README.md

Bash-Scripting

A bash shell script is a plain text file containing a set of various commands that we usually type in the command line.It can be used to automate software development tasks such as code compilation,debugging source code,change management and software testing.

Bash-Bourne again shell

Getting started

Let's start and create a file named hello-world.sh and add this code

#!/bin/bash

echo "Hello world"

The first line of the script #!/bin/bash is called shebang.It tells the operating system which interpreter to use to run the script. It instructs the operating system to use bin/bash ,the Bash shell,passing it the script's path as an argument.

Example

/bin/bash hello-world.sh

The second line echo is a builtin command that writes the arguement it recieves to the standard output.

executing the script

The script can be executed in couple of ways:

./hello-world.sh

bash hello-world.sh

warning: To run the script through this method you need to give the permissions chmod +x hello-world.sh.

variables

A variable is a container for storing values.As the name indicates variables contain a value that can be changed or modified based on the condition or the information passed to the script.

Note: spaces cannot be used around the '=' assignment operator

example:

#!/bin/bash

#declaring the variable name
name="Rahul"

echo "$name"

#output-Rahul

#reassigning the value to the variable name

name="Supraa"

echo "#name"
#output-Supraa

The variables value can be accessed using $ or ${}.

example

#!/bin/bash

name="Rahul"

echo "$name"
#or echo "${name}".

Internal variables

Bash has builtin variables which can be really helpful when writing script.Any task like knowing the current working directory or knowing the hostname can be done using the internal variables.

example:

#!/bin/bash

# display the current working directory.
echo $PWD

# prints the hostname.
echo $HOSTNAME

# prints the path environment variables.
echo $PATH

comments

The comments are added for the readable explanation of the code. Anything written after the hash sign # is considered as comment by the interpreter.Any text written in this line will not be executed.

example:

#!/bin/bash

# this is a comment.
# this is also a comment.

read

The readcommand is used to take input from the user.The way input() is used in python or cin<< is used in c++.we can use read in bash scripting.

example:

#!/bin/bash

echo "enter your name:"
read name

#the user input will be stored in the name variable.

echo "Hello,$name"

Mathematical operations

Just like other programming languages we can perform operations on bash as well.

example:

#!/bin/bash

echo "$(( 2 + 4 ))"

Arithmetic Operators

• + - addition
• - - substraction
• * - multiplication
• % - modulus
• / - division

example:

#!/bin/bash

num1=4
num2=5

# addition

echo "$(( num1 + num2 ))"

# substraction

echo "$(( num1 - num2 ))"

# multiplication

echo "$(( num1 * num 2 ))"

# division

echo "$(( num1 / num2 ))"

Relational Operators

Relational operators allow to make comparisons between the operators.

• = - Assignment operator
• < or -lt - Less than
• <=or -le - Less than or equal to
• > or -gt - Greator than
• >=or -ge - Greator than or equal to
• !=- Not equal to

Logical operators

Logical operators are used to combine multiple conditional statements.The outcome is generally a boolean value i'e either true or false.

• && or -a- AND
• || or -o- OR
• ! - NOT

example

if [[ 1 -lt 5 && 2 -lt 5 ]];then
  echo "You are right"
else 
  echo "You seem to be incorrect"
fi

Conditional statements

The conditional statements like if and 'else' are used to make decisions based on the specified conditions.

example:

name="Rahul"

if [[ $name == "Rahul" ]];then
    echo "hello $name"
else
    echo "Invalid user!"
fi

Switch statement

Switch statement are used to perform tasks based on certain conditions. It consists of cases which decide the flow of the program.

#!/bin/bash


echo "enter a number from 1 to 3"
read number

case $number in
	1)
	   echo "You chose 1"
          ;;

	2)
          echo "You chose 2"
          ;;

	3)
          echo "You chose 3"
          ;;
        *)
	        echo "You did not pick a correct number"
          ;;
esac

Iterative statements(loops)

The loops are used to perform a particular task for a certain number of times. There are three types of loops in Bash and they are while,until and for loop.

While Loop

#!/bin/bash

num=$1

while [ $num -lt 5 ]
do
    echo "$num"
    (( num++ ))
done

For Loop

#!/bin/bash

for i in {1..5}
do
   echo $i
done

Until Loop

#!/bin/bash

until [[ $choice == "blue" ]]
do
	echo "Pick red or blue"
	read choice
done

echo "you chose blue"

NOTE: We can use break and continue statement to break out of loops based on certain conditions.

example:

#!/bin/bash

for i in {1..10}
do
    if [[ $i == 5 ]];then
       break;
    else
       echo "$i"
    fi
done

Arrays

Arrays are used to store multiple values.We can store multiple values in the same variable and access using the array index

example:To access the first element we can do arr[0].The index starts from 0.

#!/bin/bash

arr=("Rahul" "Peter" "Tony")

echo "The first element of the array is ${arr[0]}"

echo "The whole array is ${arr[@]}"

echo "The length of the array is ${#arr[@]}"

Functions

Functions are block of code which are used to perform particular task and break the entire code into small modules. Functions make the code more readable and understandable.

example:

func_add ()
{
   local x=$1 # 1st argument to the function
   local y=$2 # 2nd argument  to the function

   result=$(( x + y ))
}

# the function can be called like this

func_add 2 5

Debugging your file

• You can use bash -x before your filename to debug your file

example

• To debug a file named hello.sh you can run it like bash -x ./hello.sh
• You can also add pointers in your script to debug

example

#!/bin/bash

set -x
echo "enter your name"
read name 

for i in [1..5]
do
	echo $i

set -x

Pro Tip: Keep practicing to get good at it.

Checkout this cheatsheet for more.