What is a group?

The concept of group as it is used here is that of a POSIX group and is a user management concept from the Linux OS (and many other OSes, not just UNIX-like systems). Groups are a useful concept to control access to data or programs for groups of users at once, using so-called group permissions. Three important use cases are:

  1. Controlling access to licensed software, e.g., when one or only some research groups pay for the license
  2. Creating a shared subdirectory to collaborate with several VSC-users on a single project
  3. Controlling access to a project allocation on clusters implementing a credit system (basically all clusters at KU Leuven)

VSC groups are managed without any interaction from the system administrators. This provides a highly flexible way for users to organise themselves. Each VSC group has members and moderators:

  • A user can become a member of a group after a moderator approves it. As a regular user, you can check all groups you belong to on the VSC account management web site account.vscentrum.be.
  • A moderator can add/delete members and moderators
    • When you create a new group, you become both the first member and moderator of that group.

Warning: You should not exaggerate in creating new groups. Mounting file systems over NFS doesn't work properly if you belong to more than 32 different groups, and so far we have not found a solution. This happens when you log on to a VSC cluster at a different site.

Managing groups

Viewing the groups you belong to

You will in fact see that you always belong to at least two groups depending on the institution from which you have your VSC account.

Join an existing group

  • Go to the VSC account management web site
  • Click on "New group"
  • Fill in the name of the group
    • The name of the group will automatically begin with the first letter of the hosting institute (a for Antwerp, b for Brussels, g for Ghent, l for Leuven)
    • If the name is wrong, it will treat the request as a new group
  • In the message field, describe who you are to motivate the request, so the moderator knows who is making the request
    • Moderators will deny all unclear requests

Create new group

  • Go to the VSC account management web site
  • Click on "New group"
  • Fill in the group name
  • You will receive a confirmation email
  • After the confirmation, you are now member and moderator of the new group

Working with file and directory permissions

  • The chgrp (from change group) command is used by users on Unix-like systems to change the group associated with a computer file. General syntax:
    chgrp [options] group target1 [target2 ..]
  • The chmod command (abbreviated from change mode) can change file system modes of files and directories. The modes include permissions and special modes. General syntax:
    chmod [options] mode[,mode] file1 [file2 ...]
  • Hints:
    • To view what the permissions currently are, type:
      $ ls -l file
    • -R: Changes the modes of directories and files recursively.
    • Setting the setgid permission on a directory (chmod g+s) causes new files and subdirectories created within it to inherit its groupID, rather than the primary groupID of the user who created the file (the ownerID is never affected, only the groupID). Newly created subdirectories inherit the setgid bit. Note that setting the setgid permission on a directory only affects the groupID of new files and subdirectories created after the setgid bit is set, and is not applied to existing entities. Setting the setgid bit on existing subdirectories must be done manually, with a command such as the following:
      [user@foo]# find /path/to/directory -type d -exec chmod g+s '{}' \;

Systems