PAX(1P) POSIX Programmer's Manual PAX(1P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the corresponding
Linux manual page for details of Linux behavior), or the interface may
not be implemented on Linux.
NAME
pax - portable archive interchange
SYNOPSIS
pax [-cdnv][-H|-L][-f archive][-s replstr]...[pattern...]
pax -r[-cdiknuv][-H|-L][-f archive][-o options]...[-p string]...
[-s replstr]...[pattern...]
pax -w[-dituvX][-H|-L][-b blocksize][[-a][-f archive][-o options]...
[-s replstr]...[-x format][file...]
pax -r -w[-diklntuvX][-H|-L][-p string]...[-s replstr]...
[file...] directory
DESCRIPTION
The pax utility shall read, write, and write lists of the members of
archive files and copy directory hierarchies. A variety of archive for‐
mats shall be supported; see the -x format option.
The action to be taken depends on the presence of the -r and -w
options. The four combinations of -r and -w are referred to as the four
modes of operation: list, read, write, and copy modes, corresponding
respectively to the four forms shown in the SYNOPSIS section.
list In list mode (when neither -r nor -w are specified), pax shall
write the names of the members of the archive file read from the
standard input, with pathnames matching the specified patterns,
to standard output. If a named file is of type directory, the
file hierarchy rooted at that file shall be listed as well.
read In read mode (when -r is specified, but -w is not), pax shall
extract the members of the archive file read from the standard
input, with pathnames matching the specified patterns. If an
extracted file is of type directory, the file hierarchy rooted
at that file shall be extracted as well. The extracted files
shall be created performing pathname resolution with the direc‐
tory in which pax was invoked as the current working directory.
If an attempt is made to extract a directory when the directory already
exists, this shall not be considered an error. If an attempt is made to
extract a FIFO when the FIFO already exists, this shall not be consid‐
ered an error.
The ownership, access, and modification times, and file mode of the
restored files are discussed under the -p option.
write In write mode (when -w is specified, but -r is not), pax shall
write the contents of the file operands to the standard output
in an archive format. If no file operands are specified, a list
of files to copy, one per line, shall be read from the standard
input. A file of type directory shall include all of the files
in the file hierarchy rooted at the file.
copy In copy mode (when both -r and -w are specified), pax shall copy
the file operands to the destination directory.
If no file operands are specified, a list of files to copy, one per
line, shall be read from the standard input. A file of type directory
shall include all of the files in the file hierarchy rooted at the
file.
The effect of the copy shall be as if the copied files were written to
an archive file and then subsequently extracted, except that there may
be hard links between the original and the copied files. If the desti‐
nation directory is a subdirectory of one of the files to be copied,
the results are unspecified. If the destination directory is a file of
a type not defined by the System Interfaces volume of
IEEE Std 1003.1-2001, the results are implementation-defined; other‐
wise, it shall be an error for the file named by the directory operand
not to exist, not be writable by the user, or not be a file of type
directory.
In read or copy modes, if intermediate directories are necessary to
extract an archive member, pax shall perform actions equivalent to the
mkdir() function defined in the System Interfaces volume of
IEEE Std 1003.1-2001, called with the following arguments:
* The intermediate directory used as the path argument
* The value of the bitwise-inclusive OR of SIRWXU, SIRWXG, and
SIRWXO as the mode argument
If any specified pattern or file operands are not matched by at least
one file or archive member, pax shall write a diagnostic message to
standard error for each one that did not match and exit with a non-zero
exit status.
The archive formats described in the EXTENDED DESCRIPTION section shall
be automatically detected on input. The default output archive format
shall be implementation-defined.
A single archive can span multiple files. The pax utility shall deter‐
mine, in an implementation-defined manner, what file to read or write
as the next file.
If the selected archive format supports the specification of linked
files, it shall be an error if these files cannot be linked when the
archive is extracted. For archive formats that do not store file con‐
tents with each name that causes a hard link, if the file that contains
the data is not extracted during this pax session, either the data
shall be restored from the original file, or a diagnostic message shall
be displayed with the name of a file that can be used to extract the
data. In traversing directories, pax shall detect infinite loops; that
is, entering a previously visited directory that is an ancestor of the
last file visited. When it detects an infinite loop, pax shall write a
diagnostic message to standard error and shall terminate.
OPTIONS
The pax utility shall conform to the Base Definitions volume of
IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines, except
that the order of presentation of the -o, -p, and -s options is signif‐
icant.
The following options shall be supported:
-r Read an archive file from standard input.
-w Write files to the standard output in the specified archive for‐
mat.
-a Append files to the end of the archive. It is implementation-
defined which devices on the system support appending. Addi‐
tional file formats unspecified by this volume of
IEEE Std 1003.1-2001 may impose restrictions on appending.
-b blocksize
Block the output at a positive decimal integer number of bytes
per write to the archive file. Devices and archive formats may
impose restrictions on blocking. Blocking shall be automatically
determined on input. Conforming applications shall not specify a
blocksize value larger than 32256. Default blocking when creat‐
ing archives depends on the archive format. (See the -x option
below.)
-c Match all file or archive members except those specified by the
pattern or file operands.
-d Cause files of type directory being copied or archived or ar‐
chive members of type directory being extracted or listed to
match only the file or archive member itself and not the file
hierarchy rooted at the file.
-f archive
Specify the pathname of the input or output archive, overriding
the default standard input (in list or read modes) or standard
output ( write mode).
-H If a symbolic link referencing a file of type directory is spec‐
ified on the command line, pax shall archive the file hierarchy
rooted in the file referenced by the link, using the name of the
link as the root of the file hierarchy. Otherwise, if a symbolic
link referencing a file of any other file type which pax can
normally archive is specified on the command line, then pax
shall archive the file referenced by the link, using the name of
the link. The default behavior shall be to archive the symbolic
link itself.
-i Interactively rename files or archive members. For each archive
member matching a pattern operand or file matching a file oper‐
and, a prompt shall be written to the file /dev/tty. The prompt
shall contain the name of the file or archive member, but the
format is otherwise unspecified. A line shall then be read from
/dev/tty. If this line is blank, the file or archive member
shall be skipped. If this line consists of a single period, the
file or archive member shall be processed with no modification
to its name. Otherwise, its name shall be replaced with the con‐
tents of the line. The pax utility shall immediately exit with a
non-zero exit status if end-of-file is encountered when reading
a response or if /dev/tty cannot be opened for reading and writ‐
ing.
The results of extracting a hard link to a file that has been renamed
during extraction are unspecified.
-k Prevent the overwriting of existing files.
-l (The letter ell.) In copy mode, hard links shall be made between
the source and destination file hierarchies whenever possible.
If specified in conjunction with -H or -L, when a symbolic link
is encountered, the hard link created in the destination file
hierarchy shall be to the file referenced by the symbolic link.
If specified when neither -H nor -L is specified, when a sym‐
bolic link is encountered, the implementation shall create a
hard link to the symbolic link in the source file hierarchy or
copy the symbolic link to the destination.
-L If a symbolic link referencing a file of type directory is spec‐
ified on the command line or encountered during the traversal of
a file hierarchy, pax shall archive the file hierarchy rooted in
the file referenced by the link, using the name of the link as
the root of the file hierarchy. Otherwise, if a symbolic link
referencing a file of any other file type which pax can normally
archive is specified on the command line or encountered during
the traversal of a file hierarchy, pax shall archive the file
referenced by the link, using the name of the link. The default
behavior shall be to archive the symbolic link itself.
-n Select the first archive member that matches each pattern oper‐
and. No more than one archive member shall be matched for each
pattern (although members of type directory shall still match
the file hierarchy rooted at that file).
-o options
Provide information to the implementation to modify the algo‐
rithm for extracting or writing files. The value of options
shall consist of one or more comma-separated keywords of the
form:
keyword[[:]=value][,keyword[[:]=value], ...]
Some keywords apply only to certain file formats, as indicated with
each description. Use of keywords that are inapplicable to the file
format being processed produces undefined results.
Keywords in the options argument shall be a string that would be a
valid portable filename as described in the Base Definitions volume of
IEEE Std 1003.1-2001, Section 3.276, Portable Filename Character Set.
Note:
Keywords are not expected to be filenames, merely to follow the
same character composition rules as portable filenames.
Keywords can be preceded with white space. The value field shall con‐
sist of zero or more characters; within value, the application shall
precede any literal comma with a backslash, which shall be ignored, but
preserves the comma as part of value. A comma as the final character,
or a comma followed solely by white space as the final characters, in
options shall be ignored. Multiple -o options can be specified; if key‐
words given to these multiple -o options conflict, the keywords and
values appearing later in command line sequence shall take precedence
and the earlier shall be silently ignored. The following keyword values
of options shall be supported for the file formats as indicated:
delete=pattern
(Applicable only to the -x pax format.) When used in write or
copy mode, pax shall omit from extended header records that it
produces any keywords matching the string pattern. When used in
read or list mode, pax shall ignore any keywords matching the
string pattern in the extended header records. In both cases,
matching shall be performed using the pattern matching notation
described in Patterns Matching a Single Character and Patterns
Matching Multiple Characters . For example:
-o delete=security.*
would suppress security-related information. See pax Extended
Header for extended header record keyword usage.
exthdr.name=string
(Applicable only to the -x pax format.) This keyword allows user
control over the name that is written into the ustar header
blocks for the extended header produced under the circumstances
described in pax Header Block . The name shall be the contents
of string, after the following character substitutions have been
made:
string
Includes: Replaced By:
%d The directory name of the file, equiva‐
lent to the result of the dirname util‐
ity on the translated pathname.
%f The filename of the file, equivalent to
the result of the basename utility on
the translated pathname.
%p The process ID of the pax process.
%% A '%' character.
Any other '%' characters in string produce undefined results.
If no -o exthdr.name= string is specified, pax shall use the
following default value:
%d/PaxHeaders.%p/%f
globexthdr.name=string
(Applicable only to the -x pax format.) When used in write or
copy mode with the appropriate options, pax shall create global
extended header records with ustar header blocks that will be
treated as regular files by previous versions of pax. This key‐
word allows user control over the name that is written into the
ustar header blocks for global extended header records. The name
shall be the contents of string, after the following character
substitutions have been made:
string
Includes: Replaced By:
%n An integer that represents the sequence
number of the global extended header
record in the archive, starting at 1.
%p The process ID of the pax process.
%% A '%' character.
Any other '%' characters in string produce undefined results.
If no -o globexthdr.name= string is specified, pax shall use the
following default value:
$TMPDIR/GlobalHead.%p.%n
where $ TMPDIR represents the value of the TMPDIR environment
variable. If TMPDIR is not set, pax shall use /tmp.
invalid=action
(Applicable only to the -x pax format.) This keyword allows user
control over the action pax takes upon encountering values in an
extended header record that, in read or copy mode, are invalid
in the destination hierarchy or, in list mode, cannot be written
in the codeset and current locale of the implementation. The
following are invalid values that shall be recognized by pax:
* In read or copy mode, a filename or link name that
contains character encodings invalid in the destina‐
tion hierarchy. (For example, the name may contain
embedded NULs.)
* In read or copy mode, a filename or link name that is
longer than the maximum allowed in the destination
hierarchy (for either a pathname component or the
entire pathname).
* In list mode, any character string value (filename,
link name, user name, and so on) that cannot be writ‐
ten in the codeset and current locale of the implemen‐
tation.
The following mutually-exclusive values of the action argument
are supported:
bypass
In read or copy mode, pax shall bypass the file, causing
no change to the destination hierarchy. In list mode, pax
shall write all requested valid values for the file, but
its method for writing invalid values is unspecified.
rename
In read or copy mode, pax shall act as if the -i option
were in effect for each file with invalid filename or
link name values, allowing the user to provide a replace‐
ment name interactively. In list mode, pax shall behave
identically to the bypass action.
UTF-8
When used in read, copy, or list mode and a filename,
link name, owner name, or any other field in an extended
header record cannot be translated from the pax UTF-8
codeset format to the codeset and current locale of the
implementation, pax shall use the actual UTF-8 encoding
for the name.
write
In read or copy mode, pax shall write the file, translat‐
ing or truncating the name, regardless of whether this
may overwrite an existing file with a valid name. In list
mode, pax shall behave identically to the bypass action.
If no -o invalid= option is specified, pax shall act as if -o
invalid= bypass were specified. Any overwriting of existing
files that may be allowed by the -o invalid= actions shall be
subject to permission ( -p) and modification time ( -u) restric‐
tions, and shall be suppressed if the -k option is also speci‐
fied.
linkdata
(Applicable only to the -x pax format.) In write mode, pax shall
write the contents of a file to the archive even when that file
is merely a hard link to a file whose contents have already been
written to the archive.
listopt=format
This keyword specifies the output format of the table of con‐
tents produced when the -v option is specified in list mode. See
List Mode Format Specifications . To avoid ambiguity, the
listopt= format shall be the only or final keyword= value pair
in a -o option-argument; all characters in the remainder of the
option-argument shall be considered part of the format string.
When multiple -o listopt= format options are specified, the for‐
mat strings shall be considered a single, concatenated string,
evaluated in command line order.
times
(Applicable only to the -x pax format.) When used in write or
copy mode, pax shall include atime, ctime, and mtime extended
header records for each file. See pax Extended Header File Times
.
In addition to these keywords, if the -x pax format is specified, any
of the keywords and values defined in pax Extended Header, including
implementation extensions, can be used in -o option-arguments, in
either of two modes:
keyword=value
When used in write or copy mode, these keyword/value pairs shall
be included at the beginning of the archive as typeflag g global
extended header records. When used in read or list mode, these
keyword/value pairs shall act as if they had been at the begin‐
ning of the archive as typeflag g global extended header
records.
keyword:=value
When used in write or copy mode, these keyword/value pairs shall
be included as records at the beginning of a typeflag x extended
header for each file. (This shall be equivalent to the equal-
sign form except that it creates no typeflag g global extended
header records.) When used in read or list mode, these key‐
word/value pairs shall act as if they were included as records
at the end of each extended header; thus, they shall override
any global or file-specific extended header record keywords of
the same names. For example, in the command:
pax -r -o "
gname:=mygroup,
" The precedence of -o keywords over various fields in the archive is
described in pax Extended Header Keyword Precedence .
-p string
Specify one or more file characteristic options (privileges).
The string option-argument shall be a string specifying file
characteristics to be retained or discarded on extraction. The
string shall consist of the specification characters a, e, m, o,
and p . Other implementation-defined characters can be included.
Multiple characteristics can be concatenated within the same
string and multiple -p options can be specified. The meaning of
the specification characters are as follows:
a
Do not preserve file access times.
e
Preserve the user ID, group ID, file mode bits (see the Base
Definitions volume of IEEE Std 1003.1-2001, Section 3.168, File
Mode Bits), access time, modification time, and any other imple‐
mentation-defined file characteristics.
m
Do not preserve file modification times.
o
Preserve the user ID and group ID.
p
Preserve the file mode bits. Other implementation-defined file
mode attributes may be preserved.
In the preceding list, "preserve" indicates that an attribute stored in
the archive shall be given to the extracted file, subject to the per‐
missions of the invoking process. The access and modification times of
the file shall be preserved unless otherwise specified with the -p
option or not stored in the archive. All attributes that are not pre‐
served shall be determined as part of the normal file creation action
(see File Read, Write, and Creation ).
If neither the e nor the o specification character is specified, or the
user ID and group ID are not preserved for any reason, pax shall not
set the SISUID and SISGID bits of the file mode.
If the preservation of any of these items fails for any reason, pax
shall write a diagnostic message to standard error. Failure to pre‐
serve these items shall affect the final exit status, but shall not
cause the extracted file to be deleted.
If file characteristic letters in any of the string option-arguments
are duplicated or conflict with each other, the ones given last shall
take precedence. For example, if -p eme is specified, file modification
times are preserved.
-s replstr
Modify file or archive member names named by pattern or file op‐
erands according to the substitution expression replstr, using
the syntax of the ed utility. The concepts of "address" and
"line" are meaningless in the context of the pax utility, and
shall not be supplied. The format shall be:
-s /old/new/[gp]
where as in ed, old is a basic regular expression and new can contain
an ampersand, '\n' (where n is a digit) backreferences, or subexpres‐
sion matching. The old string shall also be permitted to contain s.
Any non-null character can be used as a delimiter ( '/' shown here).
Multiple -s expressions can be specified; the expressions shall be
applied in the order specified, terminating with the first successful
substitution. The optional trailing 'g' is as defined in the ed util‐
ity. The optional trailing 'p' shall cause successful substitutions to
be written to standard error. File or archive member names that substi‐
tute to the empty string shall be ignored when reading and writing ar‐
chives.
-t When reading files from the file system, and if the user has the
permissions required by utime() to do so, set the access time of
each file read to the access time that it had before being read
by pax.
-u Ignore files that are older (having a less recent file modifica‐
tion time) than a pre-existing file or archive member with the
same name. In read mode, an archive member with the same name as
a file in the file system shall be extracted if the archive mem‐
ber is newer than the file. In write mode, an archive file mem‐
ber with the same name as a file in the file system shall be
superseded if the file is newer than the archive member. If -a
is also specified, this is accomplished by appending to the ar‐
chive; otherwise, it is unspecified whether this is accomplished
by actual replacement in the archive or by appending to the ar‐
chive. In copy mode, the file in the destination hierarchy shall
be replaced by the file in the source hierarchy or by a link to
the file in the source hierarchy if the file in the source hier‐
archy is newer.
-v In list mode, produce a verbose table of contents (see the STD‐
OUT section). Otherwise, write archive member pathnames to stan‐
dard error (see the STDERR section).
-x format
Specify the output archive format. The pax utility shall support
the following formats:
cpio
The cpio interchange format; see the EXTENDED DESCRIPTION sec‐
tion. The default blocksize for this format for character spe‐
cial archive files shall be 5120. Implementations shall support
all blocksize values less than or equal to 32256 that are multi‐
ples of 512.
pax
The pax interchange format; see the EXTENDED DESCRIPTION sec‐
tion. The default blocksize for this format for character spe‐
cial archive files shall be 5120. Implementations shall support
all blocksize values less than or equal to 32256 that are multi‐
ples of 512.
ustar
The tar interchange format; see the EXTENDED DESCRIPTION sec‐
tion. The default blocksize for this format for character spe‐
cial archive files shall be 10240. Implementations shall support
all blocksize values less than or equal to 32256 that are multi‐
ples of 512.
Implementation-defined formats shall specify a default block size as
well as any other block sizes supported for character special archive
files.
Any attempt to append to an archive file in a format different from the
existing archive format shall cause pax to exit immediately with a non-
zero exit status.
In copy mode, if no -x format is specified, pax shall behave as if -x
pax were specified.
-X When traversing the file hierarchy specified by a pathname, pax
shall not descend into directories that have a different device
ID ( stdev; see the System Interfaces volume of
IEEE Std 1003.1-2001, stat()).
The options that operate on the names of files or archive members ( -c,
-i, -n, -s, -u, and -v) shall interact as follows. In read mode, the
archive members shall be selected based on the user-specified pattern
operands as modified by the -c, -n, and -u options. Then, any -s and -i
options shall modify, in that order, the names of the selected files.
The -v option shall write names resulting from these modifications.
In write mode, the files shall be selected based on the user-specified
pathnames as modified by the -n and -u options. Then, any -s and -i
options shall modify, in that order, the names of these selected files.
The -v option shall write names resulting from these modifications.
If both the -u and -n options are specified, pax shall not consider a
file selected unless it is newer than the file to which it is compared.
List Mode Format Specifications
In list mode with the -o listopt= format option, the format argument
shall be applied for each selected file. The pax utility shall append a
to the listopt output for each selected file. The format
argument shall be used as the format string described in the Base Defi‐
nitions volume of IEEE Std 1003.1-2001, Chapter 5, File Format Nota‐
tion, with the exceptions 1. through 5. defined in the EXTENDED
DESCRIPTION section of printf, plus the following exceptions:
6. The sequence ( keyword) can occur before a format conversion
specifier. The conversion argument is defined by the value of
keyword. The implementation shall support the following key‐
words:
* Any of the Field Name entries in ustar Header Block and
Octet-Oriented cpio Archive Entry . The implementation may
support the cpio keywords without the leading c in addition
to the form required by Values for cpio cmode Field .
* Any keyword defined for the extended header in pax Extended
Header .
* Any keyword provided as an implementation-defined extension
within the extended header defined in pax Extended Header .
For example, the sequence "%(charset)s" is the string value of the name
of the character set in the extended header.
The result of the keyword conversion argument shall be the value from
the applicable header field or extended header, without any trailing
NULs.
All keyword values used as conversion arguments shall be translated
from the UTF-8 encoding to the character set appropriate for the local
file system, user database, and so on, as applicable.
7. An additional conversion specifier character, T, shall be used
to specify time formats. The T conversion specifier character
can be preceded by the sequence ( keyword= subformat), where
subformat is a date format as defined by date operands. The
default keyword shall be mtime and the default subformat shall
be:
%b %e %H:%M %Y
8. An additional conversion specifier character, M, shall be used
to specify the file mode string as defined in ls Standard Out‐
put. If ( keyword) is omitted, the mode keyword shall be used.
For example, %.1M writes the single character corresponding to
the field of the ls -l command.
9. An additional conversion specifier character, D, shall be used
to specify the device for block or special files, if applicable,
in an implementation-defined format. If not applicable, and (
keyword) is specified, then this conversion shall be equivalent
to %(keyword)u. If not applicable, and ( keyword) is omitted,
then this conversion shall be equivalent to .
10. An additional conversion specifier character, F, shall be used
to specify a pathname. The F conversion character can be pre‐
ceded by a sequence of comma-separated keywords:
(keyword[,keyword] ... )
The values for all the keywords that are non-null shall be concatenated
together, each separated by a '/' . The default shall be ( path) if the
keyword path is defined; otherwise, the default shall be ( prefix,
name).
11. An additional conversion specifier character, L, shall be used
to specify a symbolic line expansion. If the current file is a
symbolic link, then %L shall expand to:
"%s -> %s", ,
Otherwise, the %L conversion specification shall be the equivalent of
%F .
OPERANDS
The following operands shall be supported:
directory
The destination directory pathname for copy mode.
file A pathname of a file to be copied or archived.
pattern
A pattern matching one or more pathnames of archive members. A
pattern must be given in the name-generating notation of the
pattern matching notation in Pattern Matching Notation, includ‐
ing the filename expansion rules in Patterns Used for Filename
Expansion . The default, if no pattern is specified, is to
select all members in the archive.
STDIN
In write mode, the standard input shall be used only if no file oper‐
ands are specified. It shall be a text file containing a list of path‐
names, one per line, without leading or trailing s.
In list and read modes, if -f is not specified, the standard input
shall be an archive file.
Otherwise, the standard input shall not be used.
INPUT FILES
The input file named by the archive option-argument, or standard input
when the archive is read from there, shall be a file formatted accord‐
ing to one of the specifications in the EXTENDED DESCRIPTION section or
some other implementation-defined format.
The file /dev/tty shall be used to write prompts and read responses.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of pax:
LANG Provide a default value for the internationalization variables
that are unset or null. (See the Base Definitions volume of
IEEE Std 1003.1-2001, Section 8.2, Internationalization Vari‐
ables for the precedence of internationalization variables used
to determine the values of locale categories.)
LCALL If set to a non-empty string value, override the values of all
the other internationalization variables.
LCCOLLATE
Determine the locale for the behavior of ranges, equivalence
classes, and multi-character collating elements used in the pat‐
tern matching expressions for the pattern operand, the basic
regular expression for the -s option, and the extended regular
expression defined for the yesexpr locale keyword in the LCMES‐
SAGES category.
LCCTYPE
Determine the locale for the interpretation of sequences of
bytes of text data as characters (for example, single-byte as
opposed to multi-byte characters in arguments and input files),
the behavior of character classes used in the extended regular
expression defined for the yesexpr locale keyword in the LCMES‐
SAGES category, and pattern matching.
LCMESSAGES
Determine the locale for the processing of affirmative responses
that should be used to affect the format and contents of diag‐
nostic messages written to standard error.
LCTIME
Determine the format and contents of date and time strings when
the -v option is specified.
NLSPATH
Determine the location of message catalogs for the processing of
LCMESSAGES .
TMPDIR Determine the pathname that provides part of the default global
extended header record file, as described for the -o globexthdr=
keyword in the OPTIONS section.
TZ Determine the timezone used to calculate date and time strings
when the -v option is specified. If TZ is unset or null, an
unspecified default timezone shall be used.
ASYNCHRONOUS EVENTS
Default.
STDOUT
In write mode, if -f is not specified, the standard output shall be the
archive formatted according to one of the specifications in the
EXTENDED DESCRIPTION section, or some other implementation-defined for‐
mat (see -x format).
In list mode, when the -o listopt= format has been specified, the
selected archive members shall be written to standard output using the
format described under List Mode Format Specifications . In list mode
without the -o listopt= format option, the table of contents of the
selected archive members shall be written to standard output using the
following format:
"%s\n",
If the -v option is specified in list mode, the table of contents of
the selected archive members shall be written to standard output using
the following formats.
For pathnames representing hard links to previous members of the ar‐
chive:
"%s == %s\n", ,
For all other pathnames:
"%s\n",
where shall be the format specified by the ls utility
with the -l option. When writing pathnames in this format, it is
unspecified what is written for fields for which the underlying archive
format does not have the correct information, although the correct num‐
ber of -separated fields shall be written.
In list mode, standard output shall not be buffered more than a line at
a time.
STDERR
If -v is specified in read, write, or copy modes, pax shall write the
pathnames it processes to the standard error output using the following
format:
"%s\n",
These pathnames shall be written as soon as processing is begun on the
file or archive member, and shall be flushed to standard error. The
trailing , which shall not be buffered, is written when the
file has been read or written.
If the -s option is specified, and the replacement string has a trail‐
ing 'p', substitutions shall be written to standard error in the fol‐
lowing format:
"%s >> %s\n", ,
In all operating modes of pax, optional messages of unspecified format
concerning the input archive format and volume number, the number of
files, blocks, volumes, and media parts as well as other diagnostic
messages may be written to standard error.
In all formats, for both standard output and standard error, it is
unspecified how non-printable characters in pathnames or link names are
written.
When pax is in read mode or list mode, using the -x pax archive format,
and a filename, link name, owner name, or any other field in an
extended header record cannot be translated from the pax UTF-8 codeset
format to the codeset and current locale of the implementation, pax
shall write a diagnostic message to standard error, shall process the
file as described for the -o invalid= option, and then shall process
the next file in the archive.
OUTPUT FILES
In read mode, the extracted output files shall be of the archived file
type. In copy mode, the copied output files shall be the type of the
file being copied. In either mode, existing files in the destination
hierarchy shall be overwritten only when all permission ( -p), modifi‐
cation time ( -u), and invalid-value ( -o invalid=) tests allow it.
In write mode, the output file named by the -f option-argument shall be
a file formatted according to one of the specifications in the EXTENDED
DESCRIPTION section, or some other implementation-defined format.
EXTENDED DESCRIPTION
pax Interchange Format
A pax archive tape or file produced in the -x pax format shall contain
a series of blocks. The physical layout of the archive shall be identi‐
cal to the ustar format described in ustar Interchange Format . Each
file archived shall be represented by the following sequence:
* An optional header block with extended header records. This header
block is of the form described in pax Header Block, with a typeflag
value of x or g. The extended header records, described in pax
Extended Header, shall be included as the data for this header
block.
* A header block that describes the file. Any fields in the preceding
optional extended header shall 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 shall be two 512-byte blocks
filled with binary zeros, interpreted as an end-of-archive indicator.
A schematic of an example archive with global extended header records
and two actual files is shown in pax Format Archive Example . In the
example, the second file in the archive has no extended header preced‐
ing it, presumably because it has no need for extended attributes.
Figure: pax Format Archive Example
pax Header Block
The pax header block shall be identical to the ustar header block
described in ustar Interchange Format, except that two additional type‐
flag values are defined:
x Represents extended header records for the following file in the
archive (which shall have its own ustar header block). The for‐
mat of these extended header records shall be as described in
pax Extended Header .
g Represents global extended header records for the following
files in the archive. The format of these extended header
records shall be as described in pax Extended Header . 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. Archives cre‐
ated by pax -o linkdata shall include these data blocks with the hard
links.
pax Extended Header
A pax extended header contains values that are inappropriate for the
ustar header block because of limitations in that format: fields
requiring a character encoding other than that described in the
ISO/IEC 646:1991 standard, fields representing file attributes not
described in the ustar header, and fields whose format or length do not
fit the requirements of the ustar header. The values in an extended
header add attributes to the following file (or files; see the descrip‐
tion of the typeflag g header block) or override values in the follow‐
ing header block(s), as indicated in the following list of keywords.
An extended header shall consist of one or more records, each con‐
structed as follows:
"%d %s=%s\n", , ,
The extended header records shall be encoded according to the
ISO/IEC 10646-1:2000 standard (UTF-8). The field, ,
equals sign, and shown shall be limited to the portable char‐
acter set, as encoded in UTF-8. The and fields can be
any UTF-8 characters. The field shall be the decimal length of
the extended header record in octets, including the trailing .
The field shall be one of the entries from the following list
or a keyword provided as an implementation extension. Keywords consist‐
ing entirely of lowercase letters, digits, and periods are reserved for
future standardization. A keyword shall not include an equals sign. (In
the following list, the notations "file(s)" or "block(s)" is used to
acknowledge that a keyword affects the following single file after a
typeflag x extended header, but possibly multiple files after typeflag
g. Any requirements in the list for pax to include a record when in
write or copy mode shall apply only when such a record has not already
been provided through the use of the -o option. When used in copy mode,
pax shall behave as if an archive had been created with applicable
extended header records and then extracted.)
atime The file access time for the following file(s), equivalent to
the value of the statime 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 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 entries in the following table are
defined to refer to known standards; additional names may be
agreed on between the originator and recipient.
Formal Standard
ISO-IR 646 1990 ISO/IEC 646:1990
ISO-IR 8859 1 1998 ISO/IEC 8859-1:1998
ISO-IR 8859 2 1999 ISO/IEC 8859-2:1999
ISO-IR 8859 3 1999 ISO/IEC 8859-3:1999
ISO-IR 8859 4 1998 ISO/IEC 8859-4:1998
ISO-IR 8859 5 1999 ISO/IEC 8859-5:1999
ISO-IR 8859 6 1999 ISO/IEC 8859-6:1999
ISO-IR 8859 7 1987 ISO/IEC 8859-7:1987
ISO-IR 8859 8 1999 ISO/IEC 8859-8:1999
ISO-IR 8859 9 1999 ISO/IEC 8859-9:1999
ISO-IR 8859 10 1998 ISO/IEC 8859-10:1998
ISO-IR 8859 13 1998 ISO/IEC 8859-13:1998
ISO-IR 8859 14 1998 ISO/IEC 8859-14:1998
ISO-IR 8859 15 1999 ISO/IEC 8859-15:1999
ISO-IR 10646 2000 ISO/IEC 10646:2000
ISO-IR 10646 2000 UTF-8 ISO/IEC 10646, UTF-8 encoding
BINARY None.
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 indi‐
cates 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
field shall be ignored by pax.
ctime The file creation time for the following file(s), equivalent to
the value of the stctime member of the stat structure for a
file, as described by the stat() function. The creation time
shall be restored if the process has the appropriate privilege
required to do so. The format of the shall be as
described in pax Extended Header File Times .
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 trans‐
late 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 speci‐
fied, 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 fol‐
lowing ustar header block shall determine the type of link cre‐
ated. 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 (con‐
tents of the symbolic link) from the UTF-8 encoding to the char‐
acter 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 stmtime 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
shall be as described in pax Extended Header File Times
.
path The pathname of the following file(s). This record shall over‐
ride 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 speci‐
fied, 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 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 s;
all characters within the 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 Keyword Precedence
This section describes the precedence in which the various header
records and fields and command line options are selected to apply to a
file in the archive. When pax is used in read or list modes, it shall
determine a file attribute in the following sequence:
1. If -o delete= keyword-prefix is used, the affected attributes shall
be determined from step 7., if applicable, or ignored otherwise.
2. If -o keyword:= is used, the affected attributes shall be ignored.
3. If -o keyword := value is used, the affected attribute shall be
assigned the value.
4. If there is a typeflag x extended header record, the affected
attribute shall be assigned the . When extended header
records conflict, the last one given in the header shall take
precedence.
5. If -o keyword = value is used, the affected attribute shall be
assigned the value.
6. If there is a typeflag g global extended header record, the
affected attribute shall be assigned the . When global
extended header records conflict, the last one given in the global
header shall take precedence.
7. Otherwise, the attribute shall be determined from the ustar header
block.
pax Extended Header File Times
The pax utility shall write an mtime record for each file in write or
copy modes if the file's modification time cannot be represented
exactly in the ustar header logical record described in ustar Inter‐
change Format . This can occur if the time is out of ustar range, or if
the file system of the underlying implementation supports non-integer
time granularities and the time is not an integer. All of these 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, where the first digit is tenths of a
second and each subsequent digit is a tenth of the previous digit. 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 sup‐
ports the same time granularity.
ustar Interchange Format
A ustar archive tape or file shall contain a series of logical records.
Each logical record shall be a fixed-size logical record of 512 octets
(see below). Although this format may be thought of as being stored on
9-track industry-standard 12.7 mm (0.5 in) magnetic tape, other types
of transportable media are not excluded. Each file archived shall be
represented by a header logical record that describes the file, fol‐
lowed by zero or more logical records that give the contents of the
file. At the end of the archive file there shall be two 512-octet logi‐
cal records filled with binary zeros, interpreted as an end-of-archive
indicator.
The logical records may be grouped for physical I/O operations, as
described under the -b blocksize and -x ustar options. Each group of
logical records may be written with a single operation equivalent to
the write() function. On magnetic tape, the result of this write shall
be a single tape physical block. The last physical block shall always
be the full size, so logical records after the two zero logical records
may contain undefined data.
The header logical record shall be structured as shown in the following
table. All lengths and offsets are in decimal.
Table: ustar Header Block
Field Name Octet Offset Length (in Octets)
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 logical record shall be represented in the
coded character set of the ISO/IEC 646:1991 standard. For maximum
portability between implementations, names should be selected from
characters represented by the portable filename character set as octets
with the most significant bit zero. If an implementation supports the
use of characters outside of slash and the portable filename character
set in names for files, users, and groups, one or more implementation-
defined encodings of these characters shall be provided for interchange
purposes.
However, the pax utility shall never create filenames on the local sys‐
tem that cannot be accessed via the procedures described in
IEEE Std 1003.1-2001. If a filename is found on the medium that would
create an invalid filename, it is implementation-defined whether the
data from the file is stored on the file hierarchy and under what name
it is stored. The pax utility may choose to ignore these files as long
as it produces an error indicating that the file is being ignored.
Each field within the header logical record is contiguous; that is,
there is no padding used. Each character on the archive medium shall be
stored contiguously.
The fields magic, uname, and gname are character strings each termi‐
nated by a NUL character. The fields name, linkname, and prefix are
NUL-terminated character strings except when all characters in the
array contain non-NUL characters including the last character. The ver‐
sion field is two octets containing the characters "00" (zero-zero).
The typeflag contains a single character. All other fields are leading
zero-filled octal numbers using digits from the ISO/IEC 646:1991 stan‐
dard IRV. Each numeric field is terminated by one or more or
NUL characters.
The name and the prefix fields shall produce the pathname of the file.
A new pathname shall be formed, if prefix is not an empty string (its
first character is not NUL), by concatenating prefix (up to the first
NUL character), a slash character, and name; otherwise, name is used
alone. In either case, name is terminated at the first NUL character.
If prefix begins with a NUL character, it shall be ignored. In this
manner, pathnames of at most 256 characters can be supported. If a
pathname does not fit in the space provided, pax shall notify the user
of the error, and shall not store any part of the file-header or data-
on the medium.
The linkname field, described below, shall not use the prefix to pro‐
duce a pathname. As such, a linkname is limited to 100 characters. If
the name does not fit in the space provided, pax shall notify the user
of the error, and shall not attempt to store the link on the medium.
The mode field provides 12 bits encoded in the ISO/IEC 646:1991 stan‐
dard octal digit representation. The encoded bits shall represent the
following values:
Table: ustar mode Field
Bit Value IEEE Std 1003.1-2001 Bit Description
04000 SISUID Set UID on execution.
02000 SISGID Set GID on execution.
01000 Reserved for future standardization.
00400 SIRUSR Read permission for file owner class.
00200 SIWUSR Write permission for file owner
class.
00100 SIXUSR Execute/search permission for file
owner class.
00040 SIRGRP Read permission for file group class.
00020 SIWGRP Write permission for file group
class.
00010 SIXGRP Execute/search permission for file
group class.
00004 SIROTH Read permission for file other class.
00002 SIWOTH Write permission for file other
class.
00001 SIXOTH Execute/search permission for file
other class.
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 shall be ignored. Some of the mode bits in the
archive format are not mentioned elsewhere in this volume of
IEEE Std 1003.1-2001. If the implementation does not support those
bits, they may be ignored.
The uid and gid fields are the user and group ID of the owner and group
of the file, respectively.
The size field is the size of the file in octets. If the typeflag field
is set to specify a file to be of type 1 (a link) or 2 (a symbolic
link), the size field shall be specified as zero. If the typeflag field
is set to specify a file of type 5 (directory), the size field shall be
interpreted as described under the definition of that record type. No
data logical records are stored for types 1, 2, or 5. If the typeflag
field is set to 3 (character special file), 4 (block special file), or
6 (FIFO), the meaning of the size field is unspecified by this volume
of IEEE Std 1003.1-2001, and no data logical records shall be stored on
the medium. Additionally, for type 6, the size field shall be ignored
when reading. If the typeflag field is set to any other value, the num‐
ber of logical records written following the header shall be (
size+511)/512, ignoring any fraction in the result of the division.
The mtime field shall be the modification time of the file at the time
it was archived. It is the ISO/IEC 646:1991 standard representation of
the octal value of the modification time obtained from the stat() func‐
tion.
The chksum field shall be the ISO/IEC 646:1991 standard IRV representa‐
tion of the octal value of the simple sum of all octets in the header
logical record. Each octet in the header shall be treated as an
unsigned value. These values shall be added to an unsigned integer,
initialized to zero, the precision of which is not less than 17 bits.
When calculating the checksum, the chksum field is treated as if it
were all spaces.
The typeflag field specifies the type of file archived. If a particular
implementation does not recognize the type, or the user does not have
appropriate privilege to create that type, the file shall be extracted
as if it were a regular file if the file type is defined to have a
meaning for the size field that could cause data logical records to be
written on the medium (see the previous description for size). If con‐
version to a regular file occurs, the pax utility shall produce an
error indicating that the conversion took place. All of the typeflag
fields shall be coded in the ISO/IEC 646:1991 standard IRV:
0 Represents a regular file. For backwards-compatibility, a type‐
flag value of binary zero ( '\0' ) should be recognized as mean‐
ing a regular file when extracting files from the archive. Ar‐
chives written with this version of the archive file format cre‐
ate regular files with a typeflag value of the ISO/IEC 646:1991
standard IRV '0' .
1 Represents a file linked to another file, of any type, previ‐
ously archived. Such files are identified by each file having
the same device and file serial number. The linked-to name is
specified in the linkname field with a NUL-character terminator
if it is less than 100 octets in length.
2 Represents a symbolic link. The contents of the symbolic link
shall be stored in the linkname field.
3,4 Represent character special files and block special files
respectively. In this case the devmajor and devminor fields
shall contain information defining the device, the format of
which is unspecified by this volume of IEEE Std 1003.1-2001.
Implementations may map the device specifications to their own
local specification or may ignore the entry.
5 Specifies a directory or subdirectory. On systems where disk
allocation is performed on a directory basis, the size field
shall contain the maximum number of octets (which may be rounded
to the nearest disk block allocation unit) that the directory
may hold. A size field of zero indicates no such limiting. Sys‐
tems that do not support limiting in this manner should ignore
the size field.
6 Specifies a FIFO special file. Note that the archiving of a FIFO
file archives the existence of this file and not its contents.
7 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).
A-Z The letters 'A' to 'Z', inclusive, are reserved for custom
implementations. All other values are reserved for future ver‐
sions of IEEE Std 1003.1-2001.
Attempts to archive a socket using ustar interchange format shall pro‐
duce a diagnostic message. Handling of other file types is implementa‐
tion-defined.
The magic field is the specification that this archive was output in
this archive format. If this field contains ustar (the five characters
from the ISO/IEC 646:1991 standard IRV shown followed by NUL), the
uname and gname fields shall contain the ISO/IEC 646:1991 standard IRV
representation of the owner and group of the file, respectively (trun‐
cated to fit, if necessary). When the file is restored by a privileged,
protection-preserving version of the utility, the user and group data‐
bases shall be scanned for these names. If found, the user and group
IDs contained within these files shall be used rather than the values
contained within the uid and gid fields.
cpio Interchange Format
The octet-oriented cpio archive format shall be a series of entries,
each comprising a header that describes the file, the name of the file,
and then the contents of the file.
An archive may be recorded as a series of fixed-size blocks of octets.
This blocking shall be used only to make physical I/O more efficient.
The last group of blocks shall always be at the full size.
For the octet-oriented cpio archive format, the individual entry infor‐
mation shall be in the order indicated and described by the following
table; see also the header.
Table: Octet-Oriented cpio Archive Entry
Header Field Name Length (in Octets) Interpreted as
cmagic 6 Octal number
cdev 6 Octal number
cino 6 Octal number
cmode 6 Octal number
cuid 6 Octal number
cgid 6 Octal number
cnlink 6 Octal number
crdev 6 Octal number
cmtime 11 Octal number
cnamesize 6 Octal number
cfilesize 11 Octal number
Filename Field Name Length Interpreted as
cname cnamesize Pathname string
File Data Field Name Length Interpreted as
cfiledata cfilesize Data
cpio Header
For each file in the archive, a header as defined previously shall be
written. The information in the header fields is written as streams of
the ISO/IEC 646:1991 standard characters interpreted as octal numbers.
The octal numbers shall be extended to the necessary length by append‐
ing the ISO/IEC 646:1991 standard IRV zeros at the most-significant-
digit end of the number; the result is written to the most-significant
digit of the stream of octets first. The fields shall be interpreted as
follows:
cmagic
Identify the archive as being a transportable archive by con‐
taining the identifying value "070707" .
cdev, cino
Contains values that uniquely identify the file within the ar‐
chive (that is, no files contain the same pair of cdev and
cino values unless they are links to the same file). The values
shall be determined in an unspecified manner.
cmode Contains the file type and access permissions as defined in the
following table.
Table: Values for cpio cmode Field
File Permissions Name Value Indicates
CIRUSR 000400 Read by owner
CIWUSR 000200 Write by owner
CIXUSR 000100 Execute by owner
CIRGRP 000040 Read by group
CIWGRP 000020 Write by group
CIXGRP 000010 Execute by group
CIROTH 000004 Read by others
CIWOTH 000002 Write by others
CIXOTH 000001 Execute by others
CISUID 004000 Set uid
CISGID 002000 Set gid
CISVTX 001000 Reserved
File Type Name Value Indicates
CISDIR 040000 Directory
CISFIFO 010000 FIFO
CISREG 0100000 Regular file
CISLNK 0120000 Symbolic link
CISBLK 060000 Block special file
CISCHR 020000 Character special file
CISSOCK 0140000 Socket
CISCTG 0110000 Reserved
Directories, FIFOs, symbolic links, and regular files shall be sup‐
ported on a system conforming to this volume of IEEE Std 1003.1-2001;
additional values defined previously are reserved for compatibility
with existing systems. Additional file types may be supported; how‐
ever, such files should not be written to archives intended to be
transported to other systems.
cuid Contains the user ID of the owner.
cgid Contains the group ID of the group.
cnlink
Contains the number of links referencing the file at the time
the archive was created.
crdev Contains implementation-defined information for character or
block special files.
cmtime
Contains the latest time of modification of the file at the time
the archive was created.
cnamesize
Contains the length of the pathname, including the terminating
NUL character.
cfilesize
Contains the length of the file in octets. This shall be the
length of the data section following the header structure.
cpio Filename
The cname field shall contain the pathname of the file. The length of
this field in octets is the value of cnamesize.
If a filename is found on the medium that would create an invalid path‐
name, it is implementation-defined whether the data from the file is
stored on the file hierarchy and under what name it is stored.
All characters shall be represented in the ISO/IEC 646:1991 standard
IRV. For maximum portability between implementations, names should be
selected from characters represented by the portable filename character
set as octets with the most significant bit zero. If an implementation
supports the use of characters outside the portable filename character
set in names for files, users, and groups, one or more implementation-
defined encodings of these characters shall be provided for interchange
purposes. However, the pax utility shall never create filenames on the
local system that cannot be accessed via the procedures described pre‐
viously in this volume of IEEE Std 1003.1-2001. If a filename is found
on the medium that would create an invalid filename, it is implementa‐
tion-defined whether the data from the file is stored on the local file
system and under what name it is stored. The pax utility may choose to
ignore these files as long as it produces an error indicating that the
file is being ignored.
cpio File Data
Following cname, there shall be cfilesize octets of data. Interpreta‐
tion of such data occurs in a manner dependent on the file. If cfile‐
size is zero, no data shall be contained in cfiledata.
When restoring from an archive:
* If the user does not have the appropriate privilege to create a file
of the specified type, pax shall ignore the entry and write an error
message to standard error.
* Only regular files have data to be restored. Presuming a regular
file meets any selection criteria that might be imposed on the for‐
mat-reading utility by the user, such data shall be restored.
* If a user does not have appropriate privilege to set a particular
mode flag, the flag shall be ignored. Some of the mode flags in the
archive format are not mentioned elsewhere in this volume of
IEEE Std 1003.1-2001. If the implementation does not support those
flags, they may be ignored.
cpio Special Entries
FIFO special files, directories, and the trailer shall be recorded with
cfilesize equal to zero. For other special files, cfilesize is
unspecified by this volume of IEEE Std 1003.1-2001. The header for the
next file entry in the archive shall be written directly after the last
octet of the file entry preceding it. A header denoting the filename
TRAILER!!! shall indicate the end of the archive; the contents of
octets in the last block of the archive following such a header are
undefined.
EXIT STATUS
The following exit values shall be returned:
0 All files were processed successfully.
>0 An error occurred.
CONSEQUENCES OF ERRORS
If pax 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 -p option is specified, a diagnostic
message shall be written to standard error and a non-zero exit status
shall be returned, but processing shall continue. In the case where pax
cannot create a link to a file, pax shall not, by default, create a
second copy of the file.
If the extraction of a file from an archive is prematurely terminated
by a signal or error, pax may have only partially extracted the file or
(if the -n 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 SIRWXU mask set as well as incorrect
modification and access times.
The following sections are informative.
APPLICATION USAGE
The -p (privileges) option was invented to reconcile differences
between historical tar and cpio implementations. In particular, the two
utilities use -m in diametrically opposed ways. The -p option also pro‐
vides 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 are most commonly used:
-p e ``Preserve everything". This would be used by the historical
superuser, someone with all the appropriate privileges, to pre‐
serve all aspects of the files as they are recorded in the ar‐
chive. The e flag is the sum of o and p, and other implementa‐
tion-defined attributes.
-p p ``Preserve" 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.
The one pathname per line format of standard input precludes pathnames
containing s. Although such pathnames violate the portable
filename guidelines, they may exist and their presence may inhibit
usage of pax within shell scripts. This problem is inherited from his‐
torical archive programs. The problem can be avoided by listing file‐
name arguments on the command line instead of on standard input.
It is almost certain that appropriate privileges are required for pax
to accomplish parts of this volume of IEEE Std 1003.1-2001. Specifi‐
cally, creating files of type block special or character special,
restoring file access times unless the files are owned by the user (the
-t option), or preserving file owner, group, and mode (the -p option)
all probably require appropriate privileges.
In read mode, implementations are permitted to overwrite files when the
archive has multiple members with the same name. This may fail if per‐
missions on the first version of the file do not permit it to be over‐
written.
The cpio and ustar formats can only support files up to 8589934592
bytes (8 * 2^30) in size.
EXAMPLES
The following command:
pax -w -f /dev/rmt/1m .
copies the contents of the current directory to tape drive 1, medium
density (assuming historical System V device naming procedures-the his‐
torical BSD device name would be /dev/rmt9).
The following commands:
mkdir newdirpax -rw olddir newdir
copy the olddir directory hierarchy to newdir.
pax -r -s ',^//*usr//*,,' -f a.pax
reads the archive a.pax, with all files rooted in /usr in the archive
extracted relative to the current directory.
Using the option:
-o listopt="%M %(atime)T %(size)D %(name)s"
overrides the default output description in Standard Output and instead
writes:
-rw-rw- Jan 12 15:53 1492 /usr/foo/bar
Using the options:
-o listopt='%L\t%(size)D\n%.7' \
-o listopt='(name)s\n%(ctime)T\n%T'
overrides the default output description in Standard Output and instead
writes:
/usr/foo/bar -> /tmp 1492
/usr/fo
Jan 12 1991
Jan 31 15:53
RATIONALE
The pax utility was new for the ISO POSIX-2:1993 standard. It repre‐
sents a peaceful compromise between advocates of the historical tar and
cpio utilities.
A fundamental difference between cpio and tar was in the way directo‐
ries were treated. The cpio utility did not treat directories differ‐
ently from other files, and to select a directory and its contents
required that each file in the hierarchy be explicitly specified. For
tar, a directory matched every file in the file hierarchy it rooted.
The pax utility offers both interfaces; by default, directories map
into the file hierarchy they root. The -d option causes pax to skip any
file not explicitly referenced, as cpio historically did. The tar -
style behavior was chosen as the default because it was believed that
this was the more common usage and because tar is the more commonly
available interface, as it was historically provided on both System V
and BSD implementations.
The data interchange format specification in this volume of
IEEE Std 1003.1-2001 requires that processes with "appropriate privi‐
leges" shall always restore the ownership and permissions of extracted
files exactly as archived. If viewed from the historic equivalence
between superuser and "appropriate privileges", there are two problems
with this requirement. First, users running as superusers may unknow‐
ingly set dangerous permissions on extracted files. Second, it is need‐
lessly limiting, in that superusers cannot extract files and own them
as superuser unless the archive was created by the superuser. (It
should be noted that restoration of ownerships and permissions for the
superuser, by default, is historical practice in cpio, but not in tar.)
In order to avoid these two problems, the pax specification has an
additional "privilege" mechanism, the -p option. Only a pax invocation
with the privileges needed, and which has the -p option set using the e
specification character, has the "appropriate privilege" to restore
full ownership and permission information.
Note also that this volume of IEEE Std 1003.1-2001 requires that the
file ownership and access permissions shall be set, on extraction, in
the same fashion as the creat() function when provided with the mode
stored in the archive. This means that the file creation mask of the
user is applied to the file permissions.
Users should note that directories may be created by pax while extract‐
ing files with permissions that are different from those that existed
at the time the archive was created. When extracting sensitive informa‐
tion into a directory hierarchy that no longer exists, users are
encouraged to set their file creation mask appropriately to protect
these files during extraction.
The table of contents output is written to standard output to facili‐
tate pipeline processing.
An early proposal had hard links displaying for all pathnames. This was
removed because it complicates the output of the case where -v is not
specified and does not match historical cpio usage. The hard-link
information is available in the -v display.
The description of the -l option allows implementations to make hard
links to symbolic links. IEEE Std 1003.1-2001 does not specify any way
to create a hard link to a symbolic link, but many implementations pro‐
vide this capability as an extension. If there are hard links to sym‐
bolic links when an archive is created, the implementation is required
to archive the hard link in the archive (unless -H or -L is specified).
When in read mode and in copy mode, implementations supporting hard
links to symbolic links should use them when appropriate.
The archive formats inherited from the POSIX.1-1990 standard have cer‐
tain restrictions that have been brought along from historical usage.
For example, there are restrictions on the length of pathnames stored
in the archive. When pax is used in copy( -rw) mode (copying directory
hierarchies), the ability to use extensions from the -x pax format
overcomes these restrictions.
The default blocksize value of 5120 bytes for cpio was selected because
it is one of the standard block-size values for cpio, set when the -B
option is specified. (The other default block-size value for cpio is
512 bytes, and this was considered to be too small.) The default block
value of 10240 bytes for tar was selected because that is the standard
block-size value for BSD tar. The maximum block size of 32256 bytes
(2**15-512 bytes) is the largest multiple of 512 bytes that fits into a
signed 16-bit tape controller transfer register. There are known limi‐
tations in some historical systems that would prevent larger blocks
from being accepted. Historical values were chosen to improve compati‐
bility with historical scripts using dd or similar utilities to manipu‐
late archives. Also, default block sizes for any file type other than
character special file has been deleted from this volume of
IEEE Std 1003.1-2001 as unimportant and not likely to affect the struc‐
ture of the resulting archive.
Implementations are permitted to modify the block-size value based on
the archive format or the device to which the archive is being written.
This is to provide implementations with the opportunity to take advan‐
tage of special types of devices, and it should not be used without a
great deal of consideration as it almost certainly decreases archive
portability.
The intended use of the -n option was to permit extraction of one or
more files from the archive without processing the entire archive. This
was viewed by the standard developers as offering significant perfor‐
mance advantages over historical implementations. The -n option in
early proposals had three effects; the first was to cause special char‐
acters in patterns to not be treated specially. The second was to cause
only the first file that matched a pattern to be extracted. The third
was to cause pax to write a diagnostic message to standard error when
no file was found matching a specified pattern. Only the second behav‐
ior is retained by this volume of IEEE Std 1003.1-2001, for many rea‐
sons. First, it is in general not acceptable for a single option to
have multiple effects. Second, the ability to make pattern matching
characters act as normal characters is useful for parts of pax other
than file extraction. Third, a finer degree of control over the spe‐
cial characters is useful because users may wish to normalize only a
single special character in a single filename. Fourth, given a more
general escape mechanism, the previous behavior of the -n option can be
easily obtained using the -s option or a sed script. Finally, writing
a diagnostic message when a pattern specified by the user is unmatched
by any file is useful behavior in all cases.
In this version, the -n was removed from the copy mode synopsis of pax;
it is inapplicable because there are no pattern operands specified in
this mode.
There is another method than pax for copying subtrees in
IEEE Std 1003.1-2001 described as part of the cp utility. Both methods
are historical practice: cp provides a simpler, more intuitive inter‐
face, while pax offers a finer granularity of control. Each provides
additional functionality to the other; in particular, pax maintains the
hard-link structure of the hierarchy while cp does not. It is the
intention of the standard developers that the results be similar (using
appropriate option combinations in both utilities). The results are not
required to be identical; there seemed insufficient gain to applica‐
tions to balance the difficulty of implementations having to guarantee
that the results would be exactly identical.
A single archive may span more than one file. It is suggested that
implementations provide informative messages to the user on standard
error whenever the archive file is changed.
The -d option (do not create intermediate directories not listed in the
archive) found in early proposals was originally provided as a comple‐
ment to the historic -d option of cpio. It has been deleted.
The -s option in early proposals specified a subset of the substitution
command from the ed utility. As there was no reason for only a subset
to be supported, the -s option is now compatible with the current ed
specification. Since the delimiter can be any non-null character, the
following usage with single spaces is valid:
pax -s " foo bar " ...
The -t description is worded so as to note that this may cause the
access time update caused by some other activity (which occurs while
the file is being read) to be overwritten.
The default behavior of pax with regard to file modification times is
the same as historical implementations of tar. It is not the historical
behavior of cpio.
Because the -i option uses /dev/tty, utilities without a controlling
terminal are not able to use this option.
The -y option, found in early proposals, has been deleted because a
line containing a single period for the -i option has equivalent func‐
tionality. The special lines for the -i option (a single period and the
empty line) are historical practice in cpio.
In early drafts, a -e charmap option was included to increase portabil‐
ity of files between systems using different coded character sets. This
option was omitted because it was apparent that consensus could not be
formed for it. In this version, the use of UTF-8 should be an adequate
substitute.
The -k option was added to address international concerns about the
dangers involved in the character set transformations of -e (if the
target character set were different from the source, the filenames
might be transformed into names matching existing files) and also was
made more general to protect files transferred between file systems
with different {NAMEMAX} values (truncating a filename on a smaller
system might also inadvertently overwrite existing files). As stated,
it prevents any overwriting, even if the target file is older than the
source. This version adds more granularity of options to solve this
problem by introducing the -o invalid= option-specifically the UTF-8
action. (Note that an existing file that is named with a UTF-8 encoding
is still subject to overwriting in this case. The -k option closes that
loophole.)
Some of the file characteristics referenced in this volume of
IEEE Std 1003.1-2001 might not be supported by some archive formats.
For example, neither the tar nor cpio formats contain the file access
time. For this reason, the e specification character has been provided,
intended to cause all file characteristics specified in the archive to
be retained.
It is required that extracted directories, by default, have their
access and modification times and permissions set to the values speci‐
fied in the archive. This has obvious problems in that the directories
are almost certainly modified after being extracted and that directory
permissions may not permit file creation. One possible solution is to
create directories with the mode specified in the archive, as modified
by the umask of the user, with sufficient permissions to allow file
creation. After all files have been extracted, pax would then reset the
access and modification times and permissions as necessary.
The list-mode formatting description borrows heavily from the one
defined by the printf utility. However, since there is no separate op‐
erand list to get conversion arguments, the format was extended to
allow specifying the name of the conversion argument as part of the
conversion specification.
The T conversion specifier allows time fields to be displayed in any of
the date formats. Unlike the ls utility, pax does not adjust the format
when the date is less than six months in the past. This makes parsing
the output more predictable.
The D conversion specifier handles the ability to display the
major/minor or file size, as with ls, by using %-8(size)D.
The L conversion specifier handles the ls display for symbolic links.
Conversion specifiers were added to generate existing known types used
for ls.
pax Interchange Format
The new POSIX data interchange format was developed primarily to sat‐
isfy international concerns that the ustar and cpio formats did not
provide for file, user, and group names encoded in characters outside a
subset of the ISO/IEC 646:1991 standard. The standard developers real‐
ized that this new POSIX data interchange format should be very exten‐
sible because there were other requirements they foresaw in the near
future:
* Support international character encodings and locale information
* Support security information (ACLs, and so on)
* Support future file types, such as realtime or contiguous files
* Include data areas for implementation use
* Support systems with words larger than 32 bits and timers with sub‐
second granularity
The following were not goals for this format because these are better
handled by separate utilities or are inappropriate for a portable for‐
mat:
* Encryption
* Compression
* Data translation between locales and codesets
* inode storage
The format chosen to support the goals is an extension of the ustar
format. Of the two formats previously available, only the ustar format
was selected for extensions because:
* It was easier to extend in an upwards-compatible way. It offered
version flags and header block type fields with room for future
standardization. The cpio format, while possessing a more flexible
file naming methodology, could not be extended without breaking some
theoretical implementation or using a dummy filename that could be a
legitimate filename.
* Industry experience since the original " tar wars" fought in devel‐
oping the ISO POSIX-1 standard has clearly been in favor of the
ustar format, which is generally the default output format selected
for pax implementations on new systems.
The new format was designed with one additional goal in mind: reason‐
able behavior when an older tar or pax utility happened to read an ar‐
chive. Since the POSIX.1-1990 standard mandated that a "format-reading
utility" had to treat unrecognized typeflag values as regular files,
this allowed the format to include all the extended information in a
pseudo-regular file that preceded each real file. An option is given
that allows the archive creator to set up reasonable names for these
files on the older systems. Also, the normative text suggests that rea‐
sonable file access values be used for this ustar header block. Making
these header files inaccessible for convenient reading and deleting
would not be reasonable. File permissions of 600 or 700 are suggested.
The ustar typeflag field was used to accommodate the additional func‐
tionality of the new format rather than magic or version because the
POSIX.1-1990 standard (and, by reference, the previous version of pax),
mandated the behavior of the format-reading utility when it encountered
an unknown typeflag, but was silent about the other two fields.
Early proposals of the first revision to IEEE Std 1003.1-2001 contained
a proposed archive format that was based on compatibility with the
standard for tape files (ISO 1001, similar to the format used histori‐
cally on many mainframes and minicomputers). This format was overly
complex and required considerable overhead in volume and header
records. Furthermore, the standard developers felt that it would not be
acceptable to the community of POSIX developers, so it was later
changed to be a format more closely related to historical practice on
POSIX systems.
The prefix and name split of pathnames in ustar was replaced by the
single path extended header record for simplicity.
The concept of a global extended header ( typeflag g) was controver‐
sial. If this were applied to an archive being recorded on magnetic
tape, a few unreadable blocks at the beginning of the tape could be a
serious problem; a utility attempting to extract as many files as pos‐
sible from a damaged archive could lose a large percentage of file
header information in this case. However, if the archive were on a
reliable medium, such as a CD-ROM, the global extended header offers
considerable potential size reductions by eliminating redundant infor‐
mation. Thus, the text warns against using the global method for unre‐
liable media and provides a method for implanting global information in
the extended header for each file, rather than in the typeflag g
records.
No facility for data translation or filtering on a per-file basis is
included because the standard developers could not invent an interface
that would allow this in an efficient manner. If a filter, such as
encryption or compression, is to be applied to all the files, it is
more efficient to apply the filter to the entire archive as a single
file. The standard developers considered interfaces that would invoke a
shell script for each file going into or out of the archive, but the
system overhead in this approach was considered to be too high.
One such approach would be to have filter= records that give a pathname
for an executable. When the program is invoked, the file and archive
would be open for standard input/output and all the header fields would
be available as environment variables or command-line arguments. The
standard developers did discuss such schemes, but they were omitted
from IEEE Std 1003.1-2001 due to concerns about excessive overhead.
Also, the program itself would need to be in the archive if it were to
be used portably.
There is currently no portable means of identifying the character
set(s) used for a file in the file system. Therefore, pax has not been
given a mechanism to generate charset records automatically. The only
portable means of doing this is for the user to write the archive using
the -o charset= string command line option. This assumes that all of
the files in the archive use the same encoding. The "implementation-
defined" text is included to allow for a system that can identify the
encodings used for each of its files.
The table of standards that accompanies the charset record description
is acknowledged to be very limited. Only a limited number of character
set standards is reasonable for maximal interchange. Any character set
is, of course, possible by prior agreement. It was suggested that
EBCDIC be listed, but it was omitted because it is not defined by a
formal standard. Formal standards, and then only those with reasonably
large followings, can be included here, simply as a matter of practi‐
cality. The s represent names of officially registered character
sets in the format required by the ISO 2375:1985 standard.
The normal comma or -separated list rules are not followed in
the case of keyword options to allow ease of argument parsing for
getopts.
Further information on character encodings is in pax Archive Character
Set Encoding/Decoding .
The standard developers have reserved keyword name space for vendor
extensions. It is suggested that the format to be used is:
VENDOR.keyword
where VENDOR is the name of the vendor or organization in all uppercase
letters. It is further suggested that the keyword following the period
be named differently than any of the standard keywords so that it could
be used for future standardization, if appropriate, by omitting the
VENDOR prefix.
The field in the extended header record was included to make
it simpler to step through the records, even if a record contains an
unknown format (to a particular pax) with complex interactions of spe‐
cial characters. It also provides a minor integrity checkpoint within
the records to aid a program attempting to recover files from a damaged
archive.
There are no extended header versions of the devmajor and devminor
fields because the unspecified format ustar header field should be suf‐
ficient. If they are not, vendor-specific extended keywords (such as
VENDOR.devmajor) should be used.
Device and i-number labeling of files was not adopted from cpio; files
are interchanged strictly on a symbolic name basis, as in ustar.
Just as with the ustar format descriptions, the new format makes no
special arrangements for multi-volume archives. Each of the pax archive
types is assumed to be inside a single POSIX file and splitting that
file over multiple volumes (diskettes, tape cartridges, and so on),
processing their labels, and mounting each in the proper sequence are
considered to be implementation details that cannot be described
portably.
The pax format is intended for interchange, not only for backup on a
single (family of) systems. It is not as densely packed as might be
possible for backup:
* It contains information as coded characters that could be coded in
binary.
* It identifies extended records with name fields that could be omit‐
ted in favor of a fixed-field layout.
* It translates names into a portable character set and identifies
locale-related information, both of which are probably unnecessary
for backup.
The requirements on restoring from an archive are slightly different
from the historical wording, allowing for non-monolithic privilege to
bring forward as much as possible. In particular, attributes such as
"high performance file" might be broadly but not universally granted
while set-user-ID or chown() might be much more restricted. There is
no implication in IEEE Std 1003.1-2001 that the security information be
honored after it is restored to the file hierarchy, in spite of what
might be improperly inferred by the silence on that topic. That is a
topic for another standard.
Links are recorded in the fashion described here because a link can be
to any file type. It is desirable in general to be able to restore part
of an archive selectively and restore all of those files completely. If
the data is not associated with each link, it is not possible to do
this. However, the data associated with a file can be large, and when
selective restoration is not needed, this can be a significant burden.
The archive is structured so that files that have no associated data
can always be restored by the name of any link name of any link, and
the user may choose whether data is recorded with each instance of a
file that contains data. The format permits mixing of both types of
links in a single archive; this can be done for special needs, and pax
is expected to interpret such archives on input properly, despite the
fact that there is no pax option that would force this mixed case on
output. (When -o linkdata is used, the output must contain the dupli‐
cate data, but the implementation is free to include it or omit it when
-o linkdata is not used.)
The time values are included as extended header records for those
implementations needing more than the eleven octal digits allowed by
the ustar format. Portable file timestamps cannot be negative. If pax
encounters a file with a negative timestamp in copy or write mode, it
can reject the file, substitute a non-negative timestamp, or generate a
non-portable timestamp with a leading '-' . Even though some implemen‐
tations can support finer file-time granularities than seconds, the
normative text requires support only for seconds since the Epoch
because the ISO POSIX-1 standard states them that way. The ustar format
includes only mtime; the new format adds atime and ctime for symmetry.
The atime access time restored to the file system will be affected by
the -p a and -p e options. The ctime creation time (actually inode
modification time) is described with "appropriate privilege" so that it
can be ignored when writing to the file system. POSIX does not provide
a portable means to change file creation time. Nothing is intended to
prevent a non-portable implementation of pax from restoring the value.
The gid, size, and uid extended header records were included to allow
expansion beyond the sizes specified in the regular tar header. New
file system architectures are emerging that will exhaust the 12-digit
size field. There are probably not many systems requiring more than 8
digits for user and group IDs, but the extended header values were
included for completeness, allowing overrides for all of the decimal
values in the tar header.
The standard developers intended to describe the effective results of
pax with regard to file ownerships and permissions; implementations are
not restricted in timing or sequencing the restoration of such, pro‐
vided the results are as specified.
Much of the text describing the extended headers refers to use in "
write or copy modes". The copy mode references are due to the normative
text: "The effect of the copy shall be as if the copied files were
written to an archive file and then subsequently extracted ...". There
is certainly no way to test whether pax is actually generating the
extended headers in copy mode, but the effects must be as if it had.
pax Archive Character Set Encoding/Decoding
There is a need to exchange archives of files between systems of dif‐
ferent native codesets. Filenames, group names, and user names must be
preserved to the fullest extent possible when an archive is read on the
receiving platform. Translation of the contents of files is not within
the scope of the pax utility.
There will also be the need to represent characters that are not avail‐
able on the receiving platform. These unsupported characters cannot be
automatically folded to the local set of characters due to the chance
of collisions. This could result in overwriting previous extracted
files from the archive or pre-existing files on the system.
For these reasons, the codeset used to represent characters within the
extended header records of the pax archive must be sufficiently rich to
handle all commonly used character sets. The fields requiring transla‐
tion include, at a minimum, filenames, user names, group names, and
link pathnames. Implementations may wish to have localized extended
keywords that use non-portable characters.
The standard developers considered the following options:
* The archive creator specifies the well-defined name of the source
codeset. The receiver must then recognize the codeset name and per‐
form the appropriate translations to the destination codeset.
* The archive creator includes within the archive the character map‐
ping table for the source codeset used to encode extended header
records. The receiver must then read the character mapping table and
perform the appropriate translations to the destination codeset.
* The archive creator translates the extended header records in the
source codeset into a canonical form. The receiver must then perform
the appropriate translations to the destination codeset.
The approach that incorporates the name of the source codeset poses the
problem of codeset name registration, and makes the archive useless to
pax archive decoders that do not recognize that codeset.
Because parts of an archive may be corrupted, the standard developers
felt that including the character map of the source codeset was too
fragile. The loss of this one key component could result in making the
entire archive useless. (The difference between this and the global
extended header decision was that the latter has a workaround-duplicat‐
ing extended header records on unreliable media-but this would be too
burdensome for large character set maps.)
Both of the above approaches also put an undue burden on the pax ar‐
chive receiver to handle the cross-product of all source and destina‐
tion codesets.
To simplify the translation from the source codeset to the canonical
form and from the canonical form to the destination codeset, the stan‐
dard developers decided that the internal representation should be a
stateless encoding. A stateless encoding is one where each codepoint
has the same meaning, without regard to the decoder being in a specific
state. An example of a stateful encoding would be the Japanese Shift-
JIS; an example of a stateless encoding would be the ISO/IEC 646:1991
standard (equivalent to 7-bit ASCII).
For these reasons, the standard developers decided to adopt a canonical
format for the representation of file information strings. The obvious,
well-endorsed candidate is the ISO/IEC 10646-1:2000 standard (based in
part on Unicode), which can be used to represent the characters of vir‐
tually all standardized character sets. The standard developers ini‐
tially agreed upon using UCS2 (16-bit Unicode) as the internal repre‐
sentation. This repertoire of characters provides a sufficiently rich
set to represent all commonly-used codesets.
However, the standard developers found that the 16-bit Unicode repre‐
sentation had some problems. It forced the issue of standardizing byte
ordering. The 2-byte length of each character made the extended header
records twice as long for the case of strings coded entirely from his‐
torical 7-bit ASCII. For these reasons, the standard developers chose
the UTF-8 defined in the ISO/IEC 10646-1:2000 standard. This multi-byte
representation encodes UCS2 or UCS4 characters reliably and determinis‐
tically, eliminating the need for a canonical byte ordering. In addi‐
tion, NUL octets and other characters possibly confusing to POSIX file
systems do not appear, except to represent themselves. It was realized
that certain national codesets take up more space after the encoding,
due to their placement within the UCS range; it was felt that the use‐
fulness of the encoding of the names outweighs the disadvantage of size
increase for file, user, and group names.
The encoding of UTF-8 is as follows:
UCS4 Hex Encoding UTF-8 Binary Encoding
00000000-0000007F 0xxxxxxx
00000080-000007FF 110xxxxx 10xxxxxx
00000800-0000FFFF 1110xxxx 10xxxxxx 10xxxxxx
00010000-001FFFFF 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
00200000-03FFFFFF 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
04000000-7FFFFFFF 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
where each 'x' represents a bit value from the character being trans‐
lated.
ustar Interchange Format
The description of the ustar format reflects numerous enhancements over
pre-1988 versions of the historical tar utility. The goal of these
changes was not only to provide the functional enhancements desired,
but also to retain compatibility between new and old versions. This
compatibility has been retained. Archives written using the old ar‐
chive format are compatible with the new format.
Implementors should be aware that the previous file format did not
include a mechanism to archive directory type files. For this reason,
the convention of using a filename ending with slash was adopted to
specify a directory on the archive.
The total size of the name and prefix fields have been set to meet the
minimum requirements for {PATHMAX}. If a pathname will fit within the
name field, it is recommended that the pathname be stored there without
the use of the prefix field. Although the name field is known to be too
small to contain {PATHMAX} characters, the value was not changed in
this version of the archive file format to retain backwards-compatibil‐
ity, and instead the prefix was introduced. Also, because of the ear‐
lier version of the format, there is no way to remove the restriction
on the linkname field being limited in size to just that of the name
field.
The size field is required to be meaningful in all implementation
extensions, although it could be zero. This is required so that the
data blocks can always be properly counted.
It is suggested that if device special files need to be represented
that cannot be represented in the standard format, that one of the
extension types ( A- Z) be used, and that the additional information
for the special file be represented as data and be reflected in the
size field.
Attempting to restore a special file type, where it is converted to
ordinary data and conflicts with an existing filename, need not be spe‐
cially detected by the utility. If run as an ordinary user, pax should
not be able to overwrite the entries in, for example, /dev in any case
(whether the file is converted to another type or not). If run as a
privileged user, it should be able to do so, and it would be considered
a bug if it did not. The same is true of ordinary data files and simi‐
larly named special files; it is impossible to anticipate the needs of
the user (who could really intend to overwrite the file), so the behav‐
ior should be predictable (and thus regular) and rely on the protection
system as required.
The value 7 in the typeflag field is intended to define how contiguous
files can be stored in a ustar archive. IEEE Std 1003.1-2001 does not
require the contiguous file extension, but does define a standard way
of archiving such files so that all conforming systems can interpret
these file types in a meaningful and consistent manner. On a system
that does not support extended file types, the pax utility should do
the best it can with the file and go on to the next.
The file protection modes are those conventionally used by the ls util‐
ity. This is extended beyond the usage in the ISO POSIX-2 standard to
support the "shared text" or "sticky" bit. It is intended that the con‐
formance document should not document anything beyond the existence of
and support of such a mode. Further extensions are expected to these
bits, particularly with overloading the set-user-ID and set-group-ID
flags.
cpio Interchange Format
The reference to appropriate privilege in the cpio format refers to an
error on standard output; the ustar format does not make comparable
statements.
The model for this format was the historical System V cpio -c data
interchange format. This model documents the portable version of the
cpio format and not the binary version. It has the flexibility to
transfer data of any type described within IEEE Std 1003.1-2001, yet is
extensible to transfer data types specific to extensions beyond
IEEE Std 1003.1-2001 (for example, contiguous files). Because it
describes existing practice, there is no question of maintaining
upwards-compatibility.
cpio Header
There has been some concern that the size of the cino field of the
header is too small to handle those systems that have very large inode
numbers. However, the cino field in the header is used strictly as a
hard-link resolution mechanism for archives. It is not necessarily the
same value as the inode number of the file in the location from which
that file is extracted.
The name cmagic is based on historical usage.
cpio Filename
For most historical implementations of the cpio utility, {PATHMAX}
octets can be used to describe the pathname without the addition of any
other header fields (the NUL character would be included in this
count). {PATHMAX} is the minimum value for pathname size, documented
as 256 bytes. However, an implementation may use cnamesize to deter‐
mine the exact length of the pathname. With the current description of
the header, this pathname size can be as large as a number
that is described in six octal digits.
Two values are documented under the cmode field values to provide for
extensibility for known file types:
0110 000
Reserved for contiguous files. The implementation may treat the
rest of the information for this archive like a regular file.
If this file type is undefined, the implementation may create
the file as a regular file.
This provides for extensibility of the cpio format while allowing for
the ability to read old archives. Files of an unknown type may be read
as "regular files" on some implementations. On a system that does not
support extended file types, the pax utility should do the best it can
with the file and go on to the next.
FUTURE DIRECTIONS
None.
SEE ALSO
Shell Command Language, cp, ed, getopts, ls, printf(), the Base Defini‐
tions volume of IEEE Std 1003.1-2001, , the System Interfaces
volume of IEEE Std 1003.1-2001, chown(), creat(), mkdir(), mkfifo(),
stat(), utime(), write()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online
at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 PAX(1P)