7-Zip Archive (7z)

7-Zip is a high-compression file archiver that uses the LZMA and LZMA2 compression algorithms to achieve excellent compression ratios. The 7z format is the native archive format of the 7-Zip application, offering advanced features and superior compression performance.

Overview

7-Zip was developed by Igor Pavlov and first released in 1999. The 7z format supports multiple compression algorithms, strong encryption, large file sizes, and Unicode filenames, making it one of the most versatile and efficient archive formats available.

File Characteristics

• File Extension: .7z
• MIME Type: application/x-7z-compressed
• Compression: LZMA, LZMA2, PPMd, BZIP2, DEFLATE
• Maximum File Size: 16 EiB (Exbibytes)
• Maximum Archive Size: Virtually unlimited
• Unicode Support: Full Unicode filename support

Compression Algorithms

LZMA (Lempel-Ziv-Markov chain Algorithm)

• Primary compression method for 7z
• Excellent compression ratio
• Dictionary-based compression
• Variable compression levels (0-9)

LZMA2

• Improved version of LZMA
• Better multithreading support
• Enhanced compression for certain data types
• Default algorithm in newer versions

Additional Algorithms

• PPMd: Prediction by Partial Matching
• BZIP2: Block-sorting compression
• DEFLATE: Same algorithm used in ZIP
• Copy: Store files without compression

7-Zip Command Line Usage

Creating Archives

# Basic archive creation
7z a archive.7z file1.txt file2.txt

# Compress entire directory
7z a backup.7z /path/to/directory/

# Maximum compression
7z a -mx=9 compressed.7z largefile.iso

# Password protection
7z a -p"password" secure.7z confidential.doc

# Exclude files by pattern
7z a archive.7z *.txt -x!temp.txt

Extracting Archives

# Extract all files
7z x archive.7z

# Extract to specific directory
7z x archive.7z -o/path/to/extract/

# Extract specific files
7z x archive.7z file1.txt file2.txt

# List archive contents
7z l archive.7z

# Test archive integrity
7z t archive.7z

Advanced Operations

# Update existing archive
7z u archive.7z newfile.txt

# Delete files from archive
7z d archive.7z oldfile.txt

# Add files with compression method
7z a -m0=lzma2 archive.7z data/

# Split archive into volumes
7z a -v100m archive.7z largefile.iso

Archive Structure

Header Format

[Signature Header]
[Additional Headers]
[Archive Properties]
[File Headers]
[Encoded Headers]

File Attributes

• Filename and path
• File size (compressed/uncompressed)
• Creation/modification timestamps
• File attributes and permissions
• CRC32 checksum

Compression Options

Compression Levels

• 0: Store (no compression)
• 1: Fastest compression
• 3: Fast compression
• 5: Normal compression (default)
• 7: Maximum compression
• 9: Ultra compression

Dictionary Size

# Set dictionary size (affects memory usage and compression)
7z a -md=64m archive.7z files/  # 64MB dictionary
7z a -md=1g archive.7z files/   # 1GB dictionary (better compression)

Word Size

# Set word size for LZMA
7z a -mfb=32 archive.7z files/  # 32-bit word size
7z a -mfb=64 archive.7z files/  # 64-bit word size

Encryption and Security

AES-256 Encryption

# Encrypt files and filenames
7z a -p"password" -mhe=on secure.7z sensitive/

# Encrypt only file content
7z a -p"password" secure.7z sensitive/

Security Features

• Strong AES-256 encryption
• Password-based encryption
• Header encryption (hide filenames)
• Digital signatures (with certificates)

Programming Integration

Python with py7zr

import py7zr

# Create archive
with py7zr.SevenZipFile('archive.7z', 'w') as archive:
    archive.writeall('/path/to/directory')

# Extract archive
with py7zr.SevenZipFile('archive.7z', 'r') as archive:
    archive.extractall('/path/to/extract')

# List contents
with py7zr.SevenZipFile('archive.7z', 'r') as archive:
    print(archive.getnames())

Java Integration

import org.apache.commons.compress.archivers.sevenz.*;

// Create archive
try (SevenZOutputFile output = new SevenZOutputFile(new File("archive.7z"))) {
    SevenZArchiveEntry entry = output.createArchiveEntry(file, "filename.txt");
    output.putArchiveEntry(entry);
    output.write(fileData);
    output.closeArchiveEntry();
}

// Extract archive
try (SevenZFile sevenZFile = new SevenZFile(new File("archive.7z"))) {
    SevenZArchiveEntry entry;
    while ((entry = sevenZFile.getNextEntry()) != null) {
        // Process entry
    }
}

Performance Optimization

Memory Usage

# Limit memory usage
7z a -mmemuse=512m archive.7z files/

# Solid compression (better ratio, more memory)
7z a -ms=on archive.7z files/

# Non-solid compression (less memory, faster)
7z a -ms=off archive.7z files/

Multithreading

# Set number of threads
7z a -mmt=4 archive.7z files/

# Auto-detect CPU cores
7z a -mmt=on archive.7z files/

Format Comparison

7z vs ZIP

• Compression: 7z typically 30-70% better compression
• Speed: ZIP faster for decompression
• Compatibility: ZIP more widely supported
• Features: 7z supports more compression algorithms

7z vs RAR

• Compression: Similar compression ratios
• Open Source: 7z is open source, RAR is proprietary
• Features: Both support strong encryption and recovery
• Performance: RAR slightly faster decompression

File Recovery

Archive Repair

# Test and repair archive
7z t archive.7z

# Extract damaged archive (skip errors)
7z x -y archive.7z

Recovery Records

7z doesn't have built-in recovery records, but can be used with:

• External parity files (PAR2)
• File system redundancy
• Regular backup verification

Best Practices

Archive Creation

1. Choose Appropriate Compression: Balance size vs. speed
2. Use Solid Compression: For similar files
3. Set Reasonable Dictionary Size: Based on available RAM
4. Test Archives: Verify integrity after creation
5. Document Passwords: Securely store encryption passwords

Performance Optimization

1. Adjust Thread Count: Match to CPU cores
2. Monitor Memory Usage: Prevent system overload
3. Use Appropriate Algorithms: LZMA2 for general use
4. Consider Archive Size: Split large archives if needed

Security Considerations

1. Strong Passwords: Use complex passwords for encryption
2. Header Encryption: Hide filenames for sensitive data
3. Secure Deletion: Properly delete original files if needed
4. Access Control: Protect archive files from unauthorized access

Cross-Platform Support

Operating Systems

• Windows: Native 7-Zip application
• Linux: p7zip package
• macOS: Keka, The Unarchiver, p7zip
• Mobile: Various third-party apps

GUI Applications

• 7-Zip: Windows native application
• PeaZip: Cross-platform archive manager
• Keka: macOS archive utility
• File Roller: Linux GNOME archive manager

The 7z format represents one of the most advanced and efficient compression formats available, offering excellent compression ratios and robust features for both personal and professional use.