Understanding Linux Permissions: Read, Write, Execute Explained

1) The Three Types of Permissions

• Read (r): View the contents of a file or list files in a directory.
• Write (w): Modify or delete a file; add or remove files inside a directory.
• Execute (x): Run a file as a program or access the contents of a directory.

Example output from ls -l:

-rwxr-xr-- 1 user group 1024 Jan 18 10:00 script.sh

This means:

• Owner: read, write, execute
• Group: read, execute
• Others: read

2) Permission Structure

The 10-character string from ls -l is structured like this:

[-][rwx][rwx][rwx]
|   |    |    |
|   |    |    +-- others
|   |    +------- group
|   +------------ owner
+---------------- file type (- = file, d = directory, l = link)

3) Changing Permissions

Use chmod to adjust file permissions.

# Add execute permission for owner
chmod u+x script.sh

# Remove write for group
chmod g-w file.txt

# Grant all permissions to everyone
chmod 777 test.txt

Numeric mode:

chmod 755 script.sh

Breakdown: 7 = rwx, 5 = r-x, 5 = r-x.

4) Changing Ownership

# Change owner
sudo chown alice file.txt

# Change owner and group
sudo chown alice:devs file.txt

chgrp changes only the group:

sudo chgrp devs file.txt

5) Special Permissions

• Setuid (s): File runs with owner privileges.
• Setgid (s): File runs with group privileges; new files in directory inherit group.
• Sticky bit (t): On a directory, only the file’s owner can delete it (e.g., /tmp).

# Add sticky bit to /shared
sudo chmod +t /shared

6) Best Practices

• Use least privilege: avoid 777.
• Restrict sensitive files to root only (600 for configs).
• Audit with find / -perm -4000 for setuid binaries.