1. Chapter 2 User Management
CSNB113 SYSTEM ADMINISTRATION
College of Information Technology
Universiti Tenaga Nasional (UNITEN)

2. Objectives Examine /etc/group and /etc/passwd
Create, modify, and delete user accounts with useradd, usermod, and userdel
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3. Types of account in Unix
Root account
This is also called superuser and
Would have complete and unfettered control of the system
Can run any commands without any restriction.
This user should be assumed as a system administrator
System accounts
Needed for the operation of system-specific components for example mail accounts and the sshd accounts.
Usually needed for some specific function on your system, and any modifications to them could adversely affect the system
User accounts
Provide interactive access to the system for users and groups of users.
General users are typically assigned to these accounts and have limited access to critical system files and directories.
Unix supports a concept of Group Account(groups a number of accounts)
Every account would be a part of another group account.
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4. User ID (derives from actual name of the user)
Overview
Allocate
System Administrator
User Account
User ID (derives from actual name of the user)
Consists
/etc/group
/etc/passwd
Where is every user is commonly represented?
/etc/gshadow
/etc/shadow
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5. /etc/group Contains the group information for each account
Group database is maintained in /etc/group
Contains GID; number and name
A group compromises one or more members having a separate set of privileges
A user has one primary group, AND may have one or more supplementary group
All the default groups are system account specific groups and it is not recommended to use them for ordinary accounts
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6. root::0:root /etc/group Run command : cat /etc/group Field 1: name GID
Field 2: blank or x  hardly used today
Field 3: numeric GID
Field 4: list of user for which this is their supplementary group
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7. /etc/group
root user’s supplementary group
root::0:root
Line1
staff::1:
Line2
bin::2:root,bin,daemon
Line3
sys::3:root,bin,sys,adm
Line4
lp::8:root,lp,adm
Line5
Line 1: num. GID = 0, name GID = root, supplementary group for root user
Line 2: num. GID = 1, name GID = staff, no user has staff as supplementary group.
Line 3: num. GID = 2, name GID = bin, supplementary group for root, bin and daemon user
Line 4 & 5?
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8. /etc/shadow & /etc/gshadow
/etc/shadow − Holds the encrypted password of the corresponding account. Not all the systems support this file.
/etc/gshadow − This file contains secure group account information.
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9. /etc/shadow
For every line in /etc/passwd, there’s a corresponding entry in /etc/shadow
sn010101:2u6VExtjjXHFk:12285::::::
Password
Encryption
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10. /etc/passwd Store all user information except password encryption
/etc/passwd and /etc/shadow – use by login and passwd programs – for login authentication
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11. /etc/passwd Numeric UID Comment or GCOS Login shell Username
sn010101:x:210:241:Surizal Nazeri:/home/sn010101:bin/ksh
Password field
(not used)
Numeric
GID
Home Directory
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12. /etc/passwd Username: name use to log on
sn010101:x:210:241:Surizal Nazeri:/home/sn010101:bin/ksh
Username: name use to log on
Password: no longer store user password. Replaced by x
UID: numerical user identification (unique)
GID: user’s numerical group identification
Comment or GCOS: user details
Home directory: default initial location when logging in
Login shell: the first program executed after logging in
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13. Adding a User useradd command: add user to the system
Also creates user’s mailbox – set MAIL variable to point to /var/mail or /var/spool/mail
# useradd –u 210 –g dba –c “Surizal Nazeri” \
–d /home/sn –s /bin/ksh –m sn010101
Options:
-u: UID
-g: GID
-c: Comment or GCOS
-d: default initial location when logging in
-s: Login shell
-m: ensure create home directory
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14. Adding a group groupadd command: add group to the system
# groupadd -g 123 mygroup
Options:
-g: GID
-o: This option permits to add group with non-unique GID
-r: This flag instructs groupadd to add a system account
-f: This option causes to just exit with success status, if the specified group already exists.
g: If the specified GID already exists, other (unique) GID is chosen
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15. Modifying Users
usermod command: modify some of the parameters set with useradd – for example login shell (/bin/bash)
Same arguments as the useradd command, plus the -l argument, which allows you to change the account name
# usermod -d /home/sn m -l sn sn020202
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16. Modifying group To modify a group, use the groupmod syntax
Here is how you will change the mygroup GID to 234
# groupmod -n new_groupname old_groupname
# groupmod -g 234 mygroup
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17. Removing Users
userdel command: remove user from the system including data in /etc/passwd and /etc/group
Remove user
Remove user together with their home directory
# userdel sn010101
# userdel –r sn010101
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18. Removing group
Delete an existing group, all you need is the groupdel command and the group name
# groupdel mygroup
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19. References
Das, S. (2012). Your UNIX/LINUX The Ultimate Guide: Third Edition. McGraw-Hill
Hahn, H. (2008). Harley Hahn's Guide to Unix and Linux. California: McGraw-Hill Higher Education
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