Introduction to the Linux Command Line

1. Introduction to Multiuser Unix Systems

Unix systems support multiple users and concurrent tasks. Users can log in locally or remotely via SSH. Graphical interfaces can even be displayed remotely using the X Window System. This multiuser architecture stems from Unix's roots in shared academic computing.

2. Ownership and Group Structure

Each file in Unix is assigned:

• User ID (UID): representing the file owner
• Group ID (GID): linking to a group
• Group memberships: user may belong to multiple groups

id
# uid=500(me) gid=500(me) groups=500(me)

User, group, and shadow information are stored in /etc/passwd, /etc/group, and /etc/shadow.

3. File Permission Types

• Read (r): view contents or list directory
• Write (w): modify contents or add/delete files
• Execute (x): run scripts or enter directories

File Type Indicators

Indicator	File Type
-	Regular file
d	Directory
l	Symbolic link
c	Character device
b	Block device

Example Output

-rw-rw-r-- 1 me me 0 foo.txt

Interpretation: owner and group have read/write access; others have read-only access.

4. Changing Permissions with chmod

Permissions can be changed using:

• Octal notation: e.g., chmod 600 foo.txt
• Symbolic notation: e.g., chmod u+x script.sh

chmod 600 foo.txt
chmod u+x script.sh
chmod g+w shared.txt

5. Default Permissions with umask

The umask subtracts from default base permissions:

• Files: 666
• Directories: 777

umask 0002
# Results in: -rw-rw-r--

6. Special Permission Bits

Bit	Description	Example
Setuid (4000)	Run with owner’s privileges	-rwsr-xr-x
Setgid (2000)	Preserve group in sub-files	drwxrwsr-x
Sticky bit (1000)	Restrict deletion to owners	drwxrwxrwt

chmod g+s shared_dir
chmod +t public_dir

7. User Identity Control: su vs sudo

• su: switches user, prompts for target user’s password
• sudo: executes with elevated privileges, using current password and /etc/sudoers config

sudo backup_script

Note: Ubuntu disables root account by default, preferring sudo access.

8. Changing Ownership

sudo chown tony: file.txt        # Change user to tony
sudo chgrp music file.mp3        # Change group to 'music'

9. Setting Up a Shared Directory

1. Create group music
2. Add users (e.g., bill and karen) to group
3. Create shared directory /usr/local/share/Music
4. Assign ownership and permissions:

sudo chown :music /usr/local/share/Music
sudo chmod 775 /usr/local/share/Music
sudo chmod g+s /usr/local/share/Music

Ensure umask 0002 to allow group write access.

10. Managing Passwords

passwd               # Change current user password
sudo passwd username # Change another user's password

Changes are stored in /etc/shadow and follow system policies.

Conclusion

Unix’s permission system enables reliable multiuser environments. With tools like chmod, umask, sudo, chown, and passwd, administrators and users can customize access and ownership for secure resource sharing.