User administration in Linux

Add remove modify users


Only root can do that(need to login in root)


utilities such as useradd,userdel and usermod


(they are in usr/sbin)


default config files in


/etc/skel


useradd -D


by default the user will be


Group=100
Home=/home
Inactive=-1
Expire=
shell=/bin/bash
Skel=/etc/skel


after expiration the user account and the files will not be deleted


inactive period : after this amount of time after expiration everything about the user will be deleted


when creating an user 2 files are modified /etc/shadow and /etc/passwd


in the home directory the user's profile is created


in the user's home directory there are files such as


.bash_logout .bash_profile .bashrc .mozilla .emacs .zshrc


they are all user config files,they start with a (.)


in the /etc/skel directory you will find exactly same files


when creating an user the files from /etc/skel is copied in to the user's home directory


set the password of an user (for username lisa),only root can do that


passwd lisa


/etc/passwd stores info about user accounts


/etc/shadow stores the passwords in encrypted format


cat /etc/passwd


the last line would be
kennedy:x:507:507::/home/kennedy:/bin/bash


x is for password,it means password is showed in /etc/shadow file


first 507 is kennedy's userid


second 507 is kennedy's group id


however the default group id is 100,but red hat gives every user an user private group,here it's id is 507


:: -> this blank space is for an comment which is absent


to add the comment to a user


useradd -c " comment string" sourav(for user sourav)


/home/kennedy is the default home directory for kennedy


/bin/bash is the default shell for kennedy


to delete an user


the command is userdel


userdel -r kennedy


userdel -r will delete the user with it's home directory and all the files in it's home directory


also the /etc/passwd file entry for kennedy will also be deleted


usermod command can alter existing account features




usermod -c "sougata Kundu" sougata




to create multiple user's password at once


cat > password


useradd kuntal


useradd kunal


useradd kundu


ctrl + z




chpasswd < password


the passwords will be set


to see all the groups


cat /etc/group


here we will see entry such as


lisa:x:501:


the first field is the name of the group


the second field x shows the password is stored in a different file


for groups the passwords are saved in /etc/gshadow


501 is the group identification number


next will be users in the group(currentle there are none)


to add a group


to add a regular group instead of a user private group


groupadd -r experts


that will tell the system that we want this group to have a identification number under 500,over 500 numbers are reserved for user private groups


to add an user to this group


usermod -G experts lisa


we can verify in the /etc/group file that in the expert group(which has an id of 102 so it's not an user private group,now has lisa on this group)


let's add another group


groupadd -r adults


to add lisa in this group


if we use


usermod -G adults lisa


lisa will be in adults group but not in experts group anymore


to keep lisa in both groups


usermod -G experts,adulls lisa


to see if a particular user in which groups


the command is


groups lisa