Creating a shared directory for a group of users

TLDR: the following command creates a group and adds two users. We, then, link the group to the directory and set permissions. Users (userone/usertwo) now will have access to read/write/execute permission to the directory. Newly created folder and files will be assigned to the same parent group.

groupadd sharedgroup
usermod -a -G sharedgroup userone
usermod -a -G sharedgroup usertwo

mkdir shareddir
chgrp -R sharedgroup ./shareddir
chmod -R g+rwx  ./shareddir/
chmod g+s shareddir

Notes:

• The best way to get permission is to assign permissions to groups of users instead of to individuals.
• Permissions are assigned in World Permissions, Group Permissions, and Owner Permissions.
• The three numbers that are shown in the ls -l output are the owner’s permissions, group permissions, and world permissions respectively, in that order.
• You must be the owner of blob to change its permissions. Enter ls -la to see who has file permissions.
• Directory Permission sets default permission for the files and folders created in the directory.

Setting directory permissions

The command chmod <permission> <directory>is used to set permissions for the files and folders. For instance to set a file permission to be readable by others but only modifiable by the owner of the file, you could issue:

chmod 755 myfile.txt

Few common permission codes from Wikipedia

Symbolic    Numeric Description
----------	0000	no permissions
-rwx------	0700	read, write, & execute only for owner
-rwxrwx---	0770	read, write, & execute for owner and group
-rwxrwxrwx	0777	read, write, & execute for owner, group and others
---x--x--x	0111	execute
--w--w--w-	0222	write
--wx-wx-wx	0333	write & execute
-r--r--r--	0444	read
-r-xr-xr-x	0555	read & execute
-rw-rw-rw-	0666	read & write
-rwxr-----	0740	owner can read, write, & execute; group can only read; others have no permissions

A POSIX (Portable Operating System Interface for Unix) file permission model is used, with the first digit representing file type, and permissions for read (4), write (2), and execute (1). Read more