Provides a function that emulates bash 5.0's mapfile

mapfile.bash

#!/bin/bash

shopt -s extglob
[[ BASH_VERSINFO -ge 5 ]] && set -u

#### Implementation Snippet Begins ####
function mapfile.error {
	printf '%s\n' "$1" "$@"
	exit "${2-1}"
}

function mapfile {
	local -a __opts __delim __callback __array
	local __max=0 __origin=0 __skip=0 __remove_trailing=false __fd=0 __quantum=5000 __append=
	local __i __j __

	__opts=("$@")
	__array=()

	while [[ $# -gt 0 ]]; do
		case $1 in
		-d)
			__delim=$2
			shift
			;;
		-n)
			[[ ${2-} == +([[:digit:]]) ]] || mapfile.error "Invalid argument to '$1': ${2-}"
			__max=$2
			shift
			;;
		-O)
			[[ ${2-} == +([[:digit:]]) ]] || mapfile.error "Invalid argument to '$1': ${2-}"
			__origin=$2
			shift
			;;
		-s)
			[[ ${2-} == +([[:digit:]]) ]] || mapfile.error "Invalid argument to '$1': ${2-}"
			__skip=$2
			shift
			;;
		-t)
			__remove_trailing=true
			;;
		-u)
			[[ ${2-} == +([[:digit:]]) ]] || mapfile.error "Invalid argument to '$1': ${2-}"
			__fd=$2
			shift
			;;
		-C)
			[[ ${2+.} ]] || mapfile.error "Invalid argument to '$1': ${2-}"
			__callback=$2
			shift
			;;
		-c)
			[[ ${2-} == +([[:digit:]]) && $2 -gt 0 ]] || \
				mapfile.error "Invalid argument to '$1': ${2-}"
			__quantum=$2
			shift
			;;
		--)
			__array=("${__array[@]}" "${@:2}")
			break
			;;
		-*)
			mapfile.error "Invalid option: $1"
			;;
		*)
			__array=("${__array[@]}" "$1")
			;;
		esac

		shift
	done

	for __ in "${__array[@]}"; do
		[[ $__ == +([[:alpha:]_])*([[:alnum:]_]) ]] || \
			mapfile.error "Invalid non-variable argument: $__"
	done

	[[ ${#__array[@]} -gt 1 ]] && mapfile.error "Too many variables specified."

	if [[ -z ${__delim+.} && BASH_VERSINFO -ge 4 ]]; then
		builtin mapfile "${__opts[@]}"
		return
	fi

	__delim=${__delim-$'\n'}
	__array=${__array-MAPFILE}
	eval "$__array=()"

	if [[ __skip -gt 0 ]]; then
		while (( __skip-- > 0 )); do
			IFS= read -rd "$__delim" -u "$__fd" __ || return
		done
	fi

	[[ __max -eq 0 ]] && __max=-1
	[[ $__remove_trailing == false ]] && __append=$__delim

	for (( __i = ${__origin-0}, __j = 0; __j++ != __max; )); do
		IFS= read -rd "$__delim" -u "$__fd" __ || break
		__=$__$__append
		(( __j % __quantum == 0 )) && [[ ${__callback+.} ]] && "$__callback" "$__i" "$__"
		eval "$__array[__i++]=\$__"
	done
}

function readarray {
	mapfile "$@"
}
#### Implementation Snippet Ends ####

mapfile "$@"
declare -p MAPFILE