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NAME
pax — portable archive interchange
SYNOPSIS
pax [−dv] [−c|−n] [−H|−L] [−o options] [−f archive] [−s replstr]...
[pattern...]
pax −r[−c|−n] [−dikuv] [−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] [−o options]... [−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 and each entry in this
list shall be processed as if it had been a file operand on the command line. A file of type
directory shall include all of the files in the file hierarchy rooted at the file.
copy In copy mode (when both −r and −w are specified), pax shall copy the file operands to the
destination directory.
If no file operands are specified, a list of files to copy, one per line, shall be read from
the standard input. A file of type directory shall include all of the files in the file
hierarchy rooted at the file.
The effect of the copy shall be as if the copied files were written to a pax format 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 subdirectory of one of the files to be
copied, the results are unspecified. If the destination directory is a file of a type not
defined by the System Interfaces volume of POSIX.1‐2008, 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
POSIX.1‐2008, called with the following 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 diagnostic message to standard error for each one that did not match and exit with a non-
zero exit status.
The archive formats described in the EXTENDED DESCRIPTION section shall be automatically detected on
input. The default output archive format shall be implementation-defined.
A single archive can span multiple files. The pax utility shall 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 POSIX.1‐2008, Section 12.2, Utility
Syntax Guidelines, 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 appending. Additional file formats unspecified by this volume of POSIX.1‐2008
may impose restrictions on appending.
−b blocksize
Block the output at a positive decimal integer number of bytes per write to the archive file.
Devices and archive formats may impose restrictions on blocking. Blocking shall be
automatically determined on input. Conforming applications shall not specify a blocksize value
larger than 32256. Default blocking when 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 hierarchy. Otherwise, if a symbolic link referencing a file
of any other file type which pax can normally archive is specified on the command line, then
pax shall archive the file referenced by the link, using the name of the link. The default
behavior, when neither −H or −L are specified, 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 contents of the line. The pax utility shall immediately exit with a non-zero
exit status if end-of-file is encountered when reading a response or if /dev/tty cannot be
opened for reading and 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 whenever possible. If specified in conjunction with −H or −L, when a symbolic
link is encountered, the hard link created in the destination file hierarchy shall be to the
file referenced by the symbolic link. If specified when neither −H nor −L is specified, when a
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 traversal of a
file hierarchy, pax shall archive the file referenced by the link, using the name of the link.
The default behavior, when neither −H or −L are specified, 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 inapplicable to the file format being processed produces undefined results.
Keywords in the options argument shall be a string that would be a valid portable filename as
described in the Base Definitions volume of POSIX.1‐2008, Section 3.278, Portable Filename
Character Set.
Note: Keywords are not expected to be filenames, merely to follow the same character
composition rules as portable filenames.
Keywords can be preceded with white space. The value field shall 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 keywords and values appearing later in command line sequence shall take
precedence and the earlier shall be silently ignored. The following keyword values of options
shall be supported for the file formats as indicated:
delete=pattern
(Applicable only to the −x pax format.) When used in write or copy mode, pax shall omit
from extended header records that it produces any keywords matching the string pattern.
When used in read or list mode, pax shall ignore any keywords matching the string pattern
in the extended header records. In both cases, matching shall be performed using the
pattern matching notation described in Section 2.13.1, Patterns Matching a Single
Character and Section 2.13.2, Patterns Matching Multiple Characters. For example:
−o delete=security.*
would suppress security-related information. See pax Extended Header for extended header
record keyword usage.
When multiple −odelete=pattern options are specified, the patterns shall be additive; all
keywords matching the specified string patterns shall be omitted from extended header
records that pax produces.
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, │
│ │ equivalent to the result of the │
│ │ dirname utility 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 substitutions have been made:
┌──────────┬────────────────────────────────────────┐
│ string │ │
│Includes: │ Replaced by: │
├──────────┼────────────────────────────────────────┤
│%n │ An integer that represents the │
│ │ sequence number of the global extended │
│ │ header record in the archive, starting │
│ │ at 1. │
│%p │ The process ID of the pax process. │
│%% │ A '%' character. │
└──────────┴────────────────────────────────────────┘
Any other '%' characters in string produce undefined results.
If no −o globexthdr.name=string is specified, pax shall use the following default value:
$TMPDIR/GlobalHead.%p.%n
where $TMPDIR represents the value of the TMPDIR environment variable. If TMPDIR is not
set, pax shall use /tmp.
invalid=action
(Applicable only to the −x pax format.) This keyword allows user control over the action
pax takes upon encountering values in an extended header record that, in read or copy
mode, are invalid in the destination hierarchy or, in list mode, cannot be written in the
codeset and current locale of the implementation. The following are invalid values that
shall be recognized by pax:
-- In read or copy mode, a filename or link name that contains character encodings
invalid in the destination 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:
binary In write mode, pax shall generate a hdrcharset=BINARY extended header record
for each file with a filename, link name, group name, owner name, or any other
field in an extended header record that cannot be translated to the UTF‐8
codeset, allowing the archive to contain the files with unencoded extended
header record values. In read or copy mode, pax shall use the values specified
in the header without translation, regardless of whether this may overwrite an
existing file with a valid name. In list mode, pax shall behave identically to
the bypass action.
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. If a hdrcharset extended
header record is in effect for this file, the character set specified by that
record shall be used instead of UTF‐8. If a hdrcharset=BINARY extended header
record is in effect for this file, no translation shall be performed.
write In read or copy mode, pax shall write the file, translating 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 −oinvalid=bypass were
specified. Any overwriting of existing files that may be allowed by the −oinvalid=
actions shall be subject to permission (−p) and modification 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
considered part of the format string. When multiple −olistopt=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 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 <equals-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 specifying file characteristics to be retained or discarded on extraction. The
string shall consist of the specification characters a, e, m, o, and p. Other implementation-
defined characters can be included. Multiple characteristics can be concatenated within the
same string and multiple −p options can be specified. The meaning of the specification
characters are as follows:
a Do not preserve file access times.
e Preserve the user ID, group ID, file mode bits (see the Base Definitions volume of
POSIX.1‐2008, Section 3.169, 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 Section 1.1.1.4, 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) back-references, or subexpression matching. The old string shall also be
permitted to contain <newline> characters.
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 written 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 member with the same name. In read mode, an archive member with the same name
as a file in the file system shall be extracted if the archive 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 specified, 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 pathnames 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 character 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 character 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 character special archive files.
Any attempt to append to an archive file in a format different from the existing archive format
shall cause pax to exit immediately with a non-zero exit status.
−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
POSIX.1‐2008, stat()).
Specifying more than one of the mutually-exclusive options −H and −L shall not be considered an error and
the last option specified shall determine the behavior of the utility.
The options that operate on the names of files or archive members (−c, −i, −n, −s, −u, and −v) shall
interact as follows. In read mode, the archive members shall be selected based on the user-specified
pattern operands as modified by the −c, −n, and −u options. Then, any −s and −i options shall modify, in
that order, the names of the selected files. The −v option shall write names resulting from these
modifications.
In write mode, the files shall be selected based on the user-specified pathnames as modified by the −n
and −u options. Then, any −s and −i options shall modify, in that order, the names of these selected
files. The −v option shall write names resulting from these modifications.
If both the −u and −n options are specified, pax shall not consider a file selected unless it is newer
than the file to which it is compared.
List Mode Format Specifications
In list mode with the −o listopt=format option, the format argument shall be applied for each selected
file. The pax utility shall append a <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 POSIX.1‐2008,
Chapter 5, File Format Notation, with the exceptions 1. through 6. defined in the EXTENDED DESCRIPTION
section of printf, plus the following exceptions:
7. 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 Table 4-14, ustar Header Block and Table 4-16, Octet-Oriented
cpio Archive Entry. The implementation may support the cpio keywords without the leading c_ in
addition to the form required by Table 4-16, Octet-Oriented cpio Archive Entry.
-- 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 (or
alternative encoding specified by any hdrcharset extended header record) to the character set
appropriate for the local file system, user database, and so on, as applicable.
8. 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
9. 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.
10. An additional conversion specifier character, D, shall be used to specify the device for block or
special files, if applicable, in an implementation-defined format. If not applicable, and (keyword)
is specified, then this conversion shall be equivalent to %(keyword)u. If not applicable, and
(keyword) is omitted, then this conversion shall be equivalent to <space>.
11. 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).
12. An additional conversion specifier character, L, shall be used to specify a symbolic link
expansion. If the current 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 notation of the pattern matching notation in Section 2.13, Pattern Matching
Notation, including the filename expansion rules in Section 2.13.3, Patterns Used for Filename
Expansion. The default, if no pattern is specified, is to select all members in the archive.
STDIN
In write mode, the standard input shall be used only if no file operands are specified. It shall be a
file containing a list of pathnames, each terminated by a <newline> character.
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 POSIX.1‐2008, Section 8.2, Internationalization Variables the
precedence of internationalization 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 used to process affirmative responses, and the locale used to affect the
format and contents of diagnostic messages and prompts 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 specifications in the EXTENDED DESCRIPTION section, or some other implementation-defined
format (see −x format).
In list mode, when the −olistopt=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 −olistopt=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 pathname (plus any associated information
and a <newline> terminator) 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 using the −xpax archive format, if a filename, link name, group name, owner name, or any other field
in an extended header record cannot be translated between the codeset in use for that extended header
record and the character set of the current locale, pax shall write a diagnostic message to standard
error, shall process the file as described for the −o invalid= option, and then shall continue processing
with the next file.
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 (−oinvalid=) tests allow it.
In write mode, the output file named by the −f option-argument shall be a file formatted according to one
of the specifications in the EXTENDED DESCRIPTION section, or some other implementation-defined format.
EXTENDED DESCRIPTION
pax Interchange Format
A pax archive tape or file produced in the −xpax format shall contain a series of blocks. The physical
layout of the archive shall be identical to the ustar format described in ustar Interchange Format. Each
file archived shall be represented by the following sequence:
* An optional header block with extended header records. This header block is of the form described in
pax Header Block, with a typeflag value of x or g. The extended header records, described in pax
Extended Header, shall be included as the data for this header block.
* A header block that describes the file. Any fields in the preceding optional extended header shall
override the associated fields in this header block for this file.
* Zero or more blocks that contain the contents of the file.
At the end of the archive file there shall be two 512-byte blocks filled with binary zeros, interpreted
as an end-of-archive indicator.
A schematic of an example archive with global extended header records and two actual files is shown in
Figure 4-1, 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 4-1: 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 representing file attributes not described in the ustar header, and
fields whose format or length do not fit the requirements of the ustar header. The values in an extended
header add attributes to the following file (or files; see the 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
encoding. The <length> field, <blank>, <equals-sign>, and <newline> shown shall be limited to the
portable character set, as encoded in UTF‐8. The <keyword> 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>. If there is a hdrcharset extended header in effect for a file, the value field for
any gname, linkpath, path, and uname extended header records shall be encoded using the character set
specified by the hdrcharset extended header record; otherwise, the value field shall be encoded using
UTF‐8. The value field for all other keywords specified by POSIX.1‐2008 shall be encoded using UTF‐8.
The <keyword> field shall be one of the entries from the following list or a keyword provided as an
implementation extension. 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 structure for a file, as described by the stat() function. The access time shall be
restored if the process has appropriate privileges 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 POSIX.1‐2008, it shall not translate the file data into any other encoding. The
BINARY entry indicates unencoded binary data.
When used in write or copy mode, it is implementation-defined whether pax includes a charset
extended header record for a file.
comment A series of characters used as a comment. All characters in the <value> field shall be ignored
by pax.
gid The group ID of the group that owns the file, expressed as a decimal number using digits from
the ISO/IEC 646:1991 standard. This record shall override the gid field in the following header
block(s). When used in write or copy mode, pax shall include a gid extended header record for
each file whose group ID is greater than 2097151 (octal 7777777).
gname The group of the file(s), formatted as a group name in the group database. This record shall
override the gid and gname fields in the following header block(s), and any gid extended header
record. When used in read, copy, or list mode, pax shall translate the name from the encoding
in the header record to the character set appropriate for the group database on the receiving
system. If any of the characters cannot be translated, and if neither the −oinvalid=UTF‐8
option nor the −oinvalid=binary option is 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.
hdrcharset
The name of the character set used to encode the value field of the gname, linkpath, path, and
uname pax extended header records. The entries in the following table are defined to refer to
known standards; additional names may be agreed between the originator and the recipient.
┌────────────────────────┬───────────────────────────────┐
│ <value> │ Formal Standard │
├────────────────────────┼───────────────────────────────┤
│ISO-IR 10646 2000 UTF-8 │ ISO/IEC 10646, UTF-8 encoding │
│BINARY │ None. │
└────────────────────────┴───────────────────────────────┘
If no hdrcharset extended header record is specified, the default character set used to encode
all values in extended header records shall be the ISO/IEC 10646‐1:2000 standard UTF‐8
encoding.
The BINARY entry indicates that all values recorded in extended headers for affected files are
unencoded binary data from the underlying system.
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 encoding in the header to the
character set appropriate for the local file system. When used in write or copy mode, pax shall
include a linkpath extended header record for each link whose pathname cannot be represented
entirely with the members of the portable character set other than NUL.
mtime The file modification time of the following file(s), equivalent to the value of the st_mtime
member of the stat structure for a file, as described in the stat() function. This record shall
override the mtime field in the following header block(s). The modification time shall be
restored if the process has appropriate privileges 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 encoding in the header 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
encoding in the header record to the character set appropriate for the user database on the
receiving system. If any of the characters cannot be translated, and if neither the
−oinvalid=UTF‐8 option nor the −oinvalid=binary option is 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 Table 4-14, 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 −odelete=keyword-prefix is used, the affected attributes shall be determined from step 7., if
applicable, or ignored otherwise.
2. If −okeyword:= is used, the affected attributes shall be ignored.
3. If −okeyword:=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 −okeyword=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 granularities and the time is not an integer. All of
these time records shall be formatted as a decimal representation of the time in seconds since the Epoch.
If a <period> ('.') decimal point character is present, the digits to the right of the point shall
represent the units of a subsecond timing granularity, where the first digit is tenths of a second and
each subsequent digit is a tenth of the previous digit. In read or copy mode, the pax utility shall
truncate the time of a file to the greatest value that is not greater than the input header file time. In
write or copy mode, the pax utility shall output a time exactly if it can be represented exactly as a
decimal number, and otherwise shall generate only enough digits so that the same time shall be recovered
if the file is extracted on a system whose underlying implementation supports the same time granularity.
ustar Interchange Format
A ustar archive tape or file shall contain a series of logical records. Each logical record shall be a
fixed-size logical record of 512 octets (see below). Although this format may be thought of as being
stored on 9-track industry-standard 12.7 mm (0.5 in) magnetic tape, other types of transportable media
are not excluded. Each file archived shall be represented by a header logical record that describes the
file, followed by zero or more logical records that give the contents of the file. At the end of the
archive file there shall be two 512-octet 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 −bblocksize 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 4-14: ustar Header Block
┌───────────┬──────────────┬────────────────────┐
│Field Name │ Octet Offset │ Length (in Octets) │
├───────────┼──────────────┼────────────────────┤
│name │ 0 │ 100 │
│mode │ 100 │ 8 │
│uid │ 108 │ 8 │
│gid │ 116 │ 8 │
│size │ 124 │ 12 │
│mtime │ 136 │ 12 │
│chksum │ 148 │ 8 │
│typeflag │ 156 │ 1 │
│linkname │ 157 │ 100 │
│magic │ 257 │ 6 │
│version │ 263 │ 2 │
│uname │ 265 │ 32 │
│gname │ 297 │ 32 │
│devmajor │ 329 │ 8 │
│devminor │ 337 │ 8 │
│prefix │ 345 │ 155 │
└───────────┴──────────────┴────────────────────┘
All characters in the header logical record shall be represented in the coded character set of the
ISO/IEC 646:1991 standard. For maximum portability between implementations, names should be selected from
characters represented by the portable filename character set as octets with the most significant bit
zero. If an implementation supports the use of characters outside of <slash> and the portable filename
character set in names for files, users, and groups, one or more implementation-defined encodings of
these characters shall be provided for interchange purposes.
However, the pax utility shall never create filenames on the local system that cannot be accessed via the
procedures described in POSIX.1‐2008. If a filename is found on the medium that would create an invalid
filename, it is implementation-defined whether the data from the file is stored on the file hierarchy and
under what name it is stored. The pax utility may choose to ignore these files as long as it produces an
error indicating that the file is being ignored.
Each field within the header logical record is contiguous; that is, there is no padding used. Each
character on the archive medium shall be stored contiguously.
The fields magic, uname, and gname are character strings each 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> character, and name; otherwise, name is used alone. In either case, name is
terminated at the first NUL character. If prefix begins with a NUL character, it shall be ignored. In
this manner, pathnames of at most 256 characters can be supported. If a pathname does not fit in the
space provided, pax shall notify the user of the error, and shall not store any part of the file—header
or data—on the medium.
The linkname field, described below, shall not use the prefix to 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 │ POSIX.1‐2008 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 privileges are required to set one of these mode bits, and the user restoring the files
from the archive does not have appropriate privileges, the mode bits for which the user does not have
appropriate privileges shall be ignored. Some of the mode bits in the archive format are not mentioned
elsewhere in this volume of POSIX.1‐2008. 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 POSIX.1‐2008, and no data logical records shall be stored
on the medium. Additionally, for type 6, the size field shall be ignored when reading. If the typeflag
field is set to any other value, the 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 standard 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 <space> characters.
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 privileges 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 produce an error indicating that
the conversion took place. All of the typeflag fields shall be coded in the ISO/IEC 646:1991 standard
IRV:
0 Represents a regular file. For backwards-compatibility, a typeflag value of binary zero ('\0')
should be recognized 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 having the same device and file serial numbers, and pathnames that refer to
different directory entries. All such files shall be archived as linked files. 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 POSIX.1‐2008. 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. Implementations without such extensions should treat this file as a regular file (type
0).
A‐Z The letters 'A' to 'Z', inclusive, are reserved for custom implementations. All other values are
reserved for future versions of this standard.
It is unspecified whether files with pathnames that refer to the same directory entry are archived as
linked files or as separate files. If they are archived as linked files, this means that attempting to
extract both pathnames from the resulting archive will always cause an error (unless the −u option is
used) because the link cannot be created.
It is unspecified whether files with the same device and file serial numbers being appended to an archive
are treated as linked files to members that were in the archive before the append.
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 contain 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 4-16: 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 4-17: Values for cpio c_mode Field
│ ─────┬─────────┬────────────────────────┬──────────────────
--
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
Caution is advised when using the −a option to append to a cpio format archive. If any of the files being
appended happen to be given the same c_dev and c_ino values as a file in the existing part of the
archive, then they may be treated as links to that file on extraction. Thus, it is risky to use −a with
cpio format except when it is done on the same system that the original archive was created on, and with
the same pax utility, and in the knowledge that there has been little or no file system activity since
the original archive was created that could lead to any of the files appended being given the same c_dev
and c_ino values as an unrelated file in the existing part of the archive. Also, when (intentionally)
appending additional links to a file in the existing part of the archive, the c_nlink values in the
modified archive can be smaller than the number of links to the file in the archive, which may mean that
the links are not preserved on extraction.
The −p (privileges) option was invented to reconcile differences between historical tar and cpio
implementations. In particular, the two utilities use −m in diametrically opposed ways. The −p option
also provides a consistent means of extending the ways in which future file attributes can be addressed,
such as for enhanced security systems or high-performance files. Although it may seem complex, there are
really two modes that are most commonly used:
−p e ``Preserve everything''. This would be used by the historical superuser, someone with all
appropriate privileges, 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> characters.
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 POSIX.1‐2008. 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.
When archives containing binary header information are listed , the filenames printed may cause strange
behavior on some terminals.
When all of the following are true:
1. A file of type directory is being placed into an archive.
2. The ustar archive format is being used.
3. The pathname of the directory is less than or equal to 155 bytes long (it will fit in the prefix
field in the ustar header block).
4. The last component of the pathname of the directory is longer than 100 bytes long (it will not fit in
the name field in the ustar header block).
some implementations of the pax utility will place the entire directory pathname in the prefix field, set
the name field to an empty string, and place the directory in the archive. Other implementations of the
pax utility will give an error under these conditions because the name field is not large enough to hold
the last component of the directory name. This standard allows either behavior. However, when extracting
a directory from a ustar format archive, this standard requires that all implementations be able to
extract a directory even if the name field contains an empty string as long as the prefix field does not
also contain an empty string.
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 newdir
pax −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 2003 1492 /usr/foo/bar
Using the options:
−o listopt='%L\t%(size)D\n%.7' \
−o listopt='(name)s\n%(atime)T\n%T'
overrides the default output description in Standard Output and instead writes:
/usr/foo/bar −> /tmp 1492
/usr/fo
Jan 12 15:53 1991
Jan 31 15:53 2003
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 hierarchy be explicitly specified. For tar, a directory matched every file
in the file hierarchy it rooted.
The pax utility offers both interfaces; by default, directories map into the file hierarchy they root.
The −d option causes pax to skip any file not explicitly referenced, as cpio historically did. The tar
−style behavior was chosen as the default because it was believed that this was the more common usage and
because tar is the more commonly available interface, as it was historically provided on both System V
and BSD implementations.
The data interchange format specification in this volume of POSIX.1‐2008 requires that processes with
``appropriate 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 superuser. (It should be
noted that restoration of ownerships and permissions for the superuser, by default, is historical
practice in cpio, but not in In order to avoid these two problems, the pax specification has an
additional ``privilege'' mechanism, the −p option. Only a pax invocation with the privileges needed, and
which has the −p option set using the e specification character, has appropriate privileges to restore
full ownership and permission information.
Note also that this volume of POSIX.1‐2008 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. Earlier
versions of this standard did not specify any way to create a hard link to a symbolic link, but many
implementations provided this capability as an extension. If there are hard links to symbolic links when
an archive is created, the implementation is required to archive the hard link in the archive (unless −H
or −L is specified). When in read mode and in copy mode, implementations supporting hard links to
symbolic links should use them when appropriate.
The archive formats inherited from the POSIX.1‐1990 standard have 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 −xpax format overcomes these restrictions.
The default blocksize value of 5120 bytes for cpio was selected because it is one of the standard block-
size values for cpio, set when the −B option is specified. (The other default block-size value for cpio
is 512 bytes, and this was considered to be too small.) The default block value of 10240 bytes for tar
was selected because that is the standard block-size value for BSD tar. The maximum block size of 32256
bytes (215−512 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 POSIX.1‐2008 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 archive 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 portability.
The intended use of the −n option was to permit extraction of one or more files from the archive without
processing the entire archive. This was viewed by the standard developers as offering significant
performance advantages over historical implementations. The −n option in early proposals had three
effects; the first was to cause special characters in patterns to not be treated specially. The second
was to cause only the first file that matched a pattern to be extracted. The third was to cause pax to
write a diagnostic message to standard error when no file was found matching a specified pattern. Only
the second behavior is retained by this volume of POSIX.1‐2008, 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
characters 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 POSIX.1‐2008 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 appropriate 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
identical.
A single archive may span more than one file. It is suggested that implementations provide informative
messages to the user on standard error whenever the archive file is changed.
The −d option (do not create intermediate directories not listed in the archive) found in early proposals
was originally provided as a 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 reason for only a subset to be supported, the −s option is now compatible with the current
ed specification. Since the delimiter can be any non-null character, the following usage with single
<space> characters 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 practice in cpio.
In early drafts, a −echarmap 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 version, the use of UTF‐8 should be an adequate substitute.
The ISO POSIX‐2:1993 standard and ISO POSIX‐1 standard requirements for pax, however, made it very
difficult to create a single archive containing files created using extended characters provided by
different locales. This version adds the hdrcharset keyword to make it possible to archive files in
these cases without dropping files due to translation errors.
Translating filenames and other attributes from a locale's encoding to UTF‐8 and then back again can lose
information, as the resulting filename might not be byte-for-byte equivalent to the original. To avoid
this problem, users can specify the −o hdrcharset=binary option, which will cause the resulting archive
to use binary format for all names and attributes. Such archives are not portable among hosts that use
different native encodings (e.g., EBCDIC versus ASCII-based encodings), but they will allow interchange
among the vast majority of POSIX file systems in practical use. Also, the −o hdrcharset=binary option
will cause pax in copy mode to behave more like other standard utilities such as cp.
If the values specified by the −o exthdr.name=value, −o globexthdr.name=value, or by $TMPDIR (if −o
globexthdr.name is not specified) require a character encoding other than that described in the
ISO/IEC 646:1991 standard, a path extended header record will have to be created for the file. If a
hdrcharset extended header record is active for such headers, it will determine the codeset used for the
value field in these extended path header records. These path extended header records always need to be
created when writing an archive even if hdrcharset=binary has been specified and would contain the same
(binary) data that appears in the ustar header record prefix and name fields. (In other words, an
extended header path record is always required to be generated if the prefix or name fields contain non-
ASCII characters even when hdrcharset=binary is also in effect for that file.)
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 −oinvalid=option—specifically the UTF‐8 and binary actions. (Note that an
existing file is still subject to overwriting in this case. The −k option closes that loophole.)
Some of the file characteristics referenced in this volume of POSIX.1‐2008 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 permissions 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 conversion argument as part of the conversion specification.
The T conversion specifier allows time fields to be displayed in any of the date formats. Unlike the ls
utility, pax does not adjust the format when the date is less than six months in the past. This makes
parsing the output more predictable.
The D conversion specifier handles the ability to display the major/minor or file size, as with ls, by
using %−8(size)D.
The L conversion specifier handles the ls display for symbolic links.
Conversion specifiers were added to generate existing known types used for ls.
pax Interchange Format
The new POSIX data interchange format was developed primarily to 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 filename.
* 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 systems.
The new format was designed with one additional goal in mind: reasonable behavior when an older tar or
pax utility happened to read an archive. Since the POSIX.1‐1990 standard mandated that a ``format-reading
utility'' had to treat unrecognized typeflag values as regular files, this allowed the format to include
all the extended information in a pseudo-regular file that preceded each real file. An option is given
that allows the archive creator to set up reasonable names for these files on the older systems. Also,
the normative text suggests that 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 version because the POSIX.1‐1990 standard (and, by reference, the previous version of pax),
mandated the behavior of the format-reading utility when it encountered an unknown typeflag, but was
silent about the other two fields.
Early proposals for the first version of this standard contained a proposed archive format that was based
on compatibility 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. Furthermore, the standard developers felt that it would not be acceptable to
the community of POSIX developers, so it was later changed to be a format more closely related to
historical practice on POSIX systems.
The prefix and name split of pathnames in ustar was replaced by the single path extended header record
for simplicity.
The concept of a global extended header (typeflagg) 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 standard developers considered interfaces that would invoke a
shell script for each file going into or out of the archive, but the system overhead in this approach was
considered to be too high.
One such approach would be to have filter= records that give a pathname for an executable. When the
program is invoked, the file and archive would be open for standard input/output and all the header
fields would be available as environment variables or command-line arguments. The standard developers did
discuss such schemes, but they were omitted from POSIX.1‐2008 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 −ocharset=string command line
option. This assumes that all of the files in the archive use the same encoding. The ``implementation-
defined'' text is included to allow for a system that can identify the encodings used for each of its
files.
The table of standards that accompanies the charset record description is acknowledged to be very
limited. Only a limited number of character set standards is reasonable for maximal interchange. Any
character set is, of course, possible by prior agreement. It was suggested that EBCDIC be listed, but it
was omitted because it is not defined by a formal standard. Formal standards, and then only those with
reasonably large followings, can be included here, simply as a matter of 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 argument parsing for getopts.
Further information on character encodings is in pax Archive Character Set Encoding/Decoding.
The standard developers have reserved keyword name space for vendor extensions. It is suggested that the
format to be used is:
VENDOR.keyword
where VENDOR is the name of the vendor or organization in all uppercase letters. It is further suggested
that the keyword following the <period> be named differently than any of the standard keywords so that it
could be used for future standardization, if appropriate, by omitting the VENDOR prefix.
The <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 probably unnecessary for backup.
The requirements on restoring from an archive are slightly different from the historical wording,
allowing for non-monolithic privilege to bring forward as much as possible. In particular, attributes
such as ``high performance file'' might be broadly but not universally granted while set-user-ID or
chown() might be much more restricted. There is no implication in POSIX.1‐2008 that the security
information be honored after it is restored to the file hierarchy, in spite of what might be improperly
inferred by the silence on that topic. That is a topic for another standard.
Links are recorded in the fashion described here because a link can be to any file type. It is desirable
in general to be able to restore part of an archive selectively and restore all of those files
completely. If the data is not associated with each link, it is not possible to do this. However, the
data associated with a file can be large, and when selective restoration is not needed, this can be a
significant burden. The archive is structured so that files that have no associated data can always be
restored by the name of any link name of any link, and the user may choose whether data is recorded with
each instance of a file that contains data. The format permits mixing of both types of links in a single
archive; this can be done for special needs, and pax is expected to interpret such archives on input
properly, despite the fact that there is no pax option that would force this mixed case on output. (When
−o linkdata is used, the output must contain the 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 digits allowed by the ustar format. Portable file timestamps cannot be negative. If pax
encounters a file with a negative timestamp in copy or write mode, it can reject the file, substitute a
non-negative timestamp, or generate a non-portable timestamp with a leading '−'. Even though some
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 modification time) is described with appropriate privileges so that it can be ignored
when writing to the file system. POSIX does not provide a portable means to change file creation time.
Nothing is intended to prevent a non-portable implementation of pax from restoring the value.
The gid, size, and uid extended header records were included to allow expansion beyond the sizes
specified in the regular tar header. New file system architectures are emerging that will exhaust the
12-digit size field. There are probably not many systems requiring more than 8 digits for user and group
IDs, but the extended header values were included for completeness, allowing overrides for all of the
decimal values in the tar header.
The standard developers intended to describe the effective results of pax with regard to file ownerships
and permissions; implementations are not restricted in timing or sequencing the restoration of such,
provided the results are as specified.
Much of the text describing the extended headers refers to use in ``write or copy modes''. The copy mode
references are due to the normative text: ``The effect of the copy shall be as if the copied files were
written to an archive file and then subsequently extracted ...''. There is certainly no way to test
whether pax is actually generating the extended headers in copy mode, but the effects must be as if it
had.
pax Archive Character Set Encoding/Decoding
There is a need to exchange archives of files between systems of 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. Translation of the contents of files is not within the scope of the pax utility.
There will also be the need to represent characters that are not 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 codeset 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 translations to the destination codeset.
* The archive creator translates the extended header records in the source codeset into a canonical
form. The receiver must then perform the appropriate translations to the destination codeset.
The approach that incorporates the name of the source codeset poses the problem of codeset name
registration, and makes the archive useless to pax archive decoders that do not recognize that codeset.
Because parts of an archive may be corrupted, the standard developers felt that including the character
map of the source codeset was too fragile. The loss of this one key component could result in making the
entire archive useless. (The difference between this and the global extended header decision was that the
latter has a workaround—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 sufficiently 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 possibly confusing to
POSIX file systems do not appear, except to represent themselves. It was realized that certain national
codesets take up more space after the encoding, due to their placement within the UCS range; it was felt
that the 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 version 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 filename, 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. POSIX.1‐2008 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 privileges 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 POSIX.1‐2008, yet is extensible to transfer data types
specific to extensions beyond POSIX.1‐2008 (for example, contiguous files). Because it describes existing
practice, there is no question of maintaining upwards-compatibility.
cpio Header
There has been some concern that the size of the 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 number 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
Chapter 2, Shell Command Language, cp, ed, getopts, ls, printf
The Base Definitions volume of POSIX.1‐2008, Section 3.169, File Mode Bits, Chapter 5, File Format
Notation, Chapter 8, Environment Variables, Section 12.2, Utility Syntax Guidelines, <cpio.h>
The System Interfaces volume of POSIX.1‐2008, chown(), creat(), fstatat(), mkdir(), mkfifo(), utime(),
write()
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition,
Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc
and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .