Defensive Bash

Defensive Bash Scripting for Server Automation

This skill provides expertise in writing safe, reliable, and maintainable Bash scripts for server administration, Docker automation, and Moodle operations.

Core Principles

1. Script Safety Headers

ALWAYS start scripts with:

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

Explanation:

• set -e: Exit on any error
• set -u: Exit on undefined variable
• set -o pipefail: Fail if any command in a pipeline fails
• IFS: Prevent word splitting issues

2. Error Handling

ALWAYS implement proper error handling:

# Error handler function
error_exit() {
    echo "ERROR: $1" >&2
    echo "Line: ${BASH_LINENO[0]}, Function: ${FUNCNAME[1]}" >&2
    exit "${2:-1}"
}

# Trap errors
trap 'error_exit "Script failed at line $LINENO"' ERR

# Usage
some_command || error_exit "Command failed" 1

3. Input Validation

ALWAYS validate inputs:

# Check arguments
if [[ $# -lt 1 ]]; then
    echo "Usage: $0 <argument>" >&2
    exit 1
fi

# Validate argument types
if ! [[ "$1" =~ ^[0-9]+$ ]]; then
    error_exit "Argument must be a number"
fi

# Check file/directory existence
if [[ ! -f "$CONFIG_FILE" ]]; then
    error_exit "Config file not found: $CONFIG_FILE"
fi

4. Safe File Operations

ALWAYS use safe file handling:

# Create temporary files safely
readonly TMPDIR="$(mktemp -d)"
trap 'rm -rf "$TMPDIR"' EXIT

# Backup before modifying
backup_file() {
    local file="$1"
    local backup="${file}.backup.$(date +%Y%m%d_%H%M%S)"
    cp -a "$file" "$backup" || error_exit "Backup failed for $file"
    echo "$backup"
}

# Atomic writes
atomic_write() {
    local content="$1"
    local target="$2"
    local tmpfile="${target}.tmp.$$"

    echo "$content" > "$tmpfile" || error_exit "Write failed"
    mv "$tmpfile" "$target" || error_exit "Atomic move failed"
}

5. Logging

ALWAYS implement comprehensive logging:

# Logging setup
readonly LOG_FILE="/var/log/$(basename "$0" .sh).log"
readonly LOG_LEVEL="${LOG_LEVEL:-INFO}"

log() {
    local level="$1"
    shift
    local message="$*"
    local timestamp
    timestamp="$(date '+%Y-%m-%d %H:%M:%S')"

    echo "[${timestamp}] [${level}] ${message}" | tee -a "$LOG_FILE"
}

log_info() { log "INFO" "$@"; }
log_warn() { log "WARN" "$@"; }
log_error() { log "ERROR" "$@"; }
log_debug() { [[ "$LOG_LEVEL" == "DEBUG" ]] && log "DEBUG" "$@"; }

6. Idempotency

ALWAYS make operations idempotent:

# Check before creating
if [[ ! -d "$TARGET_DIR" ]]; then
    mkdir -p "$TARGET_DIR"
    log_info "Created directory: $TARGET_DIR"
else
    log_debug "Directory already exists: $TARGET_DIR"
fi

# Safe service restart
restart_service() {
    local service="$1"

    if systemctl is-active --quiet "$service"; then
        systemctl restart "$service"
        log_info "Restarted service: $service"
    else
        systemctl start "$service"
        log_info "Started service: $service"
    fi
}

7. Signal Handling

ALWAYS handle signals gracefully:

# Cleanup function
cleanup() {
    local exit_code=$?
    log_info "Cleaning up (exit code: $exit_code)..."

    # Cleanup operations
    [[ -d "$TMPDIR" ]] && rm -rf "$TMPDIR"
    [[ -n "$LOCKFILE" ]] && rm -f "$LOCKFILE"

    log_info "Cleanup complete"
    exit "$exit_code"
}

# Trap signals
trap cleanup EXIT
trap 'log_warn "Received SIGINT, exiting..."; exit 130' INT
trap 'log_warn "Received SIGTERM, exiting..."; exit 143' TERM

8. Locking Mechanism

ALWAYS prevent concurrent execution:

# Lock file management
readonly LOCKFILE="/var/run/$(basename "$0" .sh).lock"

acquire_lock() {
    if [[ -f "$LOCKFILE" ]]; then
        local pid
        pid=$(<"$LOCKFILE")
        if kill -0 "$pid" 2>/dev/null; then
            error_exit "Script already running (PID: $pid)"
        else
            log_warn "Removing stale lock file"
            rm -f "$LOCKFILE"
        fi
    fi

    echo $$ > "$LOCKFILE"
}

release_lock() {
    rm -f "$LOCKFILE"
}

trap release_lock EXIT
acquire_lock

Docker-Specific Patterns

Safe Container Execution

# Execute command in container with error handling
docker_exec() {
    local container="$1"
    shift
    local cmd="$*"

    if ! docker ps --format '{{.Names}}' | grep -q "^${container}$"; then
        error_exit "Container not running: $container"
    fi

    log_debug "Executing in $container: $cmd"
    docker exec "$container" bash -c "$cmd" || {
        error_exit "Command failed in container $container: $cmd"
    }
}

# Wait for container to be healthy
wait_for_container() {
    local container="$1"
    local timeout="${2:-60}"
    local elapsed=0

    log_info "Waiting for container: $container"

    while [[ $elapsed -lt $timeout ]]; do
        if docker ps --filter "name=${container}" --filter "status=running" | grep -q "$container"; then
            log_info "Container ready: $container"
            return 0
        fi
        sleep 2
        ((elapsed += 2))
    done

    error_exit "Container failed to start: $container"
}

Service Health Checks

# Check service availability
check_service() {
    local service="$1"
    local container="${2:-moodle-dev}"

    log_debug "Checking service: $service in $container"

    if docker_exec "$container" "systemctl is-active --quiet $service"; then
        log_info "Service running: $service"
        return 0
    else
        log_error "Service not running: $service"
        return 1
    fi
}

# HTTP endpoint check
check_http() {
    local url="$1"
    local expected_code="${2:-200}"

    log_debug "Checking HTTP: $url"

    local response_code
    response_code=$(curl -s -o /dev/null -w '%{http_code}' "$url" || echo "000")

    if [[ "$response_code" == "$expected_code" ]]; then
        log_info "HTTP check passed: $url ($response_code)"
        return 0
    else
        log_error "HTTP check failed: $url (got $response_code, expected $expected_code)"
        return 1
    fi
}

Moodle-Specific Patterns

Multi-Version Moodle Operations

# Execute Moodle CLI across versions
moodle_cli() {
    local version="$1"
    local script="$2"
    shift 2
    local args="$*"

    local php_cmd moodle_dir

    case "$version" in
        "4.1")
            php_cmd="php8.1"
            moodle_dir="/opt/moodle-MOODLE_401_STABLE"
            ;;
        "4.5")
            php_cmd="php8.2"
            moodle_dir="/opt/moodle-MOODLE_405_STABLE"
            ;;
        "5.1")
            php_cmd="php8.3"
            moodle_dir="/opt/moodle-MOODLE_501_STABLE"
            ;;
        "dh-prod")
            php_cmd="php8.1"
            moodle_dir="/workspace/moodle-dh-prod"
            ;;
        *)
            error_exit "Invalid Moodle version: $version"
            ;;
    esac

    local full_script="${moodle_dir}/admin/cli/${script}"

    if [[ ! -f "$full_script" ]]; then
        error_exit "Script not found: $full_script"
    fi

    log_info "Running Moodle $version: $script $args"
    docker_exec moodle-dev "$php_cmd $full_script $args"
}

# Purge all caches
purge_all_caches() {
    local versions=("4.1" "4.5" "5.1" "dh-prod")

    for version in "${versions[@]}"; do
        log_info "Purging cache for Moodle $version"
        moodle_cli "$version" "purge_caches.php" || log_error "Cache purge failed for $version"
    done
}

Best Practices Summary

1. Always use set -euo pipefail at script start
2. Validate all inputs before using them
3. Log all significant actions with timestamps
4. Handle errors explicitly with meaningful messages
5. Make operations idempotent when possible
6. Clean up resources in trap handlers
7. Use locks for critical sections
8. Test before production use
9. Document assumptions and requirements
10. Version control all scripts

Common Anti-Patterns to Avoid

❌ Don't:

# No error checking
docker exec moodle-dev php script.php

# Unquoted variables
file=$1
cat $file

# Ignoring errors
command || true

# No validation
rm -rf $DIR/*

✅ Do:

# Proper error checking
docker_exec moodle-dev "php script.php" || error_exit "PHP script failed"

# Quoted variables
file="$1"
cat "$file"

# Explicit error handling
command || {
    log_error "Command failed"
    return 1
}

# Validation before destructive operations
if [[ -z "$DIR" ]] || [[ ! -d "$DIR" ]]; then
    error_exit "Invalid directory: $DIR"
fi
rm -rf "${DIR:?}/"*

Testing Scripts

Always test with:

# ShellCheck static analysis
shellcheck script.sh

# Bash strict mode
bash -n script.sh  # Syntax check

# Debug mode
bash -x script.sh  # Trace execution

# Test with invalid inputs
./script.sh ""
./script.sh "../../etc/passwd"
./script.sh "$(printf '\0')"

Apply these patterns consistently for reliable, maintainable server automation scripts.