Week 8 - Shell Scripts Part 2

Question 1

Loops

Answer 1

Perform a set of commands repeatedly

Looping statements:
while statement
- interpreter continues executing
the code in the while loop portion of the script as long as the condition is true

Question 2

The while loop

Answer 2

– Repeats commands between
do and done statements
– As long as the tested condition is
true
* When the command after the
while statement returns an exit status code greater than 0
– while statement fails
– Program executes commands after
done statement

Question 3

Basic Arithmetic Expression

Answer 3

Arithmetic expression should be place inside
double parenthesis: (( expression ))

Question 4

The for Loop

Answer 4

• Repeats the commands between
  do and done a specified number of times

– Each time the script carries out the commands in the loop, a new value is given to a variable

– Assign this value in the command with positional parameters

Question 5

Shell scripts loops used

Answer 5

While and For loops are used
Loops are used to repeat actions on different variables

Question 6

Viewing file permissions command

Answer 6

> ls -l

Question 7

Permissions User

Answer 7

The file owner

Question 8

Permissions Group

Answer 8

A group of users; users are divided into groups to facilitate administrative tasks.

• Each user assigned to a primary group e.g. “users”

– Users can be members of other groups ( secondary groups )

– Each file is owned by a user and a group

– If you create a file, it is owned by
you and your primary group

– Anyone who is a member of that group (primary or secondary), has file permissions specified by that files group permission.

Question 9

Permissions Other

Answer 9

Everyone else on Linux System

Question 10

Linux file permissions

Answer 10

Mechanism to provide protection to OS
– Users don’t misuse file system resources
– Permission protects the entities of the file system so users cannot misuse entities owned by another user
* Permissions specify who can access a file and the type of access.
– File and Directory owned by a user

Question 11

Read, Write and Excecute

Answer 11

Functions of these permissions differ, depending on whether they’re applied to files or directories

Question 12

r (Read)

Answer 12

Gives users permission to open a file and view its contents

Allows users to list a directory’s contents with commands such as ls

Question 13

w (Write)

Answer 13

Gives users permission to open and file and edit its contents

Question 14

w (Write)

Answer 14

Gives users permission to open and file and edit its contents

Allows users to add or remove files and subdirectories

Question 15

x (Execute)

Answer 15

Allows users to run the file (as long as it is a program or a script)

Allows users to switch to directory with cd command to read a directory’s contents and add/remove files and subdirectories, you must have execute permission

Question 16

Example Read, Write, Execute

Answer 16

-rw r– r– 1 alex users 0 2012-04-11 09:20 file1

• User category of permissions is set to rw
  *Hyphen (–) represents no permission
  *Read, write, not execute
• Group category is set to r
  *Group has read permission but not write and execute permissions
• Other category is set to r
• Every user on the system has read permission but not write or execute

Question 17

Permission commands chmod

Answer 17

chmod (change mode) command
changes permissions on files and directories

syntax:
chmod permissions file/directory

Permissions argument
* Information used to change permissions

File/directory argument
* Specifies the file or directory you want to change

Question 18

Symbolic notation

Answer 18

Uses criteria such as categories and operators to change file permissions
Example: chmod o-wx file4

u (user) + add to existing permissions
g (group) - remove from existing permissions
o (other) = assign absolute permissions
a (all) one of the preceding operators

o-wx = other remove from existing permissions write and execute

Question 19

Numeric Notation

Answer 19

Uses numbers from 0 to 7 to represent file permissions
Example: `chmod 774 file1’

— 0
–x 1
-w- 2
-wx 3
r– 4
r-x 5
rw- 6
rwx 7

Question 20

Managing users

Answer 20

Authentication
– Process of verifying someone’s identity by checking his or her username and password against a central user database

• Linux user database consists of two configuration files:
  /etc/passwd
  /etc/shadow
  User account information is stored in thee 2 database configuration files.

Question 21

The /etc/passwd File

Answer 21

Contains user account information

Any user can read it

Each entry has seven fields separated by colons

Question 22

The /etc/shadow File

Answer 22

Configuration file containing passwords and password expiration information

For all user accounts

Can be read only by the root user

Explanation of fields:
– Username

– Encrypted password

– Last password change - number of days since January 1, 1970 that the password was last changed

– Minimum number of days before the password can be changed

– Expiration number of days before the password must be changed

• Warning——number number of days remaining until the of days remaining until the password must be changed

– Disabled——number number of days after a password has of days after a password has expired until the user account is disabled

• Disabled date——number number of days since January 1, of days since January 1, 1970 that the account has been disabled

Question 23

User account information

Answer 23

User information stored in two configuration files:
– /etc / useradd
– /etc login.defs
* Used to store default values when user is created
* Files contain:
–User’s PATH variable
–Password expiration information
–Default primary group
–Location of the home directory

Question 24

Creating user accounts

Answer 24

useradd command
– Create user accounts
– Update default information for new users
– E.g useradd user1

After creating a user, a password must be assigned

New user who doesn’t have a password yet:

user1:user1:!!:14745:0:99999:

! character in the encrypted password field means no password has been set for this user

Only the root user can set passwords for new users

passwd command
– Set a password for user1:
sudo passwd user1: sudo passwd user1

Question 25

passwd command

Answer 25

Enter a new password and then enter it again to confirm

Question 26

Modifying user accounts

Answer 26

usermod command
– Change user account information
– Only the root user can issue this command

For a currently logged in user, cannot change:
– Name
– UID
– Home directory

Question 27

Modifying user accounts

Answer 27

chage command

Only root user can issue

Modify information

Number of days between allowed and required password changes

Expiration date

Warning information

chage command used without options
- Interactive mode

Question 28

Deleting user accounts

Answer 28

userdel command
– Delete user accounts
– Remove all entries from user database files
– Doesn’t remove the user’s home directory

• r option
  – Remove user’s home directory
• When a user is deleted
  – All files he or she owned are then owned by the UID
  – If new user created with same UID, new user owns files

Question 29

Managing groups

Answer 29

Groups
– Helpful for streamlining the process of designating which users can perform certain tasks
* Every user is a member of at least one group
–Called the primary group
* Root user can assign an unlimited number of additional groups
– Called secondary groups
* /etc /group
– Group database configuration file

Question 30

id command

Answer 30

– View all the groups you belong to and their corresponding GIDs

gid: primary group of user
* All other groups are secondary groups and are separated by commas

Question 31

The /etc/group File

Answer 31

Configuration file that stores group information
* Can be read by everyone on the system

Question 32

Creating groups

Answer 32

groupadd command
– Add a group account
– Only the root user has permission to use this command

newgrp command
– Change a user’s primary group temporarily
– Stays in effect until the user logs out
- Opens a new instance of the BASH shell
– Changes the user’s primary group from users to projects

Question 33

Modifying groups

Answer 33

groupmod command
- modify existing groups
- only the root user has permission to use this command