Parser for simplest S-expression or array-expression of parentheses by using noncanonical 1-character input in POSIX Shell

swrap.sh

#!/bin/sh

# One token input function with look-ahead caching
stty -icanon
LF="$(printf \\012)"
get_c1 () {
  case "$LH" in ("") GC1R=$(dd bs=1 count=1 2>/dev/null)
  ;;(*) GC1R="$LH"; LH="" ;;esac
}
skip_spaces () {
  get_c1
  while [ "$GC1R" = " " ] || [ "$GC1R" = "$LF" ]; do get_c1; done
  LH="$GC1R"
}
get_token () {
  get_c1; T=""
  while : ; do
    case "$GC1R" in (" "|"$LF") skip_spaces
      case "$T" in ("") get_c1 ;;(*) GTR="$T"; break ;;esac
    ;;("(") skip_spaces; GTR="("; break
    ;;(")")
      case "$T" in ("") GTR=")" ;;(*) LH=")"; GTR="$T" ;;esac
      break
    ;;(*) T="$T$GC1R"; get_c1 ;;esac
  done
}

# Simple S-expression parser
pusht () { eval STACK"$SNUM"="\$1"; SNUM="$((SNUM+1))"; }
popt  () { SNUM="$((SNUM-1))"; eval POPTR="\$STACK$SNUM"; }
slist () {
  get_token
  case "$GTR" in (")") SLISTR=")"
  ;;("(")
    pusht "$GTR"; slist; popt; GTR="$POPTR"; pusht "$SLISTR"
    pusht "$GTR"; slist; popt; GTR="$POPTR"; popt
    case "$SLISTR" in (")") SLISTR="($POPTR)"
    ;;(*) SLISTR="($POPTR $SLISTR" ;;esac
  ;;(*)
    pusht "$GTR"; slist; popt;
    case "$SLISTR" in (")") SLISTR="$POPTR)"
    ;;(*) SLISTR="$POPTR $SLISTR" ;;esac
  esac
}
sread () {
  get_token
  case "$GTR" in ("(") slist; SREADR="($SLISTR"
  ;;(*) SREADR="$GTR" ;;esac
}

LH=""; SNUM=0
sread; printf "\n$SREADR\n"