List hidden files in Linux

Answer:

When your directory contains hidden files, e.g. filename start with a dot ., ls command by default will not to show them.

To list all files, include hidden files,

E.g.

# ls -a

Add an existing user to an existing group

Answer:

Add an existing user to an existing group (supplemental group)

# sudo usermod -a -G www-data john

john will be added to the www-data group