Linux File Permissions | the Magic of Restrictions Hidden in Plain Sight

I think it’s been around 5 years, I switched to linux. I also try to understand linux deeply, through config, unix tools, using CLI tools and so on… But I never tried to truly understand how linux file permission works. when i want to execute a file, I always used:

chmod +x <script_file>

But recently I explored on how there permissions works and why is it important, and why one should learn about it

What are Permissions and Ownership?

Permission goes hand in hand with ownership. Permission define who can access a file or directory and what actions they are allowed to perform.

Types of Permission

• read(r): These permissions decide whether a file can or cannot be read.
• write(w): These permissions determine whether a file can be modified.
• execute(x): These permissions determine whether a file can be executed as a program, required for scripts and binaries.

Without execute permission on a directory, you cannot traverse or access files inside it, even if you know their names.

Executable binaries require execute permission to be run.

Why permissions matters

I always wondered and sometimes annoyed to change a file permission and work with it. But as I work on many things for some time, I understand the importance of permissions. It’s always recommended to just give the minimum/enough permission to avoid unwanted complications. Because, minimum required permissions allow an account to work on the requirements, but also restricts the accounts from accessing unwanted files or operations they shouldn’t perform.

Always restrict the account to only work on what they needed to work on

Types of accounts

• owner: The owner is the account, which have access to modify the permission of a file.
• group: A group is like a pool/group of users, who share same permissions.
• others: others are all other users, who are neither the current user nor a user belongs the same user group of the file.

An example of permissions

The first column in the images shows how every type of account have different versions of permissions:

• The first ./-/d defines, whether a node is a file or a directory. ./- defines a file, while d defines a directory
• The next three represents the permission allowed for the owner, like whether the owner have read, write and execute access.
• The next three represents the permission allowed for the user group.
• The next three represents the permission allowed for the others.

How to change permissions?

When dealing with changing permissions in linux, it can be handled in 2 ways:

• whether to change the ownership of the file.
• whether to change the permission of the file without affecting the ownership.

Changing the ownership of a file

To change the ownership of a file, we use:

chown <user_name> <file_name>               # to change the ownership to a user
# or
chown <user_name>:<group_name> <file_name>  # to change the ownership to a user alongside the user group

Changing the permission of a file

To understand how to change the permission of a file, first we need to understand the linus permission numbering system. linux uses a numerical method to identify file permissions

• 4 — read access
• 2 — write access
• 1 — execute access

Any combination of these can be used, like 0(---), 6(rw-), 5(r-x), 3(-wx).

now to change the permission, use:

chmod
    <4/2/1/6/5/3>\ # owner permission
    <4/2/1/6/5/3>\ # group permission
    <4/2/1/6/5/3>\ # others permission
<file_name>

Common file permissions

some of common file permissions include:

• 644 - File Baseline
• 755 - Directory Baseline
• 400 - Key Pair

Conclusion

Understanding permissions helped me understand why restrictions play such an important role in any system.