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.