Resolve absolute path to script dir

script_dir.sh

#!/bin/sh
set -eu

# Returns the absolute directory of this script, resolving file-level symlinks.
# Usage:
#   script_dir="$(get_script_dir)" || exit 1
#   printf 'script_dir=%s\n' "$script_dir"

get_script_dir() {
  # Ensure readlink exists (portable use; no -f).
  command -v readlink >/dev/null 2>&1 || { printf '%s\n' "readlink not found" >&2; return 1; }

  # 1) Seed from call path if available; else PATH
  case "$0" in
    */*) script=$0 ;;
    *)   script=$(command -v "$0" 2>/dev/null) || { printf '%s\n' "cannot resolve: $0" >&2; return 1; } ;;
  esac
  case "$script" in
    */*) ;;  # we have a pathname
    *)   printf '%s\n' "could not determine path for $0" >&2; return 1 ;;
  esac

  # 2) Follow file-level symlinks (guard against cycles)
  max=100
  while [ -L "$script" ] && [ "$max" -gt 0 ]; do
    target=$(readlink "$script") || break
    case "$target" in
      /*) script=$target ;;
      *)  script="${script%/*}/$target" ;;
    esac
    max=$((max-1))
  done
  [ "$max" -eq 0 ] && { printf '%s\n' "symlink loop detected: $0" >&2; return 1; }

  # 3) Canonicalize directory and print
  dir=${script%/*}
  script_dir=$(
    CDPATH= cd -P -- "$dir" 2>/dev/null && pwd -P
  ) || { printf '%s\n' "failed to cd to script dir: $dir" >&2; return 1; }

  printf '%s\n' "$script_dir"
}

# Example:
# script_dir="$(get_script_dir)" || exit 1
# printf 'script_dir=%s\n' "$script_dir"