User Managmenet & Permissions

adduser "username"
userdel "username"
killall -U "username"
sudo usermod  -a -G sudo "username"

Permissions

+———+———–+———–+———–+
| Symbol | Permission| Owner | Group | Other users |
+———+———–+———–+———–+
| d | Directory | r (read) | r (read) | r (read) |
| w | Write | w (write) | - | - |
| r | Read | x (execute)| x (execute) | - |
| x | Execute | r (read) | r (read) | x (execute)|
| - | No access | - | - | - |
| x | Execute | - | - | x (execute)|
+———+———–+———–+———–+
| | | | | |
| | | User | Group | Other |
+———+———–+———–+———–+

For example, dwrx(UserOwner)-xr(Group)-x(Others)

User Management

# Add a new user
adduser armanriazi

# View account status and expiration information
chage -l armanriazi

# Remove SUID permission from a file
chmod u-s file1

# Add Sticky Bit to a file
chmod +t /test

# Add write permission for others to a file
chmod o+w /test

# Remove Sticky Bit from a file
chmod -t /test

Explanation of File Permissions Commands

  1. adduser armanriazi: Adds a new user to the system. This command prompts for various details such as full name, room number, shell, etc., unless you provide arguments.

  2. chage -l username: Displays the last change information for a user’s account. This includes password expiration details, minimum days, maximum days, warning period, and last password change.

  3. chmod u-s file1: Removes the SUID (Set User ID) bit from the file. This prevents the file from automatically inheriting the privileges of the owner when executed.

  4. chmod +t /test: Adds the Sticky Bit to the file. The Sticky Bit prevents users from deleting or renaming files owned by other users, even if they have write permissions.

  5. chmod o+w /test: Adds write permissions for others to the file.

  6. chmod -t /test: Removes the Sticky Bit from the file.

Additional Information on Permissions

  • SUID (Set User ID): When set on an executable file, it runs with the privileges of the owner rather than the executing user.
  • SGID (Set Group ID): Similar to SUID but applies to group ownership.
  • Sticky Bit: Prevents deletion or renaming of files owned by other users, even for those with write permissions.

These commands demonstrate various ways to modify file permissions in Linux systems. The chmod command is versatile and can be used with both symbolic notation (u-s, -t, o+w) and numeric notation (e.g., 755).

Remember that modifying permissions should be done carefully, especially when dealing with system files or directories. Always ensure you have the necessary privileges before making changes to file permissions.

Citations: [1] https://www.redhat.com/en/blog/suid-sgid-sticky-bit [2] https://www.cbtnuggets.com/blog/technology/system-admin/linux-file-permissions-understanding-setuid-setgid-and-the-sticky-bit [3] https://www.redhat.com/en/blog/linux-file-permissions-explained [4] https://linuxhandbook.com/suid-sgid-sticky-bit/ [5] https://www.howtogeek.com/656646/how-to-use-suid-sgid-and-sticky-bits-on-linux/ [6] https://www.geeksforgeeks.org/setuid-setgid-and-sticky-bits-in-linux-file-permissions/ [7] https://www.linuxfoundation.org/blog/blog/classic-sysadmin-understanding-linux-file-permissions [8] https://www.liquidweb.com/blog/how-do-i-set-up-setuid-setgid-and-sticky-bits-on-linux/ [9] https://learning.lpi.org/en/learning-materials/010-160/5/5.3/5.3_01/