Understand File Permissions

• In Linux / Unix, everything is a file
  • Files
  • Directories
  • Devices (usually referenced via nodes)
• All files in the system have permissions that allow or prevent others from:
  • Viewing (read)
  • Modifying (write)
  • Executing (execute)
• If a file is a directory, permissions affect different actions than regular files or device nodes.

Superuser

• Superuser (root) has access to any file on the system.

Ownership & Permissions

• Each file has access restrictions based on:
  • Owner
  • Group
  • Others
• Permissions are referred to as bits.

Permission bits

Each file/directory has three permissions:

• r = read
• w = write
• x = execute

Administrative Access

• To change or edit files that are owned by root, sudo must be used
  • See man sudo for details

Type of Access Permissions

There are three types of access permissions:

Type of User Restrictions

There are three types of users:

• Owner
• Group
• Other

ls output example:

-rwxrwxrwx

• First 3 bits → Owner
• Next 3 bits → Group
• Last 3 bits → Other

Example: Permission

If owner read & execute bits are on, then the permissions are:

-r-x------

Scope of Restrictions

• Restriction scope is not inherited
• File owner is not affected by restrictions set for:
  • Group
  • Others

Directory Permissions

• Directories have directory permissions
• Directory permissions restrict different actions than files or device nodes

Explain Permission values

Linux adds permission values together:

• r = 4
• w = 2
• x = 1

Examples:

• rwx = 4 + 2 + 1 = 7
• r-x = 4 + 0 + 1 = 5
• rw- = 4 + 2 + 0 = 6

So:

chmod 755 file

Means:

• Owner → rwx (7)
• Group → r-x (5)
• Other → r-x (5)

Directory Permissions (Meaning of r, w, x on folders)

⚠️ Without execute (x) on a directory, you cannot access files inside, even if you know their names.

Directory read permission

• Allows viewing directory contents
• Example:

ls directory_name

Directory write permission

• Allows creating and deleting files inside the directory
• ⚠️ Important concept:
  • Write permission on a directory allows deleting a file even if the user does NOT have write permission on the file itself

Directory execute permission

• Allows entering (changing into) the directory
• Example:

cd directory_name

Critical Rule (⚠️)

All folders (directories) MUST have execute (x) permission

• If a directory does not have execute permission:
  • You cannot access files inside it
  • It may disappear from file browsers
  • It will not function as a directory

Real Example:

-rw-r--r-- 1 root root 288 Jan 13 19:24 /etc/hosts

sudo nano /etc/hosts

Key Concepts (Exam)

rw-------

rwx------

One-line golden rule ⭐

For directories, execute (x) is MORE important than read (r)

Change Permissions

chmod

• Used to modify file or directory permissions
• There are two ways to change permissions:
  1. With letters (easier to understand)
  2. With numbers (octal method)

⚠️ Change permissions very carefully, otherwise security can be breached

Important Security File

• /etc/shadow
  • Stores encrypted user passwords
  • Must NOT be accessible to regular users

Example 1:

cd /etc
ls -l | grep shadow

chmod with Letters (Symbolic mode)

Syntax

chmod [options] filename

User options

Permission options

Operators

Examples:

chmod u+x script.sh

chmod g-w file.txt

chmod o=r file.txt

chmod a+r file.txt

Numeric (Octal) reminder

Example:

chmod 644 file.txt

• Owner → rw-
• Group → r–
• Others → r–

Why /etc/shadow is mentioned (exam trap ⚠️)

-rw-r----- 1 root shadow /etc/shadow

• Only root can read/write
• Group shadow can read
• Others → NO access

❌ Never do:

chmod 644 /etc/shadow

✔ Correct:

-rw-r-----  (640)

Key exam points ⭐

• chmod changes permissions, not ownership
• Wrong permissions can expose sensitive files
• Symbolic mode = readable
• Numeric mode = faster

One-line golden rule ⭐

Never loosen permissions on system files unless you fully understand the impact

chmod with Letters (Symbolic Mode)

Examples:

chmod u+x file
ls -l   # verify

-rw-r--r--

-rwxr--r--

chmod o+wx file

chmod o+w,o+x file

-rw-r--rwx

chmod g-r file

-rw-r--r--  

-rw----r--

chmod a+rwx file

-rwxrwxrwx

chmod with Numbers (Numeric / Octal Mode)

chmod XYZ file

chmod 666 file

-rw-rw-rw-

chmod 755 file

-rwxr-xr-x

chmod 600 file

-rw-------

Mapping numbers to permissions

Key exam & practical notes ⭐

• Symbolic mode (u+x) is safer and easier to read
• Numeric mode (755) is faster
• chmod does not change ownership
• a+rwx gives full access to everyone → ⚠️ risky
• Always verify with:

ls -l

One-line golden rule ⭐

Use the least permissions necessary — never more

Permission Values Recap with Multiple Examples

chmod 667 file1

-rw-rw-rwx

chmod 251 file1

--w-r-x--x

chmod 644 file1 file2 file3

-rw-r--r--

chmod 644 file1 file2 file3

-rw-r--r--

chmod u+x file

-rwxr--r--

chmod 744 file1 file2 file3

-rwxr--r--

chmod 747 file1 file2 file3

-rwxr--rwx

-rw-r--r--

chmod 604 file

-rw----r--

chmod g-r file

chmod 777 file1 file2 file3

ls -l

chmod with sudo

• If you do not own a file, you cannot change its permissions
• Use sudo to modify permissions of files owned by root
• Be extremely careful: wrong permissions can break the system

Example:

sudo chmod o+x /usr/local/bin/file

Recursive Permission Changes

Purpose

To change permissions of multiple files and directories at once

⚠️ Warning: Recursive chmod can be dangerous if used incorrectly

Recursive chmod using -R

sudo chmod -R 777 /test/test1

• Changes permissions of:
  • /test/test1
  • All subdirectories
  • All files inside

Verify

ls -l

Explanation in simple terms

Why sudo chmod is risky ⚠️

• System files (e.g. /bin, /usr, /etc) depend on strict permissions
• Accidentally running:

sudo chmod -R 777 /

❌ Will destroy your system

Best Practice

Use symbolic permissions

sudo chmod -R u+rwX,go+rX /some/dir

This:

• Adds execute only to directories
• Avoids making regular files executable

Common safe patterns

Give owner full control, others read-only

chmod -R 755 directory

Private directory

chmod -R 700 directory

Key exam & practical rules ⭐

• sudo chmod → only when you are not the owner
• -R → applies to everything inside
• chmod 777 → ❌ insecure
• Always verify:

ls -l

Golden rules (⭐)

1. Never use recursive chmod on system directories
2. Never use 777 unless you fully understand why
3. Prefer symbolic mode when using -R
4. Test on one file first

Recursive chmod using find, pipes & sudo

Problem with chmod -R

chmod -R 755 directory

❌ This applies the same permissions to:

• files
• directories

But:

• Files usually should be 644
• Directories usually should be 755

So we need more control.

Correct Permissions

Files → 644

Owner  : read + write
Group  : read
Others: read

Directories → 755

Owner  : read + write + execute
Group  : read + execute
Others: read + execute

👉 Execute (x) on directories means enter/access directory

Solution: Use find + chmod

Change permissions of only files

sudo find /path/to/directory -type f -print0 | xargs -0 sudo chmod 644

Explanation:

• find → finds files
• -type f → only files
• -print0 → safe for filenames with spaces
• xargs -0 → passes them safely
• chmod 644 → applies permission

Change permissions of only directories

sudo find /path/to/directory -type d -print0 | xargs -0 sudo chmod 755

Why not use plain chmod -R?

Combined Real-World Fix

sudo find /project -type f -exec chmod 644 {} \;
sudo find /project -type d -exec chmod 755 {} \;

✔ Correct
✔ Secure
✔ Industry standard

Warning with recursive chmod ⚠️

Before running any recursive command:

✔ Double-check the path
✔ Test on a small directory
❌ Never run on /, /usr, /etc, /bin

🚨 THIS COMMAND WILL DESTROY A SYSTEM

sudo chmod -R 777 /

Key exam points ⭐

• chmod -R applies same permissions to everything
• find -type f → files only
• find -type d → directories only
• -print0 | xargs -0 → handles spaces safely
• Preferred over recursive chmod

One-line summary ⭐

Use chmod -R only for simple cases.
Use find when files and directories need different permissions.

Changing the File Owner and Group

In Linux, every file has:

• Owner (user)
• Group

To change them, you usually need root (sudo) privileges.

1️⃣ Changing the file owner (chown)

Syntax

sudo chown user file

Example

sudo chown vivek file3

✔ Changes the owner of file3 to vivek
✔ Group remains unchanged

Change owner of multiple files / directories

sudo chown vivek file1 file2
sudo chown vivek /test

Recursive owner change

sudo chown -R vivek /test

✔ Changes owner of all files and folders inside /test

2️⃣ Changing the group owner (chgrp)

Syntax

sudo chgrp group file

Example

sudo chgrp tgsgroup file1

✔ Changes only the group
✔ Owner remains unchanged

Recursive group change

sudo chgrp -R tgsgroup /test

✔ Changes group owner of all files and folders inside /test

3️⃣ Changing owner AND group together (single command)

Syntax

sudo chown USER:GROUP file

Example

sudo chown vivek:tgsgroup file

✔ Owner → vivek
✔ Group → tgsgroup

Recursive owner + group change

sudo chown -R vivek:tgsgroup /test

✔ Changes owner and group of all files and folders inside /test

Change group owner by chown command

sudo chown :tgsgroup file

Important Notes (Exam-Relevant ⭐)

🔹 Why sudo is required?

• Normal users cannot change ownership
• Only root can change owner/group

✔ That’s why:

By default you must use sudo to change a file owner or group

Summary Table ⭐

One-line exam summary 📝

chown changes owner (and optionally group), chgrp changes only group, and both usually require sudo.

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