The -i option exists on BSD systems, giving applications and
users a way to avoid accidentally removing files when copying.
Although the 4.3 BSD version does not prompt if the standard
input is not a terminal, the standard developers decided that use
of -i is a request for interaction, so when the destination path
exists, the utility takes instructions from whatever responds on
standard input.
The exact format of the interactive prompts is unspecified. Only
the general nature of the contents of prompts are specified
because implementations may desire more descriptive prompts than
those used on historical implementations. Therefore, an
application using the -i option relies on the system to provide
the most suitable dialog directly with the user, based on the
behavior specified.
The -p option is historical practice on BSD systems, duplicating
the time of last data modification and time of last access. This
volume of POSIX.1‐2017 extends it to preserve the user and group
IDs, as well as the file permissions. This requirement has
obvious problems in that the directories are almost certainly
modified after being copied. This volume of POSIX.1‐2017 requires
that the modification times be preserved. The statement that the
order in which the characteristics are duplicated is unspecified
is to permit implementations to provide the maximum amount of
security for the user. Implementations should take into account
the obvious security issues involved in setting the owner, group,
and mode in the wrong order or creating files with an owner,
group, or mode different from the final value.
It is unspecified whether cp writes diagnostic messages when the
user and group IDs cannot be set due to the widespread practice
of users using -p to duplicate some portion of the file
characteristics, indifferent to the duplication of others.
Historic implementations only write diagnostic messages on errors
other than [EPERM].
Earlier versions of this standard included support for the -r
option to copy file hierarchies. The -r option is historical
practice on BSD and BSD-derived systems. This option is no longer
specified by POSIX.1‐2008 but may be present in some
implementations. The -R option was added as a close synonym to
the -r option, selected for consistency with all other options in
this volume of POSIX.1‐2017 that do recursive directory descent.
The difference between -R and the removed -r option is in the
treatment by cp of file types other than regular and directory.
It was implementation-defined how the - option treated special
files to allow both historical implementations and those that
chose to support -r with the same abilities as -R defined by this
volume of POSIX.1‐2017. The original -r flag, for historic
reasons, did not handle special files any differently from
regular files, but always read the file and copied its contents.
This had obvious problems in the presence of special file types;
for example, character devices, FIFOs, and sockets.
When a failure occurs during the copying of a file hierarchy, cp
is required to attempt to copy files that are on the same level
in the hierarchy or above the file where the failure occurred. It
is unspecified if cp shall attempt to copy files below the file
where the failure occurred (which cannot succeed in any case).
Permissions, owners, and groups of created special file types
have been deliberately left as implementation-defined. This is to
allow systems to satisfy special requirements (for example,
allowing users to create character special devices, but requiring
them to be owned by a certain group). In general, it is strongly
suggested that the permissions, owner, and group be the same as
if the user had run the historical mknod, ln, or other utility to
create the file. It is also probable that additional privileges
are required to create block, character, or other implementation-
defined special file types.
Additionally, the -p option explicitly requires that all set-
user-ID and set-group-ID permissions be discarded if any of the
owner or group IDs cannot be set. This is to keep users from
unintentionally giving away special privilege when copying
programs.
When creating regular files, historical versions of cp use the
mode of the source file as modified by the file mode creation
mask. Other choices would have been to use the mode of the source
file unmodified by the creation mask or to use the same mode as
would be given to a new file created by the user (plus the
execution bits of the source file) and then modify it by the file
mode creation mask. In the absence of any strong reason to change
historic practice, it was in large part retained.
When creating directories, historical versions of cp use the mode
of the source directory, plus read, write, and search bits for
the owner, as modified by the file mode creation mask. This is
done so that cp can copy trees where the user has read
permission, but the owner does not. A side-effect is that if the
file creation mask denies the owner permissions, cp fails. Also,
once the copy is done, historical versions of cp set the
permissions on the created directory to be the same as the source
directory, unmodified by the file creation mask.
This behavior has been modified so that cp is always able to
create the contents of the directory, regardless of the file
creation mask. After the copy is done, the permissions are set to
be the same as the source directory, as modified by the file
creation mask. This latter change from historical behavior is to
prevent users from accidentally creating directories with
permissions beyond those they would normally set and for
consistency with the behavior of cp in creating files.
It is not a requirement that cp detect attempts to copy a file to
itself; however, implementations are strongly encouraged to do
so. Historical implementations have detected the attempt in most
cases.
There are two methods of copying subtrees in this volume of
POSIX.1‐2017. The other method is described as part of the pax
utility (see pax(1p)). Both methods are historical practice. The
cp utility 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.
The wording allowing cp to copy a directory to implementation-
defined file types not specified by the System Interfaces volume
of POSIX.1‐2017 is provided so that implementations supporting
symbolic links are not required to prohibit copying directories
to symbolic links. Other extensions to the System Interfaces
volume of POSIX.1‐2017 file types may need to use this loophole
as well.
