pax (1p) - Linux Manuals
pax: portable archive interchange
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.NAME
pax - portable archive interchange
SYNOPSIS
pax [-cdnv][-H|-L][-f archive][-s
replstr]...[pattern...]
pax -r[-cdiknuv][-H|-L][-f
archive][-o
options]...[-p string]...
pax -w[-dituvX][-H|-L][-b
blocksize][[-a][-f archive][-o
options]...
pax -r -w[-diklntuvX][-H|-L][-p
string]...[-s replstr]...
DESCRIPTION
The pax utility shall read, write, and write lists of the members of archive files and copy directory hierarchies. A variety of archive formats shall be supported; see the -x format option.
The action to be taken depends on the presence of the -r and -w options. The four combinations of -r and -w are referred to as the four modes of operation: list, read, write, and copy modes, corresponding respectively to the four forms shown in the SYNOPSIS section.
- list
- In list mode (when neither -r nor -w are specified), pax shall write the names of the members of the archive file read from the standard input, with pathnames matching the specified patterns, to standard output. If a named file is of type directory, the file hierarchy rooted at that file shall be listed as well.
- read
- In read mode (when -r is specified, but -w is not), pax shall extract the members of the archive file read from the standard input, with pathnames matching the specified patterns. If an extracted file is of type directory, the file hierarchy rooted at that file shall be extracted as well. The extracted files shall be created performing pathname resolution with the directory in which pax was invoked as the current working directory.
If an attempt is made to extract a directory when the directory already exists, this shall not be considered an error. If an attempt is made to extract a FIFO when the FIFO already exists, this shall not be considered an error.
The ownership, access, and modification times, and file mode of the restored files are discussed under the -p option.
- write
- In write mode (when -w is specified, but -r is not), pax shall write the contents of the file operands to the standard output in an archive format. If no file operands are specified, a list of files to copy, one per line, shall be read from the standard input. A file of type directory shall include all of the files in the file hierarchy rooted at the file.
- copy
- In copy mode (when both -r and -w are specified), pax shall copy the file operands to the destination directory.
If no file operands are specified, a list of files to copy, one per line, shall be read from the standard input. A file of type directory shall include all of the files in the file hierarchy rooted at the file.
The effect of the copy shall be as if the copied files were written to an archive file and then subsequently extracted, except that there may be hard links between the original and the copied files. If the 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 IEEE Std 1003.1-2001, the results are implementation-defined; otherwise, it shall be an error for the file named by the directory operand not to exist, not be writable by the user, or not be a file of type directory.
In read or copy modes, if intermediate directories are necessary to extract an archive member, pax shall perform actions equivalent to the mkdir() function defined in the System Interfaces volume of IEEE Std 1003.1-2001, called with the 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 IEEE Std 1003.1-2001, Section 12.2, Utility Syntax Guidelines, except that the order of presentation of the -o, -p, and -s options is 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 IEEE Std 1003.1-2001 may impose restrictions on appending.
- -b
blocksize - Block the output at a positive decimal integer number of bytes per write to the archive file. Devices and archive formats may impose restrictions on blocking. Blocking shall be automatically determined on input. Conforming applications shall not specify a blocksize value larger than 32256. Default blocking when 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 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 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 IEEE Std 1003.1-2001, Section 3.276, Portable Filename Character Set.
- Note:
-
- Keywords are not expected to be filenames, merely to follow the same character composition rules as portable filenames.
Keywords can be preceded with white space. The value field shall 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 Patterns Matching a Single Character and Patterns Matching Multiple Characters . For example:
-
-o delete=security.*
would suppress security-related information. See pax Extended Header for extended header record keyword usage.
-
-
- exthdr.name=string
-
-
(Applicable only to the -x pax format.) This keyword allows user control over the name that is written into the ustar header blocks for the extended header produced under the circumstances described in pax Header Block . The name shall be the contents of string, after the following character substitutions have been made:
string Includes: Replaced By: %d The directory name of the file, 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:
- bypass
-
- In read or copy mode, pax shall bypass the file, causing no change to the destination hierarchy. In list mode, pax shall write all requested valid values for the file, but its method for writing invalid values is unspecified.
- rename
-
- In read or copy mode, pax shall act as if the -i option were in effect for each file with invalid filename or link name values, allowing the user to provide a replacement name interactively. In list mode, pax shall behave identically to the bypass action.
- UTF-8
-
- When used in read, copy, or list mode and a filename, link name, owner name, or any other field in an extended header record cannot be translated from the pax UTF-8 codeset format to the codeset and current locale of the implementation, pax shall use the actual UTF-8 encoding for the name.
- write
-
- In read or copy mode, pax shall write the file, translating or truncating the name, regardless of whether this may overwrite an existing file with a valid name. In list mode, pax shall behave identically to the bypass action.
If no -o invalid= option is specified, pax shall act as if -o invalid= bypass were specified. Any overwriting of existing files that may be allowed by the -o invalid= actions shall be subject to permission ( -p) and 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 -o listopt= format options are specified, the format strings shall be considered a single, concatenated string, evaluated in command line order.
-
- times
-
-
(Applicable only to the -x pax format.) When used in write or copy mode, pax shall include atime, ctime, and mtime extended header records for each file. See pax Extended Header File Times .
-
In addition to these keywords, if the -x pax format is specified, any of the keywords and values defined in pax Extended Header, including implementation extensions, can be used in -o option-arguments, in either of two modes:
- keyword=value
-
-
When used in write or copy mode, these keyword/value pairs shall be included at the beginning of the archive as typeflag g global extended header records. When used in read or list mode, these keyword/value pairs shall act as if they had been at the beginning of the archive as typeflag g global extended header records.
-
- keyword:=value
-
-
When used in write or copy mode, these keyword/value pairs shall be included as records at the beginning of a typeflag x extended header for each file. (This shall be equivalent to the equal-sign form except that it creates no typeflag g global extended header records.) When used in read or list mode, these keyword/value pairs shall act as if they were included as records at the end of each extended header; thus, they shall override any global or file-specific extended header record keywords of the same names. For example, in the command:
-
pax -r -o " gname:=mygroup, " <archive
the group name will be forced to a new value for all files read from the archive.
-
-
The precedence of -o keywords over various fields in the archive is described in pax Extended Header Keyword Precedence .
- -p
string - Specify one or more file characteristic options (privileges). The string option-argument shall be a string specifying file characteristics to be retained or discarded on extraction. The string shall consist of the specification characters a, e, m, o, and p . Other implementation-defined characters can be included. Multiple characteristics can be concatenated within the same string and multiple -p options can be specified. The meaning of the specification characters are as follows:
- a
-
- Do not preserve file access times.
- e
-
- Preserve the user ID, group ID, file mode bits (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 3.168, File Mode Bits), access time, modification time, and any other implementation-defined file characteristics.
- m
-
- Do not preserve file modification times.
- o
-
- Preserve the user ID and group ID.
- p
-
- Preserve the file mode bits. Other implementation-defined file mode attributes may be preserved.
In the preceding list, "preserve" indicates that an attribute stored in the archive shall be given to the extracted file, subject to the permissions of the invoking process. The access and modification times of the file shall be preserved unless otherwise specified with the -p option or not stored in the archive. All attributes that are not preserved shall be determined as part of the normal file creation action (see File Read, Write, and Creation ).
If neither the e nor the o specification character is specified, or the user ID and group ID are not preserved for any reason, pax shall not set the S_ISUID and S_ISGID bits of the file mode.
If the preservation of any of these items fails for any reason, pax shall write a diagnostic message to standard error. Failure to preserve these items shall affect the final exit status, but shall not cause the extracted file to be deleted.
If file characteristic letters in any of the string option-arguments are duplicated or conflict with each other, the ones given last shall take precedence. For example, if -p eme is specified, file modification times are preserved.
- -s
replstr -
Modify file or archive member names named by pattern or file
operands according to the substitution expression
replstr, using the syntax of the ed utility. The concepts
of "address" and
"line" are meaningless in the context of the pax utility, and
shall not be supplied. The format shall be:
-
-s /old/new/[gp]
-
where as in ed, old is a basic regular expression and new can contain an ampersand, '\n' (where n is a digit) backreferences, or subexpression matching. The old string shall also be permitted to contain <newline>s.
Any non-null character can be used as a delimiter ( '/' shown here). Multiple -s expressions can be specified; the expressions shall be applied in the order specified, terminating with the first successful substitution. The optional trailing 'g' is as defined in the ed utility. The optional trailing 'p' shall cause successful substitutions to be 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.
In copy mode, if no -x format is specified, pax shall behave as if -x pax were specified.
- -X
-
When traversing the file hierarchy specified by a pathname, pax
shall not descend into directories that have a different
device ID ( st_dev; see the System Interfaces volume of IEEE Std 1003.1-2001,
stat()).
The options that operate on the names of files or archive members ( -c, -i, -n, -s, -u, and -v) shall interact as 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 IEEE Std 1003.1-2001, Chapter 5, File Format Notation, with the exceptions 1. through 5. defined in the EXTENDED DESCRIPTION section of printf, plus the following exceptions:
- 6.
-
The sequence ( keyword) can occur before a format conversion
specifier. The conversion argument is defined by the value
of keyword. The implementation shall support the following keywords:
-
- *
- Any of the Field Name entries in ustar Header Block and Octet-Oriented cpio Archive Entry . The implementation may support the cpio keywords without the leading c_ in addition to the form required by Values for cpio c_mode Field .
- *
- Any keyword defined for the extended header in pax Extended Header .
- *
- Any keyword provided as an implementation-defined extension within the extended header defined in pax Extended Header .
-
For example, the sequence "%(charset)s" is the string value of the name of the character set in the extended header.
The result of the keyword conversion argument shall be the value from the applicable header field or extended header, without any trailing NULs.
All keyword values used as conversion arguments shall be translated from the UTF-8 encoding to the character set appropriate for the local file system, user database, and so on, as applicable.
- 7.
-
An additional conversion specifier character, T, shall be used
to specify time formats. The T conversion
specifier character can be preceded by the sequence ( keyword=
subformat), where subformat is a date format as
defined by date operands. The default keyword shall be
mtime and the
default subformat shall be:
-
%b %e %H:%M %Y
-
- 8.
- An additional conversion specifier character, M, shall be used to specify the file mode string as defined in ls Standard Output. If ( keyword) is omitted, the mode keyword shall be used. For example, %.1M writes the single character corresponding to the <entry type> field of the ls -l command.
- 9.
- An additional conversion specifier character, D, shall be used to specify the device for block or special files, if applicable, in an implementation-defined format. If not applicable, and ( keyword) is specified, then this conversion shall be equivalent to %(keyword)u. If not applicable, and ( keyword) is omitted, then this conversion shall be equivalent to <space>.
- 10.
-
An additional conversion specifier character, F, shall be used
to specify a pathname. The F conversion
character can be preceded by a sequence of comma-separated keywords:
-
(keyword[,keyword] ... )
-
The values for all the keywords that are non-null shall be concatenated together, each separated by a '/' . The default shall be ( path) if the keyword path is defined; otherwise, the default shall be ( prefix, name).
- 11.
-
An additional conversion specifier character, L, shall be used
to specify a symbolic line expansion. If the 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 Pattern Matching Notation, including
the filename
expansion rules in Patterns Used for Filename Expansion . The
default, if no
pattern is specified, is to select all members in the archive.
STDIN
In write mode, the standard input shall be used only if no file operands are specified. It shall be a text file containing a list of pathnames, one per line, without leading or trailing <blank>s.
In list and read modes, if -f is not specified, the standard input shall be an archive file.
Otherwise, the standard input shall not be used.
INPUT FILES
The input file named by the archive option-argument, or standard input when the archive is read from there, shall be a file formatted according to one of the specifications in the EXTENDED DESCRIPTION section or some other implementation-defined format.
The file /dev/tty shall be used to write prompts and read responses.
ENVIRONMENT VARIABLES
The following environment variables shall affect the execution of pax:
- LANG
- Provide a default value for the internationalization variables that are unset or null. (See the Base Definitions volume of IEEE Std 1003.1-2001, Section 8.2, Internationalization Variables for the precedence of 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 for the processing of affirmative responses that should be used to affect the format and contents of diagnostic messages written to standard error.
- LC_TIME
- Determine the format and contents of date and time strings when the -v option is specified.
- NLSPATH
- Determine the location of message catalogs for the processing of LC_MESSAGES .
- TMPDIR
- Determine the pathname that provides part of the default global extended header record file, as described for the -o globexthdr= keyword in the OPTIONS section.
- TZ
-
Determine the timezone used to calculate date and time strings when
the -v option is specified. If TZ is unset or
null, an unspecified default timezone shall be used.
ASYNCHRONOUS EVENTS
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 -o listopt= format has been specified, the selected archive members shall be written to standard output using the format described under List Mode Format Specifications . In list mode without the -o listopt= format option, the table of contents of the selected archive members shall be written to standard output using the following format:
-
"%s\n", <pathname>
If the -v option is specified in list mode, the table of contents of the selected archive members shall be written to standard output using the following formats.
For pathnames representing hard links to previous members of the archive:
-
"%s == %s\n", <ls -l listing>, <linkname>
For all other pathnames:
-
"%s\n", <ls -l listing>
where <ls
In list mode, standard output shall not be buffered more than
a line at a time.
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:
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:
In all operating modes of pax, optional messages of unspecified
format concerning the input archive format and volume
number, the number of files, blocks, volumes, and media parts as well
as other diagnostic messages may be written to standard
error.
In all formats, for both standard output and standard error, it is
unspecified how non-printable characters in pathnames or link
names are written.
When pax is in read mode or list mode, using the
-x pax archive format, and a filename, link
name, owner name, or any other field in an extended header record
cannot be translated from the pax UTF-8 codeset format to
the codeset and current locale of the implementation, pax shall
write a diagnostic message to standard error, shall process
the file as described for the -o invalid= option, and
then shall process the next file in the archive.
In read mode, the extracted output files shall be of the archived
file type. In copy mode, the copied output files
shall be the type of the file being copied. In either mode, existing
files in the destination hierarchy shall be overwritten only
when all permission ( -p), modification time ( -u), and
invalid-value ( -o invalid=) tests allow
it.
In write mode, the output file named by the -f option-argument
shall be a file formatted according to one of the
specifications in the EXTENDED DESCRIPTION section, or some other
implementation-defined format.
A pax archive tape or file produced in the -x pax
format shall contain a series of blocks. The physical
layout of the archive shall be identical to the ustar format
described in ustar Interchange
Format . Each file archived shall be represented by the following
sequence:
At the end of the archive file there shall be two 512-byte blocks
filled with binary zeros, interpreted as an end-of-archive
indicator.
A schematic of an example archive with global extended header records
and two actual files is shown in pax
Format Archive Example . In the example, the second file in the archive
has no extended header preceding it, presumably because
it has no need for extended attributes.
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:
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.
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:
The extended header records shall be encoded according to the ISO/IEC 10646-1:2000
standard (UTF-8). The
<length> field, <blank>, equals sign, and <newline> shown shall
be limited to the portable character set,
as encoded in UTF-8. The <keyword> and <value> fields
can be any UTF-8 characters. The
<length> field shall be the decimal length of the extended header
record in octets, including the trailing
<newline>.
The <keyword> field shall be one of the entries from the following
list or a keyword provided as an implementation
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.)
The encoding is included in an extended header for information only;
when pax is used as described in
IEEE Std 1003.1-2001, it shall not translate the file data into
any other encoding. The BINARY entry indicates
unencoded binary data.
When used in write or copy mode, it is implementation-defined
whether pax includes a charset
extended header record for a file.
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.
If the <value> field is zero length, it shall delete any header
block field, previously entered extended header
value, or global extended header value of the same name.
If a keyword in an extended header record (or in a -o option-argument)
overrides or deletes a corresponding field in the
ustar header block, pax shall ignore the contents of that
header block field.
Unlike the ustar header block fields, NULs shall not delimit
<value>s; all characters within the
<value> field shall be considered data for the field. None of
the length limitations of the ustar header block
fields in ustar Header Block shall apply to the extended header records.
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:
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.
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 -b blocksize and -x
ustar options. Each group of logical records may be written
with a single operation equivalent to the write() function.
On magnetic tape, the result of this write shall be a single tape
physical
block. The last physical block shall always be the full size, so logical
records after the two zero logical records may contain
undefined data.
The header logical record shall be structured as shown in the following
table. All lengths and offsets are in decimal.
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 IEEE Std 1003.1-2001. If a filename is found on the
medium that would create an invalid filename, it is
implementation-defined whether the data from the file is stored on
the file hierarchy and under what name it is stored. The
pax utility may choose to ignore these files as long as it produces
an error indicating that the file is being ignored.
Each field within the header logical record is contiguous; that is,
there is no padding used. Each character on the archive
medium shall be stored contiguously.
The fields magic, uname, and gname are character
strings each 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:
When appropriate privilege is required to set one of these mode bits,
and the user restoring the files from the archive does not
have the appropriate privilege, the mode bits for which the user does
not have appropriate privilege shall be ignored. Some of the
mode bits in the archive format are not mentioned elsewhere in this
volume of IEEE Std 1003.1-2001. If the implementation
does not support those bits, they may be ignored.
The uid and gid fields are the user and group ID of the
owner and group of the file, respectively.
The size field is the size of the file in octets. If the typeflag
field is set to specify a file to be of type 1
(a link) or 2 (a symbolic link), the size field shall be specified
as zero. If the typeflag field is set to specify a
file of type 5 (directory), the size field shall be interpreted
as described under the definition of that record type. No
data logical records are stored for types 1, 2, or 5. If the typeflag
field is set to 3 (character special file), 4 (block
special file), or 6 (FIFO), the meaning of the size field is
unspecified by this volume of IEEE Std 1003.1-2001,
and no data logical records shall be stored on the medium. Additionally,
for type 6, the size field shall be ignored when
reading. If the typeflag field is set to any other value, the
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 spaces.
The typeflag field specifies the type of file archived. If a
particular implementation does not recognize the type, or
the user does not have appropriate privilege to create that type,
the file shall be extracted as if it were a regular file if the
file type is defined to have a meaning for the size field that
could cause data logical records to be written on the medium
(see the previous description for size). If conversion to a
regular file occurs, the pax utility shall 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:
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.
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.
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:
Directories, FIFOs, symbolic links, and regular files shall be supported
on a system conforming to this volume of
IEEE Std 1003.1-2001; additional values defined previously are reserved
for compatibility with existing systems.
Additional file types may be supported; however, such files should
not be written to archives intended to be transported to other
systems.
The c_name field shall contain the pathname of the file. The
length of this field in octets is the value of
c_namesize.
If a filename is found on the medium that would create an invalid
pathname, it is implementation-defined whether the data from
the file is stored on the file hierarchy and under what name it is
stored.
All characters shall be represented in the ISO/IEC 646:1991 standard
IRV. For maximum portability between implementations,
names should be selected from characters represented by the portable
filename character set as octets with the most significant bit
zero. If an implementation supports the use of characters outside
the portable filename character set in names for files, users,
and groups, one or more implementation-defined encodings of these
characters shall be provided for interchange purposes. However,
the pax utility shall never create filenames on the local system
that cannot be accessed via the procedures described
previously in this volume of IEEE Std 1003.1-2001. If a filename
is found on the medium that would create an invalid
filename, it is implementation-defined whether the data from the file
is stored on the local file system and under what name it is
stored. The pax utility may choose to ignore these files as
long as it produces an error indicating that the file is being
ignored.
Following c_name, there shall be c_filesize octets of
data. Interpretation of such data occurs in a manner
dependent on the file. If c_filesize is zero, no data shall
be contained in c_filedata.
When restoring from an archive:
FIFO special files, directories, and the trailer shall be recorded
with c_filesize equal to zero. For other special
files, c_filesize is unspecified by this volume of IEEE Std 1003.1-2001.
The header for the next file entry in the
archive shall be written directly after the last octet of the file
entry preceding it. A header denoting the filename
TRAILER!!! shall indicate the end of the archive; the contents
of octets in the last block of the archive following such a
header are undefined.
The following exit values shall be returned:
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.
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:
The one pathname per line format of standard input precludes pathnames
containing <newline>s. Although such pathnames
violate the portable filename guidelines, they may exist and their
presence may inhibit usage of pax within shell scripts.
This problem is inherited from historical archive programs. The problem
can be avoided by listing filename arguments on the command
line instead of on standard input.
It is almost certain that appropriate privileges are required for
pax to accomplish parts of this volume of
IEEE Std 1003.1-2001. Specifically, creating files of type block
special or character special, restoring file access
times unless the files are owned by the user (the -t option),
or preserving file owner, group, and mode (the -p
option) all probably require appropriate privileges.
In read mode, implementations are permitted to overwrite files
when the archive has multiple members with the same name.
This may fail if permissions on the first version of the file do not
permit it to be overwritten.
The cpio and ustar formats can only support files up to
8589934592 bytes (8 * 2^30) in size.
The following command:
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:
copy the olddir directory hierarchy to newdir.
reads the archive a.pax, with all files rooted in /usr
in the archive extracted relative to the current
directory.
Using the option:
overrides the default output description in Standard Output and instead
writes:
Using the options:
overrides the default output description in Standard Output and instead
writes:
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 IEEE Std 1003.1-2001
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 tar.) In order to avoid these two problems, the pax
specification has an additional "privilege" mechanism, the
-p option. Only a pax invocation with the privileges needed,
and which has the -p option set using the
e specification character, has the "appropriate privilege" to
restore full ownership and permission information.
Note also that this volume of IEEE Std 1003.1-2001 requires that
the file ownership and access permissions shall be
set, on extraction, in the same fashion as the creat() function
when provided with the
mode stored in the archive. This means that the file creation mask
of the user is applied to the file permissions.
Users should note that directories may be created by pax while
extracting files with permissions that are different from
those that existed at the time the archive was created. When extracting
sensitive information into a directory hierarchy that no
longer exists, users are encouraged to set their file creation mask
appropriately to protect these files during extraction.
The table of contents output is written to standard output to facilitate
pipeline processing.
An early proposal had hard links displaying for all pathnames. This
was removed because it complicates the output of the case
where -v is not specified and does not match historical cpio
usage. The hard-link information is available in the
-v display.
The description of the -l option allows implementations to make
hard links to symbolic links.
IEEE Std 1003.1-2001 does not specify any way to create a hard link
to a symbolic link, but many implementations provide
this capability as an extension. If there are hard links to symbolic
links when an archive is created, the implementation is
required to 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 -x pax format
overcomes these restrictions.
The default blocksize value of 5120 bytes for cpio was
selected because it is one of the standard block-size
values for cpio, set when the -B option is specified.
(The other default block-size value for cpio is 512
bytes, and this was considered to be too small.) The default block
value of 10240 bytes for tar was selected because that is
the standard block-size value for BSD tar. The maximum block
size of 32256 bytes (2**15-512 bytes)
is the largest multiple of 512 bytes that fits into a signed 16-bit
tape controller transfer register. There are known limitations
in some historical systems that would prevent larger blocks from being
accepted. Historical values were chosen to improve
compatibility with historical scripts using dd or similar utilities
to manipulate
archives. Also, default block sizes for any file type other than character
special file has been deleted from this volume of
IEEE Std 1003.1-2001 as unimportant and not likely to affect the
structure of the resulting archive.
Implementations are permitted to modify the block-size value based
on the archive format or the device to which the 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 IEEE Std 1003.1-2001,
for many reasons. First, it is in general not
acceptable for a single option to have multiple effects. Second, the
ability to make pattern matching 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 IEEE Std 1003.1-2001
described as part of the cp utility. Both methods are historical
practice: cp
provides a simpler, more intuitive interface, while pax offers
a finer granularity of control. Each provides additional
functionality to the other; in particular, pax maintains the
hard-link structure of the hierarchy while cp does not. It is
the intention of the standard developers that the results be similar
(using
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 spaces is valid:
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 -e charmap option was included to increase
portability of files between systems using different
coded character sets. This option was omitted because it was apparent
that consensus could not be formed for it. In this version,
the use of UTF-8 should be an adequate substitute.
The -k option was added to address international concerns about
the dangers involved in the character set transformations
of -e (if the target character set were different from the source,
the filenames might be transformed into names matching
existing files) and also was made more general to protect files transferred
between file systems with different {NAME_MAX} values
(truncating a filename on a smaller system might also inadvertently
overwrite existing files). As stated, it prevents any
overwriting, even if the target file is older than the source. This
version adds more granularity of options to solve this problem
by introducing the -o invalid= option-specifically the
UTF-8 action. (Note that an existing file that is named with a
UTF-8 encoding is still subject to overwriting in this case. The -k
option closes that loophole.)
Some of the file characteristics referenced in this volume of IEEE Std 1003.1-2001
might not be supported by some
archive formats. For example, neither the tar nor cpio
formats contain the file access time. For this reason, the
e specification character has been provided, intended to cause
all file characteristics specified in the archive to be
retained.
It is required that extracted directories, by default, have their
access and modification times and permissions set to the
values specified in the archive. This has obvious problems in that
the directories are almost certainly modified after being
extracted and that directory permissions may not permit file creation.
One possible solution is to create directories with the mode
specified in the archive, as modified by the umask of the user,
with sufficient
permissions to allow file creation. After all files have been extracted,
pax would then reset the access and modification
times and 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.
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:
The following were not goals for this format because these are better
handled by separate utilities or are inappropriate for a
portable format:
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:
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 of the first revision to IEEE Std 1003.1-2001 contained
a proposed archive format that was based on
compatibility with the standard for tape files (ISO 1001, similar
to the format used 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 ( typeflag g)
was controversial. If this were applied to an archive being
recorded on magnetic tape, a few unreadable blocks at the beginning
of the tape could be a serious problem; a utility attempting to
extract as many files as possible from a damaged archive could lose
a large percentage of file header information in this case.
However, if the archive were on a reliable medium, such as a CD-ROM,
the global extended header offers considerable potential size
reductions by eliminating redundant information. Thus, the text warns
against using the global method for unreliable media and
provides a method for implanting global information in the extended
header for each file, rather than in the typeflag
g records.
No facility for data translation or filtering on a per-file basis
is included because the standard developers could not invent
an interface that would allow this in an efficient manner. If a filter,
such as encryption or compression, is to be applied to all
the files, it is more efficient to apply the filter to the entire
archive as a single file. The standard developers considered
interfaces that would invoke a shell script for each file going into
or out of the archive, but the system overhead in this
approach was considered to be too high.
One such approach would be to have filter= records that give
a pathname for an executable. When the program is invoked,
the file and archive would be open for standard input/output and all
the header fields would be available as environment variables
or command-line arguments. The standard developers did discuss such
schemes, but they were omitted from
IEEE Std 1003.1-2001 due to concerns about excessive overhead. Also,
the program itself would need to be in the archive
if it were to be used portably.
There is currently no portable means of identifying the character
set(s) used for a file in the file system. Therefore,
pax has not been given a mechanism to generate charset records
automatically. The only portable means of doing this is for
the user to write the archive using the -o charset= string
command line option. This assumes that all of the
files in the archive use the same encoding. The "implementation-defined"
text is included to allow for a system that can identify
the encodings used for each of its files.
The table of standards that accompanies the charset record description
is acknowledged to be very limited. Only a limited number
of character set standards is reasonable for maximal interchange.
Any character set is, of course, possible by prior agreement. It
was suggested that EBCDIC be listed, but it was omitted because it
is not defined by a formal standard. Formal standards, and then
only those with reasonably large followings, can be included here,
simply as a matter of 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:
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:
The requirements on restoring from an archive are slightly different
from the historical wording, allowing for non-monolithic
privilege to bring forward as much as possible. In particular, attributes
such as "high performance file" might be broadly but
not universally granted while set-user-ID or chown() might be
much more restricted.
There is no implication in IEEE Std 1003.1-2001 that the security
information be honored after it is restored to the file
hierarchy, in spite of what might be improperly inferred by the silence
on that topic. That is a topic for another standard.
Links are recorded in the fashion described here because a link can
be to any file type. It is desirable in general to be able
to restore part of an archive selectively and restore all of those
files completely. If the data is not associated with each link,
it is not possible to do this. However, the data associated with a
file can be large, and when selective restoration is not needed,
this can be a significant burden. The archive is structured so that
files that have no associated data can always be restored by
the name of any link name of any link, and the user may choose whether
data is recorded with each instance of a file that contains
data. The format permits mixing of both types of links in a single
archive; this can be done for special needs, and pax is
expected to interpret such archives on input properly, despite the
fact that there is no pax option that would force this
mixed case on output. (When -o linkdata is used, the output
must contain the 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 privilege" so that it can
be ignored when writing to the file system. POSIX does not provide
a portable means to change file creation time. Nothing is
intended to prevent a non-portable implementation of pax from
restoring the value.
The gid, size, and uid extended header records were
included to allow expansion beyond the sizes specified
in the regular tar header. New file system architectures are
emerging that will exhaust the 12-digit size field. There are
probably not many systems requiring more than 8 digits for user and
group IDs, but the extended header values were included for
completeness, allowing overrides for all of the decimal values in
the tar header.
The standard developers intended to describe the effective results
of pax with regard to file ownerships and permissions;
implementations are not restricted in timing or sequencing the restoration
of such, 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.
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 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:
where each 'x' represents a bit value from the character being
translated.
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.
IEEE Std 1003.1-2001 does not require the contiguous file extension,
but does define a standard way of archiving such
files so that all conforming systems can interpret these file types
in a meaningful and consistent manner. On a system that does
not support extended file types, the pax utility should do the
best it can with the file and go on to the next.
The file protection modes are those conventionally used by the ls
utility. This is
extended beyond the usage in the ISO POSIX-2 standard to support
the "shared text" or "sticky" bit. It is intended that
the conformance document should not document anything beyond the existence
of and support of such a mode. Further extensions are
expected to these bits, particularly with overloading the set-user-ID
and set-group-ID flags.
The reference to appropriate privilege in the cpio format refers
to an error on standard output; the ustar format
does not make comparable statements.
The model for this format was the historical System V cpio -c
data interchange format. This model documents the
portable version of the cpio format and not the binary version.
It has the flexibility to transfer data of any type
described within IEEE Std 1003.1-2001, yet is extensible to transfer
data types specific to extensions beyond
IEEE Std 1003.1-2001 (for example, contiguous files). Because it
describes existing practice, there is no question of
maintaining upwards-compatibility.
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.
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:
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.
Shell Command Language, cp, ed, getopts,
ls, printf(), the Base Definitions volume of IEEE Std 1003.1-2001,
<cpio.h>, the System Interfaces volume of IEEE Std 1003.1-2001,
chown(), creat(), mkdir(), mkfifo(), stat(),
utime(), write()
STDERR
"%s\n", <pathname>
"%s >> %s\n", <original pathname>, <new pathname>
OUTPUT FILES
EXTENDED DESCRIPTION
pax Interchange Format
pax Header Block
pax Extended Header
"%d %s=%s\n", <length>, <keyword>, <value>
<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.
pax Extended Header Keyword Precedence
pax Extended Header File Times
ustar Interchange Format
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
Bit Value IEEE Std 1003.1-2001 Bit
Description
04000 S_ISUID
Set UID on execution.
02000 S_ISGID
Set GID on execution.
01000 <reserved>
Reserved for future standardization.
00400 S_IRUSR
Read permission for file owner class.
00200 S_IWUSR
Write permission for file owner class.
00100 S_IXUSR
Execute/search permission for file owner class.
00040 S_IRGRP
Read permission for file group class.
00020 S_IWGRP
Write permission for file group class.
00010 S_IXGRP
Execute/search permission for file group class.
00004 S_IROTH
Read permission for file other class.
00002 S_IWOTH
Write permission for file other class.
00001 S_IXOTH
Execute/search permission for file other class.
cpio Interchange Format
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
File Permissions Name Value Indicates C_IRUSR 000400 Read by owner C_IWUSR 000200 Write by owner C_IXUSR 000100 Execute by owner C_IRGRP 000040 Read by group C_IWGRP 000020 Write by group C_IXGRP 000010 Execute by group C_IROTH 000004 Read by others C_IWOTH 000002 Write by others C_IXOTH 000001 Execute by others C_ISUID 004000 Set uid C_ISGID 002000 Set gid C_ISVTX 001000 Reserved File Type Name Value Indicates C_ISDIR 040000 Directory C_ISFIFO 010000 FIFO C_ISREG 0100000 Regular file C_ISLNK 0120000 Symbolic link C_ISBLK 060000 Block special file C_ISCHR 020000 Character special file C_ISSOCK 0140000 Socket C_ISCTG 0110000 Reserved
cpio Filename
cpio File Data
cpio Special Entries
EXIT STATUS
CONSEQUENCES OF ERRORS
APPLICATION USAGE
EXAMPLES
pax -w -f /dev/rmt/1m .
mkdir newdirpax -rw olddir newdir
pax -r -s ',^//*usr//*,,' -f a.pax
-o listopt="%M %(atime)T %(size)D %(name)s"
-rw-rw--- Jan 12 15:53 1492 /usr/foo/bar
-o listopt='%L\t%(size)D\n%.7' \
-o listopt='(name)s\n%(ctime)T\n%T'
/usr/foo/bar -> /tmp 1492
/usr/fo
Jan 12 1991
Jan 31 15:53
RATIONALE
pax -s " foo bar " ...
pax Interchange Format
VENDOR.keyword
pax Archive Character Set Encoding/Decoding
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
ustar Interchange Format
cpio Interchange Format
cpio Header
cpio Filename
FUTURE DIRECTIONS
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
SEE ALSO