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I have 2 users user1 and user2 in my Ubuntu PC. user1 was the one I created during installation. I created user2 to execute a python program. Now, this program is present in the path /home/user1/programs/. So, user2 is executing a program located in the home directory of user1. During the execution, I'm unable to create files inside the output directory /home/user1/programs/Output/. So, I went ahead to check the permissions and found the following:

$ ls -l /home/user1/programs/
drwxrwxr-x 10 user1 user1 4096 Sep  6 11:30 Output
drwxrwxr-x  2 user1 user1 4096 Sep  5 17:50 source

$ cd Output

$ mkdir dummy1

$ ls -l
drwxrwxr-x 2 user1  user1  4096 Sep  6 11:30 dummy1

$ cd dummy1

$ vi a.txt

$ ls -l
-rw-rw-r-- 1 user1 user1 7 Sep  6 11:31 a.txt

$ cd ..

$ sudo mkdir dummy2

$ ls -l
drwxr-xr-x 2 root root 4096 Sep  6 12:37 dummy2

It's clear that other users like user2 don't have write permissions within the home directory of user1.

Now, I want to add the users user1 and user2 into the same group with same permissions so that user2 can get a rwx (7)? How to create such a group?

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sudo -i
addgroup mygroup
addgroup user1 mygroup
addgroup user2 mygroup
chgrp -R mygroup /home/user1/programs/Output/
chmod -R ug+rwX  /home/user1/programs/Output/
find /home/user1/programs/Output/ -type d -exec chmod g+s {} +
exit

This adds a group called mygroup, adds both users to that group, and changes the group-ownership of /home/user1/programs/Output/ (and all files & sub-directories) to mygroup.

It and then sets the permissions of the directory to read, write, execute and all files in it to allow read and write. Again, this is applied recursively to all sub-directories of ../Output/

Finally it sets the setgid bit on the directories with chmod g+s so that all files and sub-directories created underneath /home/user1/programs/Output/ will be in mygroup.

All of this needs to be done as root, hence the sudo -i to start a root shell first and the exit at the end to quit the root shell.

  • Looks neat. Trying this out now. Any possible negative side-effects if I add the entire home directory of both users (by doing steps 5 and 6 for user1 and user2)? Just being curious. – skrowten_hermit Sep 9 at 7:36
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    no side-effect, but the effect will be that user1 and user2 will have full read and write access to each other's files and sub-directories (including ~/mail, ~/.gnupg, ~/.ssh/ and everything else. actually ssh and gnupg would probably complain about permissions, by default they're fussy about perms). I wouldn't recommend it. – cas Sep 9 at 7:42
  • If a file in /home/user1/programs/Output/ has a description of drwxrwxr-x, won't adding user1 and user2 to mygroup automatically give user2 write and execute access? – skrowten_hermit Sep 9 at 10:03
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    yes, if the file is is owned by group mygroup and both those users are in that group. All the chmod + chgrp stuff in my answer is to ensure that that is the case for that directory. – cas Sep 9 at 10:30

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