Apropos

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man -k

$ apropos passwd
chgpasswd (8)        - update group passwords in batch mode
chpasswd (8)         - update passwords in batch mode
Crypt::SmbHash (3pm) - Perl-only implementation of lanman ...
fgetpwent_r (3)      - get passwd file entry reentrantly
getpwent_r (3)       - get passwd file entry reentrantly
gpasswd (1)          - administer /etc/group and /etc/gshadow
grub-mkpasswd-pbkdf2 (1) - generate hashed password for GRUB
htpasswd (1)         - Manage user files for basic authentication
pam_localuser (8)    - require users to be listed in /etc/passwd
passwd (1)           - change user password
passwd (1ssl)        - compute password hashes
passwd (5)           - the password file
passwd2des (3)       - RFS password encryption
SSL_CTX_set_default_passwd_cb (3ssl) - set passwd callback for encrypted PEM file handling
SSL_CTX_set_default_passwd_cb_userdata (3ssl) - set passwd callback for encrypted PEM file handling
update-passwd (8)    - safely update /etc/passwd, /etc/shadow and /etc/group