Basic Knowledge of bash scripting

Introduction to Bash Scripting

Bash scripting is the process of writing scripts or programs using the Bash shell, a command-line interpreter found in Linux and other Unix systems. Bash scripting provides developers with a powerful scripting language f
or creating complex, multi-step programs or scripts. It also reduces complexity by shortening the syntax of commands as opposed to typing them out each time.

Bash scripting has a number of advantages over other scripting languages. It is relatively easy to learn, even for those who have no coding experience. This is because it is a command-based language and is easy to read and write. It is also flexible and can be used to solve a variety of tasks, most notably to automate processes or run programs.

In addition, Bash scripting is an open-source language, meaning that anyone can freely view the source code and can also make modifications and improvements as needed. This encourages innovation and development of the language while also making it cost-effective. It also integrates with other programming and scripting languages, making it easy to implement complex tasks without needing to learn the other language.

Use cases of Bash scripting include system administration, managing network services, automating disk, network, and environment backups, creating complex scripts, and a variety of other tasks.

The Bash shell is a command-line interpreter that can be used to communicate with the operating system. It interprets the user-entered commands, executes them, and interprets the output to the user. The Bash shell is an interactive feature that allows the user to get a list of available commands, execute those commands, and customize them according to their needs.

Getting Started with Bash

• Installation of Bash on different operating systems

• Windows: Windows 10 supports the Windows Subsystem for Linux (WSL) which allows users to install various Linux distributions directly from the Microsoft Store. To install Bash on Windows, users can search for “Ubuntu” or “Debian” in the Microsoft store. Once the installation process is complete, users can open Bash by searching for “Bash” in their Start menu.

• Mac OS X: The latest version of Mac OS X (10.15) includes the built-in Bash shell. To open it, users can launch the Terminal program from the Applications/Utilities folder.

• Linux: Most Linux distributions come with Bash installed by default. To open the Bash shell, users can launch the Terminal program from the menu bar.

2. Accessing the Bash shell

• On Windows: Bash can be opened from the Start menu by searching for “Bash”.

• On Mac OS X: Bash can be opened from the Terminal program located within the Applications/Utilities folder.

• On Linux: Bash can be opened from the menu bar, or from the terminal program.

3. Basic commands and navigation in Bash

• pwd: shows the current working directory

• cd: change the current working directory

• ls: list the contents of the current directory

• mkdir: create a new directory

• cp: copy files or directories

• mv: move files or directories

• rm: delete files or directories

• cat: display the contents of a file

• grep: search for patterns in text files

• exit: exit the shell.

Variables and Data Types

• Variables in Bash scripting are used to store data values so they can be changed and accessed by scripts and programs. There are several different types of data that can be stored in variables, including numbers, strings, arrays, and dictionaries.

• Number variables store numeric data, and can be used to perform mathematical operations.

• String variables store information in the form of text, which can be used to create messages and display information.

• Array variables store information in an indexed collection of data. These are useful for storing lists of items or performing iteration over a set of values.

• Dictionary variables store data in a keyword-value format, which allows for quick retrieval of data by providing a key.

• Variable assignment is the process of assigning an existing value to a variable. This is done using the equal sign (=). For example, the code:

my_variable=5

assigns the value 5 to the variable my_variable.

Variable manipulation is the process of altering the value associated with a variable. Common examples of manipulation include adding or subtracting values, changing the order of the elements in the variable, or appending new elements to the end of a list.

Conditional Statements

IF, ELSE, and ELIF Statements: IF, ELSE, and ELIF (ELSE IF) are programming statements in the Bash language (an acronym for “Bourne Again Shell”) that can be used for decision-making. An IF statement is a command that executes an action based on a certain condition. ELSE is used after an IF statement to execute an alternative action if the condition evaluated by the IF statement is false. ELIF (ELSE IF) is used when several different conditions need to be evaluated.

Comparison Operators and their Usage: Comparison operators are operators in Bash that allow you to compare two or more values or variables. Comparison operators are very important for decision-making and control flow. The most commonly used comparison operators are == (equal to), != (not equal to), > (greater than), < (less than), >= (greater than or equal to), and <= (less than or equal to).

Nesting and Logical Operators: Nesting is used when two or more IF, ELSE, or ELIF commands are used in conjunction. This can be used to create a hierarchy of conditions and commands that will be executed. Logical operators are used when forming nests and can be combined with comparison operators. Logical operators include && (AND operator), || (OR operator), and ! (NOT operator).

Loops in Bash

Looping is a fundamental functionality of any programming language. While there are numerous varieties of loop constructs, Bash supports three primary loop structures: for, while, and until.

The for loop lets you traverse a set of values one by one over a range of values. Syntax for the for loop construct is as follows:

for i in <list>
do
<commands>
done

The for loop construct must start with for, followed by a variable to hold the current item in the loop. This is followed by in a keyword and then a list of values. Each of the items in the list will be accessed one at a time and stored in the variable.

For example, the following loop prints out all the elements in an array one by one:

arr=(apple banana orange)
for i in "${arr[@]}"
do
echo "$i"
done

This will print out each of the array elements (apple, banana, orange) on a separate line.

While Loop:

The while loop runs a code block as long as the condition given is true. The syntax for the while loop is as follows:

while [ <condition> ]
do
<commands>
done

The while loop works similarly to an if statement, except the code block continues to loop until the condition returns false.

For example, the following loop will run until the count is greater than 10:

count=0
while [ $count -le 10 ]
do
echo "$count"
count=$((count+1))
done

This will print out numbers from 0 to 10 on separate lines.

Until Loop:

The until loop runs a code block until the condition given is true. The syntax for the until the loop is as follows:

until [ <condition> ]

do
<commands>
done

The until loop runs similarly to a while loop, but it keeps looping until the condition returns true. For example, the following loop will keep looping until count is equal to 10:

count=0
until [ $count -eq 10 ]
do
echo "$count"
count=$((count+1))
done