rough equivalent of `realpath -eq` in POSIX Shell

realpath.sh

# Similar to `realpath -eq`
realpath() {
    if [ "$#" -ne 1 ]; then
        return 1
    elif ! OLD_CD="$(pwd -P)" > /dev/null 2>&1 ; then
        # This depends on `pwd -P` working, and if it's not, we can't proceed anyways
        return 1
    elif [ -z "${1%%/*}" ]; then
        # Removing everything after the first / leaves nothing, so the path starts with /, and is absolute
        THE_PATH="$1"
    else
        # The path is not absolute, so add the path to the current directory to that path
        THE_PATH="${OLD_CD}/$1"
    fi

    if [ -f "${THE_PATH}" ]; then
        # It's a file, so remove everything after and including last slash
        DIR="${THE_PATH%/*}"
        # Remove everything up to and including the last slash before the file name
        FILE="${THE_PATH##*/}"
    elif [ -d "${THE_PATH}" ]; then
        # It's a directory, so remove trailing slash, if there is one
        DIR="${THE_PATH%/}"
    else
        # It's neither a directory, nor a file, so we can't get the realpath to it
        return 1
    fi

    if [ -z "${DIR}" ]; then
        # The path given was at the root of the filesystem (i.e. consisted of one / which was removed earlier)
        echo "/${FILE:-""}"
        return 0
    fi

    if ! cd "${DIR}" > /dev/null 2>&1 ; then
        # Can't cd to the directory, either because it doesn't exist, or we don't have access
        return 1
    fi
    # If it didn't fail, we've changed directory, and can echo the absolute path to where we are
    if ! ABSOLUTE_PATH="$(pwd -P)" > /dev/null 2>&1 ; then
        # Why would it failing now and not before? I don't know, but that's an error
        return 1
    fi
    # If this was a path to a file...
    if [ -n "${FILE:-""}" ]; then
        # ...add the file back to the directory
        echo "${ABSOLUTE_PATH}/${FILE}"
    else
        echo "${ABSOLUTE_PATH}"
    fi

    if ! cd "${OLD_CD}" > /dev/null 2>&1 ; then
        # We can't cd back to the directory we started in, and that's very weird
        return 1
    fi

    # Unset variables used
    unset -v OLD_CD > /dev/null 2>&1 || true
    unset -v THE_PATH > /dev/null 2>&1 || true
    unset -v FILE > /dev/null 2>&1 || true
    unset -v DIR > /dev/null 2>&1 || true
    unset -v ABSOLUTE_PATH > /dev/null 2>&1 || true
    return 0
}
export realpath