Certain commands can operate destructively on entire hierarchies.
For example, if a user with appropriate privileges mistakenly runs
‘rm -rf / tmp/junk’, that may remove
all files on the entire system. Since there are so few
legitimate uses for such a command,
GNU rm normally declines to operate on any directory
that resolves to /. If you really want to try to remove all
the files on your system, you can use the --no-preserve-root
option, but the default behavior, specified by the
--preserve-root option, is safer for most purposes.
The commands chgrp, chmod and chown
can also operate destructively on entire hierarchies, so they too
support these options. Although, unlike rm, they don’t
actually unlink files, these commands are arguably more dangerous
when operating recursively on /, since they often work much
more quickly, and hence damage more files before an alert user can
interrupt them. Tradition and POSIX require these commands
to operate recursively on /, so they default to
--no-preserve-root, but using the --preserve-root
option makes them safer for most purposes. For convenience you can
specify --preserve-root in an alias or in a shell function.
The --preserve-root option also ensures
that chgrp and chown do not modify /
even when dereferencing a symlink pointing to /.