#Linux Fundamentals

1# View current user
2whoami
3
4# View user details
5id
6
7# View all users
8cat /etc/passwd
9
10# View all groups
11cat /etc/group

1# Create user
2sudo useradd -m -s /bin/bash username
3
4# Create user with home directory and groups
5sudo useradd -m -s /bin/bash -G sudo,docker devops
6
7# Set password
8sudo passwd username
9
10# Delete user
11sudo userdel -r username
12
13# Modify user
14sudo usermod -aG docker username  # Add to group

1# Create group
2sudo groupadd developers
3
4# Add user to group
5sudo usermod -aG developers username
6
7# Remove user from group
8sudo gpasswd -d username developers
9
10# View group members
11getent group developers

1# Edit sudoers file (safe way)
2sudo visudo
3
4# Add user to sudoers
5username ALL=(ALL:ALL) ALL
6
7# Allow passwordless sudo
8username ALL=(ALL) NOPASSWD:ALL
9
10# Add to sudo group (Debian/Ubuntu)
11sudo usermod -aG sudo username
12
13# Add to wheel group (RHEL/CentOS)
14sudo usermod -aG wheel username

1# Using numeric mode
2chmod 755 script.sh
3chmod 644 config.yaml
4
5# Using symbolic mode
6chmod u+x script.sh        # Add execute for owner
7chmod g+w file.txt         # Add write for group
8chmod o-r secret.txt       # Remove read for others
9chmod a+r public.txt       # Add read for all
10
11# Recursive
12chmod -R 755 directory/

1# Change owner
2sudo chown username file.txt
3
4# Change owner and group
5sudo chown username:groupname file.txt
6
7# Recursive
8sudo chown -R username:groupname directory/

1# Update package list
2sudo apt update
3
4# Upgrade packages
5sudo apt upgrade
6
7# Install package
8sudo apt install nginx
9
10# Remove package
11sudo apt remove nginx
12
13# Remove with config files
14sudo apt purge nginx
15
16# Search for packages
17apt search nginx
18
19# Show package info
20apt show nginx
21
22# Clean up
23sudo apt autoremove
24sudo apt clean

1# Update packages
2sudo dnf update
3
4# Install package
5sudo dnf install nginx
6
7# Remove package
8sudo dnf remove nginx
9
10# Search for packages
11dnf search nginx
12
13# List installed packages
14dnf list installed
15
16# Show package info
17dnf info nginx
18
19# Clean cache
20sudo dnf clean all

1# Update package list
2apk update
3
4# Install package
5apk add nginx
6
7# Remove package
8apk del nginx
9
10# Search packages
11apk search nginx
12
13# Upgrade all packages
14apk upgrade

1# Start service
2sudo systemctl start nginx
3
4# Stop service
5sudo systemctl stop nginx
6
7# Restart service
8sudo systemctl restart nginx
9
10# Reload configuration
11sudo systemctl reload nginx
12
13# Check status
14sudo systemctl status nginx
15
16# Enable at boot
17sudo systemctl enable nginx
18
19# Disable at boot
20sudo systemctl disable nginx
21
22# List all services
23systemctl list-units --type=service
24
25# View logs
26journalctl -u nginx
27journalctl -u nginx -f  # Follow logs
28journalctl -u nginx --since "1 hour ago"

1# /etc/systemd/system/myapp.service
2[Unit]
3Description=My Application
4After=network.target
5
6[Service]
7Type=simple
8User=appuser
9WorkingDirectory=/opt/myapp
10ExecStart=/opt/myapp/bin/myapp
11Restart=always
12RestartSec=5
13
14[Install]
15WantedBy=multi-user.target

1# Reload daemon after creating/editing unit file
2sudo systemctl daemon-reload
3
4# Enable and start
5sudo systemctl enable --now myapp