Modify a tarball's header content

index.js

const fs = require('fs');
const Header = require('tar/lib/header');
const fd = fs.openSync('./service.tar', 'r+');
const childProcess = require('child_process');

const args = require('yargs')
  .option('u', {
    alias: 'user',
    demandOption: true,
    describe: 'the user to set permission bits to',
    type: 'string'
  })
  .argv

const file = args._[0];

if(!file) {
  throw new Error('filename is required');
}

// Ensure file exists
fs.statSync(file);

// Resolve username to uid/gid/group
const user = args.user;
const uid = childProcess.execSync(`id -u ${user}`).toString().trim();
const gid = childProcess.execSync(`id -g ${user}`).toString().trim();
const group = childProcess.execSync(`getent group ${gid}`).toString().trim().split(':')[0];
console.log(user, uid, group, gid);

// Tar stores uid and gid in octal
const uidString = Number(uid).toString(8);
const gidString = Number(uid).toString(8);

// Tar blocks are 512 bytes each. Anything that doesn't fill a block gets zero
// padded.
let block = 0;
let BLOCK_SIZE = 512;
const buffer = Buffer.alloc(BLOCK_SIZE);
let exit = false;
while(true) {
  const bytesRead = fs.readSync(fd, buffer, 0, BLOCK_SIZE, block * BLOCK_SIZE);
  const header = new Header(buffer);
  if(header.nullBlock === false && header.cksumValid === false) {
    console.log(header);
    throw new Error('Invalid checksum!')
  }
  if(header.nullBlock === true && exit === true) {
    break;
  }
  if(header.nullBlock === true) {
    exit = true;
    continue;
  }
  const oldUid = header.uid;
  const oldGid = header.gid;

  // Zero out the uid/user/gid/group blocks in the header
  buffer.fill(0, 108, 124) // uid and gid
  buffer.fill(0, 265, 329) // username and groupname
  buffer.fill(0, 148, 156) // chksum
  buffer.write(uidString, 108, 8, 'ascii');
  buffer.write(gidString, 116, 8, 'ascii');
  buffer.write(user, 265, 32, 'ascii');
  buffer.write(group, 297, 32, 'ascii');
  // Recompute chksum
  let sum = 256
  for (let i = 0; i < 0 + 512; i++) {
    sum += buffer[i]
  }
  buffer.write(sum.toString(8), 148, 8, 'ascii');
  const newHeader = new Header(buffer);
  console.log(`${header.path} user: ${header.uid} => ${newHeader.uid} && group: ${header.gid} => ${newHeader.gid} && cksum: ${header.cksum} => ${newHeader.cksum}`);
  // Update the header on disk
  fs.writeSync(fd, buffer, 0, BLOCK_SIZE, block * BLOCK_SIZE);
  let seekTo = ( block * BLOCK_SIZE ) + header.size;
  while(block * BLOCK_SIZE < seekTo) {
    block = block + 1;
  }
  block = block + 1;
}