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User Management

 
User management :

Whenever user is added to system, information regarding the same is stored in
three main files which are
 

/etc/passwd  :-  Account information
/etc/shadow  :-  stores encrypted password
/etc/group   :-  stores groups information

 
Lets get practical and get started with basi command for adding user
 
useradd : adds user to the system, entry of the account is made in the 
         /etc/passwd file and password for the account is stored in password /etc/shadow
 
Example :
 
 
Lets look how the entry is stored in /etc/passwd file
 
bash# cat /etc/passwd
 
root:x:0:1:Super-User:/:/sbin/sh
daemon:x:1:1::/:
bin:x:2:2::/usr/bin:
sys:x:3:3::/:
adm:x:4:4:Admin:/var/adm:
lp:x:71:8:Line Printer Admin:/usr/spool/lp:
uucp:x:5:5:uucp Admin:/usr/lib/uucp:
nuucp:x:9:9:uucp Admin:/var/spool/uucppublic:/usr/lib/uucp/uucico
smmsp:x:25:25:SendMail Message Submission Program:/:
listen:x:37:4:Network Admin:/usr/net/nls:
nobody:x:60001:60001:Nobody:/:
noaccess:x:60002:60002:No Access User:/:
nobody4:x:65534:65534:SunOS 4.x Nobody:/:
 
Each of those file are
loginname:x:UID:GID:comment:home_directory:login_shell
 
where x  specfied the password is encrypted and stored in /etc/shadow file
UID is the user-id of the user, GID is the group id
 
Note :
account file i.e /etc/passwd file can be read by anyone
however /etc/shadow can be read by only root
 
Now the user has been created you could create  password for it by using 'passwd command
bash#  passwd user1
after that it prompts for password, which is then encrypted and stored in /etc/shadow
 
If we were to modify setting for existing users, we have following commands
 
useradd  : Adds a new user account to the local system
usermod  : Modifies a user’s account on the local system
userdel  : Deletes a user’s account from the local system
 
 
Groups
 
/etc/group  : Stores the information regarding the groups configured on the system.
 

# cat /etc/group
root::0:root
other::1:
bin::2:root,bin,daemon
sys::3:root,bin,sys,adm
adm::4:root,adm,daemon
uucp::5:root,uucp
mail::6:root
tty::7:root,tty,adm
lp::8:root,lp,adm
nuucp::9:root,nuucp
staff::10:
daemon::12:root,daemon
sysadmin::14:lister,torey
nobody::60001:
noaccess::60002:
nogroup::65534:
#

Lets look at each field

<group name>:<optional password>:<GID>:<comma delimited members >
 
Commands
 
groupadd : Adds (creates) a new group account on the system
groupmod : Modifies a group account on the system
groupdel : Deletes a group account from the system
 

 
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