Passwd
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- The title of this article is incorrect because of technical limitations. The correct title is passwd.
passwd is a tool on most Unix and Linux systems used to change a user's password. A key derivation function is used to encrypt the new password. Only the encrypted version is stored.
passwd generally works by manipulating the local username and password storage file (typically /etc/passwd, or /etc/shadow when shadow passwords are in use), which means that the changed password applies only for the computer on which the command was run. If PAM is in use, however, as it can be on Linux, Solaris, Mac OS X, and FreeBSD, then the passwd command can be used to change the password of whichever authentication mechanism is in use at the time, be that Sun's NIS, Kerberos, LDAP, or any other scheme for which there is a PAM module.
Prior to the advent of PAM, it was necessary to have different commands to change passwords in different authentication schemes. For example, the command to change a NIS password was yppasswd. This required users to be trained on different methods to change passwords for different systems, and also the wasteful duplication of code in the various programs that performed the same functions with different backends.
passwd can also be used as a noun, to refer generically to any password. It is most often used to refer to passwords on UNIX systems, as its etymology is owed to the command mentioned above.