Permission types (read, write, execute) in Linux CLI - Commands & Configuration

Introduction

Files and folders on Linux have permissions that control who can read, change, or run them. These permissions help keep your data safe and your system working properly.

When you want to allow someone to open and read a file but not change it

When you need to let a script run as a program

When you want to prevent others from deleting or modifying your files

When setting up shared folders where some users can only view files

When securing sensitive files by restricting access

Commands

This command lists the file details including its permissions so you can see who can read, write, or execute the file.

Terminal

ls -l example.txt

Expected Output

-rw-r--r-- 1 user user 123 Apr 27 12:00 example.txt

→

-l - Shows detailed information including permissions

This command adds execute permission for the file owner so they can run the file as a program or script.

Terminal

chmod u+x example.txt

Expected Output

No output (command runs silently)

→

u+x - Adds execute permission for the user (owner)

Check the permissions again to confirm the execute permission was added for the owner.

Terminal

ls -l example.txt

Expected Output

-rwxr--r-- 1 user user 123 Apr 27 12:00 example.txt

→

-l - Shows detailed information including permissions

This command removes write permission for group and others to prevent them from changing the file.

Terminal

chmod go-w example.txt

Expected Output

No output (command runs silently)

→

go-w - Removes write permission for group and others

Verify the write permission was removed for group and others.

Terminal

ls -l example.txt

Expected Output

-rwxr--r-- 1 user user 123 Apr 27 12:00 example.txt

→

-l - Shows detailed information including permissions

Key Concept

If you remember nothing else from this pattern, remember: read lets you view, write lets you change, and execute lets you run a file.

Common Mistakes

Trying to run a script without execute permission

The system will not allow the file to run and shows a permission denied error

Use chmod u+x filename to add execute permission for the owner before running

Giving write permission to everyone on sensitive files

Anyone can change or delete the file, risking data loss or security issues

Remove write permission for group and others using chmod go-w filename

Confusing permission letters and symbols when using chmod

Incorrect commands may not change permissions as intended or cause errors

Use u (user), g (group), o (others) with + (add), - (remove), = (set exactly) carefully

Summary

Use ls -l to see file permissions and understand who can read, write, or execute a file.

Use chmod with u, g, o and +, -, = to add, remove, or set permissions for user, group, and others.

Remember: read means view, write means change, and execute means run the file.

Practice

1. Which permission allows you to open and view the contents of a file in Linux?

easy

A. Read permission

B. Write permission

C. Execute permission

D. Delete permission

Solution

1. Step 1: Understand the meaning of read permission

   Read permission allows a user to open and view the contents of a file without changing it.

2. Step 2: Compare with other permissions

   Write permission allows changing the file, execute permission allows running the file as a program, and delete is not a standard permission type.

3. Final Answer:

   Read permission -> Option A

4. Quick Check:

   Read = open/view file [OK]

Hint: Read means open and see file contents [OK]

Common Mistakes:

• Confusing write with read permission
• Thinking execute lets you open files
• Assuming delete is a permission type

2. Which of the following is the correct symbolic representation for write permission in Linux file permissions?

easy

A. w

B. e

C. x

D. r

Solution

1. Step 1: Recall symbolic letters for permissions

   In Linux, 'r' stands for read, 'w' for write, and 'x' for execute permission.

2. Step 2: Identify the write permission symbol

   The letter 'w' is used to represent write permission.

3. Final Answer:

   w -> Option A

4. Quick Check:

   Write permission symbol = w [OK]

Hint: Write permission is always 'w' in Linux [OK]

Common Mistakes:

• Mixing 'r' and 'w' symbols
• Using 'e' which is not a permission symbol
• Confusing execute 'x' with write

3. What will be the output of the command ls -l script.sh if the permissions shown are -rwxr--r--?

medium

A. Owner can read only; group can write only; others can execute only

B. Owner can read and write only; group can execute only; others can write only

C. Owner can execute only; group can read and write; others can execute only

D. Owner can read, write, execute; group can read only; others can read only

Solution

1. Step 1: Break down the permission string

   The string '-rwxr--r--' means: owner has read (r), write (w), execute (x); group has read (r) only; others have read (r) only.

2. Step 2: Match permissions to options

   Owner can read, write, execute; group can read only; others can read only correctly describes these permissions. Other options incorrectly assign write or execute permissions to group or others.

3. Final Answer:

   Owner can read, write, execute; group can read only; others can read only -> Option D

4. Quick Check:

   rwx r-- r-- = Owner rwx, group r, others r [OK]

Hint: r = read, w = write, x = execute in order [OK]

Common Mistakes:

• Misreading group or others permissions
• Confusing execute with write
• Ignoring the first dash meaning file type

4. You run chmod u+x file.txt but still cannot execute the file. What is the most likely reason?

medium

A. The file is not owned by you

B. The file does not have execute permission for others

C. The file is not a script or executable program

D. You forgot to add read permission

Solution

1. Step 1: Understand what chmod u+x does

   This command adds execute permission for the file owner (user).

2. Step 2: Consider why execution might still fail

   If the file is not a script or executable program, adding execute permission won't let you run it.

3. Final Answer:

   The file is not a script or executable program -> Option C

4. Quick Check:

   Execute permission needs executable content [OK]

Hint: Execute permission needs executable content, not just flag [OK]

Common Mistakes:

• Assuming ownership blocks execution
• Thinking others' execute permission is needed
• Confusing read permission with execute

5. You want to allow a group to modify a file but not execute it. Which chmod command sets read and write permissions for the group only, leaving others unchanged?

hard

A. chmod o=rw file.txt

B. chmod g=rw file.txt

C. chmod g+rwx file.txt

D. chmod u=rw file.txt

Solution

1. Step 1: Understand chmod symbolic mode

   "g=rw" sets group permissions to read and write only, removing execute if present.

2. Step 2: Check other options

   chmod g+rwx file.txt adds execute permission, which is not wanted. chmod o=rw file.txt changes others' permissions, and chmod u=rw file.txt changes user permissions.

3. Final Answer:

   chmod g=rw file.txt -> Option B

4. Quick Check:

   g=rw sets group read/write only [OK]

Hint: Use g=rw to set group read/write without execute [OK]

Common Mistakes:

• Using g+rwx adds execute permission
• Changing user or others instead of group
• Using + instead of = to overwrite permissions