Shell Script Development Best Practices

Shebang & Shell

• •Always start with #!/usr/bin/env bash for portability (never #!/bin/bash).
• •For POSIX compliance, use #!/bin/sh when features are basic.
• •For zsh-specific scripts, use #!/usr/bin/env zsh.

Error Handling

• •Always use set -Eeuo pipefail at the top after shebang:
  • •-E: Inherit ERR trap in functions
  • •-e: Exit on any command failure
  • •-u: Exit on undefined variables
  • •-o pipefail: Exit if any pipeline command fails
• •Use || true only when explicitly ignoring failures.
• •Informative error messages to stderr:
  bash

  if ! command_that_might_fail; then
      echo "Error: Command failed - check your configuration" >&2
      exit 1
  fi
  

Variables & Quoting

• •Always quote variables: "$variable", not $variable.
• •Use ${variable} when concatenating: "${prefix}_${suffix}".
• •readonly for constants: readonly SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)".
• •local for function variables: local temp_file="/tmp/script.$$".
• •Avoid global variables; prefer function parameters and return values.

Functions

• •Define before use.
• •local for all function variables.
• •Return meaningful exit codes (0 success, non-zero failure).
• •Document with purpose, parameters, return values:
  bash

  # Description: Downloads and verifies a file
  # Arguments:
  #   $1 - URL to download from
  #   $2 - destination path
  # Returns: 0 on success, 1 on failure
  download_file() {
      local url="${1}"
      local dest="${2}"
      [[ -n "$url" ]] || { echo "Error: URL required" >&2; return 1; }
      [[ -n "$dest" ]] || { echo "Error: Destination required" >&2; return 1; }
  }
  

Command Substitution

• •Use $() not backticks: result="$(command)".
• •Quote command substitutions: file_count="$(find . -type f | wc -l)".
• •Avoid unnecessary subshells.

File Operations

• •Check existence: [[ -f "$file" ]] for files, [[ -d "$dir" ]] for directories.
• •Use mktemp for temp files: temp_file="$(mktemp)".
• •Always cleanup with trap: trap 'rm -f "$temp_file"' EXIT.
• •chmod 600 for sensitive files.

Loops & Conditionals

• •Use [[ ]] not [ ].
• •Quote variables in conditionals: [[ "$var" == "value" ]].
• •Safe file iteration:
  bash

  while IFS= read -r -d '' file; do
      echo "$file"
  done < <(find . -name "*.txt" -print0)
  

Arrays

• •Declare explicitly: declare -a my_array=().
• •Access: "${array[@]}" for all, "${#array[@]}" for length.
• •Associative arrays: declare -A config=().

Input/Output

• •Use printf over echo for formatted output: printf '%s\n' "$message".
• •Errors to stderr: echo "Error message" >&2.
• •Heredocs for multi-line strings:
  bash

  cat << 'EOF' > config.txt
  multi-line content
  EOF
  

Security & Safety

• •Validate all user inputs.
• •Use -- to separate options from arguments: rm -- "$user_file".
• •Avoid eval; prefer arrays or functions.
• •Secure permissions for created files.

Portability

• •Avoid GNU-specific options when possible.
• •Test macOS vs Linux compatibility.
• •Check required tools: command -v git >/dev/null || { echo "git required" >&2; exit 1; }.

Logging

log_info() { printf '[INFO] %s\n' "$*"; }
log_warn() { printf '[WARN] %s\n' "$*" >&2; }
log_error() { printf '[ERROR] %s\n' "$*" >&2; }

Performance

• •Avoid excessive subprocess calls in loops.
• •Use built-in string operations over external tools when possible.

Testing & Validation

• •Use shellcheck for static analysis.
• •Include usage info: usage() { echo "Usage: $0 [options] arguments"; }.
• •Validate required arguments with helpful error messages.

Signal Handling

cleanup() {
    local exit_code=$?
    # cleanup logic
    exit $exit_code
}
trap cleanup EXIT INT TERM

Common Patterns

• •Root check: [[ $EUID -eq 0 ]].
• •Script directory: SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)".
• •Option parsing:
  bash

  while getopts "hv" opt; do
      case $opt in
          h) usage; exit 0 ;;
          v) verbose=1 ;;
          *) usage; exit 1 ;;
      esac
  done