Users and Permissions

Question 1

Which file stores passwords in a salted hash

Answer 1

/etc/shadow

Question 2

What is UID

Answer 2

User id

Question 3

What is GID

Answer 3

Group id

Question 4

What are the contents of the /etc/shadow file

Answer 4

• Username
• Password as a salted hash (* or ! on the password could denote that the account has been locked possibly because of invalid login attempts)
• Date of last password change
• Days until a change is allowed
• Days before a change is required
• Days of warning before expiration
• Days between expiration and deactivation
• Expiration date
• Special flag

Question 5

What is the difference between an expired and deactivated account

Answer 5

An expired account will force you to change your password on the next login before you can complete the login.

A deactivated account will require an admin to reactivate the account before you can login again.

Question 6

Create a new group

Answer 6

newgroup

Question 7

How to show current user

Answer 7

whoami

Question 8

How to show all ids for the current user (user id, group ids)

Answer 8

id

Question 9

How to show all ids for the current user (user id, group ids) or a specific user

Answer 9

id
when entered alone it will display info for the current user

id username will show info for the specified user account

Question 10

Create a new user

Answer 10

sudo adduser username

Question 11

Create a new user

Answer 11

sudo adduser username
this works like a wizard and allows to enter more info
OR
sudo useradd -s /bin/bash -d /home/jason -m -G groupName userName
sudo passwd userName

Question 12

Search for users on the system

Answer 12

grep ‘^username’ -ne /etc/passwd

will find a line in the passwd file that starts with username

Question 13

Change/Set password for a user

Answer 13

sudo passwd username

Question 14

See password info for a user

Answer 14

sudo passwd -S username

or

sudo chage -l username

Question 15

Which command is used to set password policies for a user

Answer 15

sudo chage username

use man chage to see details

Question 16

change username

Answer 16

sudo usermod -l userNewName userOldName

Question 17

change group

Answer 17

sudo groupmod -n groupNewName groupOldName

Question 18

remove user

Answer 18

sudo deluser –remove-home username

sudo userdel -r username

Question 19

check log of users deleted

Answer 19

cat /var/log/auth.log | tail -15
OR
tail -15 /var/log/auth.log
OR 
grep 'userdel' /var/log/auth.log

tail -15 just limits the output to the most recent items

Question 20

how to add groups

Answer 20

sudo groupadd groupName

Question 21

how to add user

Answer 21

sudo useradd -m tim -p password

• m makes home directory match username
• p is for password

Question 22

add user to group

Answer 22

sudo usermod -a -G groupname username

Question 23

search for a group

Answer 23

grep groupName /etc/group

Question 24

rename a group

Answer 24

sudo groupmod -n newGroupName oldGroupName

Question 25

how to disable user

Answer 25

sudo passwd -l userName

Question 26

how to enable user

Answer 26

sudo passwd userName

Question 27

command to change owner of file or directory

Answer 27

sudo chown userName tim

must use root because you must have access to the current owner and the new owner

Question 28

command to change group of file or directory

Answer 28

sudo chown userName:groupName fileOrDirName
sudo chown :groupName fileOrDirName

Need more info for this below
sudo chgrp
can do it without sudo (root) if you are in both groups

Question 29

change the default permissions of created files and directories for a user

Answer 29

umask

default files are created with 666 permissions
default directories are created with 777 permissions

Question 30

Which command option tells Linux to run the program with the permissions of whoever owns the file rather than with the permissions of the user who runs the program.

Answer 30

suid

Question 31

What are two special permission bits exist, similar to the sticky bit?

Answer 31

SUID and GUID

The SUID and SGID are special permission bits. SUID (Set owner User ID up on execution) is a special type of file permissions given to a file. Normally in Linux/Unix when a program runs, it inherits access permissions from the logged in user. SUID is defined as giving temporary permissions to a user to run a program/file with the permissions of the file owner rather that the user who runs it. SGID permission is similar to the SUID permission, the only difference is – when the script or command with SGID on is run, it runs as if it were a member of the same group in which the file is a member.