Book 13 · Operating SystemsPage 11 of 37

File Permissions

In Turkish: Dosya İzinleri

Updated September 30, 20263 min read

In short

File permissions are rules stored with each file that decide which users may read it, change it, or run it, enforced by the operating system on every access.

What are file permissions?

File permissions are the operating system's way of controlling who can do what with each file and directory. On Unix-like systems every file has an owning user, an owning group, and three sets of permissions: one for the owner, one for members of the group, and one for everyone else. Each set can allow reading (r), writing (w), and executing (x).

Running ls -l shows permissions as a string such as -rwxr-xr--: the first character is the file type, followed by the owner, group, and others triplets. The same permissions can be written in octal, where read is 4, write is 2, and execute is 1, so 754 means the owner can do everything, the group can read and execute, and others can only read. On directories the letters mean something slightly different: read lists the names, write allows creating, deleting, and renaming entries, and execute allows entering the directory. You change permissions with chmod and ownership with chown, and the kernel checks them every time a program opens a file.

Permissions are like keys in an office building: the owner has a key to their own office, the team shares a key to the project room, and visitors can only look through the window. They matter in daily work: SSH refuses to use a private key that other users can read, a script needs the execute bit before you can run it, and a web server should not be able to overwrite its own code. Windows and many Linux systems also support access control lists (ACLs), which allow more fine-grained rules for specific users.

File permissions are sometimes confused with authorization inside applications, such as role-based access control. File permissions are enforced by the operating system kernel for files and directories, while application authorization decides what a logged-in user may do inside that app. A common mistake is running chmod 777 to fix an access error, which lets every user modify the file; following the principle of least privilege and granting only what is needed is much safer.

Key takeaways

Each file has an owner, a group, and permissions for owner, group, and others.
The basic permissions are read, write, and execute.
Octal notation adds read (4), write (2), and execute (1), as in 755 or 600.
chmod changes permissions and chown changes ownership.
Avoid chmod 777; grant the least access that works.

Example

Reading and changing permissions in a Unix shellbash

# Type, then owner (rwx), group (r-x), and others (r--)
ls -l deploy.sh
# -rwxr-xr-- 1 ada devs 512 Sep 30 10:00 deploy.sh

# Make a script executable for its owner
chmod u+x deploy.sh

# Owner can read and write; nobody else has access (needed for SSH keys)
chmod 600 ~/.ssh/id_ed25519

# Change the owner and the group
sudo chown ada:devs deploy.sh

Readers ask

What does chmod 755 mean?

It gives the owner read, write, and execute permission (7), and gives the group and everyone else read and execute permission (5). It is a common setting for scripts and directories that others may use but not change.

Why is chmod 777 dangerous?

It gives every user on the system permission to read, modify, and run the file. Any compromised account or process could then change it, for example to insert malicious code.

What is the difference between Unix permissions and ACLs?

Classic Unix permissions allow rules for just three groups: owner, group, and others. Access control lists add entries for any number of specific users and groups, which is how Windows manages file access and an option on most Linux file systems.