Linux System Administrator

Welcome to the part 2 of Journey to become Linux Wizard Part 1

User Management

User & Groups

• User ID [UID] use to identify user
• Group ID [GID] use to identify groups
• Other users like System Daemon, Root user/Superuser, etc.

Root User

• User who has complete access to the system.
• Its UID is 0.
• use sudo command to execute the command as the root user.
• sudo su command to become a root user.
• command you use as a root user, their history will not be saved so use them carefully.

Who are these other users in system other than me and root?

sudo cat /etc/passwd to see a list of all users,you'll find a lot of users but lets focus on first one to understand all this gibberish,

1. 'root' is the username.
2. 'x' is the password for the user, which is encrypted .and stored in /etc/shadow
3. '0' is the User ID.
4. '0' is Group ID.
5. User info which is the root user in our case.
6. user's home directory.
7. /bin/bash is the User's Shell.

/etc/shadow

type the following command to see the password details of users

1. 'root' is the username.
2. '!' is an encrypted password.
3. '19056' is the date of the last password change since 1st Jan. 1970.
4. '0' is the Minimum password age.
5. '99999' is the Maximum password age.
6. '7' is the warning period to change the password.
7. ':' is the password expiry period.
8. ':' is the Account expiration Date.
9. ':' is Reserved Field.

/etc/group

type the command

1. 'root' is the group name.
2. 'x' is the group password.
3. '0' is the group ID.
4. Last empty field is the list of users in this group.

How to create & delete a user?

sudo useradd nameofuser

sudo userdel nameofuser

How to change password and hostname?

sudo passwd userName

sudo hostname newUser

Permissions

Types

• Read (r)
• Write (w)
• Execute (e)type the following command ls -l

start with the gibberish on the left sidedrwxrwxr-x

Understand this by dividing it into four separate parts.

1. Tells about type
   • d is a directory
   • '-' is a file
   • l is a link
2. rwx are user permissions.
3. rwx are group permissions.
4. r-x are others' permission.

so, what is this 'rwx' and 'r-x' means ?

• r is readability permission
• w is writability permission
• x is executability permission
• - empty

so in the above photo, the user and group have all permissions to read, write and execute the directory but other users only have permission to read and execute

Modifying Permissions

let say i have a file named myfile.txt with permission -rw-rw-r--

change permission using chmod command

$ chmod o+w myfile.txt

o is for other users

w is to give it writing permission

+ is to add permission

- is to remove permission

Modifying Multiple Permissions

$ chmod ugo-rwx myfile.txt

• ugo is for user, group & others
• - is to remove permission
• rwx is read, write and execution permission
• we removed all three permissions for three kind of users

Numerical representation of permissions

• read (r) 4
• write(w) 2
• execute(x) 1

to grant all permission by using numerical representation

$ chmod +777 myfile.txt

• + to add permission
• 7 to add all permission to user
• 7 to add all permission to group
• 7 to add all permission to other users

Numerical presentation

• 0 No permission
• 1 execution permission
• 2 write permission
• 3 write and execution permission
• 4 read permission
• 5 read and execution permission
• 6 read and write permission
• 7 read, write and execution permission

Change user ownership of file

$ sudo chown pintu myfile.txt

so ownership will transfer to the username pintu

Change group of file

$ sudo chgrp

Set User ID (SUID)

lets type a command in your terminal

$ ls -l /usr/bin/passwd

How am i able to access this data without being root user?

you can see its permission, lets break down it to understand it better

rwsr-xr-x

we have an s in user permission,s is SUID which give us ownership and execution permission.

S will only give only ownership

Add SUID to user

$ sudo chmod u+s myfile.txt

• add permission to user

$ sudo chmod 4755 myfile.txt

• 4 is for SUID

Add Group ID (GUID)

$ sudo chmod 2755 myfile.txt

• 2 for GUID

Sticky Bit Permission

• It grants all permission but only delete access is only for user

type the command ls -ld/tmp

you will see

drwxrwxrwt

• t is Sticky Bit

Modify Sticky Bit

$ sudo chmod +t directory

$ sudo chmod 1755 directory

• 1 for sticky bit permission

_See you in the next blog of Linux Process _