asar - Electron Archive

Asar is a simple extensive archive format, it works like tar that concatenates all files together without compression, while having random access support.

Features

Support random access
Use JSON to store files’ information
Very easy to write a parser

Command line utility

Install

This module requires Node 10 or later.

$ npm install --engine-strict asar

Usage

$ asar --help

  Usage: asar [options] [command]

  Commands:

    pack|p <dir> <output>
       create asar archive

    list|l <archive>
       list files of asar archive

    extract-file|ef <archive> <filename>
       extract one file from archive

    extract|e <archive> <dest>
       extract archive


  Options:

    -h, --help     output usage information
    -V, --version  output the version number

Excluding multiple resources from being packed

Given:

    app
(a) ├── x1
(b) ├── x2
(c) ├── y3
(d) │   ├── x1
(e) │   └── z1
(f) │       └── x2
(g) └── z4
(h)     └── w1

Exclude: a, b

$ asar pack app app.asar --unpack-dir "{x1,x2}"

Exclude: a, b, d, f

$ asar pack app app.asar --unpack-dir "**/{x1,x2}"

Exclude: a, b, d, f, h

$ asar pack app app.asar --unpack-dir "{**/x1,**/x2,z4/w1}"

Using programatically

Example

const asar = require('asar');

const src = 'some/path/';
const dest = 'name.asar';

await asar.createPackage(src, dest);
console.log('done.');

Please note that there is currently no error handling provided!

Transform

You can pass in a transform option, that is a function, which either returns nothing, or a stream.Transform. The latter will be used on files that will be in the .asar file to transform them (e.g. compress).

const asar = require('asar');

const src = 'some/path/';
const dest = 'name.asar';

function transform (filename) {
  return new CustomTransformStream()
}

await asar.createPackageWithOptions(src, dest, { transform: transform });
console.log('done.');

Using with grunt

There is also an unofficial grunt plugin to generate asar archives at bwin/grunt-asar.

Format

Asar uses Pickle to safely serialize binary value to file, there is also a node.js binding of Pickle class.

The format of asar is very flat:

| UInt32: header_size | String: header | Bytes: file1 | ... | Bytes: file42 |

The header_size and header are serialized with Pickle class, and header_size’s Pickle object is 8 bytes.

The header is a JSON string, and the header_size is the size of header’s Pickle object.

Structure of header is something like this:

{
   "files": {
      "tmp": {
         "files": {}
      },
      "usr" : {
         "files": {
           "bin": {
             "files": {
               "ls": {
                 "offset": "0",
                 "size": 100,
                 "executable": true,
                 "integrity": {
                   "algorithm": "SHA256",
                   "hash": "...",
                   "blockSize": 1024,
                   "blocks": ["...", "..."]
                 }
               },
               "cd": {
                 "offset": "100",
                 "size": 100,
                 "executable": true,
                 "integrity": {
                   "algorithm": "SHA256",
                   "hash": "...",
                   "blockSize": 1024,
                   "blocks": ["...", "..."]
                 }
               }
             }
           }
         }
      },
      "etc": {
         "files": {
           "hosts": {
             "offset": "200",
             "size": 32,
             "integrity": {
                "algorithm": "SHA256",
                "hash": "...",
                "blockSize": 1024,
                "blocks": ["...", "..."]
              }
           }
         }
      }
   }
}

offset and size records the information to read the file from archive, the offset starts from 0 so you have to manually add the size of header_size and header to the offset to get the real offset of the file.

offset is a UINT64 number represented in string, because there is no way to precisely represent UINT64 in JavaScript Number. size is a JavaScript Number that is no larger than Number.MAX_SAFE_INTEGER, which has a value of 9007199254740991 and is about 8PB in size. We didn’t store size in UINT64 because file size in Node.js is represented as Number and it is not safe to convert Number to UINT64.

integrity is an object consisting of a few keys:

A hashing algorithm, currently only SHA256 is supported.
A hex encoded hash value representing the hash of the entire file.
An array of hex encoded hashes for the blocks of the file. i.e. for a blockSize of 4KB this array contains the hash of every block if you split the file into N 4KB blocks.
A integer value blockSize representing the size in bytes of each block in the blocks hashes above

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