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Linux Session #3.md

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Session #3

  • Show all environment variables whether they are empty or not {In line #970}
man bash
  • Show all environment variables that have values (not empty)
printenv

OR

env

Users and Groups Administration :

  • User account is a systematic approach to track and monitor the usage of system resources. Each user account contains two unique identifires -> username & UID

Types of users :

  • Root user -> UID is 0 {Has All privilege }

  • Regular user -> UID >= 1000

  • Service accounts -> created when packages are installed {ex: network,audio,docker,... } -> 0 < UID <= 999

  • Managing files and groups are done by those files:

    • /etc/passwd {7 fileds} --- contains all configuration f system
    • /etc/shadow {9 fields} --- contains encrypted data of passwords in system
    • /etc/group {4 fields} --- gname:password:GID:group_members

  • Add a user in system

sudo useradd arafat
  • Rest valus will be automatice allocated OR
sudo useradd -u 1000 -g 2000 -c "Ahmed Arafat" -md /home/arafat -s /bin/bash arafat

  • -u : UID

  • -g : GID

  • -c : comment

  • -md : Home Directory

  • -s : shell

  • Then username {arafat in this command}

  • To change password or assign a password for a user for the first time

sudo passwd <username>
  • To switch to a specific user
su <username>

you will then enter his password

  • To go back to previous username -> enter exit OR Ctrl + d

    • In Line #31, shadow has 9 fields :

      • Username
      • Encrypted Password {One-way encryption}
      • Date when password was last changed
      • Number of days before password can be changed (minimum password age)
      • Number of days after password must be changed (maximum password age)
      • Number of days before password expiry date to display the warning
      • Number of days to disable account after password expire
      • Number of days since the account is disabled
      • Reserved field {Not commonly used}

    • To display [shadow] field for a specific user

    sudo chage -l <Username>

    -l : list fields

  • Some flags used with chage

    • l (small L) -> view ageing information about account
    • d -> change last password change date (-d 2021-12-31)
    • E -> set when account expires (-E 2022-12-31)
    • M {Max} {OR} m {Min} -> changes password max or min age {-M 5}
    • w -> Number of days to give prior warning before the password expires {-w 2}
    • l (capital i) -> specify number of days account should be inactive after its expiry {-I 5}

  • Example: sudo chage -m 5 -w 2 arafat

    • Modify a user data
    usermod -l arafat AhmedArafat

    -l : To change name of username

    • Note : usermod uses same flags as useradd

    • For group you can have a :

      • Primary group (Only one) -> {your names group}
      • Secondary group (Many) -> {Any other group}

    • To create an empty group

    sudo groupadd <GroupName>

    you can add [-g] -> GID

  • To add a user to group

    usermod -G <GrouoName> <Username>

  • To remove a user

sudo userdel -r <UserName>

-r : To remove his home Directory + mails

  • To remove a group
sudo groupdel <GroupName>
  • Edit group data
sudo groupmod -n zodiac Zodiacs -g 1001

-n : To change name
-g : To change UID

  • [.password] file is a backup file for [password] file in path [/etc] as it is a critical file

File Permission and Ownership :

  • User X is not in the sudoers file -> to fix this [/etc/sudoers]

  • vi sudo -> search for it in google

  • -|rwx|r-x|--- 1 arafat Zodiacs 0 Oct 31 11:06 Test.txt

  • [-] --> file type {- -> file // d -> Dir.}

  • [rwx] -> user permission

  • [r-w] -> Group permission

  • [---] -> other permission

  • [1] --> # of hard link (will discuss it in next sessions)


  • ahmed -> user owner

  • Zodiacs -> group owner

  • 0 -> file size

  • Oct 31 11:06 -> last modified time

  • Test.txt -> name of file

  • VIP Octa Numbers:

    • r {Read} = 4
    • w {Write} = 2
    • x {Execute} = 1

  • Directory:

    • [r] for Dir. -> to [ls] this Dir.
    • [w] for Dir. -> to copy/create file/Dir. inside it
    • [-x-] for Dir. -> to go to [cd] this Dir. {enter it}

  • File:

    • [r] for Dir. -> read its content
    • [w] for Dir. -> to edit its content
    • [-x-] for Dir. -> to execute executable files as .py/.cpp

  • To show permission/ownership for a file

ls -l <FileName>
  • To show permission/ownership for a Directory
ls -ld <DirName>
  • To change permission of file/Dir.
chmode <FileName>

Example:

chmod u=rwx,g=rw,o=r file.txt

OR Example:

chmod 764 file.txt

r -> 7 -> 4+2+1
w -> 6 -> 4+2
x -> 4 -> 4

  • Note : In [$ chmod] command u-rw -> remove from user r,w permissions {now user = x, assuming that he already has x}

  • Note: Root user can [r/w/x] any file, if its permission is not allowed, as he has all privilege

  • Note: you cannot prevent a user from deleting a file even if it has [---], but there is a way to protect it using the Dir. it exist in

  • VIP:

    • To copy a file I have to have [r] permission
    • To copy a file to a Dir. , Dir. should have [w][x] permission
    • To copy a Dir. to another Dir. , copied Dir. should have [r][x] permission while Dir. which will contain copied one should have [w][x] permission

Initialization Files :

  • ~/.bashrc OR ~/.bash_profile: are local files for each user so, environment variables will be shown only for one user

  • /etc/profile OR /etc/bashrc OR /etc/bash.bashrc {Global files} : if you put inside it an environment variable, it will affect all users in system

Linux Package Manager (apt) : Just goole program you want to download

sudo apt policy git

sudo apt update

sudo apt upgrade

wget

docker --version

docker -compose