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7: Working with Linux Users and Groups
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You need to control who can access a Linux system and what they can do
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To authenticate to a system, a user must supply a username and password
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Linux restores user-specific information when a user logs in, such as a home
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User home directories are created in /home by default The root user s home directory is in /root You can use the finger command to view information about a user account Every Linux user account has a unique user ID (UID) number assigned to it The root user s UID is 0 The starting UID for standard users is 1000 on some distributions and 500 on
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You can use the id command to view a user s UID You can use many different authentication methods with a Linux system For your Linux+ exam, you need to know how to use local authentication Using local authentication, user accounts are stored in /etc/passwd and
/etc/shadow
The /etc/passwd file stores user account information The /etc/shadow file stores encrypted user passwords You can use the pwck utility to verify that /etc/passwd and /etc/shadow are
synchronized
You can use the pwconv utility to copy missing users from /etc/passwd to
/etc/shadow
You can use the useradd utility to add users to a Linux system When used without any options, useradd uses the system defaults contained
in /etc/default/useradd and /etc/logindefs to create user accounts
You can use the passwd utility to set a user s password The passwd utility can also be used to check the status of a user account
Two-Minute Drill
You can use the usermod utility to modify an existing user account You can use the userdel utility to delete an existing user account By default, userdel will not remove a user s home directory unless you specify
the r option with the command
Linux groups can be used to ease administration by grouping like user
accounts together
User accounts are stored in /etc/group Some distributions store group passwords in /etc/gshadow You use the groupadd utility to add a new group to your system You use the groupmod utility to add or remove users to an existing group You use the groupdel utility to delete an existing group
Manage Ownership, Permissions, and Quotas
Ownership defines which user and group owns a particular file or directory in
the file system
You can use the ls l command to view ownership You can use the chown utility to configure user and group ownership of a file
or directory
You can use the chgrp utility to change group ownership You must be logged in as root to change user ownership You must be logged in as root or as the file/directory owner to change group
ownership
Permissions are used to define what users may or may not do with files or
directories in the file system
Linux uses the read, write, and execute permissions for files and directories Linux permissions are assigned to Owner, Group, and Others Linux permissions are additive The permissions assigned to Owner, Group, and Others constitute the file or
directory s mode
Permissions can be represented numerically: read=4, write=2, and execute=1 Summing all permissions assigned to an entity, such as Owner, allows you to
represent all assigned permissions with a single number
7: Working with Linux Users and Groups
You use the chmod utility to modify permissions Linux assigns rw rw rw permissions by default to new files and rwxrwxrwx
permissions to new directories
These permissions are too relaxed for most situations, so the umask variable is
used to subtract specific permissions from the defaults
The default value of umask is 022, which subtracts the write permission (2)
from Group and Others
You can modify the value of umask to change the default permissions assigned
upon creation
Linux also includes three default special permissions: Sticky Bit, SUID, and
SGID
You assign special permissions with chmod by adding an additional digit
before the Owner digit in the command
You can use the quota package to implement disk quotas in the file system Quotas prevent users from consuming too much disk space You can set quotas for the number of blocks a user is allowed to consume
(disk space) and the number of inodes a user may consume (number of files)
You can set hard and soft limits A user may temporarily exceed soft limits for a time you define as the grace
period
A user may not exceed a hard limit You can enter the repquota av command at the shell prompt to view a
report displaying hard and soft limits as well as current user space usage
Self Test
SELF TEST
Manage Users and Groups
1 Which of the following commands will display the UID of a user named dcoughanour when entered at the shell prompt A B C D id dcoughanour finger dcoughanour UID dcoughanhour info dcoughanour
2 Which of the following files is used to store user accounts on a Linux system that has been configured to use local authentication A B C D /etc/shadow /etc/users /etc/passwd /etc/local/accounts
3 Which of the following files is used to store user passwords on a Linux system that has been configured to use local authentication A B C D /etc/shadow /etc/users /etc/passwd /etc/local/accounts
4 Consider the following entry in /etc/passwd:
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