изменить режимы файлов (change the file modes)
Обоснование (Rationale)
The functionality of chmod is described substantially through
references to concepts defined in the System Interfaces volume of
POSIX.1‐2017. In this way, there is less duplication of effort
required for describing the interactions of permissions. However,
the behavior of this utility is not described in terms of the
chmod() function from the System Interfaces volume of
POSIX.1‐2017 because that specification requires certain side-
effects upon alternate file access control mechanisms that might
not be appropriate, depending on the implementation.
Implementations that support mandatory file and record locking as
specified by the 1984 /usr/group standard historically used the
combination of set-group-ID bit set and group execute bit clear
to indicate mandatory locking. This condition is usually set or
cleared with the symbolic mode perm symbol l instead of the perm
symbols s and x so that the mandatory locking mode is not changed
without explicit indication that that was what the user intended.
Therefore, the details on how the implementation treats these
conditions must be defined in the documentation. This volume of
POSIX.1‐2017 does not require mandatory locking (nor does the
System Interfaces volume of POSIX.1‐2017), but does allow it as
an extension. However, this volume of POSIX.1‐2017 does require
that the ls and chmod utilities work consistently in this area.
If ls -l file indicates that the set-group-ID bit is set, chmod
g-s file must clear it (assuming appropriate privileges exist to
change modes).
The System V and BSD versions use different exit status codes.
Some implementations used the exit status as a count of the
number of errors that occurred; this practice is unworkable since
it can overflow the range of valid exit status values. This
problem is avoided here by specifying only 0 and >0 as exit
values.
The System Interfaces volume of POSIX.1‐2017 indicates that
implementation-defined restrictions may cause the S_ISUID and
S_ISGID bits to be ignored. This volume of POSIX.1‐2017 allows
the chmod utility to choose to modify these bits before calling
chmod() (or some function providing equivalent capabilities) for
non-regular files. Among other things, this allows
implementations that use the set-user-ID and set-group-ID bits on
directories to enable extended features to handle these
extensions in an intelligent manner.
The X perm symbol was adopted from BSD-based systems because it
provides commonly desired functionality when doing recursive (-R
option) modifications. Similar functionality is not provided by
the find utility. Historical BSD versions of chmod, however, only
supported X with op+; it has been extended in this volume of
POSIX.1‐2017 because it is also useful with op=. (It has also
been added for op- even though it duplicates x, in this case,
because it is intuitive and easier to explain.)
The grammar was extended with the permcopy non-terminal to allow
historical-practice forms of symbolic modes like o=u -g (that is,
set the ``other'' permissions to the permissions of ``owner''
minus the permissions of ``group'').
