File Permissions

In this exercise, we will explore how file permissions work in Linux and learn how to view and modify them. Understanding file permissions is crucial for managing access to files and directories in a Linux environment.

Background

Every file and directory in Linux has three permission groups:

Group	Who
Owner (u)	The user who owns the file
Group (g)	Users in the file's group
Others (o)	Everyone else

Each group has three permission bits:

Bit	Symbol	Binary	Octal
Read	r	100	4
Write	w	010	2
Execute	x	001	1

Permissions are combined by adding the values. For example rwx = 4+2+1 = 7, r-x = 4+0+1 = 5, r-- = 4+0+0 = 4.

A full permission string like rwxr-xr-- breaks into three octal digits: 7 5 4.

rwxr-xr--
^^^        owner:  rwx = 111 = 7
   ^^^     group:  r-x = 101 = 5
      ^^^  others: r-- = 100 = 4

So chmod 754 file sets those exact permissions.

Goal

Practice reading and setting file permissions using both symbolic (chmod u+x) and octal (chmod 755) notation.

Step 1: View Permissions

Create a file and inspect its default permissions:

touch myfile.txt
ls -l myfile.txt

The output will look like:

-rw-r--r-- 1 user group 0 Jan 1 12:00 myfile.txt

The leading - means it is a regular file (a d would mean directory). The next 9 characters are the permission bits: rw- (owner), r-- (group), r-- (others).

Question: What is the octal representation of rw-r--r--?

Answer

rw- = 110 = 6, r-- = 100 = 4, r-- = 100 = 4 → 644

Step 2: Symbolic Mode

Use symbolic notation to modify permissions:

# Add execute for the owner
chmod u+x myfile.txt
ls -l myfile.txt

# Remove write for group
chmod g-w myfile.txt
ls -l myfile.txt

# Add read+write for others
chmod o+rw myfile.txt
ls -l myfile.txt

Question: After all three commands, what is the symbolic permission string and its octal equivalent?

Answer

rwx (owner) + r-- (group) + rw- (others) = rwxr--rw-
Octal: 7 4 6 → 746

Step 3: Octal Mode

Reset and apply permissions directly with octal values:

chmod 644 myfile.txt   # rw-r--r--  (default for files)
chmod 755 myfile.txt   # rwxr-xr-x  (executable / directory)
chmod 700 myfile.txt   # rwx------  (private executable)
chmod 600 myfile.txt   # rw-------  (private file)

For each command above, write out the binary and symbolic form before running it to verify your understanding.

Question: What octal value gives rwxrwxrwx (full permissions for everyone)?

Answer

111 111 111 = 7 7 7 → 777

Step 4: Directory Permissions

Permissions behave slightly differently for directories:

Bit	Meaning on a directory
r	List contents (ls)
w	Create/delete files inside
x	Enter the directory (cd)

mkdir mydir
ls -ld mydir

# Remove execute — try to enter
chmod u-x mydir
cd mydir   # should fail

# Restore
chmod u+x mydir