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-l This option is used to lock the specified account and it is
available to root only. The locking is performed by rendering
the encrypted password into an invalid string (by prefixing the
encrypted string with an !).
-u This is the reverse of the -l option - it will unlock the
account password by removing the ! prefix. This option is avail-
able to root only. By default passwd will refuse to create a
passwordless account (it will not unlock an account that has
only "!" as a password). The force option -f will override this
protection.