Shell scripting

My Two-Week Journey Into Shell Scripting: Week 1 Highlights

I recently decided to take on a two-week challenge to learn shell scripting. It’s been one week so far, and I wanted to share what I’ve learned and accomplished. Here’s a quick recap of my experience so far.

Week 1: Getting Started with Shell Scripting

This first week was all about learning the basics and getting comfortable with the core concepts. Shell scripting might look simple, but it’s incredibly powerful for handling tasks efficiently. Here’s what I focused on:

1. Learning the Basics:

Tried out different shell environments like Bash.
Learned how to write and run basic shell scripts.
Figured out how to use variables, loops, and conditions in scripts.

2. Functions in Shell Scripting:

Learned how to break scripts into smaller, reusable parts using functions.
Practiced writing functions to keep my scripts clean and organized.
Added basic error-handling to make sure the scripts didn’t crash unexpectedly.

The Highlight: My First AWS Services Script

To test out what I learned, I made a simple shell script that lists AWS services available by region and service name. Here’s how it worked:

What the Script Does:
- It checks what AWS services are available in different regions.
- It shows the services in a clear, organized way.
How I Built It:
- Used AWS CLI commands to get the list of services.
- Processed the output to make it easy to read.
- Added options to let users pick specific regions or services.
Challenges I Faced:
- Figuring out how to work with JSON output from AWS CLI was tricky at first, but tools like jq made it easier.
- Making sure the script worked on different systems took some extra effort.
What I Learned:
- How to use loops and conditions in a real script.
- Combining shell scripting with other tools like AWS CLI can save a lot of time.

Additional Shell Scripting Concepts and Examples

Step-by-Step Guide to Creating and Running a Shell Script

Step 1: Create a script.sh file and use the nano command to edit it. There are also other ways to edit files, but I find nano is the easiest.

After executing these commands, you will enter the editor where we will write the bash script.

Step 2: First, we'll write shebang or hashbang, #! which is used to tell the kernel which interpreter should be used to run the bash script. This #!/bin/bash statement tells the kernel to interpret it as a bash script. Then we'll print "Hello World" as follows:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 12/01/2025
#Description : Basic shell scripting
#####################################
echo "Hello World"

Step 3: Now, to save this file, hit CTRL + X, then hit Y for yes, and finally press Enter to save the file.

Step 4: Finally, run bash script.sh to execute the file. You’ll see "Hello World" in the terminal.

Variables

Variables are used to store data that can be used in shell scripts. To create a variable, simply assign a value to it using the = sign.

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 12/01/2025
#Description : Basic shell scripting
#####################################
NAME="John"
echo "Hello, $NAME!"

The $ sign is used to refer to the value of a variable. When you include the $ sign before the variable name, the shell interpreter replaces it with the value of the variable.

Arithmetic Operators

These operators are used to perform mathematical operations on numeric values.

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 13/01/2025
#Description : Basic shell scripting arithmetic operators
#####################################

# declare two variables
a=10
b=5

# perform arithmetic operations
sum=$((a + b))
difference=$((a - b))
product=$((a * b))
quotient=$((a / b))
remainder=$((a % b))
power=$((a ** b))

# print the results
echo "sum: $sum"
echo "difference: $difference"
echo "product: $product"
echo "quotient: $quotient"
echo "remainder: $remainder"
echo "power: $power"

Comparison Operators

These operators are used to compare values and return a Boolean value (true or false).

#!/bin/bash
# declare two variables
####################################
#Author : Parthasarathy G
#Date : 13/01/2025
#Description : Basic shell scripting compare operators
#####################################
a=10
b=5

# compare the values of the variables
if [ $a -eq $b ]
then
  echo "a is equal to b"
else
  echo "a is not equal to b"
fi

if [ $a -gt $b ]
then
  echo "a is greater than b"
else
  echo "a is not greater than b"
fi

if [ $a -lt $b ]
then
  echo "a is less than b"
else
  echo "a is not less than b"
fi

Logical Operators

These operators combine multiple conditions.

#!/bin/bash
# declare two variables
####################################
#Author : Parthasarathy G
#Date : 13/01/2025
#Description : Basic shell scripting logical operatores
#####################################
a=10
b=5

# check if a is greater than b AND less than 20
if [ $a -gt $b ] && [ $a -lt 20 ]
then
  echo "a is greater than b and less than 20"
else
  echo "a is not greater than b or not less than 20"
fi

# check if a is less than b OR greater than 20
if [ $a -lt $b ] || [ $a -gt 20 ]
then
  echo "a is less than b or greater than 20"
else
  echo "a is not less than b and not greater than 20"
fi

# check if a is NOT equal to b
if [ ! $a -eq $b ]
then
  echo "a is not equal to b"
else
  echo "a is equal to b"
fi

String Operators

These operators compare two strings.

#!/bin/bash
# declare two variables
####################################
#Author : Parthasarathy G
#Date : 14/01/2025
#Description : Basic shell scripting string operators
#####################################
name1="Rahul"
name2="Rohit"

# compare the values of the variables
if [ $name1 = $name2 ]
then
  echo "The names are the same"
else
  echo "The names are different"
fi

if [ $name1 != $name2 ]
then
  echo "The names are different"
else
  echo "The names are the same"
fi

Conditionals

If-Else Statement:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 14/01/2025
#Description : Basic shell scripting if-else statement
#####################################

num=15
# check if the number is greater than 10
if [ $num -gt 10 ]
then
   echo "The number is greater than 10"
else
   echo "The number is less than 10"
fi

Case Statement:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 14/01/2025
#Description : Basic shell scripting case statement
#####################################

fruit="apple"
# check the value of the variable
case $fruit in
  apple)
    echo "This is an apple"
    ;;
  banana)
    echo "This is a banana"
    ;;
  *)
    echo "I don't know what this is"
    ;;
esac

Loops

For Loop:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 15/01/2025
#Description : Basic shell scripting for loop
#####################################

# use a for loop to print the numbers 1 to 5

for i in 1 2 3 4 5
do
  echo $i
done

While Loop:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 15/01/2025
#Description : Basic shell scripting while loop
#####################################

# use a while loop to print the numbers 1 to 5
i=1
while [ $i -le 5 ]
do
  echo $i
  i=$((i+1))
done

Functions

Single Parameter Function:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 16/01/2025
#Description : Basic shell scripting
#####################################

# define a function to print a greeting
sayhello() {
  echo "Hello, $1!"
}
# call the sayhello function with a name parameter
sayhello "Rahul"

Function with Two Parameters:

#!/bin/bash
####################################
#Author : Parthasarathy G
#Date : 16/01/2025
#Description : Basic shell scripting
#####################################

# define a function to add two numbers
add() {
  result=$(($1 + $2))
  return $result
}

# call the add function and save the result
add 5 3
result=$?

echo "5 + 3 = $result"

Functions are a powerful feature of shell scripting and can simplify complex scripts, making them more modular and reusable.

AWS services resource-list script:

#!/bin/bash

#################################################################

# This script will list all the resources in the AWS account
# Author : Parthasarathy G
# Version: v0.0.1
# Date : 18/01/2025

# Following are the supported AWS services by the script
# 1. EC2
# 2. S3
# 3. RDS
# 4. DynamoDB
# 5. Lambda
# 6. IAM
# 7. CloudFormation
# 8. CloudWatch
# 9. VPC
# 10. SNS

#Usage: ./aws-resource-list.sh <region> <service-name>
#Example: ./aws-resource-list.sh us-east-1 ec2

#################################################################

# Check if the required arguments are passed

   if [ $# -ne 2 ]; then
   echo "Usage: ./aws-resource-list.sh <region> <service-name>"
   echo "Example: ./aws-resource-list.sh us-east-1 ec2"
    exit 1
fi

#Check if the AWS CLI is installed

if ! command -v aws &> /dev/null
then
    echo "AWS CLI could not be found. Please install the AWS CLI"
    exit
fi

#Check if the AWS CLI is configured

if [ ! -d ~/.aws ]; then
    echo "AWS CLI is not configured. Please configure the AWS CLI"
    exit
fi

#Execute the AWS CLI command based on the service name

case $2 in
    ec2)
        aws ec2 describe-instances --region $1
        ;;
    s3)
        aws s3api list-buckets --region $1
        ;;
    rds)
        aws rds describe-db-instances --region $1
        ;;
    dynamodb)
        aws dynamodb list-tables --region $1
        ;;
    lambda)
        aws lambda list-functions --region $1
        ;;
    iam)
        aws iam list-users --region $1
        ;;
    cloudformation)
        aws cloudformation list-stacks --region $1
        ;;
    cloudwatch)
        aws cloudwatch list-metrics --region $1
        ;;
    vpc)
        aws ec2 describe-vpcs --region $1
        ;;
    sns)
        aws sns list-topics --region $1
        ;;
    *)
        echo "Invalid service name. Please provide a valid service name"
        ;;
esac