Sharing a folder for different users to work on files on a CentOS/RHEL Linux machine

Task: We have a group of people who need to work on files in a shared directory. We need to set permissions for the shared folder and avoiding file permissions conflict. 

# mkdir /opt/bp-project
# groupadd bp-project
# chgrp bp-project /opt/bp-project
# chmod 2775 /opt/bp-project

Now all members of the bp-project group can create and edit files in /opt/bp-project/. Now the root or other admin users should not go ahead and change file permissions every time the users create new files. 

As you see, the group permission in changed from rwx to rws by using 2775 permission on our file. "s" is a special permission flag indicates the setgid. It also can represent setuid if it shows in the file permission section.  

setuid is usable just for executable files, when we set such a permission on an executable file it runs as the user who owns the file (instead of the user who invoked the executable file).

Note: You can put setuid flag on not executable files but it will be showed as S. The capital S informs you that this setting is probably wrong because the setuid bit is useless if the file is not executable.

Octal digit 4 represents setuid and 2 is for setgid so in the above screenshot, abc.txt file has 4744 and the bp-project directory has 2775. 

Note: If you set setuid for a directory it will be ignored by Linux. 

For more information about setuid consult Wikipedia entry.