api: YAML JSON TOON Database
version: 1.0.0
format: toon
dataset:
  id: 2893
  slug: shell-scripting-guide
  title: Shell Scripting Guide (Bash)
  description: "Comprehensive reference for Bash shell scripting: variables, control flow, functions, error handling, debugging, and best practices."
  summary: "A guide to Bash scripting by topic with examples: variables and expansion, control flow, functions, error handling, file operations, arrays and maps, debugging and best practices."
  use_when[2]: You are writing or reviewing a shell script.,You need a safe idiom for a common scripting task.
  caveats[2]: "Bash-specific features (arrays, [[ ]]) are not available in POSIX sh; set the shebang accordingly.","Use set -euo pipefail and quote variables, and consider a different language for complex logic."
  related[4]: file-operations,git-essential-commands,unix-text-processing-commands,cicd-pipeline-stages
  entry_count: 8
  fields[2]{section,field,type,example}:
    sections,topic,string,Variables and Expansion
    sections,examples,array,"[4 items]"
  category: Unix Commands
  category_slug: unix-commands
  tags: "bash,shell,scripting,unix,linux,automation,devops"
  view_count: 44
  created_at: 1777673262
  updated_at: 1777673262
data:
  sections[8]:
    - topic: Variables and Expansion
      examples[4]{name,code}:
        Simple assignment,"name=\"John\"\necho \"Hello, $name\""
        Command substitution,"current_dir=$(pwd)\necho \"We are in $current_dir\""
        Default values,"echo \"${PORT:-8080}\"  # prints 8080 if PORT unset\necho \"${PORT:=8080}\"  # sets PORT to 8080 if unset\necho \"${PORT:?Error: PORT required}\"  # exits if unset"
        String manipulation,"str=\"hello_world.txt\"\necho \"${str%.*}\"   # hello_world (remove shortest suffix)\necho \"${str##*.}\"  # txt (remove longest prefix)\necho \"${str/_/-}\"   # hello-world.txt (replace first _)"
    - topic: Control Flow
      examples[4]{name,code}:
        If statement,"if [ -f \"$file\" ]; then\n    echo \"File exists\"\nelif [ -d \"$file\" ]; then\n    echo \"Is a directory\"\nelse\n    echo \"Not found\"\nfi"
        For loop,"for item in \"${array[@]}\"; do\n    echo \"Processing: $item\"\ndone\n\n# Or with range\nfor i in {1..10}; do\n    echo \"Count: $i\"\ndone"
        While loop,"while IFS= read -r line; do\n    echo \"Line: $line\"\ndone < \"input.txt\""
        Case statement,"case \"$1\" in\n    start) echo \"Starting...\";;\n    stop)  echo \"Stopping...\";;\n    restart) echo \"Restarting...\";;\n    *)     echo \"Usage: $0 {start|stop|restart}\"; exit 1;;\nesac"
    - topic: Functions
      examples[3]{name,code}:
        Define and call,"greet() {\n    local name=\"$1\"\n    echo \"Hello, $name!\"\n}\n\ngreet \"World\"  # Hello, World!"
        Return values,"is_even() {\n    local num=$1\n    return $(( num % 2 ))\n}\n\nif is_even 42; then\n    echo \"Even\"\nelse\n    echo \"Odd\"\nfi"
        Output capture,"get_date() {\n    date \"+%Y-%m-%d\"\n}\n\ntoday=$(get_date)\necho \"Today is $today\""
    - topic: Error Handling
      examples[3]{name,code}:
        Exit on error,"set -e  # Exit immediately on error\nset -u  # Error on undefined variables\nset -o pipefail  # Fail pipeline on first error\n\n# Combined\nset -euo pipefail"
        Trap signals,"cleanup() {\n    echo \"Cleaning up...\"\n    rm -f /tmp/tempfile\n    exit\n}\n\ntrap cleanup EXIT SIGINT SIGTERM"
        Check command success,"if ! command -v docker &> /dev/null; then\n    echo \"Docker is not installed\"\n    exit 1\nfi"
    - topic: File Operations
      examples[3]{name,code}:
        Read file line by line,"while IFS= read -r line; do\n    echo \"$line\"\ndone < \"input.txt\""
        Check file conditions,"if [ -f \"$file\" ]; then echo \"regular file\"; fi\nif [ -d \"$dir\" ]; then echo \"directory\"; fi\nif [ -x \"$bin\" ]; then echo \"executable\"; fi\nif [ -z \"$str\" ]; then echo \"empty string\"; fi\nif [ -n \"$str\" ]; then echo \"non-empty\"; fi"
        Temp files,"tempfile=$(mktemp)\necho \"Writing to $tempfile\"\n# ... use it ...\nrm -f \"$tempfile\""
    - topic: Arrays and Maps
      examples[2]{name,code}:
        Indexed array,"fruits=(\"apple\" \"banana\" \"cherry\")\necho \"${fruits[0]}\"  # apple\necho \"${#fruits[@]}\"  # length 3\necho \"${fruits[@]}\"   # all elements"
        Associative array (Bash 4+),"declare -A config\nconfig[\"host\"]=\"localhost\"\nconfig[\"port\"]=8080\n\necho \"${config[host]}\"  # localhost"
    - topic: Debugging
      examples[3]{name,code}:
        Enable debug mode,"set -x  # Print commands and their arguments as executed\n# ... debug code ...\nset +x  # Disable debug\n\n# Or run script with:\n# bash -x script.sh"
        Check syntax without running,"bash -n script.sh\n# Returns silently if syntax is valid"
        ShellCheck static analysis,"# Install: apt install shellcheck / brew install shellcheck\n# Run: shellcheck script.sh\n# Catches: unquoted variables, missing shebang, common pitfalls"
    - topic: Best Practices
      examples[3]{name,code}:
        Script header,"#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'"
        Usage function,"usage() {\n    cat <<EOF\nUsage: $0 [options]\n  -h    Show help\n  -v    Verbose mode\n  -f    Input file\nEOF\n    exit 1\n}\n\nwhile getopts \"hf:v\" opt; do\n    case $opt in\n        h) usage;;\n        f) file=\"$OPTARG\";;\n        v) verbose=true;;\n        *) usage;;\n    esac\ndone"
        Portable scripting,"# Prefer POSIX sh -c over bashisms when portability matters\n# Use printf instead of echo for consistent behavior\n# Quote all variable expansions\n# Use $(...) over backticks"