Check if variable/function/array value is declared.

is_declared

#!/bin/bash
is_declared() {
    # options
    local _opts=''
    while getopts ':aAfirtx' _opt; do
        case "${_opt}" in
            [aAfirtx])
                _opts="${_opts}${_opt}"
                ;;
            *)
                return 255
                ;;
        esac
    done
    [[ "${_opts}" == '-' ]] && _opts='-'
    shift $((${OPTIND}-1))

    # vars
    while [[ $# -gt 0 ]]; do
        [[ -z "`declare -${_opts} -p "$1" 2>&1 | grep "^declare \-${_opts} $1"`" ]] && \
            return 1
    done
    return 0
}