bash-script

name	bash-script
description	Write robust bash scripts with proper error handling. Use when the user says "write a script", "bash script", "shell script", "automate this", "create a backup script", or asks to script a task.
allowed-tools	Read, Write, Edit, Bash, Glob

Bash Script

Write robust, maintainable bash scripts with proper error handling.

Instructions

1. Understand the task requirements
2. Design the script structure
3. Write with safety options and error handling
4. Test and validate

Script template

#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

# Description: Brief description of what this script does
# Usage: ./script.sh [options] <args>

readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly SCRIPT_NAME="$(basename "${BASH_SOURCE[0]}")"

# Default values
VERBOSE="${VERBOSE:-false}"

usage() {
    cat <<EOF
Usage: ${SCRIPT_NAME} [OPTIONS] <argument>

Description of what the script does.

Options:
    -h, --help      Show this help message
    -v, --verbose   Enable verbose output

Examples:
    ${SCRIPT_NAME} input.txt
    ${SCRIPT_NAME} --verbose /path/to/file
EOF
}

log() {
    echo "[$(date +'%Y-%m-%d %H:%M:%S')] $*" >&2
}

error() {
    log "ERROR: $*"
    exit 1
}

cleanup() {
    # Remove temp files, restore state, etc.
    rm -f "${TEMP_FILE:-}"
}
trap cleanup EXIT

main() {
    # Parse arguments
    while [[ $# -gt 0 ]]; do
        case "$1" in
            -h|--help) usage; exit 0 ;;
            -v|--verbose) VERBOSE=true; shift ;;
            --) shift; break ;;
            -*) error "Unknown option: $1" ;;
            *) break ;;
        esac
    done

    [[ $# -lt 1 ]] && { usage; exit 1; }

    local input="$1"
    [[ -f "$input" ]] || error "File not found: $input"

    # Main logic here
    log "Processing $input"
}

main "$@"

Safety options explained

Option	Purpose
set -e	Exit on error
set -u	Error on undefined variables
set -o pipefail	Pipeline fails if any command fails
IFS=$'\n\t'	Safer word splitting

Best practices

• MUST use set -euo pipefail at script start
• MUST quote all variables: "$var" not $var
• MUST use [[ for conditionals (not [)
• MUST use $() for command substitution (not backticks)
• Use readonly for constants
• Use local for function variables
• Use trap for cleanup
• Use mktemp for temp files

Common patterns

# Check if command exists
command -v docker &>/dev/null || error "docker not found"

# Default value
name="${1:-default}"

# Read file line by line
while IFS= read -r line; do
    echo "$line"
done < "$file"

# Process arguments
for arg in "$@"; do
    echo "$arg"
done

# Temp file with cleanup
TEMP_FILE="$(mktemp)"
trap 'rm -f "$TEMP_FILE"' EXIT

Rules

• MUST include set -euo pipefail
• MUST quote all variable expansions
• MUST include usage function for scripts with arguments
• MUST use trap for cleanup of temp files
• Never use eval with user input
• Never assume current directory
• Always use absolute paths for cron scripts