I have two user accounts set up on my mac. User `drew` I use for most things, but if I'm making a screencast I'll switch to the `demo` user. I know that the `demo` user has a clean desktop, and the font size is larger than usual in my terminal and text editors, making everything a bit more legible when capturing the screen. When I record a screencast as the `demo` user, I save the file to `/Users/Shared/screencasts`. As I understand it, the `/Users/Shared` directory is supposed to be accessible to all user accounts on the mac. If I created and saved a screenflow document as the `demo` user, I should be able to read and write that file when logged in as user `drew`.

That was the theory, but it didn't always work out that well in practice. I would occasionally find that a directory was only writable by one user or the other. Perhaps I'd open a screenflow document as user `drew` and attempt to export the video to the same directory, only to find that the directory was owned by `demo`, meaning that I couldn't create new files in that directory when logged in as user `drew`.

Ideally, I'd like to be able to set the permissions on the `/Users/Shared/screencasts` directory, and have all files and directories created beneath it inherit those permissions. This seems to have done the trick:

    $ chown -R demo:staff /Users/Shared/screencasts
    $ chmod -R +a "group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit" /Users/Shared/screencasts

In the first command, the owner is set to `demo` and the group is set to `staff` for the `screencasts` directory, as well as all files and subdirectories contained below it. Both `demo` and `drew` users are already in the `staff` group. If I can make it so that the `staff` group has read, write, and execute (where apt) rights for all files/directories, then I'll have what I need.

The second command assigns an access control list (ACL) for the `staff` group on the `screencasts` directory (and all of its children). I've included a lot of flags, but the ones to note are `file_inherit` and `directory_inherit`. If I create a file or directory inside of the `screencasts` directory, then it will inherit the same ACL flags.

To inspect the ACL, you can run the `ls` command with the `-e` flag. For example:

    $ ls -ale /Users/Shared/screencasts/
    total 0
    drwxr-xr-x+  2 demo  staff   68 19 Feb 18:56 .
     0: group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
    drwxr-xrwx  11 root  wheel  374 19 Feb 18:56 ..

As well as seeing the usual line of info about each listing, there's an extra row giving details about the ACL. The `screencasts` directory has `r-x` mode set for the group, so you might think that users in the `staff` group would be unable to add files and directories to the `screencasts` directory. But the ACL includes the flags: `add_file` and `add_subdirectory`, which means that users in the `staff` group *can* do those things.

I could demonstrate as user `drew`:

    $ whoami
    drew
    $ cd /Users/Shared/screencasts/
    $ mkdir -p foo
    $ touch foo/bar
    $ ls -ale foo
    total 0
    drwxr-xr-x+ 4 drew  staff  136 19 Feb 23:10 .
     0: group:staff inherited allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
    drwxr-xr-x+ 4 demo  staff  136 19 Feb 19:38 ..
     0: group:staff allow list,add_file,search,add_subdirectory,delete_child,readattr,writeattr,readextattr,writeextattr,readsecurity,file_inherit,directory_inherit
    -rw-r--r--+ 1 drew  staff    0 19 Feb 23:10 bar
     0: group:staff inherited allow read,write,execute,append,readattr,writeattr,readextattr,writeextattr,readsecurity

The directory `foo` and the file `bar` are both owned by user `drew`, but they're assigned to the group `staff`. And since the `demo` user is in the group `staff`, it gets all of the privileges listed in the ACL.