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Chapter 7 |
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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
where
userid specifies 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
newbie: debian:~#
adduser newbie
Adding 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 Dewbie
Room Number []:
Work Phone []:
Home Phone []:
Other []:
Is the information correct? [y/n]
y
debian:~#
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
where
name specifies the new name and
userid specifies 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
newbie to 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
root to change a password. Because of this, users
can change their own passwords without the help of the system
administrator. The
root user, however, can change
the password associated with any user account, as you'll see
shortly. Of course,
only
root
can do so - other users can change only their own password.
The
passwd command initiates a simple dialog
that resembles the following: $
passwd
Changing 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.
As the
root user, 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
root user 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
root user.
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
where
group specifies 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
root.
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
where
group specifies 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
where
user specifies the member and
group specifies the group to which the
member is added. For example, to add the user
newbie01 to 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. -
Password
-
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
where
user specifies 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:
-
ash
-
a version of the
sh
shell that resembles the System V shell -
csh
-
the C shell, favored by many users for
interactive use -
ksh
-
the Korn shell, the third major Unix
shell -
sh
-
the Bourne shell, a precursor of BASH,
also known as the
bsh shell -
tcsh
-
an enhanced version of
csh -
zsh
-
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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| 6.7 Configuring GNOME |
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7.2 Filesystem Administration |
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