File permisions for a group

[mhoney]

I'm having trouble figuring out how to create a directory so that no matter who creates a file in the directory, as long as they belong to the group that owns the directory, there files are set to +rw for the group.

Example as root:

addgroup marketing
usermod -a -G marketing someuser
mkdir /marketing
chown :marketing /marketing
chmod g+s marketing marketing
chmod -R g+rw /marketing
su someuser
cd /marketing
touch test

it seems to me that the file test should have the permissions -rwxrw-r-- but
that isnt what I get. I get -rwxr--r-- and it's driving me crazy that I can't get this to work... PLEASE HELP

MH

[Cabhan]

The permissions that a file has when created are determined by a user's umask. The umask works as follows:

permissions = 0666 & ~umask

In plain English, the umask is an octal number subtracted from 0666 in order to determine the initial permissions of a file created by a user. As far as I know, an individual user's umask is the only way to influence the permissions of a file created by that user. You may see your umask or change it by employing the umask command.

Your code does not work because chmod -R applies the chmod to all files that exist presently in that directory. It does not apply special magic to the directory that affects future files.

Something similar to what you are trying to accomplish would be to have a cronjob run every minute and apply your chmod -R g+rw command to the directory. This is not quite the same (the file will not have the permissions from creation), but is the closest I can think of (the file will have the permissions no later than one minute after creation).

[mhoney]

It just doesnt make sense that a file can be set to +rw for a group automatically on creation. How the heck do multiple people share files on linux?

[Zeroangel]

I'm not thoroughly acquainted with group permissions, but shouldnt a command like:

chgrp -R marketing /marketing

also be applied?

I would think, that the permissions settings of new files would be inherited from the directory. Correct me if i'm wrong.

[Cabhan]

@Zeroangel

No. Files do not inherit the permissions of the directory they were created under. Using chmod -R (or any other -R command) applies the command to all files that exist in the directory at the time the command is run: they cannot affect future files.

@mhoney

A common way of sharing files is to make your files readable by other people, if not writable. That way, they can always copy the file and do what they want, submitting a patch back to the master.

Alternatively, you could create a master user for the marketing group and make his primary group the marketing group. Make his umask 0006. Now all files created by this user will be readable and writable by the marketing group only. Creating a file could be done by:

sudo -u marketingmaster touch /marketing/file
or
su marketingmaster -c touch /marketing/file

And then modified by the user.

You could also look into something called Access Control Lists (ACLs). I don't know much about them, but they allow for much finer permissions. Enabling them does require kernel modification, however.