path: root/doc/tarlz.info

This is tarlz.info, produced by makeinfo version 4.13+ from tarlz.texi.

INFO-DIR-SECTION Data Compression
START-INFO-DIR-ENTRY
* Tarlz: (tarlz).               Archiver with multimember lzip compression
END-INFO-DIR-ENTRY


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Tarlz Manual
************

This manual is for Tarlz (version 0.8, 16 December 2018).

* Menu:

* Introduction::              Purpose and features of tarlz
* Invoking tarlz::            Command line interface
* File format::               Detailed format of the compressed archive
* Amendments to pax format::  The reasons for the differences with pax
* Examples::                  A small tutorial with examples
* Problems::                  Reporting bugs
* Concept index::             Index of concepts


   Copyright (C) 2013-2018 Antonio Diaz Diaz.

   This manual is free documentation: you have unlimited permission to
copy, distribute and modify it.


File: tarlz.info,  Node: Introduction,  Next: Invoking tarlz,  Prev: Top,  Up: Top

1 Introduction
**************

Tarlz is a small and simple implementation of the tar archiver. By
default tarlz creates, lists and extracts archives in a simplified
posix pax format compressed with lzip on a per file basis. Each tar
member is compressed in its own lzip member, as well as the end-of-file
blocks. This method is fully backward compatible with standard tar tools
like GNU tar, which treat the resulting multimember tar.lz archive like
any other tar.lz archive. Tarlz can append files to the end of such
compressed archives.

   Tarlz can create tar archives with four levels of compression
granularity; per file, per directory, appendable solid, and solid.

Of course, compressing each file (or each directory) individually is
less efficient than compressing the whole tar archive, but it has the
following advantages:

   * The resulting multimember tar.lz archive can be decompressed in
     parallel with plzip, multiplying the decompression speed.

   * New members can be appended to the archive (by removing the EOF
     member) just like to an uncompressed tar archive.

   * It is a safe posix-style backup format. In case of corruption,
     tarlz can extract all the undamaged members from the tar.lz
     archive, skipping over the damaged members, just like the standard
     (uncompressed) tar. Moreover, the option '--keep-damaged' can be
     used to recover as much data as possible from each damaged member,
     and lziprecover can be used to recover some of the damaged members.

   * A multimember tar.lz archive is usually smaller than the
     corresponding solidly compressed tar.gz archive, except when
     individually compressing files smaller than about 32 KiB.

   Tarlz protects the extended records with a CRC in a way compatible
with standard tar tools. *Note crc32::.

   Tarlz does not understand other tar formats like 'gnu', 'oldgnu',
'star' or 'v7'.

   Tarlz is intended as a showcase project for the maintainers of real
tar programs to evaluate the format and perhaps implement it in their
tools.


File: tarlz.info,  Node: Invoking tarlz,  Next: File format,  Prev: Introduction,  Up: Top

2 Invoking tarlz
****************

The format for running tarlz is:

     tarlz [OPTIONS] [FILES]

On archive creation or appending, tarlz removes leading and trailing
slashes from filenames, as well as filename prefixes containing a '..'
component. On extraction, archive members containing a '..' component
are skipped. Tarlz detects when the archive being created or enlarged
is among the files to be dumped, appended or concatenated, and skips it.

   On extraction and listing, tarlz removes leading './' strings from
member names in the archive or given in the command line, so that
'tarlz -xf foo ./bar baz' extracts members 'bar' and './baz' from
archive 'foo'.

   tarlz supports the following options:

'-h'
'--help'
     Print an informative help message describing the options and exit.

'-V'
'--version'
     Print the version number of tarlz on the standard output and exit.
     This version number should be included in all bug reports.

'-A'
'--concatenate'
     Append tar.lz archives to the end of a tar.lz archive. All the
     archives involved must be regular (seekable) files compressed as
     multimember lzip files, and the two end-of-file blocks plus any
     zero padding must be contained in the last lzip member of each
     archive. The intermediate end-of-file blocks are removed as each
     new archive is concatenated. Exit with status 0 without modifying
     the archive if no FILES have been specified. Tarlz can't
     concatenate uncompressed tar archives.

'-c'
'--create'
     Create a new archive from FILES.

'-C DIR'
'--directory=DIR'
     Change to directory DIR. When creating or appending, the position
     of each '-C' option in the command line is significant; it will
     change the current working directory for the following FILES until
     a new '-C' option appears in the command line. When extracting, all
     the '-C' options are executed in sequence before starting the
     extraction. Listing ignores any '-C' options specified. DIR is
     relative to the then current working directory, perhaps changed by
     a previous '-C' option.

'-f ARCHIVE'
'--file=ARCHIVE'
     Use archive file ARCHIVE. '-' used as an ARCHIVE argument reads
     from standard input or writes to standard output.

'-q'
'--quiet'
     Quiet operation. Suppress all messages.

'-r'
'--append'
     Append files to the end of a tar.lz archive. The archive must be a
     regular (seekable) file compressed as a multimember lzip file, and
     the two end-of-file blocks plus any zero padding must be contained
     in the last lzip member of the archive. First this last member is
     removed, then the new members are appended, and then a new
     end-of-file member is appended to the archive. Exit with status 0
     without modifying the archive if no FILES have been specified.
     Tarlz can't append files to an uncompressed tar archive.

'-t'
'--list'
     List the contents of an archive. If FILES are given, list only the
     given FILES.

'-v'
'--verbose'
     Verbosely list files processed.

'-x'
'--extract'
     Extract files from an archive. If FILES are given, extract only
     the given FILES. Else extract all the files in the archive.

'-0 .. -9'
     Set the compression level. The default compression level is '-6'.
     Like lzip, tarlz also minimizes the dictionary size of the lzip
     members it creates, reducing the amount of memory required for
     decompression.

'--asolid'
     When creating or appending to a compressed archive, use appendable
     solid compression. All the files being added to the archive are
     compressed into a single lzip member, but the end-of-file blocks
     are compressed into a separate lzip member. This creates a solidly
     compressed appendable archive.

'--dsolid'
     When creating or appending to a compressed archive, use solid
     compression for each directory especified in the command line. The
     end-of-file blocks are compressed into a separate lzip member. This
     creates a compressed appendable archive with a separate lzip
     member for each top-level directory.

'--no-solid'
     When creating or appending to a compressed archive, compress each
     file separately. The end-of-file blocks are compressed into a
     separate lzip member. This creates a compressed appendable archive
     with a separate lzip member for each file. This option allows
     tarlz revert to default behavior if, for example, tarlz is invoked
     through an alias like 'tar='tarlz --solid''.

'--solid'
     When creating or appending to a compressed archive, use solid
     compression. The files being added to the archive, along with the
     end-of-file blocks, are compressed into a single lzip member. The
     resulting archive is not appendable. No more files can be later
     appended to the archive.

'--anonymous'
     Equivalent to '--owner=root --group=root'.

'--owner=OWNER'
     When creating or appending, use OWNER for files added to the
     archive. If OWNER is not a valid user name, it is decoded as a
     decimal numeric user ID.

'--group=GROUP'
     When creating or appending, use GROUP for files added to the
     archive. If GROUP is not a valid group name, it is decoded as a
     decimal numeric group ID.

'--keep-damaged'
     Don't delete partially extracted files. If a decompression error
     happens while extracting a file, keep the partial data extracted.
     Use this option to recover as much data as possible from each
     damaged member.

'--missing-crc'
     Exit with error status 2 if the CRC of the extended records is
     missing.  When this option is used, tarlz detects any corruption
     in the extended records (only limited by CRC collisions). But note
     that a corrupt 'GNU.crc32' keyword, for example 'GNU.crc33', is
     reported as a missing CRC instead of as a corrupt record. This
     misleading 'Missing CRC' message is the consequence of a flaw in
     the posix pax format; i.e., the lack of a mandatory check sequence
     in the extended records. *Note crc32::.

'--uncompressed'
     With '--create', don't compress the created tar archive. Create an
     uncompressed tar archive instead.


   Exit status: 0 for a normal exit, 1 for environmental problems (file
not found, invalid flags, I/O errors, etc), 2 to indicate a corrupt or
invalid input file, 3 for an internal consistency error (eg, bug) which
caused tarlz to panic.


File: tarlz.info,  Node: File format,  Next: Amendments to pax format,  Prev: Invoking tarlz,  Up: Top

3 File format
*************

In the diagram below, a box like this:
+---+
|   | <-- the vertical bars might be missing
+---+

   represents one byte; a box like this:
+==============+
|              |
+==============+

   represents a variable number of bytes or a fixed but large number of
bytes (for example 512).


   A tar.lz file consists of a series of lzip members (compressed data
sets).  The members simply appear one after another in the file, with no
additional information before, between, or after them.

   Each lzip member contains one or more tar members in a simplified
posix pax interchange format; the only pax typeflag value supported by
tarlz (in addition to the typeflag values defined by the ustar format)
is 'x'. The pax format is an extension on top of the ustar format that
removes the size limitations of the ustar format.

   Each tar member contains one file archived, and is represented by the
following sequence:

   * An optional extended header block with extended header records.
     This header block is of the form described in pax header block,
     with a typeflag value of 'x'. The extended header records are
     included as the data for this header block.

   * A header block in ustar format that describes the file. Any fields
     defined in the preceding optional extended header records override
     the associated fields in this header block for this file.

   * Zero or more blocks that contain the contents of the file.

   At the end of the archive file there are two 512-byte blocks filled
with binary zeros, interpreted as an end-of-archive indicator. These EOF
blocks are either compressed in a separate lzip member or compressed
along with the tar members contained in the last lzip member.

   The diagram below shows the correspondence between each tar member
(formed by one or two headers plus optional data) in the tar archive and
each lzip member in the resulting multimember tar.lz archive: *Note
File format: (lzip)File format.

tar
+========+======+=================+===============+========+======+========+
| header | data | extended header | extended data | header | data |   EOF  |
+========+======+=================+===============+========+======+========+

tar.lz
+===============+=================================================+========+
|     member    |                      member                     | member |
+===============+=================================================+========+


3.1 Pax header block
====================

The pax header block is identical to the ustar header block described
below except that the typeflag has the value 'x' (extended). The size
field is the size of the extended header data in bytes. Most other
fields in the pax header block are zeroed on archive creation to
prevent trouble if the archive is read by an ustar tool, and are
ignored by tarlz on archive extraction. *Note flawed-compat::.

   The pax extended header data consists of one or more records, each of
them constructed as follows:
'"%d %s=%s\n", <length>, <keyword>, <value>'

   The <length>, <blank>, <keyword>, <equals-sign>, and <newline> in the
record must be limited to the portable character set. The <length> field
contains the decimal length of the record in bytes, including the
trailing <newline>. The <value> field is stored as-is, without
conversion to UTF-8 nor any other transformation.

   These are the <keyword> fields currently supported by tarlz:

'linkpath'
     The pathname of a link being created to another file, of any type,
     previously archived. This record overrides the linkname field in
     the following ustar header block. The following ustar header block
     determines the type of link created. If typeflag of the following
     header block is 1, it will be a hard link. If typeflag is 2, it
     will be a symbolic link and the linkpath value will be used as the
     contents of the symbolic link.

'path'
     The pathname of the following file. This record overrides the name
     and prefix fields in the following ustar header block.

'size'
     The size of the file in bytes, expressed as a decimal number using
     digits from the ISO/IEC 646:1991 (ASCII) standard. This record
     overrides the size field in the following ustar header block. The
     size record is used only for files with a size value greater than
     8_589_934_591 (octal 77777777777). This is 2^33 bytes or larger.

'GNU.crc32'
     CRC32-C (Castagnoli) of the extended header data excluding the 8
     bytes representing the CRC <value> itself. The <value> is
     represented as 8 hexadecimal digits in big endian order,
     '22 GNU.crc32=00000000\n'. The keyword of the CRC record is
     protected by the CRC to guarante that corruption is always detected
     (except in case of CRC collision). A CRC was chosen because a
     checksum is too weak for a potentially large list of variable
     sized records. A checksum can't detect simple errors like the
     swapping of two bytes.


3.2 Ustar header block
======================

The ustar header block has a length of 512 bytes and is structured as
shown in the following table. All lengths and offsets are in decimal.

Field Name   Offset   Length (in bytes)
name         0        100
mode         100      8
uid          108      8
gid          116      8
size         124      12
mtime        136      12
chksum       148      8
typeflag     156      1
linkname     157      100
magic        257      6
version      263      2
uname        265      32
gname        297      32
devmajor     329      8
devminor     337      8
prefix       345      155

   All characters in the header block are coded using the ISO/IEC
646:1991 (ASCII) standard, except in fields storing names for files,
users, and groups. For maximum portability between implementations,
names should only contain characters from the portable filename
character set. But if an implementation supports the use of characters
outside of '/' and the portable filename character set in names for
files, users, and groups, tarlz will use the byte values in these names
unmodified.

   The fields name, linkname, and prefix are null-terminated character
strings except when all characters in the array contain non-null
characters including the last character.

   The name and the prefix fields produce the pathname of the file. A
new pathname is formed, if prefix is not an empty string (its first
character is not null), by concatenating prefix (up to the first null
character), a <slash> character, and name; otherwise, name is used
alone. In either case, name is terminated at the first null character.
If prefix begins with a null character, it is ignored. In this manner,
pathnames of at most 256 characters can be supported. If a pathname does
not fit in the space provided, an extended record is used to store the
pathname.

   The linkname field does not use the prefix to produce a pathname. If
the linkname does not fit in the 100 characters provided, an extended
record is used to store the linkname.

   The mode field provides 12 access permission bits. The following
table shows the symbolic name of each bit and its octal value:

Bit Name   Bit value
S_ISUID    04000
S_ISGID    02000
S_ISVTX    01000
S_IRUSR    00400
S_IWUSR    00200
S_IXUSR    00100
S_IRGRP    00040
S_IWGRP    00020
S_IXGRP    00010
S_IROTH    00004
S_IWOTH    00002
S_IXOTH    00001

   The uid and gid fields are the user and group ID of the owner and
group of the file, respectively.

   The size field contains the octal representation of the size of the
file in bytes. If the typeflag field specifies a file of type '0'
(regular file) or '7' (high performance regular file), the number of
logical records following the header is (size / 512) rounded to the next
integer. For all other values of typeflag, tarlz either sets the size
field to 0 or ignores it, and does not store or expect any logical
records following the header. If the file size is larger than
8_589_934_591 bytes (octal 77777777777), an extended record is used to
store the file size.

   The mtime field contains the octal representation of the modification
time of the file at the time it was archived, obtained from the stat()
function.

   The chksum field contains the octal representation of the value of
the simple sum of all bytes in the header logical record. Each byte in
the header is treated as an unsigned value. When calculating the
checksum, the chksum field is treated as if it were all <space>
characters.

   The typeflag field contains a single character specifying the type of
file archived:

''0''
     Regular file.

''1''
     Hard link to another file, of any type, previously archived.

''2''
     Symbolic link.

''3', '4''
     Character special file and block special file respectively. In
     this case the devmajor and devminor fields contain information
     defining the device in unspecified format.

''5''
     Directory.

''6''
     FIFO special file.

''7''
     Reserved to represent a file to which an implementation has
     associated some high-performance attribute. Tarlz treats this type
     of file as a regular file (type 0).


   The magic field contains the ASCII null-terminated string "ustar".
The version field contains the characters "00" (0x30,0x30). The fields
uname, and gname are null-terminated character strings. Each numeric
field contains a leading zero-filled, null-terminated octal number using
digits from the ISO/IEC 646:1991 (ASCII) standard.


File: tarlz.info,  Node: Amendments to pax format,  Next: Examples,  Prev: File format,  Up: Top

4 The reasons for the differences with pax
******************************************

Tarlz is meant to reliably detect invalid or corrupt metadata during
extraction and to not create safety risks in the archives it creates. In
order to achieve these goals, tarlz makes some changes to the variant
of the pax format that it uses. This chapter describes these changes
and the concrete reasons to implement them.


4.1 Add a CRC of the extended records
=====================================

The posix pax format has a serious flaw. The metadata stored in pax
extended records are not protected by any kind of check sequence.
Corruption in a long filename may cause the extraction of the file in
the wrong place without warning. Corruption in a long file size may
cause the truncation of the file or the appending of garbage to the
file, both followed by a spurious warning about a corrupt header far
from the place of the undetected corruption.

   Metadata like filename and file size must be always protected in an
archive format because of the adverse effects of undetected corruption
in them, potentially much worse that undetected corruption in the data.
Even more so in the case of pax because the amount of metadata it
stores is potentially large, making undetected corruption more probable.

   Because of the above, tarlz protects the extended records with a CRC
in a way compatible with standard tar tools. *Note key_crc32::.


4.2 Remove flawed backward compatibility
========================================

In order to allow the extraction of pax archives by a tar utility
conforming to the POSIX-2:1993 standard, POSIX.1-2008 recommends
selecting extended header field values that allow such tar to create a
regular file containing the extended header records as data. This
approach is broken because if the extended header is needed because of
a long filename, the name and prefix fields will be unable to contain
the full pathname of the file. Therefore the files corresponding to
both the extended header and the overridden ustar header will be
extracted using truncated filenames, perhaps overwriting existing files
or directories. It may be a security risk to extract a file with a
truncated filename.

   To avoid this problem, tarlz writes extended headers with all fields
zeroed except size, chksum, typeflag, magic and version. This prevents
old tar programs from extracting the extended records as a file in the
wrong place.  Tarlz also sets to zero those fields of the ustar header
overridden by extended records.

   If the extended header is needed because of a file size larger than
8 GiB, the size field will be unable to contain the full size of the
file. Therefore the file may be partially extracted, and the tool will
issue a spurious warning about a corrupt header at the point where it
thinks the file ends. Setting to zero the overridden size in the ustar
header at least prevents the partial extraction and makes obvious that
the file has been truncated.


4.3 As simple as possible (but not simpler)
===========================================

The tarlz format is mainly ustar. Extended pax headers are used only
when needed because the length of a filename or link name, or the size
of a file exceed the limits of the ustar format. Adding extended
headers to each member just to record subsecond timestamps seems
wasteful for a backup format.


4.4 Avoid misconversions to/from UTF-8
======================================

There is no portable way to tell what charset a text string is coded
into.  Therefore, tarlz stores all fields representing text strings
as-is, without conversion to UTF-8 nor any other transformation. This
prevents accidental double UTF-8 conversions. If the need arises this
behavior will be adjusted with a command line option in the future.


File: tarlz.info,  Node: Examples,  Next: Problems,  Prev: Amendments to pax format,  Up: Top

5 A small tutorial with examples
********************************

Example 1: Create a multimember compressed archive 'archive.tar.lz'
containing files 'a', 'b' and 'c'.

     tarlz -cf archive.tar.lz a b c


Example 2: Append files 'd' and 'e' to the multimember compressed
archive 'archive.tar.lz'.

     tarlz -rf archive.tar.lz d e


Example 3: Create a solidly compressed appendable archive
'archive.tar.lz' containing files 'a', 'b' and 'c'.  Then append files
'd' and 'e' to the archive.

     tarlz --asolid -cf archive.tar.lz a b c
     tarlz --asolid -rf archive.tar.lz d e


Example 4: Create a compressed appendable archive containing directories
'dir1', 'dir2' and 'dir3' with a separate lzip member per directory.
Then append files 'a', 'b', 'c', 'd' and 'e' to the archive, all of
them contained in a single lzip member.  The resulting archive
'archive.tar.lz' contains 5 lzip members (including the EOF member).

     tarlz --dsolid -cf archive.tar.lz dir1 dir2 dir3
     tarlz --asolid -rf archive.tar.lz a b c d e


Example 5: Create a solidly compressed archive 'archive.tar.lz'
containing files 'a', 'b' and 'c'. Note that no more files can be later
appended to the archive.

     tarlz --solid -cf archive.tar.lz a b c


Example 6: Extract all files from archive 'archive.tar.lz'.

     tarlz -xf archive.tar.lz


Example 7: Extract files 'a' and 'c' from archive 'archive.tar.lz'.

     tarlz -xf archive.tar.lz a c


Example 8: Copy the contents of directory 'sourcedir' to the directory
'targetdir'.

     tarlz -C sourcedir -c . | tarlz -C targetdir -x


File: tarlz.info,  Node: Problems,  Next: Concept index,  Prev: Examples,  Up: Top

6 Reporting bugs
****************

There are probably bugs in tarlz. There are certainly errors and
omissions in this manual. If you report them, they will get fixed. If
you don't, no one will ever know about them and they will remain unfixed
for all eternity, if not longer.

   If you find a bug in tarlz, please send electronic mail to
<lzip-bug@nongnu.org>. Include the version number, which you can find
by running 'tarlz --version'.


File: tarlz.info,  Node: Concept index,  Prev: Problems,  Up: Top

Concept index
*************

[index]
* Menu:

* Amendments to pax format:              Amendments to pax format.
                                                                (line 6)
* bugs:                                  Problems.              (line 6)
* examples:                              Examples.              (line 6)
* file format:                           File format.           (line 6)
* getting help:                          Problems.              (line 6)
* introduction:                          Introduction.          (line 6)
* invoking:                              Invoking tarlz.        (line 6)
* options:                               Invoking tarlz.        (line 6)
* usage:                                 Invoking tarlz.        (line 6)
* version:                               Invoking tarlz.        (line 6)



Tag Table:
Node: Top223
Node: Introduction946
Node: Invoking tarlz3084
Node: File format9606
Ref: key_crc3214138
Node: Amendments to pax format19215
Ref: crc3219729
Ref: flawed-compat20753
Node: Examples23126
Node: Problems24802
Node: Concept index25328

End Tag Table


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