Mastering Linux File Permissions and User Management: From ls -l to sudo

1. Understanding the ls -l output

Running ls -l shows a line such as drwxr-xr-x 2 user user 4096 Jan 1 12:00 Documents. The first column ( drwxr-xr-x) encodes the file type and permission bits.

1st character : file type – d for directory, - for regular file.

2nd‑4th characters (rwx) : permissions for the owner .

5th‑7th characters (r-x) : permissions for the group .

8th‑10th characters (r-x) : permissions for others .

Meaning of the rwx bits

r

(Read): view file contents or list directory entries. w (Write): modify file contents or create/delete entries in a directory. x (Execute): run a file as a program or enter a directory.

2. Changing permissions with chmod

chmod

(Change Mode) modifies file permissions. Two common syntaxes are provided.

Numeric method (most common)

r = 4, w = 2, x = 1, none = 0.

Example: chmod 755 file sets owner to rwx (7), group to r-x (5), others to r-x (5).

Symbolic method

Specify target: u (user/owner), g (group), o (others), a (all).

Operation: + add, - remove, = set exactly.

chmod +x script.sh    # add execute permission for everyone (most common)
chmod u+w file.txt    # add write permission for the owner
chmod o-r secret.txt  # remove read permission from others

3. Superuser: Root and sudo

The root account can perform any action, including deleting the entire system. Ubuntu locks the root account by default; regular users obtain temporary admin rights with sudo.

When to use sudo? Installing software, changing system configuration, or editing files under /etc.

How to use it? Prefix the command with sudo, e.g.:

sudo apt update

The system prompts for the user's password (no asterisks are shown).

A practical habit: avoid sudo when possible. Many “permission denied” issues stem from incorrect file ownership or group; fix them with chown before adjusting permissions with chmod.

4. User and group management

chown (Change Owner)

Changes the owner (and optionally the group) of a file or directory.

sudo chown user:group file.txt
sudo chown -R user:user /var/www/html   # recursive change

adduser / useradd

Creates a new user. adduser provides an interactive wizard and is recommended.

sudo adduser newuser

usermod

Modifies user attributes, commonly used to add a user to the sudo group for admin privileges.

sudo usermod -aG sudo newuser

Conclusion

Understanding Linux permissions clarifies why Linux is generally more secure than Windows: a program cannot run unless the appropriate chmod bits allow it.