TAR Format - 7-Zip

TAR (Tape Archive) is a Unix standard archive format that stores multiple files and directories while preserving file metadata and permissions.

Format Overview

TAR provides:

No compression - Pure archiving format
Unix metadata preservation - Permissions, ownership, timestamps
Unlimited size - No file size restrictions
Symbolic link support - Full Unix file system support
Device file support - Character and block devices
Sparse file support - Efficient sparse file handling

TAR itself does not provide compression. It’s typically combined with GZIP (.tar.gz), BZIP2 (.tar.bz2), or XZ (.tar.xz) for compressed archives.

Format Structure

From source/CPP/7zip/Archive/Tar/TarHeader.h:11-17, TAR uses fixed-size 512-byte records:

namespace NFileHeader {
  const unsigned kRecordSize = 512;
  const unsigned kNameSize = 100;
  const unsigned kUserNameSize = 32;
  const unsigned kGroupNameSize = 32;
  const unsigned kPrefixSize = 155;
}

Each file entry consists of:

Header - 512 bytes containing metadata
Data - File content, padded to 512-byte blocks

Header Structure

Offset  Size  Field
     100   File name
   8     File mode (permissions)
   8     Owner user ID
   8     Owner group ID
   12    File size in bytes (octal)
   12    Modification time (Unix timestamp, octal)
   8     Checksum
   1     Type flag
   100   Link name
   6     Magic ("ustar")
   2     Version
   32    Owner user name
   32    Owner group name
   8     Device major number
   8     Device minor number
   155   Filename prefix

File Types

TAR supports various file types (TarHeader.h:47-73):

namespace NLinkFlag {
  const char kNormal       = '0';  // Normal file
  const char kHardLink     = '1';  // Hard link
  const char kSymLink      = '2';  // Symbolic link
  const char kCharacter    = '3';  // Character device
  const char kBlock        = '4';  // Block device
  const char kDirectory    = '5';  // Directory
  const char kFIFO         = '6';  // FIFO (named pipe)
  const char kContiguous   = '7';  // Contiguous file
}

Extended Types

GNU TAR and PAX formats add:

kGnu_LongName (‘L’) - Long filename (>100 chars)
kGnu_LongLink (‘K’) - Long link name
kPax (‘x’) - POSIX.1-2001 extended header
kGlobal (‘g’) - Global extended header

TAR Variants

USTAR Format

The POSIX standard TAR format (TarHeader.h:83-84):

namespace NMagic {
  extern const char k_Posix_ustar_00[8];  // "ustar\0" + version "00"
  extern const char k_GNU_ustar[8];        // GNU TAR variant
}

PAX Format

Modern POSIX.1-2001 format supporting:

Unlimited filename length
Unlimited file size
Extended attributes
High-precision timestamps

Usage Examples

Create TAR Archive

7z a -ttar archive.tar files/

Creates uncompressed TAR archive.

Create TAR.GZ (GZIP Compressed)

7z a -ttar archive.tar files/
7z a -tgzip archive.tar.gz archive.tar

Or in one step:

tar -czf archive.tar.gz files/  # Using system tar

Create TAR.BZ2 (BZIP2 Compressed)

7z a -ttar archive.tar files/
7z a -tbzip2 archive.tar.bz2 archive.tar

Create TAR.XZ (XZ Compressed)

7z a -ttar archive.tar files/
7z a -txz archive.tar.xz archive.tar

XZ compression typically provides the best compression ratio for TAR archives, while GZIP is fastest. BZIP2 offers a middle ground.

Extract TAR Archive

7z x archive.tar

Extracts all files preserving directory structure.

Extract TAR.GZ Archive

7z x archive.tar.gz -so | 7z x -si -ttar

Or simply:

7z x archive.tar.gz
7z x archive.tar

List TAR Contents

7z l archive.tar

Shows file list with permissions and sizes.

Preserving Unix Metadata

File Permissions

TAR stores full Unix permissions (rwxrwxrwx):

7z a -ttar archive.tar files/

On Windows, 7-Zip maps Windows attributes to Unix permissions:

Read-only → r--
Normal → rw-
Executable → rwx

Symbolic Links

TAR preserves symbolic links:

7z a -ttar -snl archive.tar files/

The -snl option ensures symbolic links are stored as links, not as file copies.

Timestamps

TAR stores modification time with second precision. Extended formats (PAX) support nanosecond precision.

Advanced Features

Long Filenames

Standard TAR limits filenames to 100 characters. GNU TAR and PAX support unlimited lengths:

# Automatically handles long filenames
7z a -ttar archive.tar very/long/path/to/file/with/name/longer/than/100/characters.txt

Sparse Files

GNU TAR efficiently handles sparse files:

const char kSparse = 'S';  // Sparse file flag

Sparse files with large zero regions are stored efficiently.

Incremental Backups

GNU TAR supports incremental backups:

const char kDumpDir = 'D';  // GNU dump directory

Hard Links

TAR stores hard links efficiently:

7z a -ttar archive.tar files/

Hard links reference the same data block, saving space.

Compression Comparison

Same Dataset (1 GB source files)

Format	Archive Size	Compression Time	Extraction Time
.tar	1000 MB	10s	8s
.tar.gz	350 MB	60s	20s
.tar.bz2	320 MB	120s	40s
.tar.xz	280 MB	180s	30s

Compression ratios and times vary significantly based on data type and system performance.

Implementation Details

TAR handler implementation:

source/CPP/7zip/Archive/Tar/
├── TarHandler.cpp      # Archive reading
├── TarHandlerOut.cpp   # Archive creation
├── TarIn.cpp          # TAR parsing
├── TarOut.cpp         # TAR writing
├── TarHeader.h        # Format constants
├── TarItem.h          # Item structures
└── TarUpdate.cpp      # Archive updating

Best Practices

For Linux/Unix Systems

Use TAR for maximum compatibility:

7z a -ttar backup.tar /home/user/
7z a -txz backup.tar.xz backup.tar

For Cross-Platform

TAR.GZ offers best compatibility:

7z a -ttar archive.tar files/
7z a -tgzip archive.tar.gz archive.tar

For Large Archives

Use XZ compression for best ratio:

7z a -ttar data.tar large_dataset/
7z a -txz -mx=9 data.tar.xz data.tar

For Fast Backup

Use GZIP for speed:

7z a -ttar backup.tar files/
7z a -tgzip -mx=1 backup.tar.gz backup.tar

Common Patterns

Backup with Timestamp

BACKUP_DATE=$(date +%Y%m%d)
7z a -ttar backup-$BACKUP_DATE.tar /data/
7z a -txz backup-$BACKUP_DATE.tar.xz backup-$BACKUP_DATE.tar
rm backup-$BACKUP_DATE.tar

Extract Specific Files

7z x archive.tar -i!specific/file.txt

Exclude Patterns

7z a -ttar archive.tar /source/ -x!*.tmp -x!*.log -x!.git

Limitations

TAR format limitations:

No built-in compression (must use external compression)
No encryption support
No error recovery or redundancy
Limited metadata on non-Unix systems
Standard TAR limited to 8 GB files (fixed in PAX/GNU)

Compatibility Notes

Windows

7-Zip handles TAR on Windows but:

File permissions are approximated
Symbolic links require admin privileges or Developer Mode
Device files are ignored

Linux/macOS

Full TAR support including:

All file types
Complete permission preservation
Extended attributes (with PAX format)

Archive Tools

TAR compatibility:

7-Zip - Full support for GNU TAR and PAX
GNU tar - Reference implementation
BSD tar - Full POSIX TAR and PAX support
Windows tar - Basic support (Windows 10+)

​Format Overview

​Format Structure

​Header Structure

​File Types

​Extended Types

​TAR Variants

​USTAR Format

​PAX Format

​Usage Examples

​Create TAR Archive

​Create TAR.GZ (GZIP Compressed)

​Create TAR.BZ2 (BZIP2 Compressed)

​Create TAR.XZ (XZ Compressed)

​Extract TAR Archive

​Extract TAR.GZ Archive

​List TAR Contents

​Preserving Unix Metadata

​File Permissions

​Symbolic Links

​Timestamps

​Advanced Features

​Long Filenames

​Sparse Files

​Incremental Backups

​Hard Links

​Compression Comparison

​Same Dataset (1 GB source files)

​Implementation Details

​Best Practices

​For Linux/Unix Systems

​For Cross-Platform

​For Large Archives

​For Fast Backup

​Common Patterns

​Backup with Timestamp

​Extract Specific Files

​Exclude Patterns

​Limitations

​Compatibility Notes

​Windows

​Linux/macOS

​Archive Tools

​See Also

Format Overview

Format Structure

Header Structure

File Types

Extended Types

TAR Variants

USTAR Format

PAX Format

Usage Examples

Create TAR Archive

Create TAR.GZ (GZIP Compressed)

Create TAR.BZ2 (BZIP2 Compressed)

Create TAR.XZ (XZ Compressed)

Extract TAR Archive

Extract TAR.GZ Archive

List TAR Contents

Preserving Unix Metadata

File Permissions

Symbolic Links

Timestamps

Advanced Features

Long Filenames

Sparse Files

Incremental Backups

Hard Links

Compression Comparison

Same Dataset (1 GB source files)

Implementation Details

Best Practices

For Linux/Unix Systems

For Cross-Platform

For Large Archives

For Fast Backup

Common Patterns

Backup with Timestamp

Extract Specific Files

Exclude Patterns

Limitations

Compatibility Notes

Windows

Linux/macOS

Archive Tools

See Also