PASSWD(1) General Commands Manual PASSWD(1)
passwd - change login password
passwd [ name ]
This command changes (or installs) a password associated with the user
name (your own name by default).
The program prompts for the old password and then for the new one. The
caller must supply both. The new password must be typed twice, to
New passwords must be at least four characters long if they use a suf-
ficiently rich alphabet and at least six characters long if monocase.
These rules are relaxed if you are insistent enough.
Only the owner of the name or the super-user may change a password; the
owner must prove he knows the old password.
login(1), passwd(5), crypt(3)
Robert Morris and Ken Thompson, Password Security: A Case History