Learning Debian GNU/LinuxBy Bill McCarty
1st Edition September 1999
1-56592-705-2, Order Number: 7052
360 pages, $34.95 , Includes CD-ROM
This chapter equips you to perform common system administration tasks. By using a few simple commands and a text editor, you can override many configuration choices made during system installation. You'll learn how to add, delete, and modify user accounts, and how to add and delete groups and change their membership. You'll learn how to configure swap space and how to cause Linux to automatically mount filesystems. This chapter doesn't deal with the configuration and administration of network facilities, such as servers. Those topics are covered in Chapters 10, 11, and 12.
In this section, you'll learn how to perform common administrative tasks affecting users and groups. Most system administration tasks require that you login as
root. Throughout this section and subsequent section, you should assume that you must be logged in as
root, unless directed otherwise.
To create a user account, you use the adduser command, which has the form:adduser userid
useridspecifies the name of the user account that you want to create. The command prompts you for the information needed to create the account.
Here's a typical example of using the command, which creates a user account named
adduser newbieAdding user newbie... Adding new group newbie (1001). Adding new user newbie (1001) with group newbie. Creating home directory /home/newbie. Copying files from /etc/skel Changing password for newbie Enter the new password (minimum of 5, maximum of 8 characters) Please use a combination of upper and lower case letters and numbers. Re-enter new password: Password changed. Changing the user information for newbie Enter the new value, or press return for the default Full Name :
Newbie DewbieRoom Number : Work Phone : Home Phone : Other : Is the information correct? [y/n]
Notice that the lines where the password was typed were overwritten by the subsequent lines. Moreover, for security, passwords are not echoed to the console as they are typed.
Notice also that several of the information fields were omitted - for example, Room Number. You can specify such information if you think it may be useful, but the system makes no use of the information and doesn't require you to provide it.
The similarly named useradd command also creates a user account, but does not prompt you for the password or other information.
When the command establishes a user account, it creates a home directory for the user. In the previous example, the command would have created the directory /home/newbie. It also places several configuration files in the home directory, copying them from the directory /etc/skel. These files generally have names beginning with the dot (.) character, so they are hidden from an ordinary ls command. Use the -a argument of ls to list the names of the files. The files are generally ordinary text files, which you can view with a text editor, such as ae. By modifying the contents of such files, you can control the operation of the associated application. For example, the .bashrc file controls the operation of the BASH shell, which you'll learn more about in Chapter 13, Conquering the BASH Shell.
You can change the name associated with a user account, by using the chfn command:chfn -f name userid
namespecifies the new name and
useridspecifies the account to be modified. If the name contains spaces or other special characters, it should be enclosed in double quotes ("). For example, to change the name associated with the account
newbieto Dewbie Newbie, you would enter the following command:chfn -f "Dewbie Newbie" newbie
From time to time, you should change your password, making it more difficult for others to break into your system. As system administrator, you may sometimes need to change the password associated with a user's account. For instance, some users have a bad habit of forgetting their password. They'll come to you, the system administrator, seeking help in accessing their account.
To change a password, you use the passwd command. To change your own password, enter a command like this one:passwd
This command changes the password associated with the current user account. You don't have to be logged in as
rootto change a password. Because of this, users can change their own passwords without the help of the system administrator. The
rootuser, however, can change the password associated with any user account, as you'll see shortly. Of course, only
rootcan do so - other users can change only their own password.
The passwd command initiates a simple dialog that resembles the following:$
passwdChanging password for newbie Old password: Enter the new password (minimum of 5, maximum of 8 characters) Please use a combination of upper and lower case letters and numbers. New password: Re-enter new password: Password changed.
Notice the restrictions governing the choice of password, which are designed to prohibit passwords that might be easily guessed. If you choose a password that violates these restrictions, the command will refuse the password, prompting you for another.
rootuser, you can change the password associated with any user account. The system doesn't ask you for the current password, it immediately prompts for the new password:debian:~# passwd newbie Changing password for newbie Enter the new password (minimum of 5, maximum of 8 characters) Please use a combination of upper and lower case letters and numbers. New password: Re-enter new password: Password changed.
Information on users is stored in the file /etc/passwd, which you can view using a text editor. Any user can read this file, though only the
rootuser can modify it. If you selected shadow passwords, passwords are encrypted and stored in the file /etc/shadow, which can be read only by the
Recall from Chapter 4, Issuing Linux Commands that Linux uses groups to define a set of related user accounts that can share access to a file or directory. You probably won't often find it necessary to configure group definitions, particularly if you use your system as a desktop system rather than a server. However, when you wish, you create and delete groups and modify their membership lists.
To create a new group, use the groupadd command:groupadd group
groupspecifies the name of the group to be added. Groups are stored in the file /etc/group, which can be read by any user but modified only by
For example, to add a group named
newbies, you would enter the following command:groupadd newbies
To delete a group, user the groupdel command:groupdel group
groupspecifies the name of the group to be deleted. For example, to delete the group named
newbies, you would enter the following command:groupdel newbies
To add a member to a group, you use a special form of the adduser command:adduser user group
userspecifies the member and
groupspecifies the group to which the member is added. For example, to add the user
newbie01to the group
newbies, you would enter the following command:adduser newbie01 newbies
Unfortunately, no command removes a user from a specified group. The easiest way to remove a member from a group is by editing the /etc/group file. Here's an excerpt from a typical /etc/group file:users:x:100: nogroup:x:65534: bmccarty:x:1000: newbies:x:1002:newbie01,newbie02,newbie03
Each line in the file describes a single group and has the same form as other lines, consisting of a series of fields separated by colons (:). The fields are:
- Group name
The name of the group.
The encrypted password associated with the group. This field is not generally used, containing an x instead.
- Group ID
The unique numeric ID associated with the group.
- Member list
A list of user accounts, with a comma (,) separating each user account from the next.
To remove a member from a group, first create a backup copy of the /etc/group file:cp /etc/group /etc/group.SAVE
The backup can prove helpful if you modify the file incorrectly. Next, open the /etc/group file in a text editor. Locate the line that describes the group and delete the user name and the following comma, if any. Save the file, exit the editor, and check your work.
To delete a user account, use the userdel command:userdel user
userspecifies the account to be deleted. If you want to delete the user's home directory, its files and subdirectories, use this form of the command:userdel -r user
WARNING: Because deleted files can't generally be recovered, you should backup potentially useful files before deleting a user account.
The BASH shell, which you met in Chapter 4, is the most popular, but not the only Linux shell. Others include:
a version of the sh shell that resembles the System V shell
the C shell, favored by many users for interactive use
the Korn shell, the third major Unix shell
the Bourne shell, a precursor of BASH, also known as the bsh shell
an enhanced version of csh
the Z shell, a feature-packed version of the Korn shell
When you create a new user, the system automatically assigns the shell (command interpreter) that Linux presents to the user when the user logs in. Debian GNU/Linux assigns the BASH shell, as specified by the file /etc/adduser.conf. However, you can assign another shell, if you prefer. The shell must be on the list of available shells, which resides in the file /etc/shells.
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