Using Groups
Like Trevor said, you can make a group, and give that group read, write, and execute, permission to that folder.
To create a group:
sudo groupadd groupname
You can use anything in the place of groupname, just remember what it is!
Next, let's create a folder in /share
sudo mkdir /share
Set that folder's group to the created one, and well as set permissions
sudo chgrp groupname /share
sudo chmod 775 /share
Remeber to change groupname to the one your created!
You can see that we change the permissions of /share to 775, here's a explaination of what that means.
7 in the first position, this gives the owner of the folder, read (lists files in folder), write (create/delete files in folder), and execute (be able to cd into that folder). In our case the owner of the folder is root.
7 in the second position, this gives the folder's group, read, write, and execute permission. In our case the folder's group is groupname or whatever you set it to.
5 in the final position, this gives the the public (whoever can logon to the system), read, and execute permission. This allows them to cd into folder and list the files using ls or a similar command. If you want to prevent others from cd'ing into the folder you can use 0 instead of 5, meaning they get no permissions.
To add a user to a group, so that can use the group permissions, we can use:
sudo usermod -a -G groupname [username]
Be sure to change groupname and [username] to their correct values.
Note: a user may only be in one primary group, but multiple secondary groups. The command I provided adds another secondary group to the user specified.
Using ACLs
A alternate solutions, which I think is more complex is to use ACLs (or Access Control Lists).
This allows you to grant specific permissions to specific users without the use of groups or changing owners. For this to work the file system must be mounted with the acl option.
Preparing to use ACL
Firstly, we must check if the drive is already mounted as ACL. Because we the folder is in the /share is in the root (/) partition, we must ensure that the root partition is already mounted with the acl option.
Note: The /home is not always inside the root partition, the /home folder can be it's own partition. This also applies to other partitions.
To determine if a partition has ACL enabled, we can use the mount command.
mount -l
For me, this is on the first line.
/dev/sda5 on / type ext4 (rw,errors=remount-ro)
We can see that the partition /dev/sda5 is mounted at /. Yours may not be on the first line, like mine. It may be on the second, third, but it should be mounted at /, because that's where our /share is.
Now, we will need to determine if we have the acl option.
tune2fs -l [partition path]
From my output of mount -l you can see that my partition path is /dev/sda5, yours may be different.
From running the command we will get something similar to this:
tune2fs 1.42.9 (4-Feb-2014)
Filesystem volume name: <none>
Last mounted on: /
Filesystem UUID: 5e7b3dcf-cda0-4a74-ab0e-67c9ce8ab044
Filesystem magic number: 0xEF53
Filesystem revision #: 1 (dynamic)
Filesystem features: has_journal ext_attr resize_inode dir_index filetype needs_recovery extent flex_bg sparse_super large_file huge_file uninit_bg dir_nlink extra_isize
Filesystem flags: signed_directory_hash
Default mount options: user_xattr acl
Filesystem state: clean
~cut off to save space~
On the default mount options line, we can see that we have acl enabled. If you don't have acl enabled, you can use Google find out how to do that.
Now, like the method with groups, you will need to create the folder:
sudo mkdir /share
One last thing, we must set the folder to 700, so only root (and people granted access by ACL) can access the folder:
sudo chmod 700 /share
Now we can set the access control.
Note: setting the permissions, without using acl, and instead using chmod, will cause acl permissions to be reset.
Setting ACL permissions
It's a bit simpler now, if you want to set permission for a user, you can use
sudo setfacl -m "u:[username]:rwx" /share
This will grant [username] rwx (read, write, execute) permissions. The sudo is required only because /share is owned by root.
For a group use:
sudo setfacl -m "g:[groupname]:rwx" /share
For a other/public (only includes people who are able to logon to the system) use:
sudo setfacl -m "o:rwx" /share
Note: you can swap rwx out for others like rx for read/execute only.
Note: if you pass a -d option while using setfacl (setfacl -d -m "u:...), this will set the default ACL value for the folder. This will cause any NEW files added to the folder to automatically have the permissions you applied.
