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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 (con-
sult 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 formats 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 directory 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 considered 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 hierar-
chy 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 destination directory is a subdi-
rectory 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; otherwise,
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 fol-
lowing arguments:
* The intermediate directory used as the path argument
* The value of the bitwise-inclusive OR of S_IRWXU, S_IRWXG, and S_IRWXO 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 diag-
nostic 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 determine, 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 contents 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 Guide-
lines, except that the order of presentation of the -o, -p, and -s options is significant.
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 format.
-a Append files to the end of the archive. It is implementation-defined which devices on the system support append-
ing. Additional file formats unspecified by this volume of IEEE Std 1003.1-2001 may impose restrictions on append-
ing.
-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 creating 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 archive 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 specified 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 hier-
archy. 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 operand, 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 encoun-
tered when reading a response or if /dev/tty cannot be opened for reading and writing.
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 when-
ever possible. If specified in conjunction with -H or -L, when a symbolic link is encountered, the hard link cre-
ated 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 symbolic 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 specified 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 tra-
versal 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 operand. 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 algorithm 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 inapplica-
ble 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 Defi-
nitions 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 file-
names.
Keywords can be preceded with white space. The value field shall consist 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 keywords given to these multiple -o options conflict, the key-
words 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 keyword 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 substitu-
tions 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 encounter-
ing 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
written in the codeset and current locale of the implementation.
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 replacement 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, translating 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 modifica-
tion time ( -u) restrictions, and shall be suppressed if the -k option is also specified.
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 contents 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 con-
sidered part of the format string. When multiple -o listopt= format options are specified, the format 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 beginning 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 keyword/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,
" <archive
the group name will be forced to a new value for all files read from the archive.
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 speci-
fying file characteristics to be retained or discarded on extraction. The string shall consist of the specifica-
tion characters a, e, m, o, and p . Other implementation-defined characters can be included. Multiple characteris-
tics can be concatenated within the same string and multiple -p options can be specified. The meaning of the spec-
ification 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 implementation-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 permissions 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 preserved 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 S_ISUID and S_ISGID 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 preserve 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 operands 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) backrefer-
ences, or subexpression matching. The old string shall also be permitted to contain <newline>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 utility. The optional trailing 'p' shall cause successful substitutions to be writ-
ten to standard error. File or archive member names that substitute to the empty string shall be ignored when reading and
writing archives.
-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 modification time) than a pre-existing file or archive mem-
ber 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 member is newer than the file. In write mode, an archive file member 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 spec-
ified, this is accomplished by appending to the archive; otherwise, it is unspecified whether this is accomplished
by actual replacement in the archive or by appending to the archive. 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 hierarchy is newer.
-v In list mode, produce a verbose table of contents (see the STDOUT section). Otherwise, write archive member path-
names to standard 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 section. The default blocksize for this format for
character special archive files shall be 5120. Implementations shall support all blocksize values less than or
equal to 32256 that are multiples of 512.
pax
The pax interchange format; see the EXTENDED DESCRIPTION section. The default blocksize for this format for char-
acter special archive files shall be 5120. Implementations shall support all blocksize values less than or equal
to 32256 that are multiples of 512.
ustar
The tar interchange format; see the EXTENDED DESCRIPTION section. The default blocksize for this format for char-
acter special archive files shall be 10240. Implementations shall support all blocksize values less than or equal
to 32256 that are multiples of 512.
Implementation-defined formats shall specify a default block size as well as any other block sizes supported for charac-
ter 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 ( st_dev; 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 fol-
lows. 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 <newline> to the listopt output for each selected file. The format argument shall be used as the
format string described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 5, File Format Notation, 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 keywords:
* 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 c_mode
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 appro-
priate 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 Output. If ( keyword) is omitted, the mode keyword shall be used. For example, %.1M writes the single
character corresponding to the <entry type> 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 con-
version shall be equivalent to %(keyword)u. If not applicable, and ( keyword) is omitted, then this conversion
shall be equivalent to <space>.
10. An additional conversion specifier character, F, shall be used to specify a pathname. The F conversion character
can be preceded 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 cur-
rent file is a symbolic link, then %L shall expand to:
"%s -> %s", <value of keyword>, <contents of link>
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 nota-
tion of the pattern matching notation in Pattern Matching Notation, including the filename expansion rules in Pat-
terns Used for Filename Expansion . The default, if no pattern is specified, is to select all members in the ar-
chive.
STDIN
In write mode, the standard input shall be used only if no file operands are specified. It shall be a text file contain-
ing a list of pathnames, one per line, without leading or trailing <blank>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 according 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 Variables for the precedence of international-
ization variables used to determine the values of locale categories.)
LC_ALL If set to a non-empty string value, override the values of all the other internationalization variables.
LC_COLLATE
Determine the locale for the behavior of ranges, equivalence classes, and multi-character collating elements used
in the pattern 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 LC_MESSAGES category.
LC_CTYPE
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 LC_MESSAGES category, and pattern
matching.
LC_MESSAGES
Determine the locale for the processing of affirmative responses that should be used to affect the format and con-
tents of diagnostic messages written to standard error.
LC_TIME
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 LC_MESSAGES .
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 speci-
fications in the EXTENDED DESCRIPTION section, or some other implementation-defined format (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", <pathname>
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 archive:
"%s == %s\n", <ls -l listing>, <linkname>
For all other pathnames:
"%s\n", <ls -l listing>
where <ls -l listing> 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 number of <blank>-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", <pathname>
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 <newline>, 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 trailing 'p', substitutions shall be written to standard
error in the following format:
"%s >> %s\n", <original pathname>, <new pathname>
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 cur-
rent 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), modification 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 specifi-
cations 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 ar-
chive shall be identical to the ustar format described in ustar Interchange Format . Each file archived shall be repre-
sented 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 asso-
ciated 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-ar-
chive 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 preceding 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 typeflag values are defined:
x Represents extended header records for the following file in the archive (which shall have its own ustar header
block). The format 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 created 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 repre-
senting 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 description
of the typeflag g header block) or override values in the following header block(s), as indicated in the following list
of keywords.
An extended header shall consist of one or more records, each constructed as follows:
"%d %s=%s\n", <length>, <keyword>, <value>
The extended header records shall be encoded according to the ISO/IEC 10646-1:2000 standard (UTF-8). The <length> field,
<blank>, equals sign, and <newline> shown shall be limited to the portable character set, as encoded in UTF-8. The <key-
word> and <value> fields can be any UTF-8 characters. The <length> field shall be the decimal length of the extended
header record in octets, including the trailing <newline>.
The <keyword> field shall be one of the entries from the following list or a keyword provided as an implementation exten-
sion. Keywords consisting 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 st_atime member of the stat struc-
ture for a file, as described by the stat() function. The access time shall be restored if the process has the
appropriate privilege required to do so. The format of the <value> shall be as described in pax Extended Header
File Times .
charset
The name of the character set used to encode the data in the following file(s). The entries in the following table
are defined to refer to known standards; additional names may be agreed on between the originator and recipient.
<value> 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 indicates unencoded
binary data.
When used in write or copy mode, it is implementation-defined whether pax includes a charset extended header record for a
file.
comment
A series of characters used as a comment. All characters in the <value> field shall be ignored by pax.
ctime The file creation time for the following file(s), equivalent to the value of the st_ctime 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 <value> 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 translate the name from the UTF-8 encoding in the header record to the character set appro-
priate for the group database on the receiving system. If any of the UTF-8 characters cannot be translated, and if
the -o invalid= UTF-8 option is not specified, the results are implementation-defined. When used in write or copy
mode, pax shall include a gname extended header record for each file whose group name cannot be represented
entirely with the letters and digits of the portable character set.
linkpath
The pathname of a link being created to another file, of any type, previously archived. This record shall override
the linkname field in the following ustar header block(s). The following ustar header block shall determine the
type of link created. If typeflag of the following header block is 1, it shall be a hard link. If typeflag is 2,
it shall be a symbolic link and the linkpath value shall be the contents of the symbolic link. The pax utility
shall translate the name of the link (contents of the symbolic link) from the UTF-8 encoding to the character set
appropriate for the local file system. When used in write or copy mode, pax shall include a linkpath extended
header record for each link whose pathname cannot be represented entirely with the members of the portable charac-
ter set other than NUL.
mtime The file modification time of the following file(s), equivalent to the value of the st_mtime member of the stat
structure for a file, as described in the stat() function. This record shall override the mtime field in the fol-
lowing header block(s). The modification time shall be restored if the process has the appropriate privilege
required to do so. The format of the <value> shall be as described in pax Extended Header File Times .
path The pathname of the following file(s). This record shall override the name and prefix fields in the following
header block(s). The pax utility shall translate the pathname of the file from the UTF-8 encoding to the character
set appropriate for the local file system.
When used in write or copy mode, pax shall include a path extended header record for each file whose pathname cannot be
represented entirely with the members of the portable character set other than NUL.
realtime.any
The keywords prefixed by "realtime." are reserved for future standardization.
security.any
The keywords prefixed by "security." are reserved for future standardization.
size The size of the file in octets, expressed as a decimal number using digits from the ISO/IEC 646:1991 standard.
This record shall override the size field in the following header block(s). When used in write or copy mode, pax
shall include a size extended header record for each file with a size value greater than 8589934591 (octal
77777777777).
uid The user ID of the file owner, expressed as a decimal number using digits from the ISO/IEC 646:1991 standard. This
record shall override the uid field in the following header block(s). When used in write or copy mode, pax shall
include a uid extended header record for each file whose owner ID is greater than 2097151 (octal 7777777).
uname The owner of the following file(s), formatted as a user name in the user database. This record shall override the
uid and uname fields in the following header block(s), and any uid extended header record. When used in read,
copy, or list mode, pax shall translate the name from the UTF-8 encoding in the header record to the character set
appropriate for the user database on the receiving system. If any of the UTF-8 characters cannot be translated,
and if the -o invalid= UTF-8 option is not specified, the results are implementation-defined. When used in write
or copy mode, pax shall include a uname extended header record for each file whose user name cannot be represented
entirely with the letters and digits of the portable character set.
If the <value> field is zero length, it shall delete any header block field, previously entered extended header value, or
global extended header value of the same name.
If a keyword in an extended header record (or in a -o option-argument) overrides or deletes a corresponding field in the
ustar header block, pax shall ignore the contents of that header block field.
Unlike the ustar header block fields, NULs shall not delimit <value>s; all characters within the <value> field shall be
considered data for the field. None of the length limitations of the ustar header block fields in ustar Header Block
shall apply to the extended header records.
pax Extended Header 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 <value>. 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 <value>. 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 Interchange 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 granulari-
ties 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 subse-
quent 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 dig-
its 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 repre-
sented by a header logical record that describes the file, followed by zero or more logical records that give the con-
tents of the file. At the end of the archive file there shall be two 512-octet logical 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 stan-
dard. For maximum portability between implementations, names should be selected from characters represented by the porta-
ble filename character set as octets with the most significant bit zero. If an implementation supports the use of char-
acters outside of slash and the portable filename character set in names for files, users, and groups, one or more imple-
mentation-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 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 ar-
chive medium shall be stored contiguously.
The fields magic, uname, and gname are character strings each terminated 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 version 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 standard IRV. Each numeric field is terminated by one or more <space> 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 char-
acter, 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 produce 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 standard 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 S_ISUID Set UID on execution.
02000 S_ISGID Set GID on execution.
01000 <reserved> Reserved for future standardization.
00400 S_IRUSR Read permission for file owner class.
00200 S_IWUSR Write permission for file owner
class.
00100 S_IXUSR Execute/search permission for file
owner class.
00040 S_IRGRP Read permission for file group class.
00020 S_IWGRP Write permission for file group
class.
00010 S_IXGRP Execute/search permission for file
group class.
00004 S_IROTH Read permission for file other class.
00002 S_IWOTH Write permission for file other
class.
00001 S_IXOTH 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 log-
ical 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 number 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 stan-
dard representation of the octal value of the modification time obtained from the stat() function.
The chksum field shall be the ISO/IEC 646:1991 standard IRV representation 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 conversion to a regular file occurs, the pax utility shall pro-
duce 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 typeflag value of binary zero ( '\0' ) should be recog-
nized as meaning a regular file when extracting files from the archive. Archives written with this version of the
archive file format create regular files with a typeflag value of the ISO/IEC 646:1991 standard IRV '0' .
1 Represents a file linked to another file, of any type, previously archived. Such files are identified by each file
having the same device and file serial number. The linked-to name is specified in the 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. Systems 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. Implemen-
tations 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 versions of IEEE Std 1003.1-2001.
Attempts to archive a socket using ustar interchange format shall produce a diagnostic message. Handling of other file
types is implementation-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 con-
tain the ISO/IEC 646:1991 standard IRV representation of the owner and group of the file, respectively (truncated to fit,
if necessary). When the file is restored by a privileged, protection-preserving version of the utility, the user and
group databases 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 information shall be in the order indicated and
described by the following table; see also the <cpio.h> header.
Table: Octet-Oriented cpio Archive Entry
Header Field Name Length (in Octets) Interpreted as
c_magic 6 Octal number
c_dev 6 Octal number
c_ino 6 Octal number
c_mode 6 Octal number
c_uid 6 Octal number
c_gid 6 Octal number
c_nlink 6 Octal number
c_rdev 6 Octal number
c_mtime 11 Octal number
c_namesize 6 Octal number
c_filesize 11 Octal number
Filename Field Name Length Interpreted as
c_name c_namesize Pathname string
File Data Field Name Length Interpreted as
c_filedata c_filesize 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 appending 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:
c_magic
Identify the archive as being a transportable archive by containing the identifying value "070707" .
c_dev, c_ino
Contains values that uniquely identify the file within the archive (that is, no files contain the same pair of
c_dev and c_ino values unless they are links to the same file). The values shall be determined in an unspecified
manner.
c_mode Contains the file type and access permissions as defined in the following table.
Table: Values for cpio c_mode Field
File Permissions Name Value Indicates
C_IRUSR 000400 Read by owner
C_IWUSR 000200 Write by owner
C_IXUSR 000100 Execute by owner
C_IRGRP 000040 Read by group
C_IWGRP 000020 Write by group
C_IXGRP 000010 Execute by group
C_IROTH 000004 Read by others
C_IWOTH 000002 Write by others
C_IXOTH 000001 Execute by others
C_ISUID 004000 Set uid
C_ISGID 002000 Set gid
C_ISVTX 001000 Reserved
File Type Name Value Indicates
C_ISDIR 040000 Directory
C_ISFIFO 010000 FIFO
C_ISREG 0100000 Regular file
C_ISLNK 0120000 Symbolic link
C_ISBLK 060000 Block special file
C_ISCHR 020000 Character special file
C_ISSOCK 0140000 Socket
C_ISCTG 0110000 Reserved
Directories, FIFOs, symbolic links, and regular files shall be supported on a system conforming to this volume of
IEEE Std 1003.1-2001; additional values defined previously are reserved for compatibility with existing systems. Addi-
tional file types may be supported; however, such files should not be written to archives intended to be transported to
other systems.
c_uid Contains the user ID of the owner.
c_gid Contains the group ID of the group.
c_nlink
Contains the number of links referencing the file at the time the archive was created.
c_rdev Contains implementation-defined information for character or block special files.
c_mtime
Contains the latest time of modification of the file at the time the archive was created.
c_namesize
Contains the length of the pathname, including the terminating NUL character.
c_filesize
Contains the length of the file in octets. This shall be the length of the data section following the header
structure.
cpio Filename
The c_name field shall contain the pathname of the file. The length of this field in octets is the value of c_namesize.
If a filename is found on the medium that would create an invalid pathname, 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 implementa-
tions, 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 pro-
vided for interchange purposes. However, the pax utility shall never create filenames on the local system that cannot be
accessed via the procedures described previously 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 implementation-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 c_name, there shall be c_filesize octets of data. Interpretation of such data occurs in a manner dependent on
the file. If c_filesize is zero, no data shall be contained in c_filedata.
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 format-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 implemen-
tation does not support those flags, they may be ignored.
cpio Special Entries
FIFO special files, directories, and the trailer shall be recorded with c_filesize equal to zero. For other special
files, c_filesize is unspecified by this volume of IEEE Std 1003.1-2001. The header for the next file entry in the ar-
chive 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 S_IRWXU 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 par-
ticular, the two utilities use -m in diametrically opposed ways. The -p option also provides a consistent means of
extending the ways in which future file attributes can be addressed, such as for enhanced security systems or high-per-
formance 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 privi-
leges, to preserve all aspects of the files as they are recorded in the archive. The e flag is the sum of o and
p, and other implementation-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 <newline>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 historical archive programs. The problem can be avoided by listing filename
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. Specifically, 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 permissions on the first version of the file do not permit it to be overwritten.
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 historical 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 represents a peaceful compromise between advocates of the
historical tar and cpio utilities.
A fundamental difference between cpio and tar was in the way directories were treated. The cpio utility did not treat
directories differently from other files, and to select a directory and its contents required that each file in the hier-
archy 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 "appropri-
ate privileges" 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 unknowingly set dangerous permissions on extracted files. Second, it is needlessly
limiting, in that superusers cannot extract files and own them as superuser unless the archive was created by the supe-
ruser. (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 "privi-
lege" 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 extracting files with permissions that are different from
those that existed at the time the archive was created. When extracting sensitive information 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 facilitate 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 provide this capability as an
extension. If there are hard links to symbolic links when an archive is created, the implementation is required to ar-
chive 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 certain 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 con-
sidered 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 limitations in some historical systems that
would prevent larger blocks from being accepted. Historical values were chosen to improve compatibility with historical
scripts using dd or similar utilities to manipulate 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 structure of the resulting archive.
Implementations are permitted to modify the block-size value based on the archive format or the device to which the ar-
chive is being written. This is to provide implementations with the opportunity to take advantage of special types of
devices, and it should not be used without a great deal of consideration as it almost certainly decreases archive porta-
bility.
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 performance advantages over historical
implementations. The -n option in early proposals had three effects; the first was to cause special characters in pat-
terns 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 behavior is retained by this volume of IEEE Std 1003.1-2001, for many reasons. First, it is in
general not acceptable for a single option to have multiple effects. Second, the ability to make pattern matching charac-
ters act as normal characters is useful for parts of pax other than file extraction. Third, a finer degree of control
over the special 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 interface, 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 appropri-
ate option combinations in both utilities). The results are not required to be identical; there seemed insufficient gain
to applications to balance the difficulty of implementations having to guarantee that the results would be exactly iden-
tical.
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 complement 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 rea-
son 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 functionality. The special lines for the -i option (a single period and the empty line) are historical prac-
tice in cpio.
In early drafts, a -e charmap option was included to increase portability 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 ver-
sion, 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
{NAME_MAX} 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 specified 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 per-
missions as necessary.
The list-mode formatting description borrows heavily from the one defined by the printf utility. However, since there is
no separate operand list to get conversion arguments, the format was extended to allow specifying the name of the conver-
sion 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 satisfy 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 realized that this new POSIX data interchange format should be very extensible 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 subsecond granularity
The following were not goals for this format because these are better handled by separate utilities or are inappropriate
for a portable format:
* 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 file-
name.
* Industry experience since the original " tar wars" fought in developing 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 sys-
tems.
The new format was designed with one additional goal in mind: reasonable behavior when an older tar or pax utility hap-
pened to read an archive. Since the POSIX.1-1990 standard mandated that a "format-reading utility" had to treat unrecog-
nized typeflag values as regular files, this allowed the format to include all the extended information in a pseudo-regu-
lar 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 reasonable 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 functionality of the new format rather than magic or ver-
sion 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 com-
patibility with the standard for tape files (ISO 1001, similar to the format used historically on many mainframes and
minicomputers). This format was overly complex and required considerable overhead in volume and header records. Further-
more, 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 controversial. 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 possible 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 information. Thus, the text warns against
using the global method for unreliable 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 stan-
dard 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 stan-
dard. Formal standards, and then only those with reasonably large followings, can be included here, simply as a matter of
practicality. The <value>s represent names of officially registered character sets in the format required by the
ISO 2375:1985 standard.
The normal comma or <blank>-separated list rules are not followed in the case of keyword options to allow ease of argu-
ment 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 standard-
ization, if appropriate, by omitting the VENDOR prefix.
The <length> 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 special 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 sufficient. 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 omitted in favor of a fixed-field layout.
* It translates names into a portable character set and identifies locale-related information, both of which are proba-
bly unnecessary for backup.
The requirements on restoring from an archive are slightly different from the historical wording, allowing for non-mono-
lithic 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 associ-
ated 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 duplicate 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 dig-
its 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 implementations 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 modi-
fication 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 implementa-
tion 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 com-
pleteness, 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, provided the results are as speci-
fied.
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 head-
ers 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 different 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. Transla-
tion 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 available 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 translation 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 code-
set name and perform the appropriate translations to the destination codeset.
* The archive creator includes within the archive the character mapping table for the source codeset used to encode
extended header records. The receiver must then read the character mapping table and perform the appropriate transla-
tions 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 dif-
ference between this and the global extended header decision was that the latter has a workaround-duplicating 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 archive receiver to handle the cross-product of all
source and destination codesets.
To simplify the translation from the source codeset to the canonical form and from the canonical form to the destination
codeset, the standard 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 virtually all standardized character sets. The standard developers initially
agreed upon using UCS2 (16-bit Unicode) as the internal representation. This repertoire of characters provides a suffi-
ciently rich set to represent all commonly-used codesets.
However, the standard developers found that the 16-bit Unicode representation 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 historical 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
deterministically, eliminating the need for a canonical byte ordering. In addition, NUL octets and other characters pos-
sibly 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 usefulness 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 translated.
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 archive 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 {PATH_MAX}. 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 {PATH_MAX} characters, the value was not changed in this ver-
sion of the archive file format to retain backwards-compatibility, and instead the prefix was introduced. Also, because
of the earlier 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 file-
name, need not be specially 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 similarly named special files; it is impossible to anticipate the needs of the user (who could really intend to
overwrite the file), so the behavior 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 utility. 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 conformance 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-com-
patibility.
cpio Header
There has been some concern that the size of the c_ino field of the header is too small to handle those systems that have
very large inode numbers. However, the c_ino 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 c_magic is based on historical usage.
cpio Filename
For most historical implementations of the cpio utility, {PATH_MAX} 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). {PATH_MAX} is the minimum
value for pathname size, documented as 256 bytes. However, an implementation may use c_namesize to determine the exact
length of the pathname. With the current description of the <cpio.h> header, this pathname size can be as large as a num-
ber that is described in six octal digits.
Two values are documented under the c_mode 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 Definitions volume of IEEE Std 1003.1-2001, <cpio.h>, 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, Copy-
right (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.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 PAX(1P)
