Linux (and therefore Ubuntu) has file permissions on each file and directory for the owner, group and everyone else. Those permissions determine if the file can be viewed, executed or edited.

Only the owner of a file or directory (or a privileged user, root for example) may change its mode.

Ownership of a file

To change the ownership of the file or directory: chown new_owner_username directory

chown john public_html

to change the ownership of directory (and all the files and folders in the directory) and also the group: chown -R new_owner_username:new_groupname directory

chown -R john:developers public_html

to change the ownership of all the files in the current directory and also the group: chown -R new_owner_username:new_groupname *

chown john:developers *

File permissions

The easiest way to set Linux file permissions is using a 3 digit sequence. The first digit designates owner permission; the second, the group permission; and the third, everyone else’s permission.

Read = 4
Write = 2
Execute = 1

The digit is the sum of those. So if you want to grant only read permission you use 4; read and execute 5; read, write and execute = 7.

chmod 775 index.html

That will set the permissions on index.html so the owner, and a user in the group specified can read, write and execute the file and everyone else can read and execute.

chmod -R 755 public_html

That will set the permissions on files and directories (recursively through all subdirectories) so the owner can read, write and execute; members of the group and everyone else can read and execute (but not write).

ls - l

That will give you a list of files and directories, in a directory, with the owner and group settings and the permissions for all 3 (those 2 and everyone else), which will look something like:

-rw-r--r-- 1 root developers   397 2008-05-25 20:33 index.html
-rw-r--r-- 1 mary developers  9177 2010-05-02 22:18 unix_file_permissions.html
...

The lines start with the permissions for the owner, group and then everyone else. There are 9 total characters, 3 for each. Taking the top line above:

rw-r--r--
rw-  (means the owner has read and write permission but not execute)
r--  (means the group has only read permission)
r--  (means everyone else has only read permission)

The next column tells you the number of hard links to the file or directory. Then column tells you the owner, then the group. Then the byte size of the file, the date it was last change and then the file name.

root
means the username of this file is named root

developers
group (means those users in the group named developers have the group permissions indicated)

Related: Ubuntu command line interface syntax examples