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asar - Atom-Shell Archive

build status dependencies npm version

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


Command line utility


bash $ npm install asar


```bash $ asar –help

Usage: asar [options] [command]


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


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


Using programatically


```js var asar = require(‘asar’);

var src = ‘some/path/’; var dest = ‘name.asar’;

asar.createPackage(src, dest, function() { console.log(‘done.’); }) ```

Please note that there is currently no error handling provided!

Using with grunt

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


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:

json { "files": { "tmp": { "files": {} }, "usr" : { "files": { "bin": { "files": { "ls": { "offset": "0", "size": 100, "executable": true }, "cd": { "offset": "100", "size": 100, "executable": true } } } } }, "etc": { "files": { "hosts": { "offset": "200", "size": 32 } } } } }

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.