Like tmpfs in Mac OSX

mount-ram.sh

#!/bin/sh

# This program has two feature.
#
# 1. Create a disk image on RAM.
# 2. Mount that disk image.
#
# Usage:
#   $0 <dir> <size>
#
#   size:
#     The `size' is a size of disk image (MB).
#
#   dir:
#     The `dir' is a directory, the dir is used to mount the disk image.
#
# See also:
#   - hdid(8)
#

mount_point=${1}
size=${2:-64}

mkdir -p $mount_point
if [ $? -ne 0 ]; then
    echo "The mount point didn't available." >&2
    exit $?
fi

sector=$(expr $size \* 1024 \* 1024 / 512)
device_name=$(hdid -nomount "ram://${sector}" | awk '{print $1}')
if [ $? -ne 0 ]; then
    echo "Could not create disk image." >&2
    exit $?
fi

newfs_hfs $device_name > /dev/null
if [ $? -ne 0 ]; then
    echo "Could not format disk image." >&2
    exit $?
fi

mount -t hfs $device_name $mount_point
if [ $? -ne 0 ]; then
    echo "Could not mount disk image." >&2
    exit $?
fi

umount-ram.sh

#!/bin/sh

# This program has two features.
#
# 1. Unmount a disk image.
# 2. Detach the disk image from RAM.
#
# Usage:
#   $0 <dir>
#
#   dir:
#     The `dir' is a directory, the dir is mounting a disk image.
#
# See also:
#   - hdid(8)
#

mount_point=$1
if [ ! -d "${mount_point}" ]; then
    echo "The mount point didn't available." >&2
    exit 1
fi
mount_point=$(cd $mount_point && pwd)

device_name=$(df "${mount_point}" 2>/dev/null | tail -1 | grep "${mount_point}" | cut -d' ' -f1)
if [ -z "${device_name}" ]; then
    echo "The mount point didn't mount disk image." >&2
    exit 1
fi

umount "${mount_point}"
if [ $? -ne 0 ]; then
    echo "Could not unmount." >&2
    exit $?
fi

hdiutil detach -quiet $device_name

hdid -nomount still means pre-allocating a fixed block of RAM.

Since macOS 15 there's tmpfs, which grows to fit what you actually put in it — no sizing guess up front.
Catch: mounting tmpfs over a folder hides whatever was already there, so you can't just overlay a working directory.

I wrote a tool around that — seeds the RAMDisk from an existing folder on mount and writes everything back on unmount: Persist — a fresh RAMDisk approach for modern macOS