Create
# Guided user creation
adduser <USERNAME>
# Scripted user creation
useradd --uid <UID> --gid <GID> -m -s /usr/bin/zsh -d /var/www/<USERNAME> --password <PASSWORD> <USERNAME>
Login
# Login as user
su - <USERNAME>
# Login as root
sudo -i
# Logout
CTRL + d
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 - Show file permissions.
chown USER:GROUP FILENAME - Change file owner.
sudo usermod -a -G <group> <user> - Add user to a group. Needs a logout.
chmod - Change file mode i.e. rwx permissions.
r - Read.
w - Write.
x - Execute.
| Permission | File Type | Owner | Group | Other | Meaning |
|---|---|---|---|---|---|
| -rwx------ | - | rwx | --- | --- | File that only the owner can read, write and execute. |
| drw-rw---- | d | rw- | rw- | r-- | Directory that owner and group can read/write, others just read |
Owner permissions trump group ones.
chmod +x FILE - Add execute permission to owner, group and others.
chmod u+w FILE - Add write permission to just the owner.
chmod o-r FILE - Remove read permission to other users.
This can get very tedious for each permission (9 commands), so instead, we can set the permissions by using octal numbers.
First number is owner, second is group, and third is others.
chmod 644 FILE - Owner read/write, group read, others read.
| Dec | Permission |
|---|---|
| 7 | Read, write, execute |
| 6 | Read, write |
| 5 | Read, execute |
| 4 | Read |
| 3 | Write, execute |
| 2 | Write |
| 1 | Execute |
| 0 | No permissions |
7, 6, 5, 4, 0 - Most used.
A binary mask is used to determine the numbers. Add the column values for each 1 and the sum is the decimal number.
| Type | Read (4) | Write (2) | Execute (1) | Decimal |
|---|---|---|---|---|
| User | 1 | 1 | 1 | 7 |
| Group | 1 | 0 | 1 | 5 |
| Other | 1 | 0 | 0 | 4 |
The number 754 would give the owner full permissions, the group read/execute, and just read for the others.