Portable shell boilerplate

boiler.sh

#!/usr/bin/env sh

# Backup of a draft with support for many shells: 
# bash, dash, zsh, posh, yash, ksh, mksh, oksh, busybox and so on. 
#
# I tested every single one of them.
#
# Boilerplate doesn't do much. It normalizes local vars, and exposes
# basic functions:
#
# _sh_has - checks whether a builtin exists
# _sh_fn_declaration - prints a function declaration compatible with the
#                      runtime shell
# _sh_printe - Prints escaping chars, equivalent to printf %b
# _sh_print - Prints raw strings, equivalent to printf %s
# _sh_walk_pathlist - The code runs on PATH=, so if you want
#                     to exec an external program, you need to walk
#                     is manually. It is faster than using command -v
#                     inside a subshell (no forks!)
#
# You should only use this if you are aiming to support multiple
# shell interpreters.
#

__pat__="${PATH:-".:"}"
__ops__="${-:-"-"}"
__eol__='
'

IFS=" "
PATH=""
set -euf

_sh_has () {
  command -v $1 >/dev/null 2>&1
}

if _sh_has command
then
  _sh_has local || alias local=typeset
  ! _sh_has setopt || setopt sh_word_split
  ! _sh_has shopt || set -o posix
fi

##############################################################################

_main () {
  :
}

##############################################################################

_sh_walk_pathlist () {
  local call="$1"
  shift

  IFS=":" ; set -- $1 ; IFS=" "
  
  while test $# -gt 0
  do $call "$1" && shift || return 0
  done
  
  return 1
}

if eval "_hasleak(){ local _hasleak=:;};_hasleak;\${_hasleak:-false}"
then
  unset -f _hasleak
  _fnlist=$(typeset +f)

  IFS="$__eol__"
  for _declaration in $_fnlist
  do
    _name=${_declaration%"()"}
    _code="$(typeset -f $_name)"
    eval "function $_name ${_code#"$_declaration"}"
    case $__ops__ in *x*) typeset -ft $_name;; esac
  done
  IFS=" "

  unset _fnlist _declaration _name _code _hasleak
  _sh_fn_declaration () { REPLY="function $1"; }
else
  _sh_fn_declaration () { REPLY="$1 ()"; }
fi

if _sh_has printf
then
  _sh_printe () { printf %b "$1"; }
  _sh_print  () { printf %s "$1"; }
elif _sh_has print
then
  _sh_printe () { print -n "$1"; }
  _sh_print  () { print -ln "$1"; }
else
  REPLY="$(echo -n \\061)"
  case "$REPLY" in
    '1')
      _sh_printe () { echo -n "$1"; }
      _sh_print  () {
        IFS='\' ; set -- $1 ; IFS=' '

        echo -n "$1" && shift

        while test $# -gt 0
        do echo -n "\\\\$1" && shift
        done
      }
      ;;
  esac
  REPLY=
fi

_main "${@:-}"