These are specified by POSIX but are not special builtins, and therefore
need to be available via execve() and utilities like time, nohup, xargs.
(Note that hash was moved from the XSI option to the base in the 2008
standard.)
Like most of the POSIX "regular builtin commands", these need to be executed
in a shell environment for full functionality, although they may still be of
some use outside one.
Unlike the POSIX special and regular builtin commands, POSIX does not
require these to be found before a PATH search, although that could be an
oversight.
Like some of the utilities already provided by usr.bin/alias, these may lead
to confusing results when invoked from csh(1).
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