I have changed owner of /var/www/html folder to www-data,. Now I want to grunt ownership to ubuntu too.

How can I do that. I don't know which group they belong to.
I login to ssh by using ubuntu username and my FTP also working with ubuntu, but now due to this change I am facing permission issue when editing file by ftp.

Solution 1

Easiest way if only that one user ubuntu and www-data need access would be

sudo chown ubuntu:www-data <path/to/file or directory>

this gives ownership to the user ubuntu but still keeps the ownership for the group www-data.

So you control what ubuntu can do by

sudo chmod u<+|- permission> <path/to/file or directory>

and you control what www-data can do by

sudo chmod g<+|- permission> <path/to/file or directory>

or simply use complete permission masks (see example below).

Example:

User ubuntu shall be able to read(r), write(w), execute(e)
www-data shall be able read(r), execute(e) but not write(w)
other users shall be able to do none of those

sudo chmod u+rwx <path/to/file or directory>

sudo chmod g-w+rx <path/to/file or directory>

sudo chmod o-rwx <path/to/file or directory>

or using a permission masks (helpful generator)

sudo chmod 750 <path/to/file or directory>

Also interresting for your case might be the -R parameter for both commands, applying ownership and permissions recursively on the content of a folder.

For more information about options and parameters see
man chown and
man chmod

Solution 2

I would rather NOT add a user to a system functional user's group as suggested in the comments like

sudo usermod -a -G www-data ubuntu an

as this is a really dirty way of granting a user permissions because he could also fuck up things...

Instead if you want you could add a completely new group using groupadd (see man groupadd)

groupadd <group name>

add all users who shall have the perissions on the file(s) to this group

sudo usermod -a -G <group name> ubuntu

sudo usermod -a -G <group name> www-data

and now set the ownership of the file(s) to this group

sudo chgrp <group name> <path/to/file or directory>

than you set the permissions as described before but this time only for the

group (read(r), write(w), execute(e))

sudo chmod g+rwx <path/to/file or directory>

sudo chmod u-rwx <path/to/file or directory>

sudo chmod o-rwx <path/to/file or directory>

or using the mask

sudo chmod 070 <path/to/file or directory>

Note that using this solution you lose the control over the different permissions for ubuntu and www-data.