Understanding File Permissions

What You'll Learn

Linux uses a permission system to control who can read, write, or execute files and directories. Understanding permissions is essential for security and troubleshooting "Permission denied" errors.

Reading Permission Strings

Run ls -l to see permissions:

-rw-r--r-- 1 alex alex 4096 Jan 15 notes.txt
drwxr-xr-x 2 alex alex 4096 Jan 15 documents/

The first column breaks down like this:

Position	Meaning
Character 1	Type: `-` = file, `d` = directory, `l` = symlink
Characters 2-4	Owner permissions (the user who owns the file)
Characters 5-7	Group permissions (users in the owning group)
Characters 8-10	Others permissions (everyone else)

Each set of three uses these letters:

r — Read: can view the file contents or list a directory.
w — Write: can modify or delete the file, or add/remove files in a directory.
x — Execute: can run the file as a program or enter a directory.

Changing Permissions with `chmod`

Numeric (Octal) Mode

Each permission has a number: read = 4, write = 2, execute = 1. Add them up:

chmod 755 script.sh       # Owner: rwx (7), Group: r-x (5), Others: r-x (5)
chmod 600 secret.txt      # Owner: rw- (6), Group: --- (0), Others: --- (0)
chmod 644 readme.md       # Owner: rw- (6), Group: r-- (4), Others: r-- (4)

Symbolic Mode

chmod +x script.sh        # Add execute permission for everyone
chmod u+w file.txt        # Add write for the owner only
chmod o-rw file.txt       # Remove read and write from others

Common Permission Patterns

Mode	String	Use Case
`755`	rwxr-xr-x	Directories and executable scripts
`700`	rwx------	Private directories (e.g., ~/.ssh)
`644`	rw-r--r--	Regular files most users can read
`600`	rw-------	Sensitive files (SSH keys, passwords)

Changing Ownership with `chown`

sudo chown alex:alex file.txt       # Change owner and group
sudo chown -R www-data:www-data /var/www   # Recursively change a web directory

Note: Only root (via sudo) can change ownership.

Practice Exercise

mkdir permissions-test
cd permissions-test
touch script.sh secret.txt readme.md
chmod 755 script.sh
chmod 600 secret.txt
chmod 644 readme.md
ls -la                    # See the permission differences

Troubleshooting

"Permission denied" when running a script: Run chmod +x script.sh.
"Permission denied" editing a system file: Use sudo nano /etc/file.conf.
SSH key rejected: Check that ~/.ssh/id_rsa is mode 600 and ~/.ssh/ is mode 700.

What's Next?

Now that you understand permissions, learn how to install and manage software:

Package Management for Beginners →