{ "api": "YAML JSON TOON Database", "version": "1.0.0", "format": "json", "dataset": { "id": 46, "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": [ "You are writing or reviewing a shell script.", "You need a safe idiom for a common scripting task." ], "caveats": [ "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": [ "file-operations", "git-essential-commands", "unix-text-processing-commands", "cicd-pipeline-stages" ], "category": "Unix Commands", "category_slug": "unix-commands", "tags": "bash,shell,scripting,unix,linux,automation,devops", "view_count": 0, "created_at": 1777673262, "updated_at": 1777673262, "entry_count": 8, "fields": [ { "section": "sections", "field": "topic", "type": "string", "example": "Variables and Expansion" }, { "section": "sections", "field": "examples", "type": "array", "example": "[4 items]" } ] }, "data": { "sections": [ { "topic": "Variables and Expansion", "examples": [ { "name": "Simple assignment", "code": "name=\"John\"\necho \"Hello, $name\"" }, { "name": "Command substitution", "code": "current_dir=$(pwd)\necho \"We are in $current_dir\"" }, { "name": "Default values", "code": "echo \"${PORT:-8080}\" # prints 8080 if PORT unset\necho \"${PORT:=8080}\" # sets PORT to 8080 if unset\necho \"${PORT:?Error: PORT required}\" # exits if unset" }, { "name": "String manipulation", "code": "str=\"hello_world.txt\"\necho \"${str%.*}\" # hello_world (remove shortest suffix)\necho \"${str##*.}\" # txt (remove longest prefix)\necho \"${str/_/-}\" # hello-world.txt (replace first _)" } ] }, { "topic": "Control Flow", "examples": [ { "name": "If statement", "code": "if [ -f \"$file\" ]; then\n echo \"File exists\"\nelif [ -d \"$file\" ]; then\n echo \"Is a directory\"\nelse\n echo \"Not found\"\nfi" }, { "name": "For loop", "code": "for item in \"${array[@]}\"; do\n echo \"Processing: $item\"\ndone\n\n# Or with range\nfor i in {1..10}; do\n echo \"Count: $i\"\ndone" }, { "name": "While loop", "code": "while IFS= read -r line; do\n echo \"Line: $line\"\ndone < \"input.txt\"" }, { "name": "Case statement", "code": "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": [ { "name": "Define and call", "code": "greet() {\n local name=\"$1\"\n echo \"Hello, $name!\"\n}\n\ngreet \"World\" # Hello, World!" }, { "name": "Return values", "code": "is_even() {\n local num=$1\n return $(( num % 2 ))\n}\n\nif is_even 42; then\n echo \"Even\"\nelse\n echo \"Odd\"\nfi" }, { "name": "Output capture", "code": "get_date() {\n date \"+%Y-%m-%d\"\n}\n\ntoday=$(get_date)\necho \"Today is $today\"" } ] }, { "topic": "Error Handling", "examples": [ { "name": "Exit on error", "code": "set -e # Exit immediately on error\nset -u # Error on undefined variables\nset -o pipefail # Fail pipeline on first error\n\n# Combined\nset -euo pipefail" }, { "name": "Trap signals", "code": "cleanup() {\n echo \"Cleaning up...\"\n rm -f /tmp/tempfile\n exit\n}\n\ntrap cleanup EXIT SIGINT SIGTERM" }, { "name": "Check command success", "code": "if ! command -v docker &> /dev/null; then\n echo \"Docker is not installed\"\n exit 1\nfi" } ] }, { "topic": "File Operations", "examples": [ { "name": "Read file line by line", "code": "while IFS= read -r line; do\n echo \"$line\"\ndone < \"input.txt\"" }, { "name": "Check file conditions", "code": "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 \"$str\" ]; then echo \"non-empty\"; fi" }, { "name": "Temp files", "code": "tempfile=$(mktemp)\necho \"Writing to $tempfile\"\n# ... use it ...\nrm -f \"$tempfile\"" } ] }, { "topic": "Arrays and Maps", "examples": [ { "name": "Indexed array", "code": "fruits=(\"apple\" \"banana\" \"cherry\")\necho \"${fruits[0]}\" # apple\necho \"${#fruits[@]}\" # length 3\necho \"${fruits[@]}\" # all elements" }, { "name": "Associative array (Bash 4+)", "code": "declare -A config\nconfig[\"host\"]=\"localhost\"\nconfig[\"port\"]=8080\n\necho \"${config[host]}\" # localhost" } ] }, { "topic": "Debugging", "examples": [ { "name": "Enable debug mode", "code": "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" }, { "name": "Check syntax without running", "code": "bash -n script.sh\n# Returns silently if syntax is valid" }, { "name": "ShellCheck static analysis", "code": "# Install: apt install shellcheck / brew install shellcheck\n# Run: shellcheck script.sh\n# Catches: unquoted variables, missing shebang, common pitfalls" } ] }, { "topic": "Best Practices", "examples": [ { "name": "Script header", "code": "#!/usr/bin/env bash\nset -euo pipefail\nIFS=$'\\n\\t'" }, { "name": "Usage function", "code": "usage() {\n cat <