Recursive Extraction

Purpose

Understanding how Excavate handles nested archives automatically.

Concepts

Recursive extraction is the process of detecting and extracting archives within archives, continuing until no more nested archives are found.

The Problem

Many archive formats contain other archives:

installer.msi
├── data1.cab      # Nested CAB archive
├── data2.cab      # Nested CAB archive
└── resources/
    └── fonts.zip  # Nested ZIP archive

Without recursive extraction, you would need to:

  1. Extract the MSI

  2. Identify the CAB files

  3. Extract each CAB

  4. Identify any further nested archives

  5. Repeat

The Solution

With recursive_packages: true, Excavate handles this automatically:

Excavate::Archive.new('installer.msi').extract(recursive_packages: true)

How It Works

The recursive extraction algorithm:

1. Extract top-level archive
2. For each extracted file:
   a. Check if file is an archive (by extension)
   b. If yes, extract it
   c. Replace archive file with extracted contents
   d. Recursively check new files
3. Repeat until no archives found

Supported Nested Formats

Excavate can recursively extract any supported format within any other:

  • MSI → CAB

  • PKG → CPIO.GZ

  • RPM → CPIO.GZ

  • ZIP → any format

  • TAR.GZ → any format

  • 7-Zip → any format

Example Flow 

Installer.pkg (XAR)
    │
    ▼ Extract XAR
Package.pkg/
├── Distribution
└── Payload (CPIO.GZ)
    │
    ▼ Extract CPIO.GZ
Applications/
└── App.app/
    └── Resources/
        └── data.zip
            │
            ▼ Extract ZIP
        config/
        └── settings.json

When to Use

Use recursive extraction when:

  • Working with installers (MSI, PKG, RPM)

  • Processing archives that may contain other archives

  • You need all files, not the intermediate archives

Use non-recursive when:

  • You only need the top-level contents

  • Processing speed is critical

  • You want to examine the nested archives themselves

Performance Impact

Recursive extraction requires:

  • Multiple extraction passes

  • Additional temporary disk space

  • More I/O operations

For large archives, consider using filters to reduce work:

# Efficient: Extract only needed files
Excavate::Archive.new('large.msi').extract(
  recursive_packages: true,
  filter: '**/*.dll'
)

See Also