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added small doc about tar 'pax header' format
This commit is contained in:
Denis Vlasenko
parent
664733f1a3
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ec0c920a78
239
docs/tar_pax.txt
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239
docs/tar_pax.txt
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'pax headers' is POSIX 2003 (iirc) addition designed to fix
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tar format limitations - older tar format has fixed fields
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for everything (filename, uid, filesize etc) which can overflow.
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pax Header Block
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The pax header block shall be identical to the ustar header block
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described in ustar Interchange Format, except that two additional
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typeflag values are defined:
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x
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Represents extended header records for the following file in
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the archive (which shall have its own ustar header block).
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g
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Represents global extended header records for the following
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files in the archive. Each value shall affect all subsequent files
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that do not override that value in their own extended header
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record and until another global extended header record is reached
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that provides another value for the same field. The typeflag g
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global headers should not be used with interchange media that
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could suffer partial data loss in transporting the archive.
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For both of these types, the size field shall be the size of the
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extended header records in octets. The other fields in the header
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block are not meaningful to this version of the pax utility.
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However, if this archive is read by a pax utility conforming to
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the ISO POSIX-2:1993 standard, the header block fields are used to
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create a regular file that contains the extended header records as
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data. Therefore, header block field values should be selected to
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provide reasonable file access to this regular file.
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A further difference from the ustar header block is that data
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blocks for files of typeflag 1 (the digit one) (hard link) may be
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included, which means that the size field may be greater than
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zero.
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pax Extended Header
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An extended header shall consist of one or more records, each
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constructed as follows:
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"%d %s=%s\n", <length>, <keyword>, <value>
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The <length> field shall be the decimal length of the extended
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header record in octets, including length string itself and the
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trailing <newline>.
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[skip]
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atime
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The file access time for the following file(s), equivalent to
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the value of the st_atime member of the stat structure for a file,
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as described by the stat() function. The access time shall be
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restored if the process has the appropriate privilege required to
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do so. The format of the <value> shall be as described in pax
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Extended Header File Times.
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charset
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The name of the character set used to encode the data in the
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following file(s).
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The encoding is included in an extended header for information
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only; when pax is used as described in IEEE Std 1003.1-2001, it
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shall not translate the file data into any other encoding. The
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BINARY entry indicates unencoded binary data.
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When used in write or copy mode, it is implementation-defined
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whether pax includes a charset extended header record for a file.
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comment
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A series of characters used as a comment. All characters in
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the <value> field shall be ignored by pax.
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gid
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The group ID of the group that owns the file, expressed as a
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decimal number using digits from the ISO/IEC 646:1991 standard.
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This record shall override the gid field in the following header
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block(s). When used in write or copy mode, pax shall include a gid
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extended header record for each file whose group ID is greater
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than 2097151 (octal 7777777).
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gname
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The group of the file(s), formatted as a group name in the
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group database. This record shall override the gid and gname
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fields in the following header block(s), and any gid extended
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header record. When used in read, copy, or list mode, pax shall
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translate the name from the UTF-8 encoding in the header record to
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the character set appropriate for the group database on the
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receiving system. If any of the UTF-8 characters cannot be
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translated, and if the -o invalid= UTF-8 option is not specified,
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the results are implementation-defined. When used in write or copy
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mode, pax shall include a gname extended header record for each
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file whose group name cannot be represented entirely with the
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letters and digits of the portable character set.
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linkpath
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The pathname of a link being created to another file, of any
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type, previously archived. This record shall override the linkname
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field in the following ustar header block(s). The following ustar
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header block shall determine the type of link created. If typeflag
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of the following header block is 1, it shall be a hard link. If
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typeflag is 2, it shall be a symbolic link and the linkpath value
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shall be the contents of the symbolic link. The pax utility shall
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translate the name of the link (contents of the symbolic link)
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from the UTF-8 encoding to the character set appropriate for the
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local file system. When used in write or copy mode, pax shall
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include a linkpath extended header record for each link whose
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pathname cannot be represented entirely with the members of the
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portable character set other than NUL.
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mtime
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The file modification time of the following file(s),
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equivalent to the value of the st_mtime member of the stat
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structure for a file, as described in the stat() function. This
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record shall override the mtime field in the following header
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block(s). The modification time shall be restored if the process
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has the appropriate privilege required to do so. The format of the
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<value> shall be as described in pax Extended Header File Times.
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path
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The pathname of the following file(s). This record shall
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override the name and prefix fields in the following header
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block(s). The pax utility shall translate the pathname of the file
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from the UTF-8 encoding to the character set appropriate for the
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local file system.
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When used in write or copy mode, pax shall include a path
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extended header record for each file whose pathname cannot be
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represented entirely with the members of the portable character
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set other than NUL.
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realtime.any
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The keywords prefixed by "realtime." are reserved for future
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standardization.
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security.any
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The keywords prefixed by "security." are reserved for future
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standardization.
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size
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The size of the file in octets, expressed as a decimal number
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using digits from the ISO/IEC 646:1991 standard. This record shall
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override the size field in the following header block(s). When
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used in write or copy mode, pax shall include a size extended
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header record for each file with a size value greater than
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8589934591 (octal 77777777777).
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uid
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The user ID of the file owner, expressed as a decimal number
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using digits from the ISO/IEC 646:1991 standard. This record shall
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override the uid field in the following header block(s). When used
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in write or copy mode, pax shall include a uid extended header
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record for each file whose owner ID is greater than 2097151 (octal
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7777777).
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uname
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The owner of the following file(s), formatted as a user name
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in the user database. This record shall override the uid and uname
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fields in the following header block(s), and any uid extended
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header record. When used in read, copy, or list mode, pax shall
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translate the name from the UTF-8 encoding in the header record to
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the character set appropriate for the user database on the
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receiving system. If any of the UTF-8 characters cannot be
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translated, and if the -o invalid= UTF-8 option is not specified,
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the results are implementation-defined. When used in write or copy
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mode, pax shall include a uname extended header record for each
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file whose user name cannot be represented entirely with the
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letters and digits of the portable character set.
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If the <value> field is zero length, it shall delete any header
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block field, previously entered extended header value, or global
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extended header value of the same name.
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If a keyword in an extended header record (or in a -o
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option-argument) overrides or deletes a corresponding field in the
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ustar header block, pax shall ignore the contents of that header
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block field.
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Unlike the ustar header block fields, NULs shall not delimit
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<value>s; all characters within the <value> field shall be
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considered data for the field. None of the length limitations of
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the ustar header block fields in ustar Header Block shall apply to
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the extended header records.
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pax Extended Header File Times
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Time records shall be formatted as a decimal representation of the
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time in seconds since the Epoch. If a period ( '.' ) decimal point
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character is present, the digits to the right of the point shall
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represent the units of a subsecond timing granularity. In read or
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copy mode, the pax utility shall truncate the time of a file to
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the greatest value that is not greater than the input header
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file time. In write or copy mode, the pax utility shall output a
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time exactly if it can be represented exactly as a decimal number,
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and otherwise shall generate only enough digits so that the same
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time shall be recovered if the file is extracted on a system whose
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underlying implementation supports the same time granularity.
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Example from Linux kernel archive tarball:
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00000000 70 61 78 5f 67 6c 6f 62 61 6c 5f 68 65 61 64 65 |pax_global_heade|
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00000010 72 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |r...............|
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00000020 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000030 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000040 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000050 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000060 00 00 00 00 30 30 30 30 36 36 36 00 30 30 30 30 |....0000666.0000|
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00000070 30 30 30 00 30 30 30 30 30 30 30 00 30 30 30 30 |000.0000000.0000|
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00000080 30 30 30 30 30 36 34 00 30 30 30 30 30 30 30 30 |0000064.00000000|
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00000090 30 30 30 00 30 30 31 34 30 35 33 00 67 00 00 00 |000.0014053.g...|
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000000a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000000f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000100 00 75 73 74 61 72 00 30 30 67 69 74 00 00 00 00 |.ustar.00git....|
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00000110 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000120 00 00 00 00 00 00 00 00 00 67 69 74 00 00 00 00 |.........git....|
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00000130 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000140 00 00 00 00 00 00 00 00 00 30 30 30 30 30 30 30 |.........0000000|
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00000150 00 30 30 30 30 30 30 30 00 00 00 00 00 00 00 00 |.0000000........|
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00000160 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000170 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000180 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000190 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001a0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001b0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001c0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001d0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001e0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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000001f0 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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00000200 35 32 20 63 6f 6d 6d 65 6e 74 3d 62 31 30 35 30 |52 comment=b1050|
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00000210 32 62 32 32 61 31 32 30 39 64 36 62 34 37 36 33 |2b22a1209d6b4763|
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00000220 39 64 38 38 62 38 31 32 62 32 31 66 62 35 39 34 |9d88b812b21fb594|
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00000230 39 65 34 0a 00 00 00 00 00 00 00 00 00 00 00 00 |9e4.............|
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00000240 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 |................|
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...
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