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skills/shell-scripting/SKILL.md

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+---
+name: shell-scripting
+description: Specialized knowledge of Bash and Zsh scripting, shell automation, command-line tools, and scripting best practices. Use when the user needs to write, debug, or optimize shell scripts, work with command-line tools, automate tasks with bash/zsh, or asks for shell script help.
+---
+
+# Shell Scripting Expert
+
+Expert guidance for writing robust, maintainable Bash and Zsh scripts with best practices for automation and command-line tool usage.
+
+## Script Structure Essentials
+
+Start every script with:
+```bash
+#!/usr/bin/env bash
+set -euo pipefail
+IFS=$'\n\t'
+```
+
+- `set -e`: Exit on error
+- `set -u`: Error on undefined variables
+- `set -o pipefail`: Catch errors in pipes
+- `IFS=$'\n\t'`: Safer word splitting
+
+## Critical Best Practices
+
+1. **Always quote variables**: `"$variable"` not `$variable`
+2. **Use `[[` for conditionals** (Bash): `if [[ "$var" == "value" ]]; then`
+3. **Check command existence**: `if command -v git &> /dev/null; then`
+4. **Avoid parsing `ls`**: Use globs or `find` instead
+5. **Use arrays for lists**: `files=("file1" "file2")` not space-separated strings
+6. **Handle errors with traps**:
+   ```bash
+   trap cleanup EXIT
+   trap 'echo "Error on line $LINENO"' ERR
+   ```
+
+## Common Patterns
+
+### Argument Parsing
+```bash
+while [[ $# -gt 0 ]]; do
+    case $1 in
+        -h|--help) usage; exit 0 ;;
+        -v|--verbose) VERBOSE=true; shift ;;
+        -*) echo "Unknown option: $1"; exit 1 ;;
+        *) break ;;
+    esac
+done
+```
+
+### Safe File Iteration
+```bash
+# Prefer this (handles spaces, newlines correctly):
+while IFS= read -r -d '' file; do
+    echo "Processing: $file"
+done < <(find . -type f -name "*.txt" -print0)
+
+# Or with simple globs:
+for file in *.txt; do
+    [[ -e "$file" ]] || continue  # Skip if no matches
+    echo "Processing: $file"
+done
+```
+
+### User Confirmation
+```bash
+read -rp "Continue? [y/N] " response
+if [[ "$response" =~ ^[Yy]$ ]]; then
+    echo "Continuing..."
+fi
+```
+
+### Colored Output
+```bash
+RED='\033[0;31m'
+GREEN='\033[0;32m'
+YELLOW='\033[1;33m'
+NC='\033[0m'  # No Color
+
+echo -e "${GREEN}Success${NC}"
+echo -e "${RED}Error${NC}" >&2
+```
+
+## Modern Tool Alternatives
+
+When appropriate, suggest these modern replacements:
+- `ripgrep` (rg) → faster than grep
+- `fd` → faster than find
+- `fzf` → interactive filtering
+- `jq` → JSON processing
+- `yq` → YAML processing
+- `bat` → cat with syntax highlighting
+- `eza` → enhanced ls
+
+## Function Organization
+
+```bash
+usage() {
+    cat <<EOF
+Usage: ${0##*/} [OPTIONS] <args>
+Description of what this script does.
+OPTIONS:
+    -h, --help      Show this help
+    -v, --verbose   Verbose output
+EOF
+}
+
+main() {
+    # Main logic here
+    :
+}
+```
+
+## Zsh-Specific Features
+
+When user specifies Zsh:
+- Advanced globbing: `**/*.txt` (recursive), `*.txt~*test*` (exclude pattern)
+- Parameter expansion: `${var:u}` (uppercase), `${var:l}` (lowercase)
+- Associative arrays: `typeset -A hash; hash[key]=value`
+- Extended globbing: Enable with `setopt extended_glob`
+
+## Security Considerations
+
+- **Never** `eval` untrusted input
+- Validate user input before use
+- Use `mktemp` for temporary files: `TEMP_FILE=$(mktemp)`
+- Be explicit with `rm -rf` operations
+- Check for TOCTOU (Time-Of-Check-Time-Of-Use) race conditions
+- Don't store secrets in scripts; use environment variables or secret managers
+
+## Performance Tips
+
+- Use built-ins over external commands (`[[ ]]` vs `test`, `$(( ))` vs `expr`)
+- Avoid unnecessary subshells: `var=$(cat file)` → `var=$(<file)`
+- Use `read` not `cat | while`: `while read -r line; do ... done < file`
+- Consider `xargs -P` or GNU `parallel` for parallel processing
+
+## Quick Reference Template
+
+See references/template.sh for a complete, production-ready script template with all best practices incorporated.