Shell Scripting Expertise

Key Principles

• Start every Bash script with

  set -euo pipefail

  to fail on errors, undefined variables, and pipeline failures
• Quote all variable expansions ("$var", "${array[@]}") to prevent word splitting and globbing surprises
• Use functions to organize logic; each function should do one thing and use local variables with

  local

• Prefer built-in string manipulation (parameter expansion) over spawning external processes for simple operations
• Write scripts that produce meaningful exit codes: 0 for success, 1 for general errors, 2 for usage errors

Techniques

• Use parameter expansion for string operations:

  ${var:-default}

  for defaults,

  ${var%.*}

  to strip extensions,

  ${var##*/}

  for basename
• Handle cleanup with

  trap 'cleanup_function' EXIT

  to ensure temporary files and resources are released on any exit path
• Parse arguments with

  getopts

  for simple flags or a

  while

  loop with

  case

  for long options and positional arguments
• Use process substitution

  <(command)

  to feed command output as a file descriptor to tools that expect file arguments
• Apply heredocs with

  <<'EOF'

  (quoted) to prevent variable expansion in template content, or

  <<EOF

  (unquoted) for interpolated templates
• Validate inputs at the top of the script: check required environment variables, verify file existence, and validate argument counts before proceeding

Common Patterns

• Idempotent Operations: Check state before acting:

  command -v tool >/dev/null 2>&1 || install_tool

  ensures the script can be run multiple times safely
• Temporary File Management: Create temp files with

  mktemp

  and register cleanup in a trap:

  tmpfile=$(mktemp) && trap "rm -f $tmpfile" EXIT

• Logging Function: Define

  log() { printf '[%s] %s\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)" "$*" >&2; }

  to send timestamped messages to stderr, keeping stdout clean for data
• Parallel Execution: Launch background jobs with

  &

  , collect PIDs, and

  wait

  for all of them; check exit codes individually for error reporting

Pitfalls to Avoid

• Do not parse

  ls

  output for file iteration; use globbing (

  for f in *.txt

  ) or

  find

  with

  -print0

  piped to

  while IFS= read -r -d '' file

  for safe filename handling
• Do not use

  eval

  with user-supplied input; it enables arbitrary code execution and is almost never necessary with modern Bash features
• Do not assume GNU coreutils are available on all systems; macOS ships BSD versions with different flags; test on target platforms or use POSIX-only features
• Do not write scripts longer than 200 lines without considering whether Python or another language would be more maintainable; shell excels at gluing commands together, not at complex logic