Understanding Permissions in Linux

🔒 Understanding Permissions in Linux

Linux permissions control who can read, write, or execute files and directories. They are essential for security and access management.

1. File Permissions: Read, Write, Execute

Each file and directory in Linux has three types of permissions:

Permission	Symbol	Meaning
Read	`r` (4)	View the contents of a file or list a directory
Write	`w` (2)	Modify the file or create/delete files in a directory
Execute	`x` (1)	Run the file (if it's a script or program) or access a directory

Each file has three permission groups:

Owner (User) - The creator of the file.
Group - A group of users who share permissions.
Others - Everyone else.

You can view file permissions using:

ls -l

Example output:

rwxr--r-- 1 user group 1234 Feb 10 12:00 myfile.sh

-rwxr--r--
- - → Regular file (or d for directory)
- rwx → Owner has read, write, execute (r=4, w=2, x=1, total = 7)
- r-- → Group has read only (r=4)
- r-- → Others have read only (r=4)

2. Changing Permissions (chmod)

chmod modifies permissions.

Symbolic Method

chmod u+x file    # Add execute (x) for the user
chmod g-w file    # Remove write (w) for the group
chmod o+r file    # Add read (r) for others
chmod ugo=rwx file  # Give full permissions to everyone

Octal Method

Each permission type has a number:

r = 4
w = 2
x = 1
No permission = 0

To set permissions:

chmod 755 file  # Owner: rwx (7), Group: r-x (5), Others: r-x (5)
chmod 644 file  # Owner: rw- (6), Group: r-- (4), Others: r-- (4)
chmod 777 file  # Everyone has full permissions (⚠️ NOT recommended)

3. Changing Ownership (chown)

chown changes the owner of a file.

sudo chown user file       # Change owner
sudo chown user:group file # Change owner and group
sudo chown -R user:group directory/  # Apply recursively

4. Changing Group Ownership (chgrp)

chgrp changes only the group of a file.

sudo chgrp group file

5. Understanding Special Permissions

Sticky Bit (`t`): Prevents users from deleting others' files in a directory.

chmod +t directory/

Example: /tmp has a sticky bit so only file owners can delete their own files.

SetUID (`s`): Runs a file with the owner's privileges.

chmod u+s file

Example: /bin/passwd lets users change their password.

SetGID (`s`): Runs a file with the group's privileges.

chmod g+s directory/

Files created in this directory inherit the group.

6. Viewing Effective Permissions

To see effective user permissions:

whoami      # Show current user
groups      # Show user's groups
id          # Show user and group IDs

To check access to a file:

namei -l /path/to/file

7. Default Permissions (umask)

umask sets the default permissions for new files.

View the current umask:
```
  umask
```
Set a new umask (e.g., 022 for 755):
```
  umask 022
```

8. Common Permission Issues & Fixes

Permission Denied (Errno 13)

sudo chmod -R 755 /path/to/directory
sudo chown -R user:user /path/to/directory

Cannot Execute Script

chmod +x script.sh
./script.sh

Can't Write to Directory

chmod u+w directory/

9. Security Best Practices

✅ Avoid using chmod 777 (full access for everyone).
✅ Use sudo only when necessary.
✅ Set directories to 755 and files to 644 unless execution is needed.
✅ Use chown carefully to prevent unauthorized access.

Sudip Bhandari

Sudip Bhandari

Understanding Permissions in Linux