Linux stuff

Common Commands with examples

These commands and their usage are used for both the LinuC Level 1 Exam 101 and Exam 102. Using these commands in various scenarios to become proficient in basic command-line tasks, file management, and system administration.

ls - List directory contents

The ls command is used to list files and directories in a specified location. It has many options to customize the output.

Examples:

• ls -l: Long listing format, showing permissions, owner, size, and modification date
• ls -a: Show all files, including hidden ones (those starting with a dot)
• ls -R: Recursively list subdirectories
• ls -lh: Use human-readable file sizes (e.g., 1K, 234M, 2G)
• ls /etc: List contents of the /etc directory

cd - Change directory

cd is used to navigate the file system. It can use absolute or relative paths.

Examples:

• cd /home/user: Change to the /home/user directory (absolute path)
• cd ..: Move up one directory level
• cd -: Return to the previous directory
• cd ~: Change to the user's home directory
• cd Documents/Projects: Change to a subdirectory (relative path)

pwd - Print working directory

pwd displays the full path of the current working directory.

Examples:

• pwd: Outputs something like /home/user/Documents
• pwd -P: Print the physical directory, without any symbolic links

mkdir - Create a new directory

mkdir is used to create one or more directories.

Examples:

• mkdir newdir: Create a directory named "newdir"
• mkdir -p parent/child/grandchild: Create nested directories
• mkdir dir1 dir2 dir3: Create multiple directories at once
• mkdir -m 755 secured_dir: Create a directory with specific permissions

touch - Create an empty file or update file timestamps

touch can create empty files or update the access and modification times of existing files.

Examples:

• touch newfile.txt: Create a new empty file or update timestamps if it exists
• touch -t 202502191200 file.txt: Set specific timestamp (YYYYMMDDhhmm)
• touch -r reference_file target_file: Use timestamps from reference_file

cp - Copy files and directories

cp is used to copy files and directories from one location to another.

Examples:

• cp file.txt /home/user/backup/: Copy a file to another directory
• cp -r dir1 dir2: Copy a directory and its contents recursively
• cp -i *.txt /destination/: Copy all .txt files, prompting before overwrite
• cp -p source dest: Preserve file attributes (mode, ownership, timestamps)

mv - Move or rename files and directories

mv can move files/directories or rename them.

Examples:

• mv file.txt newname.txt: Rename a file
• mv /home/user/doc.txt /tmp/: Move a file to another directory
• mv dir1 dir2: Rename a directory (if dir2 doesn't exist) or move dir1 into dir2
• mv -i *.jpg /home/user/Pictures/: Move all .jpg files, prompting before overwrite

rm - Remove files or directories

rm is used to delete files and directories. Use with caution!

Examples:

• rm file.txt: Delete a file
• rm -r directory/: Recursively remove a directory and its contents
• rm -f unwanted_file: Force removal without prompting
• rm -i *.log: Remove .log files with confirmation for each

rmdir - Remove empty directories

rmdir is used to remove empty directories.

Examples:

• rmdir empty_folder: Remove an empty directory
• rmdir -p parent/child/grandchild: Remove nested empty directories

shutdown - Shut down or restart the system

The shutdown command is used to safely bring down or restart a Linux system, allowing for scheduled shutdowns and custom messages to users.

Examples:

• shutdown now: Shut down the system immediately
• shutdown -r +10: Reboot the system in 10 minutes
• shutdown -h 23:30: Schedule a shutdown at 11:30 PM
• shutdown -c: Cancel a scheduled shutdown
• shutdown +15 "System maintenance in 15 minutes": Schedule a shutdown with a custom message
• shutdown -P now: Power off the system immediately

halt - Stop all CPU functions

The halt command stops all CPU functions and brings the system to a halt state. It's similar to shutdown but may not power off the system on all hardware.

Examples:

• halt: Halt the system
• halt -p: Halt the system and power off
• halt -f: Force halt without calling shutdown
• halt -w: Don't actually halt, just write a wtmp record

reboot - Restart the system

The reboot command restarts the system, similar to shutdown -r but more direct.

Examples:

• reboot: Reboot the system
• reboot -f: Force an immediate reboot without calling shutdown
• reboot -w: Don't actually reboot, just write a wtmp record
• reboot --no-wall: Don't send wall message before rebooting

poweroff - Shut down and power off the system

The poweroff command shuts down the system and turns off the power. It's equivalent to shutdown -P.

Examples:

• poweroff: Shut down and power off the system
• poweroff -f: Force power off without calling shutdown
• poweroff -w: Don't actually power off, just write a wtmp record
• poweroff --no-wall: Don't send wall message before powering off

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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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

Detailed: 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