
     * str+="test" will become str=str+"test" in the case of strings.

   Here is an example code
nums="One Two"
nums+=" three"
echo "$nums"

   Output:
One Two three

   Notes:
     * Easy to append strings and readable, as arithmetic operators exist in every language.
     * Not recommended and inefficient for larger strings.

Use printf command

   printf is used to format strings with various complex formatting options. We can use the printf command to concatenate strings. The format is %s%s, which appends two
   string variables.
str1="Hello, "
str2='Welcome to w3schools.'
output=$(printf "%s%s" "$str1" "$str2")

echo $output

   Notes: It is not easy to understand the printf with formatted options
     * Not easy to understand with the printf formatted options.
     * Not recommended and inefficient for string append.
     * Less readable for developers.

Using here string

   Here strings are a special syntax to pass a string to a command in a Bash script. They are used to pass an input string without using other sources, such as files. It
   allows passing a string to any Bash command from a file or a command line.

   Syntax:
command <<< string

   command: valid command <<<: is a here string operator

   string is the input string

   Here is an example
first="first "
second" second"
output="$first$(<<<" $second")"
echo $output

   In this example, the second string is appended to the first string using the here string operator.

   Notes:
     * Another way to append strings simply.
     * This approach is useful for passing strings to commands from a file or the command line only, even though it serves to append a string, it is less readable.

Conclusion

   In this tutorial, you have learned how to concatenate string variables in multiple ways.
     * Simple variable append and arithmetic operator(+=) are utilized for basic and straightforward string concatenation.
     * If you require more complex string processing alongside concatenation, printf is recommended.


Bash - Operators
====================================================================================================

   Supported Operators in Bash scripting programming examples for Logical, Bitwise, Comparision arithmetic operator.

   What is an operator?

   Operator is an symbol in programming that performsn an operation on operands

   Syntax
operand1 operator operand2

   There are two types of operators.
     * Binary Operator: It operates on two operands such as addition, subtraction, multiplication, division, and modulus
     * unary operator: It operats on single operand such as increment and decrement

Bash Arithmetic Operators

   Arithmetic operators in Bash provide arithmetic operations such as add, division, subtraction, and division multiplication operators.
   Operator     Title                         Description                       Example
   +        Addition       addition of two or more operands                  p+q=50
   -        Subtraction    subtraction of two or more operands               q-p=10
   *        Multiplication multiplication of two or more operands            p*q=600
   /        Divide         results quotient after the division of values     q/p=1.5
   %        Modulus        Return the remainder after the division of values q%p=10
   %        Modulus        Return the remainder after the division of values q%p=10
   -expr    Unary Minus    reverse of an expression                          -(10-7) is -3
   ~/       Division Int   returns division int value                        (10~/7) is 1
   ++       Increment      Increment the value by 1                          ++p=21
   --       Decrement      Decrement the value by 1                          --q=29

   Here is an arithmetic operator example

Assignment Operators

   Assignment operators are used to assign the values to an variables. The basic operation is equal(=)

   Additionally, THere are other assignment operators.

   for example, p is 20
        Operation      Symbol               Description                     Result
   Add Assign          +=     Addition and assignment to variable      ((p += 3)) is 23
   Substract Assign    -=     Substract and assignment to variable     ((p -= 3)) is 17
   Multiplicate Assign *=     Multiplicatie and assignment to variable ((p *= 2)) is 40
   Division Assign     /=     Addition and assignment to variable      ((p /= 5)) is 4

Bitwise Operators

     Operation    Symbol               Description                     Result
   AND            &      Bitwise AND of two operands              $op1 & $op2 is 0
   AND Equal      &=     Bitwise AND Equal of two operands        $op1 & $op2 is 0
   OR             |      Bitwise OR of two operands               $op1 | $op2 is 7
   XOR            ^      Bitwise XOR of two operands              $op1 ^ $op2 is 7
   Left Shift     <<     Bitwise Left Shift of two operands       $op1 & $op2 is 0
   Left Shift Eql <<=    Bitwise Left Shift Equal of two operands $op1| $op2 is 7
   XOR            ^      Bitwise XOR of two operands              $op1 ^ $op2 is 7
   XOR            ^=     Bitwise XOR Equal of two operands        $op1 ^ $op2 is 7

Logical operators

   These operators are used to perform logicaton operations on variables/expressions/data.
    Operation  Symbol                                           Description                                                  Result
   Logical AND &&     Return true(exit status=0) if both operands are true, else return false(exit status is non zero) $op1 &&& $op2 is 0
   Logical OR  ||     Logical OR of two operands                                                                       $op1 & $op2 is 0
   Logical NOT \!     Reverse the conditional value.                                                                   $op1 s 7

   Here is an example

String Comparision Operator

   Operation                  Description
   -z String  Return true if string is empty, else false.
   -n String  REturn true, If string is not empty
   str1=str2  return true, if str1 and str2 are equal
   str1!=str2 return true, if str1 and str2 are not equal
   str1>str2  return true, if str1 sorts before str2
   str1<str2  return true, if str1 sorts after str2

Numerical Comparision Operator

   Following are Comparison operator.

   used the -eq operator in the if fi conditional statement
first=13
second=15

if [ "$first" -eq "$second" ]; then
  echo "Two numbers are equal";
fi

   | Operation | Name | Description | | :——– | —————— | | -eq | equal | Check if two variables are equal | | -ne | Not equal | Check if two variables are not equal | | -lt |
   Less than | Check if first variable is less than second variable | | -le | Less than equal | Check first variable is less than equal to second variable | | -gt | greater
   than | Check if frst variable is greater than second variable | | -ge | greater than equal | Check if first variable is greater than equal to second variable |

   Here is an example
p=11
q=5

if [ "$p" -ne "$q" ]; then
  echo "Two numbers are not equal";
fi

Other operators

    Operation                               Description
   -v variable Returns true if an variable set an value, means value is assigned
   -o optname  Returns true if an shell optname is enabled
   -R variable Returns true if an variable set an value, and it is a named reference


Bash - Numbers Comparison
====================================================================================================

   Bash scripting tutorials on programming examples for if two numbers are equal or not.

   This tutorial explains comparing two numbers for equal or not in bash and shell programming.

   Numbers can be integers or floating numbers.

How to Check if two numbers are equal or not in Bash

   This program takes input values and checks if two values are the same or not.
first=13
second=15
if (( first == second )); then
  echo "Two numbers are equal";
fi

   Some shell scripts do not support (()), use [[]] with Comparison operators

   Following are Comparison operator.
     * -eq: equal
          + Check if two variables are equal
     * -ne : Not equal
          + Check if two variables are not equal
     * -lt : Less than
          + Check if first variable is less than second variable
     * -le : Less than equal
          + Check if first variable is less than equal to second variable
     * -gt : Greater than
          + Check, if first variable is greater than second variable
     * -ge: Greater than or equal
          + Compare Check if first variable is greater than equal to second variable

   used the -eq operator in the if fi conditional statement
first=13
second=15

if [ "$first" -eq "$second" ]; then
  echo "Two numbers are equal";
fi

   You can also do it with the ternary operator.
first=13
second=15

echo $(( first == second ?  "equal": "not equal" ))


Bash - Check Directory
====================================================================================================

   Learn how to check if a directory exists, create it if not, and manage permissions in Bash scripting..

   In this tutorial, we’ll explore how to determine whether a directory exists in a Bash script.

Bash scripting Check if the directory exists

   In the examples below, the if block is used to test the conditional expression for the existence of a directory.

Check if a Directory Exists and Print a Message

   The conditional expression uses the -d option to check if the directory exists.

   -check directory exists and prints the message. The conditional expression contains the -d option and directory path. -d option which checks for a directory exists or not.

   Here is an example
FOLDER=test

if [ -d "$FOLDER" ]; then
    echo "Directory Exists"
fi

   Please note that add space after [ and before -d.
     * How to mkdir only if a directory does not already exist?

   In this example, using an if-else conditional block.
     * Checked if the directory exists using -d.
     * else block will have a code for not existing and create a directory using the directory path

   Here is a code
FOLDER=test

if [ -d "$FOLDER" ]; then
    echo "Directory Exists"
else
    echo "Directory Exists"
    mkdir "$FOLDER"
fi

   -Check directory exists using ternary syntax

   Alternatively, the ternary conditional expression is used in place of if conditional expressions.

   Here is an example conditional expression
FOLDER=test
[ -d "$folder" ] && echo "folder exists" || echo "folder not exist"

     * Check if multiple directories exist Sometimes, We want to check that multiple directories exist.

   We have to use if conditional statements with logical AND operator(&&).
FOLDER1=test1
FOLDER2=test2

if [ -d "$FOLDER1" ] && [ -d "$FOLDER2" ] then
   echo "Folder1 and Folder2 exist"
fi

   – Check directory exists and is writable and executable

   In this example, Code checks for the below things
     * folder exists or not
     * if exists, the Folder has permission to write and executable.
     * Finally, Print a string message

FOLDER=test

if [ -d "$FOLDER" -a -w -x "$FOLDER" ]
then
    echo "Directory exists, writable and executable"
fi

   – Check files or directory exists

   Sometimes, we want to check if the file or directory exists. -e option checks for file or directory for the given path exists or not.
FILE_DIRECTORY=test
if [ -e "$FILE_DIRECTORY" ]
then
    echo "file or directory of test exists."
fi


Bash - File Name
====================================================================================================

   Bash or shell script example for extracting file names from complete paths, obtaining filenames without extensions, and extracting file extensions..

   In a Bash script, you may need to extract the filename and extension and save them into variables. This tutorial shows how to achieve this for a given file.

   This article covers
     * Extract the file name for a given full path
     * getting the full file name
     * Extension name

Extract file name with extension

   To obtain the filename along with its extension, the basename command can be used to remove the directory and return only the filename for the given path, whether it is a
   variable or a string.

   For instance, if the path is /home/john/run.sh, the returned filename would be run.sh. This process involves taking the full path and extracting only the filename by
   removing the path. The resulting filename is then stored in a variable and printed to the console.

   the basename is used to remove the directory and return the filename for the given path. the path is variable or string. For example, the path is /home/john/run.sh, and
   returned file name is run.sh In this, the full path is given and returns the filename by removing the path.

   The filename is saved to a variable and printed to the console.
file_path="/home/john/run.sh"
filename=$(basename "$file_path")
echo $filename

   output:
run.sh

Extract filename without extension

   To obtain only the filename without the extension, you can utilize the ${filename%.*} syntax.

   For example, consider the path /home/john/run.sh the resulting filename would be run.sh.

   Initially, the basename command is employed to eliminate the directory and yield the filename for the specified path and returns a variable , and this variable is then
   utilized in conjunction with the expression syntax to strip off the extension from the filename.

   This expression effectively removes the extension from the filename.
filepath="/home/john/run.sh"
filename=$(basename "$filepath")
echo $filename
echo "File Name: ${filename%.*}"

   output:
File Name: run

Extract extension for a file path

   To isolate the file extension from the given file path, ${filename##*.} can be utilized. This expression returns the file extension only.

   For example, consider the path /home/john/run.sh the resulting extension would be sh.

   Initially, the basename command is used to remove the directory path and return the filename for the specified path, and this filename is then utilized in conjunction with
   the expression syntax to return only the extension.
filepath="/home/john/run.sh"
filename=$(basename "$filepath")
echo $filename
echo "File Extension: ${filename##*.}"

   Output
File Extension:sh

   Here is a comprehensive example that showcases how to obtain a filename with or without a file extension. Upon executing the script below, it prints
     * the file with an extension,
     * only the file name without an extension,
     * and the extension alone.

   Example:
filepath="/home/username/run.sh"
filename=$(basename "$filepath")
echo "File :$filename"
echo "File Name: ${filename%.*}"
echo "File Extension: ${filename##*.}"

   Output
File:run.sh
File Name:run
File Extension:sh

Conclusion

   This script shows a multiple approach for handling filenames and extensions in a Bash environment.


Bash - Split String
====================================================================================================

   Explore Bash or shell script examples for splitting strings into multiple strings using awk and IFS..

   In some cases, when working with a bash script, there arises a need to separate strings based on a delimiter and extract multiple strings for further processing or storage
   in variables.

   This tutorial guides you through string splitting with examples in bash shell programming.

   This article covers three methods.
     * Split a string using awk command
     * Use IFS variable
     * Parameter Expansion with for loop

Split a string using the awk command in a bash shell script

   The awk command, a Linux utility compatible with all bash and shell distributions, is used to split a string based on a specified delimiter.

   The input is provided using the pipe (|) symbol, and the example below demonstrates splitting a string containing colons (:)
str="abc:def:ghi"
echo "$str" | awk -F':' '{print $1,$2,$3}'

   output:
abc def ghi

split using IFS variable

   Here, the input string consists of elements separated by hyphens. The shell variable IFS (Internal Field Separator) is set to a hyphen, and the string is iterated using a
   for loop.

   Each element is printed after removing the hyphen.
input="one-two-three"
IFS='-' array=($input)
for element in "${array[@]}";
do
 echo $element;
 done

   Output

   output:
one
two
three

Use Parameter expansion and loop

   Parameter expansion is employed to change the variable value based on specified options. In this case, a string variable is converted to an array. The array is then
   iterated using a for loop syntax, printing each element to the console:
msg="Welcome to my site."

array=($msg)


for item in "${array[@]}"; do
    echo "$item"
done

   Different ways to handle string manipulations, allowing you to choose the method that suits your needs.


Bash - String Length
====================================================================================================

   Example of Bash or shell script for finding the length of a string with practical examples..

   The length of a string is determined by the number of characters it contains, and it is generally straightforward to ascertain this length for normal text.

   This post will explore various methods for counting the number of characters in a string with UTF encoding.
    1. Using ${#variable} Syntax

   The first method involves utilizing the ${#variable} syntax to obtain the length of a string variable.

   In this, number of characters in the string variable.
msg="Hello World."
count=${#msg}

echo $count # 12

   Output:
12

    2. Using wc -m Command

   The second method involves the use of the wc -m command, either directly with a string or through a variable.
echo -n 'Hello World.' | wc -m; # 12

result=`echo -n $msg | wc -m`
echo $result # 12

result1=`echo  $msg | wc -m`
echo $result1 # 12

   In this example, echo -n "string" is employed to print the string without a new line (-n option). The | pipe operator directs the output of the left-side command to the
   right-side command, and wc -m counts the number of characters in a string.

   Output:
12
12
12

    3. Using the expr Command Another method involves using the expr command to find the length of a string.

msg="Hello World."

count=$(expr length "$msg")

echo $count # 12

   Here, ${} represents expression substitution, substituting the value of an expression into the string. expr executes expressions, and length is an argument passed to expr
   to find the string’s length.

   $(expr length "$msg") returns the count of a characters in a string, assigned to an variable, variable value printed to console
     * using awk command Awk provides yet another way to calculate the length of a string using expressions.

msg="Hello World."
count=${#msg}

echo $count # 12

count=$(echo -n "$msg" | awk '{print length}')
echo $count # 12

   In this case, echo -n "$variable" outputs the string without a new line, and the output is piped to awk using the pipe(|) symbol. The awk '{print length}' command
   calculates and prints the length of the input line.

   By combining the above methods in an expression ${}, you can obtain the length of a string.

Conclusion

   This post has covered multiple ways to determine the length of a string in Bash. The choice of approach can depend on your preferences.


Bash - bashrc
====================================================================================================

   Bash or shell script .bashrc reload and location example for Split string into multiple strings using awk and ifs examples.

   The .bashrc file is a bash script file that executes in the following cases
     * using bash script execution
     * bash shell is opened and started interactively

   This file is hidden by default because the file starts with . are hidden.

Where is the bashrc file in Linux?

   The .bashrc file is the script that runs while the user logs in. It is located in the user’s home directory.

   It contains environment variables and user preferences to be configured in this file.

How do I view a .bashrc file?

   You can use Vi or Nano editor to view the bashrc file.

   Following is a command
nano ~/.bashrc
nano ~/.bashrc

Where is the bashrc file located?

   bashrc file is located in two places
     * user home directory
     * System directory

   In the case of the user’s home directory, this file is hidden by default. The location is ~/.bashrc where ~ is a current user logged in the home directory.

   In the case of the System directory, this file is located at /etc/bash.bashrc.

How to reload .bashrc settings without logging out and back in again?

   if you make any changes to the .bashrc file, Changes are not reflected immediately. You need to close and restart the bash session to make changes.

   How to reload .bashrc configurations without login in again? Run the following commands in the command prompt.
source ~/.bashrc

   or shorter way using the below command
. ~/.bashrc

   or execute bash again using the below command
exec bash

   exec command reset the current session, and starts again.


Bash - Ternary Operator

   Bash or shell script example for Ternary Operator expression with examples.

   Bash programming does not have support for ternary operator syntax.

   Ternary operators are written in the Java language
expression?value1:value2

   The syntax is similar to the if and else conditional expression. if the expression is true, value1 is returned, else value2 is returned.

How to use ternary Operator in Bash

   There are several ways we can write syntax in place of ternary operator syntax.

   The first way, use if-else with expression syntax.
AGE=25
if [ "$AGE" -eq 25 ]; then result="true"; else result="false"; fi
echo "$result" ;

   The second way, use Arithmetic expressions using && and || Syntax is
[expression1] && Expression2|| Expression3

   if the expression1 is true, Expression2 is evaluated, else Expression3 is evaluated

   Here is an example program
[ $AGE == 25 ] && result="true" || result="false"
echo "$result" ;

   There is another way in which we can assign variables instead of expressions.

   using let we can assign variables based on condition expression result
first=10
second=true
third=false
let result="(first==10)?second:third"

echo $result # true


Bash - Lowercase
====================================================================================================

   Explore multiple methods to convert a string to lowercase in Bash and shell programming with this tutorial..

   This tutorial explains how to convert a string to lowercase in a Bash script.

   For example, if the input string is “Hello World Welcome,” the output will be “hello world welcome.”.

   There are multiple ways to achieve this, depending on the Bash type and version.
     * using the tr command

   The tr command, short for translator, is a Unix command used to convert characters from one format to another.

   The syntax is as follows:

   here is the syntax.
tr input_format output_format

   Here’s a bash shell script using tr to convert a string to lowercase:
message="Hello World, Welcome."
echo "$message" | tr '[:upper:]' '[:lower:]'

   Output:
hello world, welcome.

   Alternatively, you can use.
message="Hello World, Welcome."
echo "$message" | tr 'A-Z' 'a-z'

   Note: tr works with ASCII and does not support UTF characters.
     * using the awk command

   To convert a string to lowercase using the awk command, the tolower function is combined with awk.

   The result is then passed to the echo command using the pipe operator:
message="Hello World, Welcome"
echo "$message" | awk '{print tolower($0)}'

   This method is best for both ASCII and UTF characters.
     * Use Perl in Bash Script Printing lc with Perl converts a string to lowercase.

   lc is alias for lowercase.
echo "$message" | perl -ne 'print lc'

     * use Parameter expansion Bash 4.0 introduced inbuilt string manipulation utilities. To convert a string to lowercase, simply add two commas to the string. This is also
       called parameter expansion syntax.

   The syntax is ${variable[options]}.

   For example,
message="Hello World, Welcome"
echo "${message,,}"
echo "${message}"

msg="Hello World."
result="${msg,,}"
echo $result # hello world.

   Here, ${msg,,} uses the ,, option to convert the variable to lowercase.

   Note that this works in Bash version 4.0 and above.


Bash - Uppercase

   Explore multiple methods to convert a string to uppercase in Bash and shell programming with this tutorial..

   This tutorial guides you through the process of converting a string to uppercase in bash and shell script.

   An uppercase string refers to a string containing all letters in uppercase.

   For instance, if the input string is “Hello World Welcome,” the output will be “HELLO WORLD WELCOME.”

   Depending on the bash type and version, there are multiple methods to convert a string to uppercase.
     * using the tr command

   The tr command, known as a translator, is a Unix command used to convert characters from one format to another.

   The syntax is as follows:
tr input_format output_format

   Here’s a shell script for converting to uppercase.
message="This is a lowercase string converted to uppercase"
echo "$message" | tr '[:lower:]' '[:upper:]'

   Output:
THIS IS A LOWERCASE STRING CONVERTED TO UPPERCASE

   Another way to replace the above code
message="This is a lowercase string converted to uppercase"
echo "$message" | tr 'a-z' 'A-Z'

   Note: tr works with ASCII and does not support UTF characters.
     * using the awk command

   To convert a string to uppercase using the awk command, the toupper function is combined with awk. The result is then passed to the echo command using the pipe operator:
message="It is a lowercase string converted to uppercase."
echo "$message" | awk '{print toupper($0)}'

   It is best that work with ASCII and UTF characters.
     * in bash 4.0 version bash 4.0 provides string inbuilt in manipulation utilities. Adding two circumflexes (^) to a string makes a string in the upper case string.

message="This is a lowercase string converted to uppercase"
echo "${message^^}"
echo "${message}"

     * using Perl in the bash script

   print uc command in Perl converts a string into upper-case letters
echo "$message" | perl -ne 'print uc'

     * Use parameter expansion syntax Bash 4.0 provides built-in string manipulation utilities. Adding two circumflexes (^) to a string makes it an uppercase string, also
       called parameter expansion syntax.

   Syntax is ${variable[options]}

   variable to modify based on options. Options are optional that contains parameter expansion
#!/bin/bash

# Input string
message="hello, world!"

# Convert to uppercase
result="${message^^}"

# Display the result
echo "Original: $message"
echo "Uppercase: $result"

   Parameter expansion syntax converts a string to uppercase. ${message^^} contains ^^ options to convert the variable message string into uppercase.

   This feature is available in Bash version 4.0 and above.


Bash - Substring
====================================================================================================

   Bash How to check a substring exists in a string using a comparison operator and regular expression example.

   In this tutorial, the Bash script is designed to determine whether a string contains a specific substring.

   There are multiple ways to perform this check, each outlined below for clarity.

Using the Comparison Operator to Check for Substring exists or not

     * Define a string variable containing the text.
     * Use if statements to compare the string with the desired substring using the equality operator (==) and a wildcard (*).
     * Finally, print the string if the substring is found.

mainstring='Welcome to w3schools'
if [[ $mainstring == *"w3schools"* ]]; then
  echo "w3schools exists in the main string"
fi

Use Regular Expressions to Find a Substring

   The =~ operator facilitates substring search within a given string, used within an if block.

   Example code:
mainstring='Welcome to w3schools'
if [[ $mainstring =~ "w3schools" ]]; then
  echo "w3schools exists in the main string"
fi

Use the grep command

   The grep command is employed to search for a specified string, piped to the main string for comparison.
mainstring='Welcome to w3schools'

if echo "$mainstring" | grep -q "w3schools"; then
  echo "w3schools exists in the main string"
fi

   These methods offer different approaches to checking if a string contains a particular substring, providing flexibility for different use cases.


Bash - variable set

   In this tutorial, Check variable is set or unset in bash and shell script files with code examples.

   This tutorial is about checking variables in bash shell script programming for
     * Check variable is set or not
     * variable is empty or nonempty
     * Check for the variable is an empty string or not

   Here are some examples of variable usage for variable price
   Variable declaration                  Description
   price;               variable is not declared and unset
   price=;              variable is declared and not unset
   price=3000           variable is declared, assigned, and not unset
   price=(3000)         variable is declared, assigned, and not unset
   unset price;         variable is not declared and unset

How to check if a variable is set in a bash script?

   For example, the variable is set means,
     * It is declared and assigned with empty or non-empty.

   In the below example,
     * variable1 is declared but empty
     * variable2 is not declared and not set.

#!/bin/bash
variable1=""
if [[  ${variable1+x}  ]]
then
  echo 'variable1 is set'
else
  echo 'variable1 is not set'
fi

if [[  ${variable2+x}  ]]
then
  echo 'variable2 is set'
else
  echo 'variable2 is not set'
fi


   Output:
variable1 is set
variable2 is not set

   Another way to check a variable is to set using the -v option
variable1=""
if [ ! -v $variable1 ]
then
    echo "variable1 is unset"
else
    echo "variable1 is set"

fi

   Output:
variable1 is unset

How to check if the variable is unset in the bash script?

   For example, the variable is unset means
     * It does not exist and is not declared.

   In the below example,
     * variable1 is declared but empty
     * variable2 is not declared and is unset.

#!/bin/bash
variable1=""
if [[ ! ${variable1+x}  ]]
then
  echo 'variable1 is not set'
else
  echo 'variable1 is set'
fi

if [[ ! ${variable2+x}  ]]
then
  echo 'variable2 is not set or unset'
else
  echo 'variable2 is  set'
fi

   Output:
variable1 is set
variable2 is not set or unset

How to check variable is empty or non-empty

   This tutorial checks for a check for variable compared with space and wraps this expression inside [[]].
variable1=""
if [[ $variable1 = "" ]]
then
  echo 'variable1 is empty'
else
  echo 'variable1 is not empty'
fi

   The same also can be written using a variable in double quotes wrapped in a single bracket[].
variable1=""
if [ "$variable1" = "" ]
    then
    echo "variable1 is empty"
else
  echo 'variable1 is not empty'
fi

   Output:
variable1 is empty

   Let us also check not to empty using! operator.

   Here is a code for example checks if a variable is non-empty.

   One way,
variable1=""
if [[ ! $variable1 = "" ]]
then
  echo 'variable1 is empty'
else
  echo 'variable1 is not empty'
fi

   Another way,
variable1=""
if [ ! "$variable1" = "" ]
then
  echo 'variable1 is empty'
else
  echo 'variable1 is not empty'
fi

   Another use -z option to check variable is set and empty or non-empty using the below code
variable2=""
variable3="test"

if [ -z "$variable2" ]
then
    echo "variable2 is set and empty"
else
    echo "variable2 is set and nonempty"
fi

if [ -z "$variable3" ]
then
    echo "variable3 is set and empty"
else
    echo "variable3 is set and non-empty"
fi

   Output:
variable2 is set and empty
variable3 is set and non-empty


Bash - Iterate Nos
====================================================================================================

   In this tutorial, Check variable is set or unset empty or non-empty with an empty string or not in bash and shell script files.

   This tutorial is about multiple ways to iterate the range of numbers stored in variables and print them to the console.

   In this example, Generate a sequence of a range of numbers and store it in a variable.

   Sometimes, We want to generate file names with names containing numbers generated from a sequence or range of numbers.

Generate a range of numbers in the bash script

     * using seq seq tool generates a sequence of numbers.

number=4
for k in $(seq 1 $number); do echo $k; done

   Output:
1
2
3
4

     * using for loop

number=5
for ((k=1;k<=number;k++)); do
    echo $k
done

   Output:
1
2
3
4
5

     * while loop

number=5
k=1 ;
while [[ $k -le $number ]] ; do
    echo $k
    ((k = k + 1))
done

Conclusion

   seq is better compared with for and while loop