port bash_history to zsh, while preserving existing zsh history too!

bash-to-zsh-history.js

const fs = require('fs');

/**
 * This script transfers bash history to zsh history
 * Change bash and zsh history files, if you don't use defaults
 *
 * Usage: node bash_to_zsh_history.js
 *
 * Author: Matteo Gaggiano, Modified by Drew Petersen
 */

// change if you don't use default values
const BashHistoryFilePath = process.env.HOME + "/gtdrewp-bash_history";
const ZshHistoryFilePath = process.env.HOME + "/.zsh_history";

// Read the bash history file
const bashHistFile = fs.readFileSync(BashHistoryFilePath, 'utf8');
const bashHistFileLines = bashHistFile.split("\n");

// Open the zsh history file
const zshHistFileLines = [];

// Get timestamp required for zsh history file format and update the history file
const start = (Date.now() / 1000).toFixed(0) * 1;
bashHistFileLines.forEach((command, index) => {
  const time = start - (bashHistFileLines.length - index);
  if (command && command.trim().length > 0)
    zshHistFileLines.push(`: ${time}:0;${command}\n`);
});

const combined = fs.readFileSync(ZshHistoryFilePath, 'utf8').split('\n');
combined.push.apply(combined, zshHistFileLines);

fs.writeFileSync(ZshHistoryFilePath, combined.join(''), {encoding: 'utf8', flag: 'a+'});