diff --git a/src/commands.js b/src/commands.js
new file mode 100644
index 0000000..72f1082
--- /dev/null
+++ b/src/commands.js
@@ -0,0 +1,13 @@
+import { countHandler } from './commands/count.js'
+import { csvToJsonHandler } from './commands/csvToJson.js'
+import { jsonToCsvHandler } from './commands/jsonToCsv.js'
+import { upHandler, cdHandler, lsHandler } from './navigation.js'
+
+export const COMMAND_HANDLERS_MAP = {
+  up: upHandler,
+  cd: cdHandler,
+  ls: lsHandler,
+  'csv-to-json': csvToJsonHandler,
+  'json-to-csv': jsonToCsvHandler,
+  count: countHandler,
+}
diff --git a/src/commands/count.js b/src/commands/count.js
index e69de29..bc49c3d 100644
--- a/src/commands/count.js
+++ b/src/commands/count.js
@@ -0,0 +1,56 @@
+
+import { extname } from 'node:path';
+import { createReadStream } from 'node:fs';
+import { argParser } from '../utils/argParser.js';
+import { pathResolver } from '../utils/pathResolver.js';
+import { InvalidInputError } from '../utils/errors.js';
+
+const LINE_SEPARATOR = /\r?\n/;
+const WORD_SEPARATOR = /\s+/;
+
+const getWordsFromString = (str) => {
+  return str.split(WORD_SEPARATOR).filter(w => w.length > 0);
+}
+
+export const countHandler = async (args) => {
+  const inputArg = argParser(args, 'input', true);
+  const inputPath = pathResolver(inputArg);
+
+  const inputFileExt = extname(inputPath).toLowerCase();
+
+  if (inputFileExt !== '.txt') {
+    throw new InvalidInputError('Invalid file extention');
+  }
+
+  const readableStream = createReadStream(inputPath, { encoding: 'utf-8' });
+
+  let buffer = '';
+  let linesCount = 0;
+  let wordsCount = 0;
+  let charsCount = 0;
+
+  for await (const chunk of readableStream) {
+    buffer += chunk;
+    charsCount += chunk.length;
+
+    const lines = buffer.split(LINE_SEPARATOR);
+    buffer = lines.pop();
+
+    linesCount += lines.length;
+
+    const words = getWordsFromString(lines.join(' '));
+    wordsCount += words.length;
+  }
+
+  if (buffer.length) {
+    charsCount += buffer.length;
+    linesCount++;
+
+    const words = getWordsFromString(buffer);
+    wordsCount += words.length;
+  }
+
+  console.log(`Lines: ${linesCount}`);
+  console.log(`Words: ${wordsCount}`);
+  console.log(`Characters: ${charsCount}`);
+}
\ No newline at end of file
diff --git a/src/commands/csvToJson.js b/src/commands/csvToJson.js
index e69de29..af3c1bb 100644
--- a/src/commands/csvToJson.js
+++ b/src/commands/csvToJson.js
@@ -0,0 +1,79 @@
+import { Transform } from 'node:stream';
+import { fileConfersionHandler } from '../utils/fileConversionHandler.js';
+
+const EXTRA_HEADER_NAME = 'Extra';
+const INDENT = 2;
+const LINE_SEPARATOR = /\r?\n/;
+
+const getCsvToJsonTransformSteam = () => {
+  let transformBuffer = '';
+  let headers = [];
+  let isFirstDataRow = true;
+
+  const getDataPrefix = () => {
+    let prefix = `,\n${' '.repeat(INDENT)}`;
+    if (isFirstDataRow) {
+      prefix = ' '.repeat(INDENT);
+      isFirstDataRow = false;
+    }
+    return prefix;
+  }
+
+  const transformDataToJsonString = (data) => {
+    const obj = {};
+    let unknownHeaderCounter = 1;
+    data.forEach((d, i) => {
+      let header;
+      if (headers[i]) {
+        header = headers[i];
+      } else {
+        header = `${EXTRA_HEADER_NAME}${unknownHeaderCounter}`;
+        unknownHeaderCounter++;
+      }
+      obj[header] = d;
+    });
+    return JSON.stringify(obj);
+  }
+
+  return new Transform({
+    transform(chunk, _, callback) {
+      transformBuffer += chunk;
+
+      const lines = transformBuffer.split(LINE_SEPARATOR);
+      transformBuffer = lines.pop();
+
+      lines.forEach((line) => {
+        if (!line.trim()) {
+          return;
+        }
+
+        const parsedLineData = line.split(',');
+
+        if (!headers.length) {
+          headers = parsedLineData.map(h => h.trim());
+          this.push('[\n');
+        } else {
+          this.push(`${getDataPrefix()}${transformDataToJsonString(parsedLineData)}`);
+        }
+      });
+
+      callback();
+    },
+    flush(callback) {
+      if (!transformBuffer.trim()) {
+        return '\n]';
+      }
+
+      const parsedLineData = transformBuffer.split(',');
+      this.push(`${getDataPrefix()}${transformDataToJsonString(parsedLineData)}\n]\n`);
+
+      transformBuffer = '';
+      callback();
+    }
+  });
+}
+
+export const csvToJsonHandler = async (args) => {
+  const transformStream = getCsvToJsonTransformSteam();
+  return fileConfersionHandler(args, '.csv', transformStream);
+}
diff --git a/src/commands/jsonToCsv.js b/src/commands/jsonToCsv.js
index e69de29..34ea0ee 100644
--- a/src/commands/jsonToCsv.js
+++ b/src/commands/jsonToCsv.js
@@ -0,0 +1,38 @@
+import { Transform } from 'node:stream';
+import { fileConfersionHandler } from '../utils/fileConversionHandler.js';
+
+const getJsonToCsvTransformSteam = () => {
+  let jsonStringBuffer = '';
+
+  return new Transform({
+    transform(chunk, _, callback) {
+      jsonStringBuffer += chunk;
+      callback();
+    },
+    flush(callback) {
+      const jsonData = JSON.parse(jsonStringBuffer);
+      jsonStringBuffer = '';
+      if (!Array.isArray(jsonData)) {
+        throw new Error('Invalid json type');
+      }
+
+      const headerSet = new Set();
+      jsonData.map((row) => Object.keys(row).forEach(h => headerSet.add(h)));
+      const headers = Array.from(headerSet);
+
+      this.push(`${headers.join(',')}\n`)
+
+      jsonData.forEach(row => {
+        const valuesInHeadersOrder = headers.map((header) => row[header] || '');
+        this.push(`${valuesInHeadersOrder.join(',')}\n`);
+      })
+
+      callback();
+    }
+  });
+}
+
+export const jsonToCsvHandler = async (args) => {
+  const transformStream = getJsonToCsvTransformSteam();
+  return fileConfersionHandler(args, '.json', transformStream);
+}
diff --git a/src/cwdState.js b/src/cwdState.js
new file mode 100644
index 0000000..e118cd3
--- /dev/null
+++ b/src/cwdState.js
@@ -0,0 +1,17 @@
+import os from 'node:os';
+import { InvalidInputError } from './utils/errors.js';
+
+export const initialCwd = os.homedir();
+let cwd = initialCwd;
+
+export const getCwd = () => {
+  return cwd;
+}
+
+// dir - absolute path
+export const chCwd = (dir) => {
+  if (!dir.startsWith(initialCwd)) {
+    throw new InvalidInputError('Cannot go higher than initial directory');
+  }
+  cwd = dir;
+}
\ No newline at end of file
diff --git a/src/main.js b/src/main.js
index e69de29..acff6a3 100644
--- a/src/main.js
+++ b/src/main.js
@@ -0,0 +1,7 @@
+import { initRepl } from './repl.js';
+
+const init = () => {
+  initRepl();
+}
+
+init();
diff --git a/src/navigation.js b/src/navigation.js
index e69de29..bbac816 100644
--- a/src/navigation.js
+++ b/src/navigation.js
@@ -0,0 +1,56 @@
+import fs from 'node:fs/promises';
+import { getCwd, chCwd, initialCwd } from './cwdState.js';
+import { InvalidInputError } from './utils/errors.js';
+import { pathResolver } from './utils/pathResolver.js';
+
+
+export const upHandler = () => {
+  if (getCwd() !== initialCwd) {
+    const newDirectory = pathResolver(`${getCwd}/..`);
+    chDir(newDirectory);
+  }
+}
+
+export const cdHandler = async (args) => {
+  const pathToDirectory = args[0];
+  if (!pathToDirectory) {
+    throw new InvalidInputError('No path/to/directory');
+  }
+  const newDirPath = pathResolver(pathToDirectory);
+
+  const stats = await fs.stat(newDirPath);
+  
+  if (stats.isDirectory()) {
+    chCwd(newDirPath)
+  } else {
+    throw new InvalidInputError('Path is not a directory');
+  }
+}
+
+export const lsHandler = async () => {
+  const dirEntities = await fs.readdir(getCwd(), { withFileTypes: true });
+
+  const folders = [];
+  const files = [];
+
+  let maxEntitName = 0;
+
+  dirEntities.forEach((dirEnt) => {
+    maxEntitName = Math.max(maxEntitName, dirEnt.name.length);
+
+    if (dirEnt.isDirectory()) {
+      folders.push(dirEnt.name);
+    } else if (dirEnt.isFile()) {
+      files.push(dirEnt.name);
+    }
+  });
+
+  const formatEntityName = (entitiName) => `${entitiName}${' '.repeat(maxEntitName - entitiName.length)}`
+
+  folders.sort((a, b) => a.localeCompare(b)).forEach((ent) => {
+    console.log(`${formatEntityName(ent)}    [folder]`)
+  });
+  files.sort((a, b) => a.localeCompare(b)).forEach((ent) => {
+    console.log(`${formatEntityName(ent)}    [file]`)
+  });
+}
diff --git a/src/repl.js b/src/repl.js
index e69de29..57492f6 100644
--- a/src/repl.js
+++ b/src/repl.js
@@ -0,0 +1,81 @@
+
+
+import readline from 'node:readline';
+import process from 'node:process';
+import { getCwd } from './cwdState.js';
+import { InvalidInputError, INVALID_INPUT_ERROR_CODE } from './utils/errors.js';
+import { COMMAND_HANDLERS_MAP } from './commands.js';
+
+const WELCOME_TEXT = 'Welcome to Data Processing CLI!';
+const EXIT_TEXT = 'Thank you for using Data Processing CLI!';
+const INVALID_COMMAND_TEXT = 'Invalid input';
+const OPERATION_FAILED_TEXT = 'Operation failed';
+const CURRENT_DIRECTORY_PREFIX = 'You are currently in';
+
+const ANSI_COLORS = {
+  red: '\x1b[31m',
+  green: '\x1b[32m',
+};
+const ANSI_COLOR_RESET = '\x1b[0m';
+
+const onSuccess = () => {
+  console.log(`${ANSI_COLORS.green}${CURRENT_DIRECTORY_PREFIX} ${getCwd()}${ANSI_COLOR_RESET}`);
+}
+
+const onError = (err) => {
+  // For testing only, TODO: delete before review next line
+  console.log(err); 
+  if (err.code === INVALID_INPUT_ERROR_CODE) {
+    console.log(`${ANSI_COLORS.red}${INVALID_COMMAND_TEXT}${ANSI_COLOR_RESET}`);
+  } else {
+    console.log(`${ANSI_COLORS.red}${OPERATION_FAILED_TEXT}${ANSI_COLOR_RESET}`);
+  }
+}
+
+const handleCommand = async(command, commandArgs) => {
+  if (command in COMMAND_HANDLERS_MAP) {
+    try {
+      await COMMAND_HANDLERS_MAP[command](commandArgs);
+      onSuccess()
+    } catch (err) {
+      onError(err)
+    }
+
+  } else {
+    onError(new InvalidInputError('Invalid command'));
+  }
+}
+
+export const initRepl = () => {
+  const rl = readline.createInterface({
+    input: process.stdin, 
+    output: process.stdout,
+    prompt: '> '
+  });
+
+  rl.on('line', async (line) => {
+    const lineTrimmed = line.trim();
+    const [command, ...commandArgs] = lineTrimmed.split(' ');
+
+    if (command === '.exit' && commandArgs.length === 0) {
+      rl.close()
+    } else {
+      await handleCommand(command, commandArgs);
+      rl.prompt();
+    }
+  });
+
+  rl.on('SIGINT', () => {
+    rl.close();
+  });
+
+  rl.on('close', () => {
+    console.log(EXIT_TEXT)
+    process.exit(0);
+  });
+
+  console.log(WELCOME_TEXT);
+  // Initial display of directory
+  onSuccess();
+  rl.prompt();
+}
diff --git a/src/utils/argParser.js b/src/utils/argParser.js
index e69de29..8051e95 100644
--- a/src/utils/argParser.js
+++ b/src/utils/argParser.js
@@ -0,0 +1,32 @@
+import { InvalidInputError } from './errors.js';
+
+export const argParser = (args, argName, required = false) => {
+  const argIndex = args.findIndex(arg => arg.startsWith(`--${argName}`));
+
+  if (argIndex === -1) {
+    if (required) {
+      throw new InvalidInputError('Argument is required');
+    } else {
+      return;
+    }
+  }
+
+  const arg = args[argIndex];
+  let argValue;
+
+  if (arg.includes('=')) {
+    argValue = arg.split('=')[1];
+  } else {
+    const nextArgvValue = args[argIndex + 1];
+
+    if (nextArgvValue && !nextArgvValue.startsWith('--')) {
+      argValue = nextArgvValue;
+    }
+  }
+
+  if (!argValue && required) {
+    throw new InvalidInputError('Argument is required');
+  }
+
+  return argValue;
+}
\ No newline at end of file
diff --git a/src/utils/errors.js b/src/utils/errors.js
new file mode 100644
index 0000000..21e3055
--- /dev/null
+++ b/src/utils/errors.js
@@ -0,0 +1,11 @@
+export const INVALID_INPUT_ERROR_CODE = 'INVALID_INPUT';
+
+export class InvalidInputError extends Error {
+  constructor(message) {
+    super(message);
+    this.name = 'InvalidInputError';
+    this.code = INVALID_INPUT_ERROR_CODE;
+
+    Error.captureStackTrace(this, this.constructor);
+  }
+}
diff --git a/src/utils/fileConversionHandler.js b/src/utils/fileConversionHandler.js
new file mode 100644
index 0000000..5320df4
--- /dev/null
+++ b/src/utils/fileConversionHandler.js
@@ -0,0 +1,25 @@
+import { pipeline } from 'node:stream/promises';
+import { extname } from 'node:path';
+import { createReadStream, createWriteStream } from 'node:fs';
+import { argParser } from '../utils/argParser.js';
+import { pathResolver } from '../utils/pathResolver.js';
+import { InvalidInputError } from '../utils/errors.js';
+
+export const fileConfersionHandler = async (args, fileExtention, transformStream) => {
+  const inputArg = argParser(args, 'input', true);
+  const outputArg = argParser(args, 'output', true);
+
+  const inputPath = pathResolver(inputArg);
+  const outputPath = pathResolver(outputArg);
+
+  const inputFileExt = extname(inputPath).toLowerCase();
+
+  if (inputFileExt !== fileExtention) {
+    throw new InvalidInputError('Invalid file extention');
+  }
+
+  const inputStream = createReadStream(inputPath, { encoding: 'utf-8' });
+  const outputStream = createWriteStream(outputPath, { encoding: 'utf-8' });
+
+  await pipeline(inputStream, transformStream, outputStream);
+};
\ No newline at end of file
diff --git a/src/utils/pathResolver.js b/src/utils/pathResolver.js
index e69de29..2b2cb9c 100644
--- a/src/utils/pathResolver.js
+++ b/src/utils/pathResolver.js
@@ -0,0 +1,6 @@
+import path from 'node:path';
+import { getCwd } from '../cwdState.js';
+
+export const pathResolver = (pathArg) => {
+  return path.resolve(getCwd(), pathArg);
+}
\ No newline at end of file