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authorTomas Bzatek <tbzatek@redhat.com>2010-02-05 11:06:31 +0100
committerTomas Bzatek <tbzatek@redhat.com>2010-02-05 11:06:31 +0100
commitbaea7d877d3cf69679a39e8512a120658a478073 (patch)
tree37c9a98cb3d3a322f3f91c8ca656ccd6bd2eaebe /libarchive/libarchive-2.7.1/doc/man/tar.5
parente42a4ff3031aa1c1aaf27aa34d9395fec185924b (diff)
downloadtuxcmd-modules-baea7d877d3cf69679a39e8512a120658a478073.tar.xz
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-.TH TAR 5 "May 20, 2004" ""
-.SH NAME
-.ad l
-\fB\%tar\fP
-\- format of tape archive files
-.SH DESCRIPTION
-.ad l
-The
-\fB\%tar\fP
-archive format collects any number of files, directories, and other
-file system objects (symbolic links, device nodes, etc.) into a single
-stream of bytes.
-The format was originally designed to be used with
-tape drives that operate with fixed-size blocks, but is widely used as
-a general packaging mechanism.
-.SS General Format
-A
-\fB\%tar\fP
-archive consists of a series of 512-byte records.
-Each file system object requires a header record which stores basic metadata
-(pathname, owner, permissions, etc.) and zero or more records containing any
-file data.
-The end of the archive is indicated by two records consisting
-entirely of zero bytes.
-.PP
-For compatibility with tape drives that use fixed block sizes,
-programs that read or write tar files always read or write a fixed
-number of records with each I/O operation.
-These
-``blocks''
-are always a multiple of the record size.
-The most common block size\(emand the maximum supported by historic
-implementations\(emis 10240 bytes or 20 records.
-(Note: the terms
-``block''
-and
-``record''
-here are not entirely standard; this document follows the
-convention established by John Gilmore in documenting
-\fB\%pdtar\fP.)
-.SS Old-Style Archive Format
-The original tar archive format has been extended many times to
-include additional information that various implementors found
-necessary.
-This section describes the variant implemented by the tar command
-included in
-At v7,
-which is one of the earliest widely-used versions of the tar program.
-.PP
-The header record for an old-style
-\fB\%tar\fP
-archive consists of the following:
-.RS 4
-.nf
-struct header_old_tar {
- char name[100];
- char mode[8];
- char uid[8];
- char gid[8];
- char size[12];
- char mtime[12];
- char checksum[8];
- char linkflag[1];
- char linkname[100];
- char pad[255];
-};
-.RE
-All unused bytes in the header record are filled with nulls.
-.RS 5
-.TP
-\fIname\fP
-Pathname, stored as a null-terminated string.
-Early tar implementations only stored regular files (including
-hardlinks to those files).
-One common early convention used a trailing "/" character to indicate
-a directory name, allowing directory permissions and owner information
-to be archived and restored.
-.TP
-\fImode\fP
-File mode, stored as an octal number in ASCII.
-.TP
-\fIuid\fP, \fIgid\fP
-User id and group id of owner, as octal numbers in ASCII.
-.TP
-\fIsize\fP
-Size of file, as octal number in ASCII.
-For regular files only, this indicates the amount of data
-that follows the header.
-In particular, this field was ignored by early tar implementations
-when extracting hardlinks.
-Modern writers should always store a zero length for hardlink entries.
-.TP
-\fImtime\fP
-Modification time of file, as an octal number in ASCII.
-This indicates the number of seconds since the start of the epoch,
-00:00:00 UTC January 1, 1970.
-Note that negative values should be avoided
-here, as they are handled inconsistently.
-.TP
-\fIchecksum\fP
-Header checksum, stored as an octal number in ASCII.
-To compute the checksum, set the checksum field to all spaces,
-then sum all bytes in the header using unsigned arithmetic.
-This field should be stored as six octal digits followed by a null and a space
-character.
-Note that many early implementations of tar used signed arithmetic
-for the checksum field, which can cause interoperability problems
-when transferring archives between systems.
-Modern robust readers compute the checksum both ways and accept the
-header if either computation matches.
-.TP
-\fIlinkflag\fP, \fIlinkname\fP
-In order to preserve hardlinks and conserve tape, a file
-with multiple links is only written to the archive the first
-time it is encountered.
-The next time it is encountered, the
-\fIlinkflag\fP
-is set to an ASCII
-Sq 1
-and the
-\fIlinkname\fP
-field holds the first name under which this file appears.
-(Note that regular files have a null value in the
-\fIlinkflag\fP
-field.)
-.RE
-.PP
-Early tar implementations varied in how they terminated these fields.
-The tar command in
-At v7
-used the following conventions (this is also documented in early BSD manpages):
-the pathname must be null-terminated;
-the mode, uid, and gid fields must end in a space and a null byte;
-the size and mtime fields must end in a space;
-the checksum is terminated by a null and a space.
-Early implementations filled the numeric fields with leading spaces.
-This seems to have been common practice until the
-IEEE Std 1003.1-1988 (``POSIX.1'')
-standard was released.
-For best portability, modern implementations should fill the numeric
-fields with leading zeros.
-.SS Pre-POSIX Archives
-An early draft of
-IEEE Std 1003.1-1988 (``POSIX.1'')
-served as the basis for John Gilmore's
-\fB\%pdtar\fP
-program and many system implementations from the late 1980s
-and early 1990s.
-These archives generally follow the POSIX ustar
-format described below with the following variations:
-.RS 5
-.IP \(bu
-The magic value is
-``ustar\ \&''
-(note the following space).
-The version field contains a space character followed by a null.
-.IP \(bu
-The numeric fields are generally filled with leading spaces
-(not leading zeros as recommended in the final standard).
-.IP \(bu
-The prefix field is often not used, limiting pathnames to
-the 100 characters of old-style archives.
-.RE
-.SS POSIX ustar Archives
-IEEE Std 1003.1-1988 (``POSIX.1'')
-defined a standard tar file format to be read and written
-by compliant implementations of
-\fBtar\fP(1).
-This format is often called the
-``ustar''
-format, after the magic value used
-in the header.
-(The name is an acronym for
-``Unix Standard TAR''.)
-It extends the historic format with new fields:
-.RS 4
-.nf
-struct header_posix_ustar {
- char name[100];
- char mode[8];
- char uid[8];
- char gid[8];
- char size[12];
- char mtime[12];
- char checksum[8];
- char typeflag[1];
- char linkname[100];
- char magic[6];
- char version[2];
- char uname[32];
- char gname[32];
- char devmajor[8];
- char devminor[8];
- char prefix[155];
- char pad[12];
-};
-.RE
-.RS 5
-.TP
-\fItypeflag\fP
-Type of entry.
-POSIX extended the earlier
-\fIlinkflag\fP
-field with several new type values:
-.RS 5
-.TP
-``0''
-Regular file.
-NUL should be treated as a synonym, for compatibility purposes.
-.TP
-``1''
-Hard link.
-.TP
-``2''
-Symbolic link.
-.TP
-``3''
-Character device node.
-.TP
-``4''
-Block device node.
-.TP
-``5''
-Directory.
-.TP
-``6''
-FIFO node.
-.TP
-``7''
-Reserved.
-.TP
-Other
-A POSIX-compliant implementation must treat any unrecognized typeflag value
-as a regular file.
-In particular, writers should ensure that all entries
-have a valid filename so that they can be restored by readers that do not
-support the corresponding extension.
-Uppercase letters "A" through "Z" are reserved for custom extensions.
-Note that sockets and whiteout entries are not archivable.
-.RE
-It is worth noting that the
-\fIsize\fP
-field, in particular, has different meanings depending on the type.
-For regular files, of course, it indicates the amount of data
-following the header.
-For directories, it may be used to indicate the total size of all
-files in the directory, for use by operating systems that pre-allocate
-directory space.
-For all other types, it should be set to zero by writers and ignored
-by readers.
-.TP
-\fImagic\fP
-Contains the magic value
-``ustar''
-followed by a NUL byte to indicate that this is a POSIX standard archive.
-Full compliance requires the uname and gname fields be properly set.
-.TP
-\fIversion\fP
-Version.
-This should be
-``00''
-(two copies of the ASCII digit zero) for POSIX standard archives.
-.TP
-\fIuname\fP, \fIgname\fP
-User and group names, as null-terminated ASCII strings.
-These should be used in preference to the uid/gid values
-when they are set and the corresponding names exist on
-the system.
-.TP
-\fIdevmajor\fP, \fIdevminor\fP
-Major and minor numbers for character device or block device entry.
-.TP
-\fIprefix\fP
-First part of pathname.
-If the pathname is too long to fit in the 100 bytes provided by the standard
-format, it can be split at any
-\fI/\fP
-character with the first portion going here.
-If the prefix field is not empty, the reader will prepend
-the prefix value and a
-\fI/\fP
-character to the regular name field to obtain the full pathname.
-.RE
-.PP
-Note that all unused bytes must be set to
-.BR NUL.
-.PP
-Field termination is specified slightly differently by POSIX
-than by previous implementations.
-The
-\fImagic\fP,
-\fIuname\fP,
-and
-\fIgname\fP
-fields must have a trailing
-.BR NUL.
-The
-\fIpathname\fP,
-\fIlinkname\fP,
-and
-\fIprefix\fP
-fields must have a trailing
-.BR NUL
-unless they fill the entire field.
-(In particular, it is possible to store a 256-character pathname if it
-happens to have a
-\fI/\fP
-as the 156th character.)
-POSIX requires numeric fields to be zero-padded in the front, and allows
-them to be terminated with either space or
-.BR NUL
-characters.
-.PP
-Currently, most tar implementations comply with the ustar
-format, occasionally extending it by adding new fields to the
-blank area at the end of the header record.
-.SS Pax Interchange Format
-There are many attributes that cannot be portably stored in a
-POSIX ustar archive.
-IEEE Std 1003.1-2001 (``POSIX.1'')
-defined a
-``pax interchange format''
-that uses two new types of entries to hold text-formatted
-metadata that applies to following entries.
-Note that a pax interchange format archive is a ustar archive in every
-respect.
-The new data is stored in ustar-compatible archive entries that use the
-``x''
-or
-``g''
-typeflag.
-In particular, older implementations that do not fully support these
-extensions will extract the metadata into regular files, where the
-metadata can be examined as necessary.
-.PP
-An entry in a pax interchange format archive consists of one or
-two standard ustar entries, each with its own header and data.
-The first optional entry stores the extended attributes
-for the following entry.
-This optional first entry has an "x" typeflag and a size field that
-indicates the total size of the extended attributes.
-The extended attributes themselves are stored as a series of text-format
-lines encoded in the portable UTF-8 encoding.
-Each line consists of a decimal number, a space, a key string, an equals
-sign, a value string, and a new line.
-The decimal number indicates the length of the entire line, including the
-initial length field and the trailing newline.
-An example of such a field is:
-.RS 4
-25 ctime=1084839148.1212\en
-.RE
-Keys in all lowercase are standard keys.
-Vendors can add their own keys by prefixing them with an all uppercase
-vendor name and a period.
-Note that, unlike the historic header, numeric values are stored using
-decimal, not octal.
-A description of some common keys follows:
-.RS 5
-.TP
-\fBatime\fP, \fBctime\fP, \fBmtime\fP
-File access, inode change, and modification times.
-These fields can be negative or include a decimal point and a fractional value.
-.TP
-\fBuname\fP, \fBuid\fP, \fBgname\fP, \fBgid\fP
-User name, group name, and numeric UID and GID values.
-The user name and group name stored here are encoded in UTF8
-and can thus include non-ASCII characters.
-The UID and GID fields can be of arbitrary length.
-.TP
-\fBlinkpath\fP
-The full path of the linked-to file.
-Note that this is encoded in UTF8 and can thus include non-ASCII characters.
-.TP
-\fBpath\fP
-The full pathname of the entry.
-Note that this is encoded in UTF8 and can thus include non-ASCII characters.
-.TP
-\fBrealtime.*\fP, \fBsecurity.*\fP
-These keys are reserved and may be used for future standardization.
-.TP
-\fBsize\fP
-The size of the file.
-Note that there is no length limit on this field, allowing conforming
-archives to store files much larger than the historic 8GB limit.
-.TP
-\fBSCHILY.*\fP
-Vendor-specific attributes used by Joerg Schilling's
-\fB\%star\fP
-implementation.
-.TP
-\fBSCHILY.acl.access\fP, \fBSCHILY.acl.default\fP
-Stores the access and default ACLs as textual strings in a format
-that is an extension of the format specified by POSIX.1e draft 17.
-In particular, each user or group access specification can include a fourth
-colon-separated field with the numeric UID or GID.
-This allows ACLs to be restored on systems that may not have complete
-user or group information available (such as when NIS/YP or LDAP services
-are temporarily unavailable).
-.TP
-\fBSCHILY.devminor\fP, \fBSCHILY.devmajor\fP
-The full minor and major numbers for device nodes.
-.TP
-\fBSCHILY.dev,\fP \fBSCHILY.ino\fP, \fBSCHILY.nlinks\fP
-The device number, inode number, and link count for the entry.
-In particular, note that a pax interchange format archive using Joerg
-Schilling's
-\fBSCHILY.*\fP
-extensions can store all of the data from
-\fIstruct\fP stat.
-.TP
-\fBLIBARCHIVE.xattr.\fP \fInamespace\fP.\fIkey\fP
-Libarchive stores POSIX.1e-style extended attributes using
-keys of this form.
-The
-\fIkey\fP
-value is URL-encoded:
-All non-ASCII characters and the two special characters
-``=''
-and
-``%''
-are encoded as
-``%''
-followed by two uppercase hexadecimal digits.
-The value of this key is the extended attribute value
-encoded in base 64.
-XXX Detail the base-64 format here XXX
-.TP
-\fBVENDOR.*\fP
-XXX document other vendor-specific extensions XXX
-.RE
-.PP
-Any values stored in an extended attribute override the corresponding
-values in the regular tar header.
-Note that compliant readers should ignore the regular fields when they
-are overridden.
-This is important, as existing archivers are known to store non-compliant
-values in the standard header fields in this situation.
-There are no limits on length for any of these fields.
-In particular, numeric fields can be arbitrarily large.
-All text fields are encoded in UTF8.
-Compliant writers should store only portable 7-bit ASCII characters in
-the standard ustar header and use extended
-attributes whenever a text value contains non-ASCII characters.
-.PP
-In addition to the
-\fBx\fP
-entry described above, the pax interchange format
-also supports a
-\fBg\fP
-entry.
-The
-\fBg\fP
-entry is identical in format, but specifies attributes that serve as
-defaults for all subsequent archive entries.
-The
-\fBg\fP
-entry is not widely used.
-.PP
-Besides the new
-\fBx\fP
-and
-\fBg\fP
-entries, the pax interchange format has a few other minor variations
-from the earlier ustar format.
-The most troubling one is that hardlinks are permitted to have
-data following them.
-This allows readers to restore any hardlink to a file without
-having to rewind the archive to find an earlier entry.
-However, it creates complications for robust readers, as it is no longer
-clear whether or not they should ignore the size field for hardlink entries.
-.SS GNU Tar Archives
-The GNU tar program started with a pre-POSIX format similar to that
-described earlier and has extended it using several different mechanisms:
-It added new fields to the empty space in the header (some of which was later
-used by POSIX for conflicting purposes);
-it allowed the header to be continued over multiple records;
-and it defined new entries that modify following entries
-(similar in principle to the
-\fBx\fP
-entry described above, but each GNU special entry is single-purpose,
-unlike the general-purpose
-\fBx\fP
-entry).
-As a result, GNU tar archives are not POSIX compatible, although
-more lenient POSIX-compliant readers can successfully extract most
-GNU tar archives.
-.RS 4
-.nf
-struct header_gnu_tar {
- char name[100];
- char mode[8];
- char uid[8];
- char gid[8];
- char size[12];
- char mtime[12];
- char checksum[8];
- char typeflag[1];
- char linkname[100];
- char magic[6];
- char version[2];
- char uname[32];
- char gname[32];
- char devmajor[8];
- char devminor[8];
- char atime[12];
- char ctime[12];
- char offset[12];
- char longnames[4];
- char unused[1];
- struct {
- char offset[12];
- char numbytes[12];
- } sparse[4];
- char isextended[1];
- char realsize[12];
- char pad[17];
-};
-.RE
-.RS 5
-.TP
-\fItypeflag\fP
-GNU tar uses the following special entry types, in addition to
-those defined by POSIX:
-.RS 5
-.TP
-7
-GNU tar treats type "7" records identically to type "0" records,
-except on one obscure RTOS where they are used to indicate the
-pre-allocation of a contiguous file on disk.
-.TP
-D
-This indicates a directory entry.
-Unlike the POSIX-standard "5"
-typeflag, the header is followed by data records listing the names
-of files in this directory.
-Each name is preceded by an ASCII "Y"
-if the file is stored in this archive or "N" if the file is not
-stored in this archive.
-Each name is terminated with a null, and
-an extra null marks the end of the name list.
-The purpose of this
-entry is to support incremental backups; a program restoring from
-such an archive may wish to delete files on disk that did not exist
-in the directory when the archive was made.
-.PP
-Note that the "D" typeflag specifically violates POSIX, which requires
-that unrecognized typeflags be restored as normal files.
-In this case, restoring the "D" entry as a file could interfere
-with subsequent creation of the like-named directory.
-.TP
-K
-The data for this entry is a long linkname for the following regular entry.
-.TP
-L
-The data for this entry is a long pathname for the following regular entry.
-.TP
-M
-This is a continuation of the last file on the previous volume.
-GNU multi-volume archives guarantee that each volume begins with a valid
-entry header.
-To ensure this, a file may be split, with part stored at the end of one volume,
-and part stored at the beginning of the next volume.
-The "M" typeflag indicates that this entry continues an existing file.
-Such entries can only occur as the first or second entry
-in an archive (the latter only if the first entry is a volume label).
-The
-\fIsize\fP
-field specifies the size of this entry.
-The
-\fIoffset\fP
-field at bytes 369-380 specifies the offset where this file fragment
-begins.
-The
-\fIrealsize\fP
-field specifies the total size of the file (which must equal
-\fIsize\fP
-plus
-\fIoffset\fP).
-When extracting, GNU tar checks that the header file name is the one it is
-expecting, that the header offset is in the correct sequence, and that
-the sum of offset and size is equal to realsize.
-FreeBSD's version of GNU tar does not handle the corner case of an
-archive's being continued in the middle of a long name or other
-extension header.
-.TP
-N
-Type "N" records are no longer generated by GNU tar.
-They contained a
-list of files to be renamed or symlinked after extraction; this was
-originally used to support long names.
-The contents of this record
-are a text description of the operations to be done, in the form
-``Rename %s to %s\en''
-or
-``Symlink %s to %s\en ;''
-in either case, both
-filenames are escaped using K&R C syntax.
-.TP
-S
-This is a
-``sparse''
-regular file.
-Sparse files are stored as a series of fragments.
-The header contains a list of fragment offset/length pairs.
-If more than four such entries are required, the header is
-extended as necessary with
-``extra''
-header extensions (an older format that is no longer used), or
-``sparse''
-extensions.
-.TP
-V
-The
-\fIname\fP
-field should be interpreted as a tape/volume header name.
-This entry should generally be ignored on extraction.
-.RE
-.TP
-\fImagic\fP
-The magic field holds the five characters
-``ustar''
-followed by a space.
-Note that POSIX ustar archives have a trailing null.
-.TP
-\fIversion\fP
-The version field holds a space character followed by a null.
-Note that POSIX ustar archives use two copies of the ASCII digit
-``0''.
-.TP
-\fIatime\fP, \fIctime\fP
-The time the file was last accessed and the time of
-last change of file information, stored in octal as with
-\fImtime\fP.
-.TP
-\fIlongnames\fP
-This field is apparently no longer used.
-.TP
-Sparse \fIoffset\fP / \fInumbytes\fP
-Each such structure specifies a single fragment of a sparse
-file.
-The two fields store values as octal numbers.
-The fragments are each padded to a multiple of 512 bytes
-in the archive.
-On extraction, the list of fragments is collected from the
-header (including any extension headers), and the data
-is then read and written to the file at appropriate offsets.
-.TP
-\fIisextended\fP
-If this is set to non-zero, the header will be followed by additional
-``sparse header''
-records.
-Each such record contains information about as many as 21 additional
-sparse blocks as shown here:
-.RS 4
-.nf
-struct gnu_sparse_header {
- struct {
- char offset[12];
- char numbytes[12];
- } sparse[21];
- char isextended[1];
- char padding[7];
-};
-.RE
-.TP
-\fIrealsize\fP
-A binary representation of the file's complete size, with a much larger range
-than the POSIX file size.
-In particular, with
-\fBM\fP
-type files, the current entry is only a portion of the file.
-In that case, the POSIX size field will indicate the size of this
-entry; the
-\fIrealsize\fP
-field will indicate the total size of the file.
-.RE
-.SS Solaris Tar
-XXX More Details Needed XXX
-.PP
-Solaris tar (beginning with SunOS XXX 5.7 ?? XXX) supports an
-``extended''
-format that is fundamentally similar to pax interchange format,
-with the following differences:
-.RS 5
-.IP \(bu
-Extended attributes are stored in an entry whose type is
-\fBX\fP,
-not
-\fBx\fP,
-as used by pax interchange format.
-The detailed format of this entry appears to be the same
-as detailed above for the
-\fBx\fP
-entry.
-.IP \(bu
-An additional
-\fBA\fP
-entry is used to store an ACL for the following regular entry.
-The body of this entry contains a seven-digit octal number
-(whose value is 01000000 plus the number of ACL entries)
-followed by a zero byte, followed by the
-textual ACL description.
-.RE
-.SS Other Extensions
-One common extension, utilized by GNU tar, star, and other newer
-\fB\%tar\fP
-implementations, permits binary numbers in the standard numeric
-fields.
-This is flagged by setting the high bit of the first character.
-This permits 95-bit values for the length and time fields
-and 63-bit values for the uid, gid, and device numbers.
-GNU tar supports this extension for the
-length, mtime, ctime, and atime fields.
-Joerg Schilling's star program supports this extension for
-all numeric fields.
-Note that this extension is largely obsoleted by the extended attribute
-record provided by the pax interchange format.
-.PP
-Another early GNU extension allowed base-64 values rather
-than octal.
-This extension was short-lived and such archives are almost never seen.
-However, there is still code in GNU tar to support them; this code is
-responsible for a very cryptic warning message that is sometimes seen when
-GNU tar encounters a damaged archive.
-.SH SEE ALSO
-.ad l
-\fBar\fP(1),
-\fBpax\fP(1),
-\fBtar\fP(1)
-.SH STANDARDS
-.ad l
-The
-\fB\%tar\fP
-utility is no longer a part of POSIX or the Single Unix Standard.
-It last appeared in
-Version 2 of the Single UNIX Specification (``SUSv2'').
-It has been supplanted in subsequent standards by
-\fBpax\fP(1).
-The ustar format is currently part of the specification for the
-\fBpax\fP(1)
-utility.
-The pax interchange file format is new with
-IEEE Std 1003.1-2001 (``POSIX.1'').
-.SH HISTORY
-.ad l
-A
-\fB\%tar\fP
-command appeared in Seventh Edition Unix, which was released in January, 1979.
-It replaced the
-\fB\%tp\fP
-program from Fourth Edition Unix which in turn replaced the
-\fB\%tap\fP
-program from First Edition Unix.
-John Gilmore's
-\fB\%pdtar\fP
-public-domain implementation (circa 1987) was highly influential
-and formed the basis of
-\fB\%GNU\fP tar.
-Joerg Shilling's
-\fB\%star\fP
-archiver is another open-source (GPL) archiver (originally developed
-circa 1985) which features complete support for pax interchange
-format.