passwd - change user password
passwd [options] [LOGIN]
The passwd command changes passwords for user accounts. A normal user may only change the password for their own account, while the superuser may change the password for any account. passwd also changes the account or associated password validity period.
The user is first prompted for their old password, if one is present. This password is then encrypted and compared against the stored password. The user has only one chance to enter the correct password. The superuser is permitted to bypass this step so that forgotten passwords may be changed.
After the password has been entered, password aging information is checked to see if the user is permitted to change the password at this time. If not, passwd refuses to change the password and exits.
The user is then prompted twice for a replacement password. The second entry is compared against the first and both are required to match in order for the password to be changed.
Then, the password is tested for complexity. passwd will reject any password which is not suitably complex. Care must be taken not to include the system default erase or kill characters.
The security of a password depends upon the strength of the encryption algorithm and the size of the key space. The legacy UNIX System encryption method is based on the NBS DES algorithm. More recent methods are now recommended (see ENCRYPT_METHOD). The size of the key space depends upon the randomness of the password which is selected.
Compromises in password security normally result from careless password selection or handling. For this reason, you should not select a password which appears in a dictionary or which must be written down. The password should also not be a proper name, your license number, birth date, or street address. Any of these may be used as guesses to violate system security.
As a general guideline, passwords should be long and random. It's fine to use simple character sets, such as passwords consisting only of lowercase letters, if that helps memorizing longer passwords. For a password consisting only of lowercase English letters randomly chosen, and a length of 32, there are 26^32 (approximately 2^150) different possible combinations. Being an exponential equation, it's apparent that the exponent (the length) is more important than the base (the size of the character set).
You can find advice on how to choose a strong password on http://en.wikipedia.org/wiki/Password_strength
The options which apply to the passwd command are:
-i, --inactive INACTIVE
Note that this does not disable the account. The user may still be able to login using another authentication token (e.g. an SSH key). To disable the account, administrators should use usermod --expiredate 1 (this set the account's expire date to Jan 2, 1970).
Users with a locked password are not allowed to change their password.
-n, --mindays MIN_DAYS
-r, --repository REPOSITORY
-R, --root CHROOT_DIR
-P, --prefix PREFIX_DIR
-w, --warndays WARN_DAYS
-x, --maxdays MAX_DAYS
Passing the number -1 as MAX_DAYS will remove checking a password's validity.
Password complexity checking may vary from site to site. The user is urged to select a password as complex as he or she feels comfortable with.
Users may not be able to change their password on a system if NIS is enabled and they are not logged into the NIS server.
passwd uses PAM to authenticate users and to change their passwords.
The passwd command exits with the following values:
The following web page comically (yet correctly) compares the strength of two different methods for choosing a password: "https://xkcd.com/936/"