Code-Memo

User and group management

Users

Create a user:

sudo useradd -m -s /bin/bash alice
sudo passwd alice

Modify a user:

sudo usermod -aG sudo alice     # add to group (Debian/Ubuntu)
sudo usermod -s /bin/zsh alice  # change shell
sudo usermod -L alice           # lock account
sudo usermod -U alice           # unlock account

Remove a user:

sudo userdel alice
sudo userdel -r alice           # remove home directory too

Groups

sudo groupadd devops
sudo gpasswd -a alice devops
groups alice
id alice

Where identity data lives

/etc/passwd (user records)
/etc/shadow (password hashes, aging)
/etc/group (groups)
/etc/gshadow (group secrets/admins)

Sudo basics

Users allowed via /etc/sudoers and /etc/sudoers.d/*
Always edit with visudo to validate syntax

Permissions and ownership