Format Detection

Purpose

Understanding how Excavate identifies archive formats.

Concepts

Format detection is the process of determining which extractor to use for a given file. Excavate uses file extension as the primary detection method.

Extension-Based Detection

Excavate maps file extensions to extractors:

TYPES = {
  "cab"  => CabExtractor,
  "cpio" => CpioExtractor,
  "exe"  => SevenZipExtractor,
  "gz"   => GzipExtractor,
  "msi"  => OleExtractor,
  "pkg"  => XarExtractor,
  "rpm"  => RpmExtractor,
  "tar"  => TarExtractor,
  "xz"   => XzExtractor,
  "zip"  => ZipExtractor
}

Compound Extensions

Some formats use compound extensions:

  • .tar.gz - TAR + GZIP

  • .tar.xz - TAR + XZ

  • .tgz - Short for .tar.gz

  • .txz - Short for .tar.xz

Excavate handles these automatically by extracting layers in sequence.

Special Cases

EXE Files

EXE files may be 7-Zip or CAB self-extracting archives:

# Try 7-Zip first
# If fails, try CAB
Excavate::Archive.new('installer.exe').extract

GZIP Verification

For GZIP files, Excavate verifies magic bytes:

# Checks for \x1F\x8B magic bytes
# Prevents extracting non-GZIP files with .gz extension
Excavate::Archive.new('file.gz').extract

Magic Byte Detection

Limited magic byte detection is available via FileMagic:

format = Excavate::FileMagic.detect('archive.cab')
# => :cab

format = Excavate::FileMagic.detect('file.gz')
# => :gzip

Supported magic byte detection:

Format Magic Bytes Return

CAB

MSCF\x00\x00\x00\x00

:cab

GZIP

\x1F\x8B

:gzip

XZ

\xFD7zXZ\x00

:xz

Limitations

Extension-based detection has limitations:

  • Files must have correct extensions

  • Renamed files may not extract properly

  • Content-based detection is limited

Workarounds:

# Rename file to correct extension
File.rename('unknown.dat', 'archive.zip')
Excavate::Archive.new('archive.zip').extract

Best Practices

  • Ensure files have correct extensions

  • Use .tar.gz not .tgz for clarity

  • Verify format if extraction fails

See Also