Introduction to Linux Commands and Concepts

• The passwd command, without arguments, prompts the current user to change their password.
• The Bash shell, a default for most Linux distributions, incorporates features from both the Korn and Bourne shells.
• The clear command is optimal for clearing the terminal window's current output.
• The kernel in UNIX/Linux allocates resources and manages hardware.
• Command history allows you to execute a previously used command.
• The primary purpose of telnet is to establish a remote terminal session.
• Server-based networks rely on centralized resource management, unlike peer-to-peer networks.
• The X Window system provides a graphical user interface in UNIX/Linux.
• The command date -u outputs UTC time and date
• In "GNU/Linux," GNU stands for "GNU's Not Unix."
• The layered structure of UNIX/Linux protects hardware by restricting access to the kernel, allows simultaneous use of multiple shells, and reduces the need for use authentication.
• The shell interprets user commands in UNIX/Linux.
• The man -K option allows keyword searches across all man pages.
• The /etc/passwd file contains user account information.
• SSH is more secure than Telnet because it encrypts communications.
• Modern Linux distributions support usernames up to 32 characters long.
• Multi User capability allows multiple users to access the same resource simultaneously.
• The command cal -m displays the current month's calendar with Monday as the first day of the week.
• A user can change their current shell by typing the desired shell's name.
• The command who am idisplays the user account logged in on the terminal.
• The /mnt directory is commonly used for mounting temporary file systems.
• The command unlink removes an empty directory.
• The /proc directory contains information about running processes.
• The mount command connects a directory to the root hierarchy.
• The command cd .. moves to the parent directory.
• UNIX/Linux uses a tree-based hierarchical file system structure.
• The command chmod 750 file grants read, write, and execute permissions to the owner, read and execute to the group, and no access for others.
• Hidden files in a directory are listed using ls -a.
• The /var directory typically stores log files and variable data.
• The command find /home -name ".txt" searches for .txt files in /home and its subdirectories.
• The umask command sets default permissions for new files.
• The /dev directory contains device files.
• The dot (.) signifies the current directory in file system navigation.
• The command chown changes a directory's ownership.
• chmod 700 file protects a file from being modified or deleted by anyone except the owner.
• /documents is NOT part of the standard Linux directory hierarchy
• The /etc directory is used for system-wide configuration files.
• Inodes in a Linux file system track file permissions and data.
• The command pwd displays the present working directory path.
• The command ls -l displays files in long format with detailed information.

Vi/Nano/Emacs Editor Commands

• The dd command in vi deletes a single line.
• In vi, u undoes the last command.
• In vi, /error searches for the word "error".
• The command :%s/foo/bar/g in vi replaces "foo" with "bar" globally.
• In nano, Ctrl+o saves the current file.
• To start inserting text in vi, enter insert mode.
• To move the cursor to the end of a line in vi, use $.
• To quit vi without saving changes, use :q!.
• In nano, Ctrl+k cuts the current line.
• In Emacs, Ctrl+s searches for a specific string.
• The command :wq saves and exits a file in vi.
• To switch from insert mode back to command mode in vi, press Esc.
• In vi, yw yanks (copies) a single word.
• To delete from the current position to the end of the line in vi, use D.
• In nano, Ctrl+w searches for a string.

File Manipulation Commands

• The sort command arranges lines in alphabetical or numerical order.
• The cut command extracts specific columns from a file.
• The join command merges two files based on a common field.
• The wc command counts the number of lines, words, and characters in a file.
• To display the first 10 lines of a file, use head.
• The -i option for grep ignores case sensitivity.
• The uniq command removes duplicate lines in a file.
• The tail command displays the last few lines of a file.
• The cat command concatenates files and displays the output.
• The diff command displays the differences between two files.
• The -F option in awk specifies a field delimiter.
• To count the number of lines in a file, use wc -l.
• The sed command s/$/text/ appends text to the end of a line.
• cut -f1,3 extracts fields 1 and 3 from each line of a file.
• The paste command combines the content of two files line by line.

Piping and Special Characters

• The pipe operator | passes the output of one command as input to another.
• The grep command searches for a specific pattern in a file.
• The uniq command requires a sorted input file to work properly.
• The -n option in grep displays line numbers of matches.
• The tr command translates or deletes characters in a file.
• The sed command s/error/fixed/ replaces the word "error" with "fixed".
• The wc -w command displays the number of words in a file.
• The pr command is used to format file output into columns.
• The diff command displays the differences between two files line by line.
• The paste command combines two files horizontally (side by side).
• The comm command displays output in three columns: unique to file 1, unique to file 2, and common lines.
• The comm -3 command prints only the unique lines between two files.
• The awk '{print $2, $4}' command prints the 2nd and 4th fields of a file.
• The sed '5d' command deletes line 5 from a file.
• cut -d: -f2 extracts the second field from a file using : as the delimiter.

Acronyms

• ROM: Read-Only Memory
• EPROM: Erasable Programmable Read-Only Memory
• EEPROM: Electrically Erasable Programmable Read-Only Memory
• I/O: Input/Output
• SAN: Storage-Area Network
• HPC: High-Performance Computing
• DLM: Distributed Lock Manager
• VMM: Virtual Machine Manager
• VM: Virtual Machine
• TCP/IP: Transmission Control Protocol/Internet Protocol
• LAN: Local Area Network
• WAN: Wide Area Network
• MAN: Metropolitan Area Network
• PAN: Personal Area Network
• NEO: Network Operating System
• VoIP: Voice over IP
• SaaS: Software as a Service
• PaaS: Platform as a Service
• IaaS: Infrastructure as a Service
• DRM: Digital Rights Management
• FSF: Free Software Foundation
• GPL: GNU Public License
• UI: User Interface
• CLI: Command-Line Interface
• GUI: Graphical User Interface
• API: Application Programming Interface
• MS-DOS: Microsoft Disk Operating System
• GRUB: GRand Unified Bootloader
• PCB: Process Control Block
• PID: Process Identifier
• IPC: Interprocess Communication
• UDP: User Datagram Protocol
• MIDL: Microsoft Interface Definition Language
• XDR: External Data Representation
• NC: Network Computer
• PDA: Personal Digital Assistant
• ALU: Arithmetic Logic Unit
• HDD: Hard-Disk Drive
• NVM: Nonvolatile Memory
• NVS: Nonvolatile Storage
• DMA: Direct Memory Access
• SMP: Symmetric Multiprocessing
• SID: Security ID
• FIFO: First In First Out
• EC2: Elastic Compute Cloud
• ASICs: Application-Specific Integrated Circuits

Explore essential Linux commands like passwd, clear, and date. Understand the Bash shell, kernel functions, command history, and telnet. Learn about network models and the role of the X Window system in providing a GUI.