Shell Scripting Guide

Script Template

#!/usr/bin/env bash
set -euo pipefail   # exit on error, undefined vars, pipe fail
IFS=$'\n\t'

# Script directory
SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"

main() {
  local name="${1:-World}"
  echo "Hello, $name!"
}

main "$@"

Variables & Strings

name="Alice"
greeting="Hello, $name"

# Default value
port="${PORT:-8080}"

# String operations
file="photo.tar.gz"
echo "${file%.gz}"        # photo.tar (remove suffix)
echo "${file%%.*}"        # photo (remove all after .)
echo "${file#photo.}"     # tar.gz (remove prefix)
echo "${#file}"           # 11 (length)
echo "${file/photo/img}"  # img.tar.gz (replace)

Arrays

arr=(one two three four)
echo "${arr[0]}"          # one
echo "${arr[@]}"          # all elements
echo "${#arr[@]}"         # 4 (length)
arr+=("five")             # append

# Iterate
for item in "${arr[@]}"; do
  echo "$item"
done

# Associative array (Bash 4+)
declare -A map
map["key"]="value"
echo "${map[key]}"

Functions

log() {
  local level="$1"; shift
  echo "[$(date '+%Y-%m-%d %H:%M:%S')] [$level] $*" >&2
}

retry() {
  local attempts="${1:-3}"; shift
  local cmd=("$@")
  for ((i=1; i<=attempts; i++)); do
    "${cmd[@]}" && return 0
    log WARN "Attempt $i/$attempts failed"
    sleep $((i * 2))
  done
  return 1
}