Setgid on a directory is not giving execute permission on the file within directory

Question

I have created directory as following:

root@host [~]#  mkdir mydir
root@host [~]#  ls -ld mydir
drwxrwxr-x 2 test test 4096 Mar 2 19:36 mydir
root@host [~]# 

Then I have changed the group ownership of the directory by using the 'chgrp' command, and then also added the ‘setgid’ permission.

root@host [~]#  chgrp test2 mydir/
root@host [~]#  chmod g+s mydir
root@host [~]#  ls -ld mydir/
drwxrwsr-x 2 test test2 4096 Mar 2 19:36 mydir/
root@host [~]# 

Now, when I create a file under the directory, I see the file is missing execute permission.

root@host [~]#  touch mydir/myfile3
root@host [~]#  ls -l mydir/myfile3
-rw-rw-r-- 1 test test2 0 Mar 2 19:59 mydir/myfile3
root@host [~]#

My understanding is that the file should get exactly same permission as the parent directory. And, the parent directory as rwx permission so the file should also get rwx permission.

Answer 1

The sgid bit on directories causes files created inside them to be owned by the group owning the directory, that’s all. You can see this in your example: myfile2 is owned by group test2.

The sgid bit doesn’t determine permissions on files created inside the directory.

Again, see Understanding UNIX permissions and file types for details.