Good and easy way to share files on local machine

Access control lists

The straight answer is access control lists (ACLs). Yeah, you can find a counterexample, but they're good enough in practice (unlike mere group writability which requires that users think about it all the time). What they do require is that the system administrator (root) define the groups, if you want files to be shared only by a named group (root can choose to delegate, for example by accepting groups from LDAP, but that's another story).

You do need participating users to have a umask of 022. If they create non-world-readable files routinely, this scheme won't work. But if they have a restrictive umask, it's presumably because they don't want to share files anyway.

Enabling ACLs

Ubuntu doesn't enable ACLs by default, so there's a one-time admin requirement. Edit /etc/fstab using your favorite editor, and change every line corresponding to a filesystem where you want to share files: add acl to the options. (Make sure not to change any other line, and not to use an editor that wraps long lines.) Here's an example line with the acl option added:

UUID=5e1ec7ed-face-dead-beef-c011ec7ab1e5  /  ext4  errors=remount-ro,acl  0 1

For the option to take effect the first time, use a command like the following (for each filesystem):

sudo mount -o remount,acl /

Install the ACL tools from the acl package.

Setting up the shared directory

To have files shared by the group mygroup:

setfacl -m group:mygroup:rwx /path/to/shared/root
setfacl -d -m group:mygroup:rwx /path/to/shared/root

If people create files and copy them to the shared directory, the files will be world-readable (because of the umask) and anyone in the group can add and remove files (because the group is group-writable). People can't edit each others' files, but that's a good thing or you'd run into editing conflicts straight away.

If you don't have a unix group, you can add users one by one:

setfacl -m user:bob:rwx /path/to/shared/root
setfacl -d -m user:bob:rwx /path/to/shared/root

Version control

If you do want people to be able to edit files in place, you also need something to prevent editing conflicts. That's version control.

You don't need any of this to share a git repository. You know there are solutions like gitosis, so use them.

Simply do this:

mkdir /src/teamA
addgroup teamA
chgrp teamA /src/teamA
chmod g+rws /src/teamA

Now everybody in the teamA group can make everything inside /src/teamA

The magic is the sgid (set group id) bit on directory.