Shell Scripting Expert

Expert guidance for writing robust, maintainable Bash and Zsh scripts with best practices for automation and command-line tool usage.

Script Structure Essentials

Start every script with:

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

• set -e

  : Exit on error

• set -u

  : Error on undefined variables

• set -o pipefail

  : Catch errors in pipes

• IFS=$'\n\t'

  : Safer word splitting

Critical Best Practices

1. Always quote variables:

   "$variable"

   not

   $variable

2. Use

   [[

   for conditionals (Bash):

   if [[ "$var" == "value" ]]; then

3. Check command existence:

   if command -v git &> /dev/null; then

4. Avoid parsing

   ls

   : Use globs or

   find

   instead
5. Use arrays for lists:

   files=("file1" "file2")

   not space-separated strings
6. Handle errors with traps:

   trap cleanup EXIT
   trap 'echo "Error on line $LINENO"' ERR
   

Common Patterns

Argument Parsing

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

Safe File Iteration

# Prefer this (handles spaces, newlines correctly):
while IFS= read -r -d '' file; do
    echo "Processing: $file"
done < <(find . -type f -name "*.txt" -print0)

# Or with simple globs:
for file in *.txt; do
    [[ -e "$file" ]] || continue  # Skip if no matches
    echo "Processing: $file"
done

User Confirmation

read -rp "Continue? [y/N] " response
if [[ "$response" =~ ^[Yy]$ ]]; then
    echo "Continuing..."
fi

Colored Output

RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
NC='\033[0m'  # No Color

echo -e "${GREEN}Success${NC}"
echo -e "${RED}Error${NC}" >&2

Modern Tool Alternatives

When appropriate, suggest these modern replacements:

• ripgrep

  (rg) → faster than grep

• fd

  → faster than find

• fzf

  → interactive filtering

• jq

  → JSON processing

• yq

  → YAML processing

• bat

  → cat with syntax highlighting

• eza

  → enhanced ls

Function Organization

usage() {
    cat <<EOF
Usage: ${0##*/} [OPTIONS] <args>
Description of what this script does.
OPTIONS:
    -h, --help      Show this help
    -v, --verbose   Verbose output
EOF
}

main() {
    # Main logic here
    :
}

Zsh-Specific Features

When user specifies Zsh:

• Advanced globbing:

  **/*.txt

  (recursive),

  *.txt~*test*

  (exclude pattern)
• Parameter expansion:

  ${var:u}

  (uppercase),

  ${var:l}

  (lowercase)
• Associative arrays:

  typeset -A hash; hash[key]=value

• Extended globbing: Enable with

  setopt extended_glob

Security Considerations

• Never

  eval

  untrusted input
• Validate user input before use
• Use

  mktemp

  for temporary files:

  TEMP_FILE=$(mktemp)

• Be explicit with

  rm -rf

  operations
• Check for TOCTOU (Time-Of-Check-Time-Of-Use) race conditions
• Don't store secrets in scripts; use environment variables or secret managers

Performance Tips

• Use built-ins over external commands (

  [[ ]]

  vs

  test

  ,

  $(( ))

  vs

  expr

  )
• Avoid unnecessary subshells:

  var=$(cat file)

  →

  var=$(<file)

• Use

  read

  not

  cat | while

  :

  while read -r line; do ... done < file

• Consider

  xargs -P

  or GNU

  parallel

  for parallel processing

Quick Reference Template

See references/template.sh for a complete, production-ready script template with all best practices incorporated.