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