V7/usr/man/man1/passwd.1

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.TH PASSWD 1 
.SH NAME
passwd \- change login password
.SH SYNOPSIS
.B passwd
[ name ]
.SH DESCRIPTION
This command changes (or installs) a password
associated with the user
.IR name
(your own name by default).
.PP
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 forestall mistakes.
.PP
New passwords must be at least four characters long if they use
a sufficiently rich alphabet and at least six characters long
if monocase.
These rules are relaxed if you are insistent enough.
.PP
Only the owner of the name or the super-user may change a password;
the owner must prove he knows the old password.
.SH FILES
/etc/passwd
.SH "SEE ALSO"
login(1), passwd(5), crypt(3)
.br
Robert Morris and Ken Thompson,
.I Password Security: A Case History