chmod (change permissions) in Linux CLI - Commands & Configuration

Introduction

Sometimes files or folders are locked so you cannot open or change them. The chmod command changes who can read, write, or run a file. This helps you control access to your files safely.

When you want to let a friend open a file but not change it

When you need to make a script file runnable on your computer

When you want to stop others from deleting or editing your important files

When you share a folder and want only certain people to add files

When you fix permission errors that stop programs from working

Commands

This command adds permission for the file owner (user) to run the script.sh file. It makes the script executable by you.

Terminal

chmod u+x script.sh

Expected Output

No output (command runs silently)

→

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

This command lists the file details including permissions so you can check if the execute permission was added.

Terminal

ls -l script.sh

Expected Output

-rwxr--r-- 1 user user 123 Apr 27 12:00 script.sh

→

-l - Show detailed file information including permissions

This command removes read and write permissions for group and others on secret.txt, so only the owner can read or change it.

Terminal

chmod go-rw secret.txt

Expected Output

No output (command runs silently)

→

go-rw - Remove read and write permissions for group and others

Check the permissions of secret.txt to confirm group and others cannot read or write it.

Terminal

ls -l secret.txt

Expected Output

-rw------- 1 user user 456 Apr 27 12:05 secret.txt

→

-l - Show detailed file information including permissions

Key Concept

If you remember nothing else from chmod, remember: permissions control who can read, write, or run a file, and you change them by adding (+) or removing (-) rights for user (u), group (g), or others (o).

Common Mistakes

Using chmod without specifying user/group/others and plus/minus signs

The command will fail or not change permissions as expected because it needs clear instructions on what to add or remove

Always specify who (u/g/o) and what (+/-) permission to change, like 'chmod u+x file'

Trying to run a script without execute permission

The system will deny running the file because it lacks execute rights

Add execute permission with 'chmod u+x script.sh' before running

Removing all permissions accidentally

You may lock yourself out of the file and cannot read or change it

Check permissions carefully and avoid removing all rights from the user

Summary

chmod changes file permissions to control who can read, write, or execute files.

Use symbols u, g, o to specify user, group, or others and + or - to add or remove permissions.

Check permissions with 'ls -l' to confirm changes.

Practice

1. What does the chmod 755 filename command do in Linux?

easy

A. Removes all permissions from everyone

B. Sets read and write for owner; execute for group and others

C. Sets read, write, execute for owner; read and execute for group and others

D. Only changes the file owner

Solution

1. Step 1: Understand numeric permission 755

   The digits 7, 5, 5 represent permissions for owner, group, and others respectively. 7 means read(4)+write(2)+execute(1)=7, 5 means read(4)+execute(1)=5.

2. Step 2: Apply permissions to file

   Owner gets full permissions (rwx), group and others get read and execute (r-x). This matches the description in Sets read, write, execute for owner; read and execute for group and others.

3. Final Answer:

   Sets read, write, execute for owner; read and execute for group and others -> Option C

4. Quick Check:

   chmod 755 = rwxr-xr-x [OK]

Hint: Remember 7=rwx, 5=rx, 6=rw [OK]

Common Mistakes:

• Confusing numeric codes with letters
• Thinking 755 removes permissions
• Assuming chmod changes ownership

2. Which of the following is the correct syntax to add execute permission for the user on a file named script.sh?

easy

A. chmod +x u script.sh

B. chmod script.sh u+x

C. chmod x+u script.sh

D. chmod u+x script.sh

Solution

1. Step 1: Recall chmod symbolic syntax

   The correct format is chmod [who][operator][permission] filename. Here, 'u' means user, '+' adds permission, 'x' is execute.

2. Step 2: Match syntax to options

   chmod u+x script.sh matches the correct syntax: chmod u+x script.sh. Other options have incorrect order or misplaced arguments.

3. Final Answer:

   chmod u+x script.sh -> Option D

4. Quick Check:

   chmod u+x filename adds execute for user [OK]

Hint: Use chmod [u/g/o][+/-][r/w/x] filename [OK]

Common Mistakes:

• Placing permissions before user
• Putting filename before options
• Using spaces inside symbolic mode

3. What will be the output of ls -l file.txt after running chmod 640 file.txt on a file owned by user 'alice' and group 'staff'?

medium

A. -rw-r----- 1 alice staff 0 date file.txt

B. -rwxr-xr-x 1 alice staff 0 date file.txt

C. -rw-rw-r-- 1 alice staff 0 date file.txt

D. -r--r----- 1 alice staff 0 date file.txt

Solution

1. Step 1: Decode chmod 640 permissions

   6 = read(4)+write(2) for owner, 4 = read(4) for group, 0 = no permissions for others.

2. Step 2: Translate to ls -l format

   Owner: rw-, Group: r--, Others: ---. So the permission string is '-rw-r-----'. The rest shows ownership and file info.

3. Final Answer:

   -rw-r----- 1 alice staff 0 date file.txt -> Option A

4. Quick Check:

   chmod 640 = rw-r----- [OK]

Hint: Match digits to rwx: 6=rw-, 4=r--, 0=--- [OK]

Common Mistakes:

• Mixing group and others permissions
• Assuming execute permission is set
• Confusing numeric values

4. You ran chmod 777 myfile but ls -l myfile still shows -rw-r--r--. What is the most likely reason?

medium

A. You forgot to run the command as root or with sudo

B. The file system is mounted as read-only

C. The chmod command syntax is incorrect

D. The file is a directory, not a file

Solution

1. Step 1: Understand chmod effect and file system state

   chmod 777 should set full permissions. If permissions don't change, it may be due to file system restrictions like read-only mount.

2. Step 2: Evaluate other options

   Running as root is not always required for own files. Syntax is correct. File type does not prevent permission changes. Read-only file system blocks changes.

3. Final Answer:

   The file system is mounted as read-only -> Option B

4. Quick Check:

   Read-only FS blocks chmod changes [OK]

Hint: Check if filesystem is read-only if chmod fails [OK]

Common Mistakes:

• Assuming sudo always needed
• Blaming syntax without checking
• Ignoring file system mount options

5. You want to set permissions so that the owner can read and write, the group can only read, and others have no permissions on a file named data.txt. Which command achieves this?

hard

A. chmod 640 data.txt

B. chmod 750 data.txt

C. chmod 664 data.txt

D. chmod 600 data.txt

Solution

1. Step 1: Translate desired permissions to numeric code

   Owner: read+write = 6, Group: read only = 4, Others: no permissions = 0. So numeric code is 640.

2. Step 2: Match numeric code to options

   chmod 640 data.txt is chmod 640 data.txt, which matches the desired permissions exactly.

3. Final Answer:

   chmod 640 data.txt -> Option A

4. Quick Check:

   Owner=rw, Group=r, Others=none = 640 [OK]

Hint: Use chmod 640 for rw-r----- permissions [OK]

Common Mistakes:

• Using 750 which gives execute permission
• Using 664 which allows others read
• Using 600 which denies group read