(coreutils.info)Treating / specially

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2.11 Treating ‘/’ specially
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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.

   Note that the ‘--preserve-root’ option also ensures that ‘chgrp’ and
‘chown’ do not modify ‘/’ even when dereferencing a symlink pointing to
‘/’.