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 <