5.6. Users and groups

The users of a GNU/Linux system normally have an associated account (defined with some of their data and preferences) along with an allocated amount of space on the disk in which they can develop their files and directories. This space is allocated to the user and may only be used by the user (unless the permissions specify otherwise).

Among the accounts associated to users, we can find different types:

A user account is normally created by specifying a name (or user identifier), a password and a personal associated directory (the account).

The information on the system's users is included in the following files:

/etc/passwd /etc/shadow /etc/group /etc/gshadow

Example of some lines of the /etc/passwd:

juan:x:1000:1000:Juan Garcia,,,:/home/juan:/bin/bash root:x:0:0:root:/root:/bin/bash

where (if the :: appear together, the box is empty):

In order to avoid this, the passwords are no longer placed in this file; only an "x" is, to indicate that they are located in another file, which can only be read by the root user, /etc/shadow, the contents of which may be something similar to the following:

juan:a1gNcs82ICst8CjVJS7ZFCVnu0N2pBcn/:12208:0:99999:7:::

where the user identifier is located, along with the encrypted password. In addition, they appear as spaces separated by ":":

In addition, the encryption codes can be more difficult, as it is now possible to use a system called md5 (it usually appears as an option when installing the system) to protect the users' passwords. We will examine some more details in the unit on security.

In /etc/group we will find the information on the user groups:

jose:x:1000:

where we have:

name-group:password-group:identifier-of-group:list-users

The list of the users in the group may or may not be present; given that this information is already in /etc/passwd, it is not usually placed in /etc/group. If it is placed there, it usually appears as a list of users separated by commas. The groups may also posses an associated password (although this is not that common), as in the case of the user, there is also a shadow file: /etc/gshadow.

Other interesting files are the ones in /etc/skel directory, which contains the files that are included in each user account when it is created. We must remember that, as we saw with the interactive shells, we could have some configuration scripts that execute when we enter or exit the account. The "skeletons", which are copied in user account when they are created, are saved in the skel directory. The administrator is usually in charge of creating adequate files for the users, providing the necessary execution paths, initialising the system's variables that are needed for the software etc.

We will now see a series of useful commands for the administration of users (we will mention their functions and perform some tests in the workshop):

With regard to the administration of users and groups, what we have mentioned here refers to the local administration of one sole machine. In systems with multiple machines that the users share, a different management system is used for the information on users. These systems, generically called network information systems, such as NIS, NIS+ or LDAP, use databases for storing the information on the users and groups, effectively using servers, where the database and other client machines are stored and where this information can be consulted. This makes it possible to have one single copy of the user data (or various synchronised copies) and makes it possible for them to enter any available machine of the set administered with these systems. At the same time, these systems incorporate additional concepts of hierarchies and/or domains/machine and resource zones, that make it possible to adequately represent the resources and their use in organisations with different organisational structures for their own personnel and internal departments.

We can check whether we are in a NIS-type environment by seeing if compat appears in the passwd line and group configuration file, /etc/nsswitch.conf, if we are working with local files, or nis or nisplus according to the system on which we are working. Generally, this does not involve any modification for the simple user, as the machines are managed transparently, more so if it is combined with files shared by NFS that makes the account available, regardless of the machine used. Most of the abovementioned commands can still be used without any problem under NIS or NIS+, in which they are equivalent, except for the command for changing the password, which, instead of passwd, we usually use yppasswd (NIS) or nispasswd (NIS+); although it is typical for the administrator to rename them to passwd, (through a link), which means that users will not notice the difference.

We will look at this and other methods for configuring the network administration units.