Linux Shell Scripting Skill

A Claude Code skill for writing production-ready shell scripts — covering Bash scripting patterns, system administration automation, error handling, logging, security practices, and portable script design.

When to Use This Skill

Choose this skill when:

• Automating system administration tasks (backups, monitoring, deployments)
• Writing setup scripts for development environments
• Creating cron jobs and scheduled maintenance scripts
• Building CI/CD pipeline scripts
• Managing users, permissions, and system configurations
• Diagnosing system issues with shell commands

Consider alternatives when:

• You need complex data processing (use Python or Node.js)
• You need cross-platform compatibility (use a scripting language)
• You need interactive CLI applications (use a CLI framework)

Quick Start

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# Template for production-ready scripts
#!/usr/bin/env bash
set -euo pipefail
IFS=$'\n\t'

# Configuration
readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
readonly LOG_FILE="/var/log/$(basename "$0" .sh).log"

# Logging function
log() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] $*" | tee -a "$LOG_FILE"; }
err() { log "ERROR: $*" >&2; }

# Cleanup on exit
cleanup() { log "Script finished"; }
trap cleanup EXIT

# Main logic
main() {
  log "Starting $(basename "$0")"
  # Your script logic here
}

main "$@"

Core Concepts

Script Safety Headers

Flag	Purpose	Prevents
set -e	Exit on error	Silent failures continuing
set -u	Error on undefined variables	Typos causing empty strings
set -o pipefail	Pipe failure propagation	Hiding pipe command errors
IFS=$'\n\t'	Safe word splitting	Space-separated filename issues
trap cleanup EXIT	Guaranteed cleanup	Resource leaks on failure

Common Patterns

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# Safe file operations
tmpfile=$(mktemp)
trap "rm -f '$tmpfile'" EXIT

# Conditional execution with defaults
DB_HOST="${DB_HOST:-localhost}"
DB_PORT="${DB_PORT:-5432}"

# Array iteration
files=("file1.txt" "file2.txt" "file3.txt")
for file in "${files[@]}"; do
  process "$file"
done

# Function with error handling
backup_database() {
  local db_name="$1"
  local backup_dir="${2:-/backups}"
  local timestamp=$(date +%Y%m%d_%H%M%S)
  local backup_file="${backup_dir}/${db_name}_${timestamp}.sql.gz"

  pg_dump "$db_name" | gzip > "$backup_file" || {
    err "Failed to backup database: $db_name"
    return 1
  }

  log "Backup created: $backup_file ($(du -h "$backup_file" | cut -f1))"
}

# Process monitoring
check_service() {
  local service="$1"
  if systemctl is-active --quiet "$service"; then
    log "$service is running"
  else
    err "$service is not running — restarting"
    systemctl restart "$service"
  fi
}

Configuration

Parameter	Type	Default	Description
shell	string	"bash"	Shell interpreter: bash, sh, zsh
strict_mode	boolean	true	Enable set -euo pipefail
logging	boolean	true	Include logging functions
cleanup_trap	boolean	true	Add EXIT trap for cleanup
portability	string	"bash"	Portability: bash, posix, cross-platform
error_handling	string	"exit"	Error behavior: exit, continue, retry

Best Practices

1. Always use set -euo pipefail — these flags catch the vast majority of shell scripting bugs; without them, errors are silently ignored and undefined variables expand to empty strings.

2. Quote all variable expansions — always use "$variable" not $variable; unquoted variables break on filenames with spaces and can cause glob expansion in unexpected ways.

3. Use functions for reusable logic — declare functions with local variables to prevent pollution of the global namespace; functions make scripts testable and readable.

4. Use mktemp for temporary files — never hardcode temp file paths like /tmp/myfile; mktemp creates unique files safely, preventing race conditions and security issues.

5. Log to both stdout and a file — use tee -a "$LOG_FILE" so output is visible during interactive runs and preserved for later debugging of automated runs.

Common Issues

Script breaks on filenames with spaces — Always quote variables and use "$@" instead of $*. Set IFS=$'\n\t' to prevent space-based word splitting in loops.

Script works interactively but fails in cron — Cron runs with a minimal PATH and no shell profile. Set PATH explicitly at the top of the script and use full paths for commands.

set -e doesn't catch errors in subshells — Errors in $(command) or command | other don't trigger set -e as expected. Use set -o pipefail for pipes and check $? explicitly for critical subshell commands.