pax.html

unix 下的C开发手册,还用详细的例程。

<dt><b>00200</b><dd>Write by owner.

<dt><b>00100</b><dd>Execute/search by owner.

<dt><b>00040</b><dd>Read by group.

<dt><b>00020</b><dd>Write by group.

<dt><b>00010</b><dd>Execute/search by group.

<dt><b>00004</b><dd>Read by other.

<dt><b>00002</b><dd>Write by other.

<dt><b>00001</b><dd>Execute/search by other.

</dl>
<p>
When appropriate privilege is
required to set one of these mode bits, and the user restoring the
files from the archive does not have the appropriate privilege,
the mode bits for which the user does not have appropriate privilege
will be ignored.
Some of the mode bits in the archive format
are not mentioned elsewhere in this specification.
If the implementation
does not support those bits, they may be ignored.
<p>
The
<b>uid</b>
and
<b>gid</b>
fields are the user and group ID
of the owner and group of the file, respectively.
<p>
The
<b>size</b>
field is the size of the file in octets.
If the
<b>typeflag</b>
field is set to specify a file to be of type
1
(a link)
or
2
(reserved for symbolic links),
the
<b>size</b>
field will be specified as zero.
If the
<b>typeflag</b>
field is set to specify a file of type
5
(directory), the
<b>size</b>
field will be interpreted as described under
the definition of that record type.
No data blocks will be stored for types
1,
2
or
5.
If the
<b>typeflag</b>
field is set to
3
(character special file),
4
(block special file),
or
6
(FIFO),
the meaning of the
<b>size</b>
field is unspecified by this specification, and no data
blocks will be stored on the medium.
Additionally, for
6,
the
<b>size</b>
field is ignored when reading.
If the
<b>typeflag</b>
field is set
to any other value,
<b>(size +511)/512</b>,
the number of blocks written following the header will be
ignoring any fraction in the result of the division.
<p>
The
<b>mtime</b>
field is the modification time of the file at the time
it was archived.
It is the ISO/IEC&nbsp;646:1991 standard representation of the octal value of the
modification time obtained from the <b>XSH</b> specification
<i><a href="../xsh/stat.html">stat()</a></i>
function.
<p>
The
<b>chksum</b>
field is the ISO/IEC&nbsp;646:1991 standard IRV representation of the octal value of the
simple sum of all octets in the header block.
Each octet in the header is treated as an unsigned value.
These values will be added to an unsigned integer,
initialised to zero, the precision of which will be not
less than 17 bits.
When calculating the
checksum, the
<b>chksum</b>
field is treated as if it were all spaces.
<p>
The
<b>typeflag</b>
field specifies the type of file archived.
If a
particular implementation does not recognise the type, or the
user does not have appropriate privilege to create that type,
the file will be extracted as if it were a regular file if the
file type is defined to have a meaning for the
<b>size</b>
field that could cause data blocks to be written on the medium (see the
previous description for
<i>size ).</i>
If conversion to a regular file occurs, the
<i>pax</i>
utility will produce
an error indicating that the conversion took place.
All of the
<b>typeflag</b>
fields will be coded in the ISO/IEC&nbsp;646:1991 standard IRV:
<dl compact>

<dt><b>'0'</b><dd>Represents a regular file.
For backward compatibility, a
<b>typeflag</b>
value of binary zero
('\0')
should be recognised as meaning a regular file
when extracting files from the archive.
Archives
written with this version of the archive file format
create regular files with a
<b>typeflag</b>
value of the ISO/IEC&nbsp;646:1991 standard IRV
0.

<dt><b>'1'</b><dd>Represents a file linked to another file, of
any type, previously archived.
Such files are
identified by each file having the same device and file serial number.
The linked-to name is specified in the
<b>linkname</b>
field with a NUL-character terminator if it is less than
100 octets in length.

<dt><b>'2'</b><dd>Reserved to represent a link to another file, of
any type, whose device or file serial number differs.
This is provided for systems that support linked files
whose device or file serial numbers differ, and should
be treated as a type
1
file if this extension does not exist.

<dt><b>'3','4'</b><dd>Represent character special files
and block special files respectively.
In this case the
<b>devmajor</b>
and
<b>devminor</b>
fields contain information defining the device, the
format of which is unspecified by this specification.
Implementations may map the device specifications to their own local
specification or may ignore the entry.

<dt><b>'5'</b><dd>Specifies a directory or subdirectory.
On systems where disk allocation is performed on a
directory basis, the
<b>size</b>
field will contain the maximum
number of octets (which may be rounded to the nearest
disk block allocation unit) that the directory may hold.
A
<b>size</b>
field of zero indicates no such limiting.
Systems that do not support limiting in this manner
should ignore the
<b>size</b>
field.

<dt><b>'6'</b><dd>Specifies a FIFO special file.
Note that the archiving of a FIFO file archives the
existence of this file and not its contents.

<dt><b>'7'</b><dd>Reserved to represent a file to which an implementation
has associated some high-performance attribute.
Implementations without such extensions should treat this
file as a regular file (type
0).

<dt><b>'A'-'Z'</b><dd>The letters
A
to
Z,
inclusive,
are reserved for custom implementations.
All other values are
reserved for specification in future issues.

</dl>
<p>
The
<b>magic</b>
field is the specification that this archive was output
in this archive format.
If this field contains
<b>ustar</b>
(the five characters from the ISO/IEC&nbsp;646:1991 standard IRV shown followed by NUL),
the
<b>uname</b>
and
<b>gname</b>
fields will contain the ISO/IEC&nbsp;646:1991 standard IRV
representation of the owner and group of the file respectively
(truncated to fit, if necessary).
When the file is restored by a privileged, protection-preserving
version of the utility,
the password and group files will be scanned for these names.
If found, the user and group IDs
contained within these
files will be used rather than the values contained
within the
<b>uid</b>
and
<b>gid</b>
fields.
</blockquote><h4><a name = "tag_001_014_1701">&nbsp;</a>EXIT STATUS</h4><blockquote>
The following exit values are returned:
<dl compact>

<dt>0<dd>All files were processed successfully.

<dt>&gt;0<dd>An error occurred.

</dl>
</blockquote><h4><a name = "tag_001_014_1702">&nbsp;</a>CONSEQUENCES OF ERRORS</h4><blockquote>
If
<i>pax</i>
cannot create a file or a link when reading an archive or
cannot find a file when writing an archive, or cannot preserve
the user ID, group ID or file mode when the
<b>-p</b>
option is
specified, a diagnostic message will be written to standard
error and a non-zero exit status will be returned, but
processing will continue.
In the case where
<i>pax</i>
cannot create a link to a file,
<i>pax</i>
will not, by default, create a second
copy of the file.
<p>
If the extraction of a file from an archive is prematurely
terminated by a signal or error,
<i>pax</i>
may have only partially
extracted the file or (if the
<b>-n</b>
option was not specified) may
have extracted a file of the same name as that specified by the
user, but which is not the file the user wanted.
Additionally,
the file modes of extracted directories may have additional
bits from the S_IRWXU mask set as well as incorrect modification
and access times.
</blockquote><h4><a name = "tag_001_014_1703">&nbsp;</a>APPLICATION USAGE</h4><blockquote>
The
<b>-p</b>
(privileges) option was invented to reconcile differences
between historical
<i><a href="tar.html">tar</a></i>
and
<i><a href="cpio.html">cpio</a></i>
implementations.
In particular, the two utilities use
<b>-m</b>
in diametrically opposed ways.
The
<b>-p</b>
option also provides a consistent means of
extending the ways in which
future file attributes can be addressed,
such as for enhanced security systems
or high-performance files.
Although it may seem complex, there are really
two modes that will be most commonly used:
<dl compact>

<dt><b>-p e</b><dd>&quot;Preserve everything&quot;.
This would be used by the historical superuser,
someone with all the appropriate privileges,
to preserve all aspects of the files as
they are recorded in the archive.
The
<b>e</b>
flag is the sum of
<b>o</b>
and
<b>p</b>,
and other implementation-dependent attributes.

<dt><b>-p p</b><dd>&quot;Preserve&quot; the file mode bits.
This would be used by the user with regular
privileges who wished to preserve aspects of
the file other than the ownership.
The file times are preserved by default,
but two other flags are offered to disable these
and use the time of extraction.

</dl>
<p>
The one pathname per line format of standard input precludes
pathnames containing
newline characters.
Although such pathnames violate
the portable filename guidelines, they may exist and their
presence may inhibit usage of
<i>pax</i>
within shell scripts.
This problem is inherited from historical archive programs.
The problem can be avoided by listing
filename arguments on the command line
instead of on standard input.
<p>
It is almost certain that appropriate privileges will be
required for
<i>pax</i>
to accomplish parts of this specification.
Specifically, creating files of type block special or character
special, restoring file access times unless the files are owned
by the user (the
<b>-t</b>
option) or preserving file owner, group,
and mode (the
<b>-p</b>
option) will all probably require appropriate privileges.
<p>
In read mode,
implementations are permitted to overwrite files when the
archive has multiple members with the same name.
This may fail if permissions on the first version of the file do
not permit it to be overwritten.
<p>
The
<i><a href="cpio.html">cpio</a></i>
and
<i><a href="tar.html">tar</a></i>
formats can only support files up to 8 gigabytes in size.
<p>
The 
<i>pax</i>
utility is not able to handle arbitrary file sizes. There is
currently a proposal in ballot in the IEEE PASC Shell and Utilities Working Group to address this
issue. 
</blockquote><h4><a name = "tag_001_014_1704">&nbsp;</a>EXAMPLES</h4><blockquote>
The following command:
<pre>
<code>
pax -w -f /dev/rmt/1m .
</code>
</pre>
copies the contents of the current directory to tape drive 1, medium density
(assuming
historical System&nbsp;V device naming procedures.
The historical
BSD device name would be
/dev/rmt9).
<p>
The following commands:
<pre>
<code>
mkdir <i>newdir</i>
pax -rw <i>olddir</i> <i>newdir</i>
</code>
</pre>
copy the
<i>olddir</i>
directory hierarchy to
<i>newdir .</i>
<p>
<pre>
<code>
pax -r -s ',^//*usr//*,,' -f a.pax
</code>
</pre>
reads the archive
<b>a.pax</b>,
with all files rooted in
<b>/usr</b>
in the archive extracted relative to the current directory.
</blockquote><h4><a name = "tag_001_014_1705">&nbsp;</a>FUTURE DIRECTIONS</h4><blockquote>
It is expected that future versions of the ISO/IEC 9945-2:1993 standard will
offer additional file formats and the
<b>-o</b>
option
will be used by future issues
to specify such features as international filename
and file codeset translations, security, accounting, and so on,
related to each additional format.
<p>
The 
<i>pax</i>
utility is not able to handle arbitrary file sizes. There is
currently a proposal in ballot in the IEEE PASC Shell and Utilities Working Group to address this
issue. 
</blockquote><h4><a name = "tag_001_014_1706">&nbsp;</a>SEE ALSO</h4><blockquote>
None.
</blockquote><hr size=2 noshade>
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