Linux stuff

Links to useful Linux things

- Common Commands

SSH Stuff

ssh - Secure Shell for remote system access

ssh is used to securely log into remote systems, execute commands on remote machines, and create encrypted tunnels for various network services.

Examples:

• ssh user@example.com: Connect to example.com as "user"
• ssh -p 2222 user@example.com: Connect using a non-standard port (2222)
• ssh -i ~/.ssh/my_key user@example.com: Use a specific private key for authentication
• ssh user@example.com 'ls -l /tmp': Execute a command on the remote system without entering interactive mode
• ssh -X user@example.com: Enable X11 forwarding for GUI applications

scp - Secure Copy for file transfer

scp uses SSH to securely copy files between local and remote systems or between two remote systems.

Examples:

• scp file.txt user@example.com:/home/user/: Copy a local file to a remote system
• scp user@example.com:/home/user/file.txt .: Copy a remote file to the local system
• scp -r directory/ user@example.com:/home/user/: Recursively copy a directory to a remote system
• scp -P 2222 file.txt user@example.com:/home/user/: Use a non-standard SSH port
• scp user1@host1:/file user2@host2:/directory/: Copy a file from one remote host to another

ssh-keygen - Generate SSH key pairs

ssh-keygen creates public/private key pairs for SSH authentication, offering a more secure alternative to password-based logins.

Examples:

• ssh-keygen -t rsa: Generate an RSA key pair
• ssh-keygen -t ed25519: Generate an Ed25519 key pair (more secure and efficient)
• ssh-keygen -t rsa -b 4096: Generate an RSA key with 4096-bit strength
• ssh-keygen -f ~/.ssh/my_key: Specify a custom filename and path for the key
• ssh-keygen -p -f ~/.ssh/id_rsa: Change the passphrase of an existing private key

ssh-copy-id - Install SSH key on a remote server

ssh-copy-id securely copies your public key to a remote server's authorized_keys file, enabling key-based authentication.

Examples:

• ssh-copy-id user@example.com: Copy the default public key to the remote server
• ssh-copy-id -i ~/.ssh/my_key.pub user@example.com: Specify a particular public key to copy
• ssh-copy-id -p 2222 user@example.com: Use a non-standard SSH port

ssh-agent - Manage SSH keys

ssh-agent is a program that holds private keys used for public key authentication, allowing you to use SSH without re-entering your passphrase.

Examples:

• eval $(ssh-agent): Start the SSH agent
• ssh-add: Add the default private key to the agent
• ssh-add ~/.ssh/my_key: Add a specific private key to the agent
• ssh-add -l: List fingerprints of all identities currently represented by the agent
• ssh-add -D: Delete all identities from the agent

sftp - Secure File Transfer Protocol

sftp provides an interactive file transfer session with an interface similar to FTP but using SSH for security.

Examples:

• sftp user@example.com: Start an SFTP session
• sftp> get remote_file: Download a file from the remote system
• sftp> put local_file: Upload a file to the remote system
• sftp> ls: List directory contents on the remote system
• sftp> !ls: Execute a local command (list local directory contents)

systemd

systemctl - Control the systemd system and service manager

systemctl is the primary command-line tool for interacting with systemd, the init system and service manager for modern Linux distributions. It allows users to manage system services, check their status, and control various aspects of the system.

Examples:

• systemctl start apache2.service: Start the Apache web server service
• systemctl stop mysql.service: Stop the MySQL database service
• systemctl restart sshd.service: Restart the SSH daemon
• systemctl status nginx.service: Check the status of the Nginx web server
• systemctl enable cups.service: Enable the CUPS printing service to start at boot
• systemctl disable bluetooth.service: Prevent the Bluetooth service from starting at boot
• systemctl reload systemd-journald.service: Reload the systemd journal service configuration
• systemctl list-units --type=service: List all active service units
• systemctl is-active postgresql.service: Check if the PostgreSQL service is currently running
• systemctl is-enabled firewalld.service: Check if the firewall service is set to start at boot
• systemctl daemon-reload: Reload systemd manager configuration after modifying unit files
• systemctl list-dependencies apache2.service: Show dependencies of the Apache service
• systemctl set-default multi-user.target: Change the default system target (runlevel)
• systemctl poweroff: Shut down the system
• systemctl reboot: Restart the system

Create, monitor and terminate process

& - Run a command in the background

The & symbol is used to run a command or script in the background, allowing you to continue using the terminal.

Examples:

• long_running_script.sh &: Run the script in the background
• sleep 60 &: Run the sleep command for 60 seconds in the background
• nohup command &: Run a command in the background, immune to hangups

bg - Resume suspended jobs in the background

bg resumes execution of a suspended job in the background.

Examples:

• bg %1: Resume the first suspended job in the background
• bg %vim: Resume the suspended vim job in the background
• bg: Resume the most recently suspended job in the background

fg - Bring background jobs to the foreground

fg brings a background job into the foreground, allowing direct interaction.

Examples:

• fg %2: Bring the second background job to the foreground
• fg %emacs: Bring the background emacs job to the foreground
• fg: Bring the most recent background job to the foreground

jobs - Display status of jobs in the current session

jobs lists the active jobs in the current shell session.

Examples:

• jobs: List all jobs
• jobs -l: List jobs with process IDs
• jobs -p: List only process IDs of the active jobs

nohup - Run a command immune to hangups

nohup allows a command to continue running after the user logs out.

Examples:

• nohup long_running_script.sh &: Run a script that continues after logout
• nohup command > output.log 2>&1 &: Run a command, redirecting output and errors
• nohup python script.py &: Run a Python script that persists after terminal closure

screen - Terminal multiplexer

screen creates a virtual terminal that can be detached and reattached, allowing sessions to persist.

Examples:

• screen: Start a new screen session
• screen -S session_name: Start a named screen session
• screen -r: Reattach to a detached screen session
• screen -list: List available screen sessions

tmux - Terminal multiplexer

tmux, like screen, allows for persistent terminal sessions and window management.

Examples:

• tmux: Start a new tmux session
• tmux new -s session_name: Create a named tmux session
• tmux attach -t session_name: Attach to an existing session
• tmux ls: List active tmux sessions

top - Display and update sorted information about processes

top provides a dynamic real-time view of running system processes.

Examples:

• top: Display system processes, updating every 3 seconds
• top -u username: Show processes for a specific user
• top -p 1234,5678: Monitor specific processes by PID

ps - Report a snapshot of current processes

ps displays information about a selection of the active processes.

Examples:

• ps aux: Display detailed information about all processes
• ps -ef: Show all processes in full format
• ps --sort=-%cpu: List processes sorted by CPU usage (descending)

pstree - Display a tree of processes

pstree shows the running processes as a tree, which is helpful for understanding process relationships.

Examples:

• pstree: Display process tree
• pstree -p: Show process IDs along with the tree
• pstree -u: Show user names for each process

uptime - Show how long the system has been running

uptime displays the current time, system uptime, number of users, and load average.

Examples:

• uptime: Display system uptime and load
• uptime -p: Show uptime in a pretty format
• uptime -s: Display the system up since time

pgrep - Look up or signal processes based on name and other attributes

pgrep looks through the currently running processes and lists the process IDs which match the selection criteria.

Examples:

• pgrep firefox: List PIDs of firefox processes
• pgrep -u root: List PIDs of processes owned by root
• pgrep -l sshd: List PIDs and names of sshd processes

kill - Send a signal to a process

kill sends a signal to a process, by default the TERM signal to request termination.

Examples:

• kill 1234: Send SIGTERM to process with PID 1234
• kill -9 5678: Send SIGKILL to forcefully terminate process 5678
• kill -l: List available signals

pkill - Signal processes based on name and other attributes

pkill looks up processes based on name and other attributes and sends them a signal.

Examples:

• pkill firefox: Send SIGTERM to all firefox processes
• pkill -9 httpd: Send SIGKILL to all httpd processes
• pkill -u username: Terminate all processes owned by username

killall - Kill processes by name

killall sends a signal to all processes running any of the specified commands.

Examples:

• killall nginx: Terminate all processes named nginx
• killall -9 apache2: Forcefully kill all apache2 processes
• killall -u username bash: Kill all bash processes for a specific user

Desktop environment

startx - Start the X Window System

startx is a script that initializes the X Window System session. It sets up the environment and launches the X server along with initial clients.

Examples:

• startx: Start the X session with default settings
• startx -- :1: Start X on display :1
• startx ~/.xinitrc: Use a custom initialization script

X server - Core component of the X Window System

The X server manages the display, keyboard, and mouse, handling rendering and input for client applications.

Examples:

• Xorg: The most common X server implementation
• Xvfb: X virtual framebuffer, a display server without a physical display
• Xnest: Nested X server, allowing an X session within another X session

X client - Application that uses the X protocol

X clients are programs that use the X protocol to display their graphical user interface.

Examples:

• xterm: Terminal emulator for X
• xclock: Simple clock application
• xeyes: Demo program showing eyes that follow the mouse cursor

Display Manager - Graphical login manager

A display manager provides a graphical login interface and starts the X session after user authentication.

Examples:

• gdm: GNOME Display Manager
• lightdm: Lightweight Display Manager
• sddm: Simple Desktop Display Manager

Window Manager - Controls window placement and appearance

The window manager handles the positioning, resizing, and decorations of windows within the X environment.

Examples:

• i3: Tiling window manager
• openbox: Lightweight stacking window manager
• awesome: Highly configurable window manager

X Window System - Network-transparent windowing system

The X Window System is a complete architecture for creating graphical user interfaces on Unix-like operating systems.

Examples:

• X11: The current version of the X Window System protocol
• Xlib: C library for X Window System client-side programming
• XCB: C library for X protocol connections

Integrated Desktop Environment - Complete graphical user interface

An integrated desktop environment provides a consistent user interface, including window management, file management, and system settings.

Examples:

• GNOME: Popular desktop environment with a focus on simplicity
• KDE Plasma: Feature-rich desktop environment
• Xfce: Lightweight desktop environment for Unix-like operating systems

xauth - X authority file utility

xauth is used to edit and display the authorization information used in connecting to the X server.

Examples:

• xauth list: Display the list of authority entries
• xauth add :0 . $(mcookie): Add a new authority entry for display :0
• xauth extract - $DISPLAY | ssh remote_host xauth merge -: Transfer X authority to a remote host

DISPLAY - Environment variable for X display

The DISPLAY environment variable tells X clients which display and screen to use.

Examples:

• export DISPLAY=:0: Set the display to the first local X server
• echo $DISPLAY: Print the current DISPLAY value
• DISPLAY=:1 xterm: Run xterm on display :1

Terminal program - Text-based interface for interacting with the system

Terminal programs provide a command-line interface within the X environment.

Examples:

• xterm: Standard terminal emulator for X
• gnome-terminal: GNOME's terminal emulator
• konsole: KDE's terminal emulator