Week 8 - Shell Scripts Part 2 Flashcards
Loops
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
The while loop
– 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
Basic Arithmetic Expression
Arithmetic expression should be place inside
double parenthesis: (( expression ))
The for Loop
- 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
Shell scripts loops used
While and For loops are used
Loops are used to repeat actions on different variables
Viewing file permissions command
> ls -l
Permissions User
The file owner
Permissions Group
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.
Permissions Other
Everyone else on Linux System
Linux file permissions
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
Read, Write and Excecute
Functions of these permissions differ, depending on whether they’re applied to files or directories
r (Read)
Gives users permission to open a file and view its contents
Allows users to list a directory’s contents with commands such as ls
w (Write)
Gives users permission to open and file and edit its contents
w (Write)
Gives users permission to open and file and edit its contents
Allows users to add or remove files and subdirectories
x (Execute)
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
Example Read, Write, Execute
-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
Permission commands chmod
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
Symbolic notation
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
Numeric Notation
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
Managing users
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.
The /etc/passwd File
Contains user account information
Any user can read it
Each entry has seven fields separated by colons
The /etc/shadow File
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
User account information
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
Creating user accounts
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
passwd command
Enter a new password and then enter it again to confirm
Modifying user accounts
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
Modifying user accounts
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
Deleting user accounts
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
Managing groups
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
id command
– 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
The /etc/group File
Configuration file that stores group information
* Can be read by everyone on the system
Creating groups
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
Modifying groups
groupmod command
- modify existing groups
- only the root user has permission to use this command
