🪴 Jisang Yun

Linux Commands and Terminal Commands Summary

5 min read

Basic Shell Concepts

What is a Shell?

  • Command processor that provides interface to the kernel
  • Executes commands, navigates filesystem, sets environment variables, and handles scripting

Types of Shells

  • sh: Original Bourne Shell
  • csh: C Shell with improved interactive features
  • bash: Enhanced version of sh, default in many Linux distributions
  • zsh: Modern shell with extensive features and customization options

Useful Shell Features

  • Auto-completion: Press Tab to complete commands or filenames (double Tab shows options)
  • Command history: Access previous commands with ↑ and ↓ keys
  • Command history search: Use Ctrl+r to search through command history

File System Navigation

Directory Structure

  • / - Root directory (top-level directory)
  • Key directories:
    • /home - User home directories
    • /usr - User programs, libraries, and header files
    • /etc - System configuration files
    • /var - Variable data and logs
    • /tmp - Temporary files
    • /boot - Boot loader and kernel files
    • /dev - Device files
    • /opt - Optional/add-on software

Path Concepts

  • Current directory: Working directory (check with pwd)
  • Absolute path: Complete path from root (starts with /)
    • Example: /usr/bin/python
  • Relative path: Path relative to current location
    • . - Current directory
    • .. - Parent directory
    • ~ - Home directory
    • Example: ../bin/python (when in /usr/lib)
  • pwd - Print working directory
  • cd [directory] - Change directory
    • cd / - Go to root directory
    • cd ~ - Go to home directory
    • cd .. - Go up one directory
  • ls - List files and directories
    • ls -l - Detailed listing
    • ls -a - Show hidden files
    • ls -F - Show file types with indicators
    • ls -lh - Human-readable file sizes

File and Directory Operations

File Viewing

  • cat [file] - Display entire file content
    • cat -n [file] - Display with line numbers
  • head [file] - Display first lines (default: 10)
    • head -n 5 [file] - Show first 5 lines
  • tail [file] - Display last lines (default: 10)
    • tail -n 7 [file] - Show last 7 lines
  • less [file] - View file with scrolling
    • Navigate: ↑/↓ keys, Space for page down, q to quit

File and Directory Management

  • mkdir [directory] - Create directory
    • mkdir -p [path] - Create nested directories
  • rmdir [directory] - Remove empty directory
  • touch [file] - Create empty file or update timestamp
  • rm [file] - Remove file
    • rm -r [directory] - Remove directory and contents
    • rm -f [file] - Force removal without prompt
  • cp [source] [destination] - Copy file or directory
    • cp -r [source_dir] [destination] - Copy directory recursively
  • mv [source] [destination] - Move/rename file or directory
  • Create hard link: ln [target] [link_name]
    • References same file data (inode)
    • Cannot span different filesystems or link directories
  • Create symbolic link: ln -s [target] [link_name]
    • Like shortcuts in Windows
    • Can link to directories and across filesystems
  • find [path] [options] - Search for files
    • find . -name "filename" -print - Find by name in current directory
    • find . -name "file*" -type f -print - Find files starting with “file”
    • File types: f (file), d (directory), l (symbolic link)

Command Help and Documentation

Getting Help

  • Basic help: [command] --help
  • Manual pages: man [command]
  • Info pages: info [command]

Finding Commands

  • which [command] - Show full path of command
  • whereis [command] - Show binary, source, and manual locations
  • Command search path stored in $PATH environment variable

Text Editing with Vim

Basic Vim Usage

  • Open file: vim [filename]
  • Modes:
    • Command mode (default): For navigation and commands
    • Insert mode: Enter with i, exit with Esc
  • Saving and quitting:
    • :w - Save
    • :q - Quit
    • :wq or :x - Save and quit
    • :q! - Quit without saving

Shell Customization

Aliases

  • Create command shortcuts
  • Temporary: alias ls='ls -F'
  • Permanent: Add to ~/.bashrc

Prompt Customization

  • Controlled by PS1 environment variable
    • Check current: echo $PS1
    • Change: PS1='bash> '
  • Common placeholders:
    • \u - Username
    • \h - Hostname
    • \w - Current directory (full path)
    • \W - Current directory (basename only)
    • \d - Date
    • \t - Time
    • \$ - Shows # for root, $ for normal users

Environment Variables

  • View all: printenv
  • Set variable: export VARIABLE=value
  • Configuration files:
    • /etc/profile - System-wide settings
    • ~/.profile - User-specific login settings
    • ~/.bashrc - User-specific shell settings

File Permissions

Understanding Permissions

  • File listing shows: [type][user perms][group perms][others perms]
  • Permission types:
    • r - Read
    • w - Write
    • x - Execute
  • Example: -rw-r--r--
    • - - Regular file
    • rw- - Owner can read and write
    • r-- - Group can only read
    • r-- - Others can only read

Changing Permissions

  • Symbolic mode:
    • chmod u+w [file] - Add write permission for user
    • chmod g-x [file] - Remove execute permission for group
    • chmod o=r [file] - Set others permission to read only
  • Numeric mode:
    • chmod 644 [file] - Set to -rw-r--r--
    • chmod 755 [file] - Set to -rwxr-xr-x
    • Number calculation: read(4) + write(2) + execute(1)

Super User Operations

  • Switch to root: su
  • Execute as root: sudo [command]
  • Add user to sudo group: sudo usermod -aG sudo [username]
  • Edit sudo configuration: sudo visudo

Input/Output Redirection

Redirecting Output

  • [command] > [file] - Redirect output, overwrite file
  • [command] >> [file] - Redirect output, append to file
  • [command] 2> [file] - Redirect error output
  • [command] &> [file] - Redirect both standard and error output

Redirecting Input

  • [command] < [file] - Use file as input source

Pipes

  • [command1] | [command2] - Send output of command1 as input to command2
  • Example: ls /usr/bin/gcc* | wc -l - Count gcc files

Package Management (APT)

Basic APT Commands

  • Update package list: sudo apt update
  • Install package: sudo apt install [package]
  • Remove package: sudo apt remove [package]
  • Search for package: apt search [keyword]
  • List installed packages: dpkg -l
  • Show package info: dpkg -L [package]

Development Tools

Compiler Installation

  • Install GCC: sudo apt install -y gcc
  • Check installation path: which gcc
  • List compiler files: ls -alth $(which gcc)*
  • List toolchain tools: ls -alth /usr/bin/$(gcc --dumpmachine)*

Library Management

  • Install library: sudo apt install -y libjpeg-dev
  • Check package info: dpkg -l | grep jpeg
  • Check library location: dpkg -L libjpeg-turbo8
  • Analyze library: readelf -a /usr/lib/x86_64-linux-gnu/libjpeg.so.8.2.2 | grep SONAME

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