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

ssh - Secure Shell for remote system access

sh 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)