#!/bin/sh
set -e
Die(){
 e=$?
 trap - EXIT
 rm -f $TMP
 exit ${1:-$e}
}; trap Die INT HUP EXIT
usage(){
 printf >&2 '%s\n' 'usage: Y- FILE [CMD...]'\
 $'\033[31mWARNING!\033[0m overwrites FILE with [ nil | CMD ] <FILE'
 Die 1
}

[ $# -lt 1 ]&& {
 usage
}
# Y(f)-> f(Y(f))
# Y(Y)-> Y(Y(Y))
TMP=$(mktemp)||{
 typeset e=$?
 echo >&2 "Y-: mktemp error."
 Die $e
}
f="$1"; shift 1
cat "$f">$TMP
exec 3<$TMP #new pos:0 fd
rm -f $TMP #see /proc/$$/fd/3

case "$Y" in
''|u)
 ;;
*)
 echo >&2 'Y-: (unexpected $Y)'
 Die 1
 ;;
esac

Y=3$Y Y "$@"<&3 >"$f"

Die