htpasswd

Create a password file

htpasswd -c /path/to/passwd/file admin

To change the password for an existing user

htpasswd /path/to/passwd/file  USER_HERE

You can use -b option to specify the password as a command line option, this is useful for adding or updating users non-interactively.

htpasswd -b -c /path/to/passwd/file USER_HERE PASSWORD_HERE

To limit access, add the following to the Apache virtual host entry.

<Directory "/var/www/html">
    AuthType Basic
    AuthName "Restricted Content"
    AuthUserFile /path/to/passwd/file
    Require valid-user
</Direcory>

Apache