Example AWS CLI Command to List IAM Roles and Policies

Example AWS CLI Command to List IAM Roles and Policies: A Simple Guide for Beginners

If you're learning about AWS (Amazon Web Services) and the CLI (Command Line Interface), you might come across something called IAM roles and policies. These are really important tools that help you control who can do what in your AWS account. Don't worry if that sounds complicated—let's break it down in a way that even a 12-year-old can understand!

In this post, I'll show you some simple AWS CLI commands to list IAM roles and policies in your AWS account. We’ll explain them step by step, so you can follow along easily. You’ll also find some helpful keywords and common interview questions about IAM roles and policies at the end.

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What are IAM Roles and Policies?

Imagine you have a toy store, and you need help running it. You might give different people different tasks:

• Some people can open the store.
• Some people can run the cash register.
• Some people can restock the shelves.

Now, think of IAM roles as the people with different jobs, and IAM policies as the instructions that tell them what they are allowed to do.

IAM stands for Identity and Access Management, and it helps AWS users control access to AWS resources.

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What is AWS CLI?

The AWS CLI (Command Line Interface) is like a special tool you type commands into to control things in your AWS account. Instead of clicking around in a web browser, you can give AWS instructions using commands.

Now, let's learn how to use the AWS CLI to list IAM roles and policies!

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How to List IAM Roles Using AWS CLI

To list all the IAM roles in your AWS account, use this simple command:

aws iam list-roles

Here’s how it works:

• aws: This is the main AWS command.
• iam: This tells AWS that we want to do something with IAM (the part of AWS that handles roles and policies).
• list-roles: This command asks AWS to show us all the roles.

When you run this command, AWS will show you a list of all the roles in your account. A role is like a worker in your toy store—they have specific tasks they can do.

Example Output:

{
  "Roles": [
    {
      "RoleName": "AdminRole",
      "Arn": "arn:aws:iam::123456789012:role/AdminRole"
    },
    {
      "RoleName": "ReadOnlyRole",
      "Arn": "arn:aws:iam::123456789012:role/ReadOnlyRole"
    }
  ]
}

This means you have two roles: one called AdminRole (maybe this one can do everything) and another called ReadOnlyRole (maybe this one can only look at things without making changes).

---

How to List IAM Policies Using AWS CLI

Next, let's list the policies—these are the instructions or rules that tell the roles what they are allowed to do.

To list all the policies in your account, use this command:

aws iam list-policies

Just like the roles command, this tells AWS to show all the policies in your account.

Example Output:

{
  "Policies": [
    {
      "PolicyName": "S3ReadOnlyPolicy",
      "Arn": "arn:aws:iam::123456789012:policy/S3ReadOnlyPolicy"
    },
    {
      "PolicyName": "AdminAccess",
      "Arn": "arn:aws:iam::123456789012:policy/AdminAccess"
    }
  ]
}

Here, you have two policies:

• S3ReadOnlyPolicy: Maybe this allows someone to look at things in S3 (Amazon’s storage service) but not change anything.
• AdminAccess: This might let someone do anything in the account because they are an "Admin."

---

How to List Attached Policies to a Role

You might also want to know which policies are attached to a specific role. For example, what can your AdminRole do?

To list the policies attached to a role, use this command:

aws iam list-attached-role-policies --role-name AdminRole

This tells AWS to show all the policies connected to the AdminRole.

---

How to List Inline Policies for a Role

Sometimes, roles have custom rules written directly inside them, called inline policies. You can list these using:

aws iam list-role-policies --role-name AdminRole

This will show any special rules that are only attached to the AdminRole and nowhere else.

---

Top 10 Interview Questions About IAM Roles and Policies (with Answers)

1. What is an IAM role in AWS?
   An IAM role in AWS is a set of permissions that define what actions are allowed and denied for a specific identity in your AWS account. Unlike users, roles don't have long-term credentials and are used to delegate access to services and resources.
2. What is the difference between an IAM role and an IAM user?
   An IAM user represents a person or service and has long-term credentials (like access keys and passwords). An IAM role, on the other hand, doesn't have permanent credentials. Instead, it is assumed by users, applications, or services, and provides temporary permissions.
3. How do you attach a policy to an IAM role?
   You can attach a policy to an IAM role using the AWS CLI with the following command:

   aws iam attach-role-policy --role-name <role_name> --policy-arn <policy_arn>

4. What is a managed policy versus an inline policy?
   Managed policies are reusable policies that can be attached to multiple roles, users, or groups. These are either AWS-managed (created by AWS) or customer-managed (created by you). Inline policies, on the other hand, are directly embedded into a specific role, user, or group and are not reusable.
5. How do you check what policies are attached to an IAM role?
   You can check the attached policies of a role using the command:

   aws iam list-attached-role-policies --role-name <role_name>

6. What are the default IAM policies provided by AWS?
   AWS provides several default policies (AWS-managed policies) like AdministratorAccess, ReadOnlyAccess, and PowerUserAccess. These predefined policies allow common levels of access to AWS services without needing to create new policies from scratch.
7. What is the purpose of the IAM policy simulator?
   The IAM Policy Simulator is a tool that allows you to test and troubleshoot IAM policies. You can use it to see the effects of policies and determine what permissions are granted or denied before actually applying them to roles or users.
8. How can you restrict access to specific AWS services for a role?
   You can restrict access by attaching a custom policy to the role that defines the specific actions and services the role can access. For example, to limit access to only Amazon S3, you could create a policy like:

   {
     "Version": "2012-10-17",
     "Statement": [
       {
         "Effect": "Allow",
         "Action": "s3:*",
         "Resource": "*"
       }
     ]
   }

9. What is the difference between a role and a group in AWS IAM?
   An IAM role grants temporary access to AWS resources without requiring long-term credentials and is assumed by users or services. An IAM group is a collection of IAM users that makes it easier to manage permissions for multiple users.
10. Can an IAM role have multiple policies attached?
    Yes, an IAM role can have multiple managed and inline policies attached to it. You can combine multiple policies to define what actions are allowed or denied for the role.

---

Conclusion

Working with AWS CLI and learning to manage IAM roles and policies might seem tough at first, but with practice, it becomes easier. Using simple commands like list-roles and list-policies, you can quickly find out who has access to your AWS resources and what they are allowed to do.

Remember, IAM is all about keeping your AWS account safe and secure, so it’s important to understand who can access what! Hopefully, this guide helped you take the first step in understanding IAM roles and policies.

Let us know if you have any questions, and happy learning!

How to Read a Large Text File Line by Line Using Java Sample Code

Reading a large text file efficiently in Java is a common task in day to day software development. When dealing with files too large to load into memory at once, it's essential to read them line by line to avoid memory overflow. The topic is also discussed during interviews specically focusing on underlying implementation of below classes. In this post, we will explore different approaches for reading a large file line by line using Java:

1. Using BufferedReader (Pre-Java 8)
2. Using the Files class with Java 8 and above
3. Using java.util.Scanner
4. Using Files.newBufferedReader()
5. Using SeekableByteChannel
6. Using FileUtils.lineIterator (Apache Commons IO)

We will provide sample code for reading large files using each of these methods, making it easy for you to choose the approach that best fits your project.

1. Reading a Large File Line by Line Using BufferedReader

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class ReadLargeFileUsingBufferedReader {
    public static void main(String[] args) {
        String samplePath = "example/path/to-large-file.txt";

        try (BufferedReader br = new BufferedReader(new FileReader(samplePath))) {
            String line;
            while ((line = br.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

2. Reading a Large File Line by Line Using Java 8 Files.lines()

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.stream.Stream;

public class ReadLargeFileUsingJava8 {
    public static void main(String[] args) {
        String samplePath = "example/path/to-large-file.txt";

        try (Stream<String> lines = Files.lines(Paths.get(samplePath))) {
            lines.forEach(System.out::println);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

3. Reading a Large File Line by Line Using java.util.Scanner

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class ReadLargeFileUsingScanner {
    public static void main(String[] args) {
        String samplePath = "example/path/to-large-file.txt";

        try (Scanner scanner = new Scanner(new File(samplePath))) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                System.out.println(line);
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

4. Reading a Large File Line by Line Using Files.newBufferedReader()

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ReadLargeFileUsingNewBufferedReader {
    public static void main(String[] args) {
        String samplePath = "example/path/to-large-file.txt";

        try (BufferedReader reader = Files.newBufferedReader(Paths.get(samplePath))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

5. Reading a Large File Line by Line Using SeekableByteChannel

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.SeekableByteChannel;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class ReadLargeFileUsingSeekableByteChannel {
    public static void main(String[] args) {
        Path samplePath = Paths.get("example/path/to-large-file.txt");

        try (SeekableByteChannel sbc = Files.newByteChannel(samplePath)) {
            ByteBuffer buffer = ByteBuffer.allocate(1024);
            while (sbc.read(buffer) > 0) {
                buffer.flip();
                while (buffer.hasRemaining()) {
                    System.out.print((char) buffer.get());
                }
                buffer.clear();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

6. Reading a Large File Line by Line Using FileUtils.lineIterator (Apache Commons IO)

import org.apache.commons.io.FileUtils;
import org.apache.commons.io.LineIterator;

import java.io.File;
import java.io.IOException;

public class ReadLargeFileUsingLineIterator {
    public static void main(String[] args) {
        String samplePath = "example/path/to-large-file.txt";

        try (LineIterator it = FileUtils.lineIterator(new File(samplePath), "UTF-8")) {
            while (it.hasNext()) {
                String line = it.nextLine();
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Key Differences Between These Methods

• BufferedReader is one of the most memory-efficient options for reading large files.
• Files.lines() and Files.newBufferedReader() provide modern, concise approaches but may consume more memory for larger files.
• Scanner is flexible but slower for very large files.
• SeekableByteChannel allows random access to file data, useful when working with specific sections of a file.
• FileUtils.lineIterator() is perfect for handling extremely large files while keeping memory usage low.

Interview Questions: File Reading in Java

If you're preparing for a Java interview, understanding file handling in Java is crucial. Below are some common interview questions related to reading large files in Java:

1. What is the difference between BufferedReader and Files.lines()?

• BufferedReader reads the file line by line without loading the entire file into memory, making it more memory efficient for large files.
• Files.lines() uses a stream to read all lines from a file and can be easily processed with Java Streams API. However, it might load more data into memory, especially for very large files.

2. When would you choose Scanner over BufferedReader?

• Scanner is better suited for parsing input with custom delimiters and working with different types of data (e.g., integers, floats). However, for reading large files line by line efficiently, BufferedReader is a better option because of its lower memory consumption and faster performance.

3. What is the advantage of using SeekableByteChannel?

• SeekableByteChannel allows random access to file data. You can jump to a specific position in the file to read or write, which is not possible with BufferedReader or Scanner.

4. How does FileUtils.lineIterator() handle very large files?

• FileUtils.lineIterator() is part of Apache Commons IO and allows reading large files with very low memory usage. It loads and processes the file line by line without consuming excessive memory.

5. What are the advantages of using Files.newBufferedReader() in Java 8?

• Files.newBufferedReader() is a modern API that simplifies file handling and integrates with the Path class, offering a concise and readable way to work with files. It's an efficient alternative to BufferedReader for reading files in Java 8 and above.

Learnings:

Java provides various ways to read a large text file line by line. You can choose the right approach based on your project’s requirements, file size, and memory considerations. For most cases, BufferedReader or Files.newBufferedReader() is the best option for efficiency and simplicity. If you need to work with very large files, FileUtils.lineIterator() or SeekableByteChannel may offer better performance with minimal memory usage.

Related Keywords:

• Sample code for reading a large file using Java
• Example code for reading large file in Java
• Java BufferedReader example
• Java 8 Files.lines() example
• Apache Commons IO lineIterator example

How to List S3 Buckets and Objects Using AWS CLI

Amazon Simple Storage Service (S3) is a scalable cloud storage solution provided by AWS, widely used for storing data of all kinds. Whether you are managing backups, application files, or large datasets, the AWS CLI (Command Line Interface) is an essential tool for quickly interacting with S3. One of the most frequent tasks is listing buckets and objects in your S3 storage.

In this article, we’ll guide you through various methods of listing your S3 buckets and their contents using AWS CLI. We will explain each command and provide examples to help you get started quickly.

Prerequisites

• AWS CLI is installed: You can install it from the AWS CLI installation guide.
• AWS CLI is configured: Run the command aws configure to set up your credentials (Access Key, Secret Access Key, Region, etc.).
• Necessary permissions: Make sure your IAM user has the right permissions to access and list S3 buckets. The required permission is s3:ListBucket.

1. Listing All S3 Buckets

To list all the S3 buckets in your AWS account, use the following command:

aws s3 ls

This command will return a list of all S3 buckets with their creation dates.

Example Output:

2023-10-12 12:34:56 bucket-name-1
2023-09-10 08:21:33 bucket-name-2

2. Listing Contents of a Specific S3 Bucket

If you want to list all the objects in a specific bucket, you can append the bucket name to the command:

aws s3 ls s3://bucket-name

Replace bucket-name with the actual name of your S3 bucket.

Example Output:

2024-01-10 14:20:15    1024 file1.txt
2024-01-10 14:30:25    2048 file2.txt

3. Listing Objects in a Specific Folder

S3 buckets can contain virtual directories (folders). To list the contents of a specific folder within a bucket, specify the folder name:

aws s3 ls s3://bucket-name/folder-name/

Example Output:

2024-02-15 15:10:05    512  folder-name/file3.jpg
2024-02-16 10:12:45   1024  folder-name/file4.pdf

4. Listing Objects Recursively

To list all objects in a bucket, including those stored in subdirectories, use the --recursive option:

aws s3 ls s3://bucket-name --recursive

Example Output:

2024-01-10 14:20:15    1024 folder1/file1.txt
2024-01-10 14:30:25    2048 folder2/file2.txt
2024-01-11 09:15:10    512  folder2/subfolder/file3.jpg

5. Listing with Human-Readable File Sizes

To view file sizes in a human-readable format (e.g., KB, MB, GB), use the --human-readable option:

aws s3 ls s3://bucket-name --human-readable

Example Output:

2024-01-10 14:20:15   1.0 KiB folder1/file1.txt
2024-01-10 14:30:25   2.0 KiB folder2/file2.txt

6. Summarizing Total Files and Sizes

To get a summary of the total number of objects and their cumulative size in a bucket, use the --summarize option along with --recursive:

aws s3 ls s3://bucket-name --recursive --summarize

Example Output:

2024-01-10 14:20:15    1024 folder1/file1.txt
2024-01-10 14:30:25    2048 folder2/file2.txt

Total Objects: 2
Total Size: 3 KiB

7. Filtering Results Using Wildcards

You can filter the objects by file name patterns using wildcards:

aws s3 ls s3://bucket-name --recursive --exclude "*" --include "*.txt"

This command will only list .txt files, excluding other file types.

Common Errors and How to Fix Them

• Access Denied Error: Ensure that your IAM user has the necessary permissions to list the bucket contents. You need s3:ListBucket and possibly other permissions for more advanced actions.
• No Such Bucket: Verify that the bucket name is correct and exists in the region you’re working in.
• CLI Configuration Issues: Ensure the AWS CLI is properly configured using aws configure, and check if you’re using the correct AWS profile if necessary.

Using the AWS CLI to list S3 buckets and objects is a powerful way to interact with your storage without needing to navigate the AWS Management Console. Whether you're listing all buckets, viewing files in a folder, or summarizing the total size of a bucket, these commands provide flexibility and control over your cloud storage operations.

By mastering these CLI commands, you can streamline your cloud management processes and handle S3 tasks more efficiently, saving both time and effort.

Java Function Code to create Reverse Linked List of Singly Linked List (In memory pointer changes)

Here is a sample java Function Code to create Reverse Linked List of Singly Linked List (In memory pointer changes). Approach

• 1) Maintain a previous node pointer
• 2) maintain a current node pointer
• 3) store current-> next in temporary variable
• 4) replace the current node-> next to previous pointer
• 5) do step 3 and 4 till you reach current node is your end node
• 6) at the end update the head->next to null as the old linked list head will be your end node now
• 7) return previous node as this will be the head of your new linked list.

You can see the image below - before and after structure of linked list:

package com.sample.linkedlist;

public class ReverseLinkedList {

 static class Node {
  int data;
  Node next;

  public Node(int data) {
   this.data = data;
  }

  @Override
  public String toString() {
   return "Node[data=" + data + "]";
  }

 }

 public static Node reverse(Node head) {

  if (head == null)
   return null;

  Node currentNode = head.next;
  Node previousNode = head;
  while (currentNode != null) {
   System.out.println("previous:" + previousNode +" current:"+currentNode);
   
   Node tempCurrent = currentNode.next;

   currentNode.next = previousNode;
   previousNode = currentNode;

   /* move to next node */
   currentNode = tempCurrent;
  }
  head.next = null;

  return previousNode;
 }

 static void print(Node head) {
  while (head != null) {
   System.out.print(head + " ");
   head = head.next;
  }
  System.out.println();

 }

 public static void main(String[] args) {
  
  Node head = new Node(1);
  head.next = new Node(2);
  head.next.next = new Node(3);
  head.next.next.next=new Node(4);
  
  /*print actual linked list*/
  print(head);

  Node newHead = reverse(head);

  /*print reversed linked list*/
  print(newHead);
 }
}

Output

Node[data=1] Node[data=2] Node[data=3] Node[data=4] 
previous:Node[data=1] current:Node[data=2]
previous:Node[data=2] current:Node[data=3]
previous:Node[data=3] current:Node[data=4]
Node[data=4] Node[data=3] Node[data=2] Node[data=1] 

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