Some historical implementations of the ls utility show all
entries in a directory except dot and dot-dot when a superuser
invokes ls without specifying the -a option. When ``normal''
users invoke ls without specifying -a, they should not see
information about any files with names beginning with a <period>
unless they were named as file operands.
Implementations are expected to traverse arbitrary depths when
processing the -R option. The only limitation on depth should be
based on running out of physical storage for keeping track of
untraversed directories.
The -1 (one) option was historically found in BSD and BSD-derived
implementations only. It is required in this volume of
POSIX.1‐2017 so that conforming applications might ensure that
output is one entry per line, even if the output is to a
terminal.
The -S option was added in Issue 7, but had been provided by
several implementations for many years. The description given in
the standard documents historic practice, but does not match much
of the documentation that described its behavior. Historical
documentation typically described it as something like:
-S Sort by size (largest size first) instead of by name.
Special character devices (listed last) are sorted by
name.
even though the file type was never considered when sorting the
output. Character special files do typically sort close to the
end of the list because their file size on most implementations
is zero. But they are sorted alphabetically with any other files
that happen to have the same file size (zero), not sorted
separately and added to the end.
This volume of POSIX.1‐2017 is frequently silent about what
happens when mutually-exclusive options are specified. Except for
-R, -d, and -f, the ls utility is required to accept multiple
options from each mutually-exclusive option set without treating
them as errors and to use the behavior specified by the last
option given in each mutually-exclusive set. Since ls is one of
the most aliased commands, it is important that the
implementation perform intuitively. For example, if the alias
were:
alias ls="ls -C"
and the user typed ls -1 (one), single-text-column output should
result, not an error.
The -g, -l (ell), -n, and -o options are not mutually-exclusive
options. They all enable long format output. They work together
to determine whether the file's owner is written (no if -g is
present), file's group is written (no if -o is present), and if
the file's group or owner is written whether it is written as the
name (default) or a string representation of the UID or GID
number (if -n is present). The -C, -m, -x, and -1 (one) are
mutually-exclusive options and the first three of these disable
long format output. The -1 (one) option does not directly change
whether or not long format output is enabled, but by overriding
-C, -m, and -x, it can re-enable long format output that had been
disabled by one of these options.
Earlier versions of this standard did not describe the BSD -A
option (like -a, but dot and dot-dot are not written out). It has
been added due to widespread implementation.
Implementations may make -q the default for terminals to prevent
trojan horse attacks on terminals with special escape sequences.
This is not required because:
* Some control characters may be useful on some terminals; for
example, a system might write them as "\001" or "^A".
* Special behavior for terminals is not relevant to
applications portability.
An early proposal specified that the
<optional alternate access method flag> had to be '+' if there
was an alternate access method used on the file or <space> if
there was not. This was changed to be <space> if there is not and
a single printable character if there is. This was done for three
reasons:
1. There are historical implementations using characters other
than '+'.
2. There are implementations that vary this character used in
that position to distinguish between various alternate access
methods in use.
3. The standard developers did not want to preclude future
specifications that might need a way to specify more than one
alternate access method.
Nonetheless, implementations providing a single alternate access
method are encouraged to use '+'.
Earlier versions of this standard did not have the -k option,
which meant that the -s option could not be used portably as its
block size was implementation-defined, and the units used to
specify the number of blocks occupied by files in a directory in
an ls -l listing were fixed as 512-byte units. The -k option has
been added to provide a way for the -s option to be used
portably, and for consistency it also changes the aforementioned
units from 512-byte to 1024-byte.
The <date and time> field in the -l format is specified only for
the POSIX locale. As noted, the format can be different in other
locales. No mechanism for defining this is present in this volume
of POSIX.1‐2017, as the appropriate vehicle is a messaging
system; that is, the format should be specified as a ``message''.
