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Mitesh Shah

Linux Expert | Automation Enthusiast | Security Consultant

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Users & Group ID Numbers

  • When Files Are Stored On The Computer, The Metadata About The File Is Stored Numerically.
  • That Is, The Username And Group Affiliation Of The File Are Not Stored; Rather, The User ID And Group ID Numbers Are Stored.


[mitesh@Matrix ~]$ ls -l
drwxr-xr-x. 2 mitesh mitesh 4096 Aug 26 12:11 Desktop

[mitesh@Matrix ~]$ ls -ln
drwxr-xr-x. 2 501 501 4096 Aug 26 12:11 Desktop

Users & Group Informations Files


  • The /etc/passwd file contains the list of the system’s accounts.


Username:Password PlaceHolder:UID:GID:GECOS:Home Directory:Shell

NOTE!: GID Is The User’s Primary Group ID Number


  • The /etc/shadow file contains the users encrypted passwords and account expiration information.
  • The /etc/shadow file is not readable by anyone.


  • The /etc/group file defines the groups on the system


Groupname:Password PlaceHolder:GID:UserList

NOTE!: UserList Is The List Of Usernames That Are Members Of This Group, Separated By The Commas.


  • The /etc/gshadow file contains the groups encrypted passwords and the list of group administrators.
  • The /etc/gshadow file is not readable by anyone.

Users Management Tools


  1. useradd: Create a new user or update default new user information
  2. usermod: Modify a user account
  3. userdel[-r]: Delete a user account [Removes users Home Directory and users Mail Spool]

Graphical Tool

  1. system-config-users:
    • System -> Administration -> Users and Groups
    • Add Modify Deletes Users and Groups

System Users and Groups

  • In additions to the ordinary user accounts and the superuser root account, The number of system users and groups exist.

  • The main reason for creating system users and groups is, Runs several programs as non-priviledged users or as a particular groups. Examples: Daemon, mail, lp, nobody, web or print servers
  • Running programs in this way limits the amount of damage any single program can do to the system.

  • System Users & Groups All Have UID & GID Numbers Between The 1 & 499.


# Sample of /etc/passwd
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin

# Sample of /etc/group

Monitoring Logins

  1. w: Show who is logged on and what they are doing.

  2. last: Show listing of last logged in users and reboot history.
    Examples: last; last root; last mitesh; last reboot;

  3. lastb: Show bad login information.

  4. lastlog: Reports the most recent login of all users or of a given user
    Examples: lastlog; lastlog -u root;

Default Permissions

Files:		0666 - umask
Directory:	0777 - umask


Non-privileged user’s umask is 0002

Files:      0666 - 0002 = 0664
Directory:  0777 - 0002 = 0775

Root user’s umask is 0022

Files:      0666 - 0022 = 0644
Directory:  0777 - 0022 = 0755

Changing umask value

[mitesh@Matrix ~]$ umask 022
[mitesh@Matrix ~]$ umask 0022

NOTE!: The umask is typically set by the scripts run at the login time.
That means your umask value is set to default everytime you login into the system.

Special Permissons For Executables

  • In addition to the user, group and other permissions, An additional set of permissions exist called special permissions.

    1. 4(s) The suid - set user id bit
    2. 2(s) The sgid - set group id bit
    3. 1(t) The sticky bit

  • The special permission is displayed in the place of x.

    • Small Letter = Executable Permission + Special Permission
    • Capital Letter = No Executable Permission Only Special Permission

For Files

The SUID Permissions
  • The command will run with the authority of the owner of the file, Rather than, the authority of the user running the command.


[mitesh@Matrix ~]$ passwd

NOTE!: The passwd command changes a user’s password,
which is stored in the /etc/shadow file and it is not writable for non-privileged users.
However, since the passwd command is owned by root and runs with the suid permissions, Users running the command have the root privilege while changing their passwords.
Hence, They have the permissions to edit the /etc/shadow file.

[mitesh@Matrix ~]$ cp /usr/bin/whoami /tmp/
[mitesh@Matrix ~]$ chmod u+s /tmp/whoami
[mitesh@Matrix ~]$ /tmp/whoami
[neo@Matrix ~]$ /tmp/whoami

[mitesh@Matrix ~]$ chmod u-s /tmp/whoami
[mitesh@Matrix ~]$ /tmp/whoami
[neo@Matrix ~]$ /tmp/whoami
The SGID Permissions
  • The command will run with the authority of the group of the file.

For Directory

The SGID Permissions
  • The files created in this directory will inherit its group affiliation from the directory, Rather than inheriting it from the user.
  • The SGID Bit is commonly set for the Group Directories.


[mitesh@Matrix ~]$ chmod g+s GroupDir
[mitesh@Matrix ~]$ chmod 2775 GroupDir
[mitesh@Matrix ~]$ chmod g-s GroupDir
[mitesh@Matrix ~]$ chmod 775 GroupDir

The Sticky Bit

  • The Sticky Bit For A Directory, Sets A Special Restriction On Deletion Of Files.
  • If the sticky bit is set for the directory, Then only the owner of the files or root can delete the files - Regardless of the write permissions of the directory.
  • An Example of sticky bit set is /tmp directory


[mitesh@Matrix ~]$ chmod o+t ~/tmp/
[mitesh@Matrix ~]$ chmod 1777 ~/tmp/
[mitesh@Matrix ~]$ chmod o-t ~/tmp/
[mitesh@Matrix ~]$ chmod 777 ~/tmp/

[mitesh@Matrix ~]$ ls -ld /tmp
drwxrwxrwt. 28 root root 4096 Jun  8 14:06 /tmp

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