Create and Manage Linux Users

# Create user with home directory
sudo useradd -m -s /bin/bash username

# Set password
sudo passwd username

# All in one (Debian/Ubuntu: more interactive)
sudo adduser username

id username
cat /etc/passwd | grep username

# Add to sudo group (Ubuntu/Debian)
sudo usermod -aG sudo username

# Add to wheel group (CentOS/AlmaLinux/RHEL)
sudo usermod -aG wheel username

# Verify groups
groups username

echo "username ALL=(ALL) NOPASSWD:ALL" | sudo tee /etc/sudoers.d/username

# Create .ssh directory for new user
sudo mkdir -p /home/username/.ssh
sudo chmod 700 /home/username/.ssh

# Add the user's public key
sudo nano /home/username/.ssh/authorized_keys
# Paste the public key (e.g. content of id_rsa.pub)

# Set correct permissions
sudo chmod 600 /home/username/.ssh/authorized_keys
sudo chown -R username:username /home/username/.ssh

# Create user without shell
sudo useradd -m -s /usr/sbin/nologin sftpuser
sudo passwd sftpuser

# Create the directory the user will see
sudo mkdir -p /home/sftpuser/files
sudo chown sftpuser:sftpuser /home/sftpuser/files

# Configure SSH for SFTP chroot
sudo nano /etc/ssh/sshd_config

# Home must be owned by root for chroot
sudo chown root:root /home/sftpuser
sudo chmod 755 /home/sftpuser

sudo systemctl restart ssh

# Delete user (keep home)
sudo userdel username

# Delete user and their home
sudo userdel -r username

# Verify
id username  # should say "no such user"

# Change shell
sudo chsh -s /bin/bash username
sudo chsh -s /bin/zsh username
sudo chsh -s /usr/sbin/nologin username  # disable login

# See available shells
cat /etc/shells

# Lock (adds ! to password)
sudo usermod -L username
sudo passwd -l username

# Unlock
sudo usermod -U username
sudo passwd -u username

# Check status
sudo passwd -S username
# Output: username L ... (L = locked, P = password active)

# All system users
cat /etc/passwd | cut -d: -f1

# Only users with home (humans, not system)
awk -F: '$3 >= 1000 && $3 < 65534 {print $1}' /etc/passwd

# Who is connected now
who
w

# Last accesses
last | head -20

# Last failed attempts
sudo lastb | head -20

# Change owner
sudo chown username:username /path/file

# Change recursively
sudo chown -R www-data:www-data /var/www/html

# Standard permissions for web
sudo find /var/www/html -type f -exec chmod 644 {} \;
sudo find /var/www/html -type d -exec chmod 755 {} \;