постраничная основа (by-page basis)
Обоснование (Rationale)
The more utility, available in BSD and BSD-derived systems, was
chosen as the prototype for the POSIX file display program since
it is more widely available than either the public-domain program
less or than pg, a pager provided in System V. The 4.4 BSD more
is the model for the features selected; it is almost fully
upwards-compatible from the 4.3 BSD version in wide use and has
become more amenable for vi users. Several features originally
derived from various file editors, found in both less and pg,
have been added to this volume of POSIX.1‐2017 as they have
proved extremely popular with users.
There are inconsistencies between more and vi that result from
historical practice. For example, the single-character commands
h, f, b, and <space> are screen movers in more, but cursor movers
in vi. These inconsistencies were maintained because the cursor
movements are not applicable to more and the powerful
functionality achieved without the use of the control key
justifies the differences.
The tags interface has been included in a program that is not a
text editor because it promotes another degree of consistent
operation with vi. It is conceivable that the paging environment
of more would be superior for browsing source code files in some
circumstances.
The operating mode referred to for block-mode terminals
effectively adds a <newline> to each Synopsis line that currently
has none. So, for example, d<newline> would page one screenful.
The mode could be triggered by a command line option, environment
variable, or some other method. The details are not imposed by
this volume of POSIX.1‐2017 because there are so few systems
known to support such terminals. Nevertheless, it was considered
that all systems should be able to support more given the
exception cited for this small community of terminals because, in
comparison to vi, the cursor movements are few and the command
set relatively amenable to the optional <newline> characters.
Some versions of more provide a shell escaping mechanism similar
to the ex ! command. The standard developers did not consider
that this was necessary in a paginator, particularly given the
wide acceptance of multiple window terminals and job control
features. (They chose to retain such features in the editors and
mailx because the shell interaction also gives an opportunity to
modify the editing buffer, which is not applicable to more.)
The -p (position) option replaces the + command because of the
Utility Syntax Guidelines. The +command option is no longer
specified by POSIX.1‐2008 but may be present in some
implementations. In early proposals, it took a pattern argument,
but historical less provided the more general facility of a
command. It would have been desirable to use the same -c as ex
and vi, but the letter was already in use.
The text stating ``from a non-rewindable stream ...
implementations may limit the amount of backwards motion
supported'' would allow an implementation that permitted no
backwards motion beyond text already on the screen. It was not
possible to require a minimum amount of backwards motion that
would be effective for all conceivable device types. The
implementation should allow the user to back up as far as
possible, within device and reasonable memory allocation
constraints.
Historically, non-printable characters were displayed using the
ARPA standard mappings, which are as follows:
1. Printable characters are left alone.
2. Control characters less than \177 are represented as followed
by the character offset from the '@' character in the ASCII
map; for example, \007 is represented as 'G'.
3. \177 is represented as followed by '?'.
The display of characters having their eighth bit set was less
standard. Existing implementations use hex (0x00), octal (\000),
and a meta-bit display. (The latter displayed characters with
their eighth bit set as the two characters "M-", followed by the
seven-bit display as described previously.) The latter probably
has the best claim to historical practice because it was used
with the -v option of 4 BSD and 4 BSD-derived versions of the cat
utility since 1980.
No specific display format is required by POSIX.1‐2008.
Implementations are encouraged to conform to historic practice in
the absence of any strong reason to diverge.
