users.md

Login

# Show all users
less /etc/passwd

# Login as user
su - <USERNAME>

# Login as root
sudo -i

# Logout
CTRL + d # or exit

# Change password
sudo passwd <USERNAME>

Add

# Guided user creation
sudo adduser <USERNAME>

# Add sudo access
sudo usermod -aG sudo <USERNAME>

# remove sudo access
sudo deluser <USERNAME> sudo

# Scripted user creation
useradd --uid <UID> --gid <GID> -m -s /usr/bin/zsh -d /var/www/<USERNAME> --password <PASSWORD> <USERNAME>

Change

# Username
sudo usermod -l new-name old-name

# Password
sudo passwd

Access

# Limit access to specific directory
sudo usermod -d /home/user/directory <username>

Remove

# Remove user
sudo deluser --remove-home <USERNAME>

# Check if user exists
id <USERNAME>

# Kill all processes by user
pkill -u <USERNAME>

The root account is disabled by default in Ubuntu, so there is no root password, that's why su fails with an authentication error.

Permissions

ls -l

#    permission   links   user    group    size     changed        file
#    -rwxrwxrwx     1     foo      bar      45    Nov 13 22:51   script.js

Permission	Type	Owner	Group	Other	Meaning
-rwx------	- (file)	rwx	---	---	File only the owner can read, write and execute.
drw-rw----	d (directory)	rw-	rw-	r--	Owner and group can read/write, others just read.

# Change file owner
chown user:group file_name

# Add user to a group. Needs a logout
sudo usermod -a -G group user

chmod

# Change file mode i.e. `rwx` permissions.
chmod

# Add EXECUTE permission to owner, group and others.
chmod +x FILE

# Add WRITE permission to just the owner.
chmod u+w FILE

# Remove read permission to other users.
chmod o-r FILE

Adding permission one by one can get very tedious (9 commands), so instead, we can set the permissions by using octal numbers.

chmod 754 FILE

	Owner	Group	Others
Octal	7	5	4
Chars	rwx	r-x	r--
Permission	Read, write, execute	Read, execute	Read

A binary mask is used to determine the numbers. Add the column values for each 1 and the sum is the decimal number.

Dec	Permission	RWX	Read (4)	Write (2)	Execute (1)	Decimal
7	Read, write, execute	rwx	1	1	1	7
6	Read, write	rw-	1	1	0	6
5	Read, execute	r-x	1	0	1	5
4	Read	r--	1	0	0	4
3	Write, execute	-wx	0	1	1	3
2	Write	-w-	0	1	0	2
1	Execute	--x	0	0	1	1
0	No permissions	---	0	0	0	0