ALAN OLIVER

Key Facts – Linux Bash

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by

Alan Oliver
in

Definition and Purpose

  • Bash (Bourne Again Shell) is a command-line shell and scripting language for Unix-like operating systems, including Linux.
  • It serves as an interface between the user and the operating system, allowing command execution, file manipulation, and automation via scripts.
  • Default shell on most Linux distributions and macOS (until macOS Catalina switched to zsh).

Core Features

  • Command Execution: Run commands like ls, cd, mkdir (via aliases or built-ins).
  • Scripting: Write scripts using variables, loops, conditionals, and functions.
  • Pipes and Redirection: Use | to pipe output to another command, > to redirect output to files, < for input redirection.
  • Job Control: Manage background processes with &, fg, bg, and jobs.
  • Tab Completion: Autocomplete commands, file names, and paths.
  • History: Access previous commands with history or arrow keys.

Basic Syntax

  • Commands: command [options] [arguments] (e.g., ls -l /home).
  • Variables: Declare with name=value (e.g., MY_VAR=hello), access with $name.
  • Comments: Start with #.
  • Shebang: #!/bin/bash at the top of scripts to specify interpreter.

Essential Commands

  • File Operations: cp (copy), mv (move), rm (remove), touch (create empty file).
  • Directory Navigation: cd (change directory), pwd (print working directory).
  • Text Processing: cat (display file), grep (search), sed (edit streams), awk (pattern scanning).
  • System Info: uname, top, df (disk free), ps (processes).
  • Permissions: chmod (change mode), chown (change owner).

Scripting Basics

  • Conditionals: if [ condition ]; then ...; fi (e.g., [ "$VAR" -eq 5 ] checks if VAR equals 5).
  • Loops: for i in {1..5}; do echo $i; done or while [ condition ]; do ...; done.
  • Functions: function_name() { commands; }.
  • Exit Codes: $? returns the exit status of the last command (0 = success, non-zero = failure).

Configuration Files

  • ~/.bashrc: User-specific settings (aliases, functions, prompt customization).
  • ~/.bash_profile: Loaded on login shells (often sources .bashrc).
  • /etc/bash.bashrc: System-wide configuration.

Common Tools and Utilities

  • find: Search for files (e.g., find / -name "file.txt").
  • xargs: Pass output as arguments to another command.
  • curl/wget: Download files from the web.
  • tar: Archive files (e.g., tar -cvf archive.tar dir/).

Key Concepts

  • Environment Variables: PATH (executable search path), HOME (user’s home directory), PS1 (prompt string).
  • Globbing: Use * (any characters), ? (single character) for pattern matching (e.g., ls *.txt).
  • Quoting: Single quotes (') preserve literal value, double quotes (") allow variable expansion.
  • Subshells: Run commands in a separate shell with ( ) or backticks `.

Practical Examples

  • List files and filter: ls -l | grep "txt".
  • Simple script: 
    #!/bin/bash
for i in {1..3}; do
    echo "Number: $i"
done
  • Check disk usage: df -h | awk '{print $5}'.

Strengths and Limitations

  • Strengths: Lightweight, powerful for automation, widely supported.
  • Limitations: Slower than compiled languages for heavy computation, less intuitive for complex data structures.

Resources

Adjacent Topics

  • Other Shells: sh (Bourne Shell), zsh, fish, ksh.
  • Linux Fundamentals: File system hierarchy, permissions, processes.
  • Text Processing: Regular expressions, sed, awk.
  • DevOps Tools: Cron (scheduling), SSH, Docker (bash often used in containers).
  • Programming: Python, Perl (alternatives for scripting).

This covers the essentials of Bash for quick mastery on Linux systems. Practice with real commands and scripts is key to fluency.