The chmod utility shall change any or all of the file mode bits
of the file named by each file operand in the way specified by
the mode operand.
It is implementation-defined whether and how the chmod utility
affects any alternate or additional file access control mechanism
(see the Base Definitions volume of POSIX.1‐2017, Section 4.5,
File Access Permissions) being used for the specified file.
Only a process whose effective user ID matches the user ID of the
file, or a process with appropriate privileges, shall be
permitted to change the file mode bits of a file.
Upon successfully changing the file mode bits of a file, the
chmod utility shall mark for update the last file status change
timestamp of the file.
