passwd is a command on Unix, Plan 9, Inferno, and most Unix-like operating systems used to change a user's password.

The file's name originates from one of its initial functions as it contained the data used to verify passwords of user accounts.

Systems administrators can reduce the likelihood of brute-force attacks by making the list of hashed passwords unreadable by unprivileged users.

However, this would restrict access to other data in the file such as username-to-userid mappings, which would break many existing utilities and provisions.

Those values can be used to mount a brute force attack offline, testing possible passwords against the hashed passwords relatively quickly without alerting system security arrangements designed to detect an abnormal number of failed login attempts.

In 1987, the author of the original Shadow Password Suite, Julie Haugh, experienced a computer break-in and wrote the initial release of the Shadow Suite containing the login, passwd and su commands.

The original release, written for the SCO Xenix operating system, quickly got ported to other platforms.

This required users to be aware of the different methods to change passwords for different systems, and also resulted in wasteful duplication of code in the various programs that performed the same functions with different back ends.

In most implementations, there is now a single passwd command, and the control of where the password is actually changed is handled transparently to the user via pluggable authentication modules (PAMs).