Brainfuck interpreter in POSIX sh

brainfuck.sh

#!/bin/sh
# Brainfuck interpreter implemented in pure POSIX sh builtins only (except I/O)
# Tested in bash and busybox ash (getchar uses a bash-specific read)

# === I/O ===
getchar() {
    # bashism
    IFS= read -rN 1 a
    if [ -z "$a" ]; then
        echo $th
    else
        printf %d "'$a"
    fi
}
putchar() {
    printf "\x$(printf %x $1)"
}

# === Core BF interpreter: pure POSIX shell with builtins only from here on ===
pl=""
pr=""
while read -r line; do
    pl="$pl$line"
done <"$1"

th="0"
tl=""
tr=""

while :; do
    case "$pl" in
        '+'*)
            th=$((th+1));;
        '-'*)
            th=$((th-1));;
        '.'*)
            putchar $th;;
        ','*)
            th=$(getchar);;
        '>'*)
            tl="$th $tl"
            set -- $tr
            th=${1:-0}
            shift
            tr="$*"
            ;;
        '<'*)
            tr="$th $tr"
            set -- $tl
            th=${1:-0}
            shift
            tl="$*"
            ;;
        '['*)
            case "$th" in 0)
                i=1
                while :; do
                    tmp="${pl#?}"
                    pr="${pl%"$tmp"}$pr"
                    pl="$tmp"
                    case "$pl" in
                        '['*) i=$((i+1));;
                        ']'*) i=$((i-1));;
                    esac
                    case $i in 0) break; esac
                done
            esac
            ;;
        ']'*)
            case "$th" in 0);; *)
                i=1
                while :; do
                    tmp="${pr#?}"
                    pl="${pr%"$tmp"}$pl"
                    pr="$tmp"
                    case "$pl" in
                        ']'*) i=$((i+1));;
                        '['*) i=$((i-1));;
                    esac
                    case $i in 0) break; esac
                done
            esac
            ;;
        '')
            break
    esac
    tmp="${pl#?}"
    pr="${pl%"$tmp"}$pr"
    pl="$tmp"
done

putchar(){
    (
    O="$(printf %o $1)"'
    case O in
        ?) O=00"$O";;
        ??) O=0"$O";;
        ????*)
            printf '%s' "$0: putchar: assertion failed: (length of O)<4" 1>&2
            exit 10
            ;;
    esac
    printf '\'"$O"
    )
}

This is shorter;

printf '\'"$(printf %03o "$1")"