UNIX Groups

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About UNIX groups

You can use UNIX groups to share files with a small number of University of Delaware users. Each person who uses the central UNIX servers is associated with a list containing at least one group, and each file or directory on the central UNIX servers is associated with one group. This is usually referred to as group membership and group ownership, respectively. That is, users are in groups and files are owned by a group.

You do not need to do anything to be in a group—this is all managed for you. All users with an email account are in group 4000. Many students, registered for class, are in a group created specifically for their class section. Researchers using Strauss for computing work are in a group created for their computing projects. At the University of Delaware we also use UNIX groups for accounting purposes, and that is why the group names are usually four-digit account project codes. Each accounting project has a project director who is responsible for adding or removing members from the group. The project director is an instructor for a class project, a principal investigator for a sponsored project, or the university staff member who originally requested the project. The project director manages the members of the project by contacting Access.

All files or directories are owned by the person who creates them. In addition, each file or directory is owned by a group. Managing group ownership of files and directories requires some action by the person who created them. It is important to have group ownership correct if you want to share files with your group. Group ownership does not imply group access. You must set the file access permissions so your group can use the files. You can set permissions to restrict the type of access group members have to your directories and files. You can use different UNIX groups to share files with separate sets of users.

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UNIX group membership

Users are organized into groups. Every user is in at least one group and may be in other groups. Group membership gives you special access to files and directories that are permitted to that group.

You are in a primary group and may be in several secondary groups. You are said to be in a group if the group name is in your list of groups. You do not have to be logged in to be in a group. When you are logged in, you are assigned a group which is called your current group. This is also termed "being in a group," but it is better to say "your shell is assigned to the group." When you first log in, you are assigned your primary group, which is also called your default group. You can change your current group, i.e., start a shell with a secondary group as the current group, with the newgrp command. You can change your primary group, i.e., set a default group for your next login, from the UD Network page. You can see your group list or the group list of any user with the groups command. For example:

strauss<1>% groups dnairn anita
dnairn : 1864 0123 0191 0217 0361 0363 0379 0380 0400 0583 4000
anita : 1864 0123 0388 0400 0583 4000

lists all the groups for dnairn and anita. The first group is the primary group, and the remaining groups are in alphabetical order. If you just type groups, you will see the names of your groups.

Note: Currently, the UNIX systems are configured to allow only 16 total groups in the group list. If you see exactly 16 projects in your list, then you may be in a project, but not in the UNIX group for that project.

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Group ownership of files and directories

Every file and directory has a username and a groupname associated with it. The username is the owner, and the groupname owns the file or directory. A directory is a collection of files and possibly other sub-directories. There are commands for managing group ownership for both directories and files. In the example commands given in this document we use filename to indicate the name of a file, but in most cases you can use the same command with the name of a directory.

The long format of the listing command shows the permission modes, the owner, and the group for both files and directories. Use the ls -dl filename command to get a one-line listing for a single file or directory. The command ll (or ls -l ) will list all the files and directories in your current directory. The ones beginning with a "d" are directories.

When a file or directory is first created, it takes as its group the current group of your shell. This is the default group for all login shells, but you can start another shell for any group with the command newgrp project. If you are going to create files for a secondary group then it easier to create all these files from a shell started with the newgrp command.

If you want to change the group associated with a file or directory that already exists, use the command chgrp project filename. You must be the owner of the file filename, and you must be a member of the group project to make the change. If the long listing shows a file that is not owned by the proper group, you must contact the owner of the file and get them to change the group.

In many cases, the group ownership does not matter, but if you want to share a file with a group, then it is important that you get the ownership correct. Otherwise, you may be inviting all users to put their large files in your directory.

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Group permissions for files and directories

Just setting up a file to be owned by a group does not give your group any access to the file. You grant or limit access by setting permission modes. You can use the -dl command to see the permission modes (a set of 10 letters or dashes in the long listing of a file or directory). The -dl option on the ls command will list the information for the directory or file in long format. Without the d, all the files in the directory would be listed instead of just the directory you asked for. For example, to get a long listing for a directory with the name "kneeland," you would type:
<2>% ls -dl kneeland
drwxr-x--- 3 dnairn 0217 512 Aug 14 15:14 kneeland

The first set of characters are the mode, the following number is a count, the user name is the owner, and the 4 digit account code is the group:

mode: drwxr-x---
The mode begins with a d so it is a directory. The owner, dnairn, has permission mode rwx which is full access. Any other user in group 0217 has permission mode r-x, which gives browsing access (can read and search, but does not have permission to add, rename, or delete files in the directory). Every other user that is not dnairn and not in group 0217 has permission mode ---, which is no access.
count: 3
There are three files in this directory. The count is always one if you are listing a file.
username: dnairn
The user with login name dnairn is the owner of the file. The owner will have permission modes according the the first three codes after the d. The owner always can change permission modes with the chmod command.
groupname: 0217
The directory is said to be owned by this group. Any user in group 0217, except dnairn, will have permissions granted according to the middle three codes in the permission modes.
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Some UNIX commands for working with groups

Command Description Example
chdgrp List groups with title and remaining balance
chdgrp
groups See groups to which you belong with primary group first groups
id See current group as part of your id id
newgrp Start a shell in a different group newgrp 1234
chmod Change permissions for directories and files chmod g+rwx myfile
chgrp Change group ownership of directories and files chgrp 1234 myfile
ls List file permissions ls -l
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Troubleshooting

You can use a UNIX group to share an unlimited number of files on an ongoing basis with others who have their own central UNIX account and are members of the same UNIX group.

One of the most common mistakes in sharing files on a UNIX system is to forget to set file permissions or to set them incorrectly. If permissions are not set correctly then a user will see the following message or a similar one when they try to access your directory or files:

permission denied

  1. Make sure you have a proper group for sharing. You must have a group that both of you are in but not group 4000 since every user with an email account is in group 4000. You can check this with the command groups $USER username where the second username is the UDelNet ID of the person who got the "permission denied" message. You must pick a group that is common to both lists. For example I want to share with the user anita:
    <1>% groups $USER anita
    dnairn : 1864 0123 0191 0217 0361 0363 0379 0380 0400 0583 4000
    anita : 1864 0123 0388 0400 0583 4000
  2. Project code 1864 is a good group name to choose.

  3. Check to make sure the correct group owns the file with the ls -dl filename command. You should see the project number in the long formatted list as the group name:
  4. <2>% ls -dl myfile
    -rw-r----- 1 dnairn 1864 0 Dec 21 15:09 myfile
  5. Check to make sure the r code appears in the middle three permission modes (above). If this is not correct, type:
  6. chmod g+r myfile
  7. Finally, check to make sure every directory above your current directory has the x permission in all three locations. This is called "execute permissions for all," or symbolically "a+x." You can use the . as the current directory and .. for the parent directory to list several levels:
  8. <2>% ls -dl . .. ../.. ../../..
    drwxrwsr-x 2 dnairn 1864 512 Oct 16 10:42 .
    drwxrwsr-t 3 dnairn 1864 512 Oct 16 10:26 ..
    drwxr-xr-x 84 dnairn 1864 6656 Dec 21 11:07 ../..
    drwxr-xr-x 198 root root 9216 Aug 22 04:10 ../../..

Another common problem is to set file permissions for existing files, but to neglect to set permissions for newly created files. By default, others cannot access your files. You must give explicit permissions to each file when it is created.

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