Grep in Node.js

ngrep.js

'use strict';

const fs = require('fs');
const readline = require('readline');

const flags = {
  n: 'Show line number'
};

const printHelp = () => {
  console.log('ngrep [options] [pattern] [file]');
  console.log('');
  console.log('Options');
  Object.keys(flags).forEach(key => {
    console.log('\t', `-${key}`, '\t', flags[key]);
  });
};

const printHelpAndExitWithError = (message) => {
  console.error(message);
  console.error('');
  printHelp();
  process.exit(1);
}

const commandArguments = process.argv.slice(2);
let options = commandArguments.slice(0, -2);
let pattern = commandArguments[commandArguments.length - 2];
const filename = commandArguments[commandArguments.length - 1];

if (!process.stdin.isTTY) {
  options = commandArguments.slice(2, 1);
  pattern = commandArguments[commandArguments.length - 1];
} else {
  if (typeof pattern !== 'string') {
    printHelpAndExitWithError('Pattern expected');
  } else if (typeof filename !== 'string') {
    printHelpAndExitWithError('Filename expected');
  }
}

if (typeof pattern !== 'string') {
  printHelpAndExitWithError('Pattern expected')
}

const setFlags = {

};

options.forEach((arg) => {
  if (!/^-\w$/.test(arg)) {
    printHelpAndExitWithError(`Bad argument '${arg}'`);
  }

  const flag = arg.slice(-1);
  if (!flags[flag]) {
    printHelpAndExitWithError('Bad argument');
  }

  setFlags[flag] = true;
});

const regex = new RegExp(pattern);

let lineNumber = 1;
let found = false;

let instream = process.stdin;
if (process.stdin.isTTY) {
  instream = fs.createReadStream(filename);
}

const rl = readline.createInterface({ input: instream });

rl.on('line', (input) => {
  if (regex.test(input)) {
    found = true;
    if (setFlags.n) {
      console.log(`${lineNumber}:${input}`);
    } else {
      console.log(input);
    }
  }
  lineNumber++;
});

process.on('beforeExit', () => {
  if (!found) {
    process.exit(1);
  }
});