Bangladesh Chemical Industries Corporation, Assistant Programmer, 2022

Bangladesh Chemical Industries Corporation

Post: Assistant Programmer
Exam Date: 2022, Exam Taker: BUET

1. (a) Find the possible path to know the how data on the internet travels from your machine to the site www.bcic.gov.bd. Write down the necessary Linux command to accomplish this.

traceroute www.bcic.gov.bd

traceroute: This command traces the route packets take to reach a network host.
www.bcic.gov.bd: The destination website for which you want to trace the route.

1. (b) You want to run some specific commands at some pre-schedules time. Which command will have to be used for this?

Scheduling Commands in Linux

To run specific commands at a pre-scheduled time in Linux, the at command is used. The at command is suitable for scheduling tasks that need to be executed only once at a specific time.

Purpose of at Command

The at command allows users to schedule a command or script to run at a particular date and time in the future, unlike cron which is used for repetitive tasks.

Command Syntax

at TIME

Common Options

-l : Lists all scheduled at jobs.

-r : Removes a scheduled at job.

Example

To run a command 15 minutes from now:

at now + 15 minutes

After entering the command, type the task to be executed:

echo "Task executed" >> /home/user/task_log.txt

Press Ctrl + D to confirm. The command will run once after 15 minutes.

Linux-এ Command Scheduling

Linux-এ নির্দিষ্ট সময়ে একবার command চালানোর জন্য at command ব্যবহার করা হয়। এটি এমন task-এর জন্য ব্যবহৃত হয় যেগুলো একবারই execute করতে হয়।

at Command-এর উদ্দেশ্য

at command ব্যবহার করে ভবিষ্যতের কোনো নির্দিষ্ট date ও time-এ command বা script চালানো যায়, যেখানে cron মূলত বারবার চলা task-এর জন্য ব্যবহৃত হয়।

Command Syntax

at TIME

প্রচলিত Option

-l : সব scheduled at job-এর তালিকা দেখায়।

-r : নির্দিষ্ট at job মুছে দেয়।

উদাহরণ

১৫ মিনিট পর একটি command চালাতে:

at now + 15 minutes

এরপর যে command চালাতে চান তা লিখুন:

echo "Task executed" >> /home/user/task_log.txt

Ctrl + D চাপলে taskটি confirm হবে এবং নির্দিষ্ট সময়ে একবারই execute হবে।

1 (c) Write JavaScript's function to validate a customer number where the customer number is 3 uppercase letter district code followed by 8 digits.

function validateCustomerNumber(num) {
    var pattern = /^[A-Z]{3}[0-9]{8}$/;
    if (pattern.test(num)) {
        return true;
    } else {
        return false;
    }
}

2. (a) Show a 3-Way Handshake Protocol in TCP connection establishment using a diagram.

3-Way Handshake Protocol in TCP Connection Establishment

The 3-way handshake is a mechanism used by TCP to establish a reliable connection between a client and a server before data transmission begins.

Step 1: SYN (Synchronization)

The client initiates the connection by sending a SYN packet to the server containing an initial sequence number.

Step 2: SYN-ACK (Synchronization and Acknowledgment)

After receiving the SYN packet, the server replies with a SYN-ACK packet, acknowledging the client’s request and sending its own sequence number along with window size and maximum segment size.

Step 3: ACK (Acknowledgment)

The client sends an ACK packet to the server to confirm receipt of the SYN-ACK, after which the TCP connection is successfully established and data transfer can begin.

TCP Connection Establishment-এ 3-Way Handshake Protocol

3-way handshake হলো TCP protocol-এর একটি প্রক্রিয়া, যার মাধ্যমে client ও server-এর মধ্যে একটি reliable connection স্থাপন করা হয় data transmission শুরু করার আগে।

Step 1: SYN (Synchronization)

Client প্রথমে server-এর কাছে একটি SYN packet পাঠায়, যেখানে একটি initial sequence number থাকে।

Step 2: SYN-ACK (Synchronization and Acknowledgment)

Server SYN packet পাওয়ার পর SYN-ACK packet পাঠায়, যা client-এর request acknowledge করে এবং server-এর sequence number, window size ও maximum segment size জানায়।

Step 3: ACK (Acknowledgment)

Client SYN-ACK পাওয়ার পর ACK packet পাঠায় এবং এর মাধ্যমে TCP connection সম্পূর্ণভাবে স্থাপিত হয় ও data transmission শুরু হয়।

2 (b) Write the difference between SQL Injection and Cross Site Scripting (XSS).

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Bangladesh Chemical Industries Corporation Post: Assistant ProgrammerExam Date: 2022, Exam Taker: BUET

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Bangladesh Chemical Industries Corporation

Post: Assistant Programmer
Exam Date: 2022, Exam Taker: BUET