| Command | Description |
|---|
whoami | displays the current logged-in user |
id | shows user ID (UID), group ID (GID), and group memberships |
who | lists all users currently logged into the system |
users | lists logged-in users in a single line |
last | Shows the login history of users. |
Create New Users and Understand Primary Groups
| Command | Description |
|---|
id <username> | displays the UID, GID, and groups of a specific user |
sudo adduser <username> | creates a new user with a home directory and default settings |
sudo adduser --home <homepath> --gid <groupname> <username> | creates a new user with arguments |
sudo passwd <username> | Sets or changes a user’s password. |
Note: A user’s primary group is automatically created with the same name as the user by default.
Explore and Modify User Groups
| Command | Description |
|---|
groups username | lists all groups a user belongs to |
getent group <groupname> | get gid from a group name |
sudo groupadd <groupname> | creates a new group |
sudo usermod -aG <groupname> <username> | adds a user to an additional group without removing existing groups |
sudo usermod -G <groupname> <username> | assigns a user to a group, replacing all existing group memberships |
sudo deluser <username> <groupname> | removes a user from a specific group |
Grant Sudo Privileges to Users
| Command | Description |
|---|
sudo usermod -aG sudo <username> | adds a user to the sudo group (Debian/Ubuntu systems) |
sudo visudo | opens the sudoers file for editing |
%groupname ALL=(ALL:ALL) ALL | grants sudo privileges to all users in a group |
View and Understand File Permissions
| Command | Description |
|---|
ls -l | displays detailed file information, including permissions |
ls -ld directory/ | shows permissions for a directory |
File permission format: -rw-r--r--
- First character: Type (
- for file, d for directory)
- Next three characters: Owner permissions (read, write, execute)
- Middle three characters: Group permissions
- Last three characters: Other users’ permissions
Change File Ownership
chown: Changes the ownership of files or directories.
| Command | Description |
|---|
sudo chown user file.txt | changes the owner of file.txt to user |
sudo chown user:group file.txt | changes both the owner and group of file.txt |
sudo chown -R user directory/ | recursively changes ownership for a directory |
Modify File Permissions
chmod: Changes file permissions.
| Command | Description |
|---|
chmod 644 file.txt | sets permissions to read/write for owner, read-only for group and others |
chmod u+x file.sh | adds execute permission for the owner |
chmod -R 755 folder/ | recursively sets permissions for a directory |
Common Numeric Permissions:
777: Full permissions for everyone755: Full permissions for owner, read/execute for group and others644: Read/write for owner, read-only for group and others