Create RAM disks on MacOS that unmount cleanly and reclaim all RAM used

RAMDisk.md

[RAMDisk for macOS]

A minimal ZSH function to create and destroy APFS-formatted RAM disks on macOS.

Features

💾 Create APFS RAM disks up to 128 GiB

✅ Safe teardown - unmounts & detaches cleanly

👮🏼‍♂️ Input validation - enforces valid size and context

⚡ Fast - great for builds, testing, and temp data

Usage

Copy the ramdisk function from ramdisk.sh, paste into .zshrc or equivalent

ramdisk create <size_in_GiB>
ramdisk destroy

Example

ramdisk create 8
→ Creates /Volumes/RAMDisk backed by 8 GiB of RAM

ramdisk destroy
→ Unmounts and detaches the RAM disk cleanly, reclaiming all RAM used

Mount path is always: /Volumes/RAMDisk

Format is always: APFS

Created with: hdiutil and diskutil

ramdisk.sh

# ramdisk: create or destroy a macOS APFS RAM disk
ramdisk() {
  local cmd size blocks device backing mp="/Volumes/RAMDisk"

  # validate command
  if [[ $1 != create && $1 != destroy ]]; then
    echo "Usage: ramdisk create <size_in_GiB> | destroy" >&2
    return 1
  fi
  cmd=$1

  # Create 
  if [[ $cmd == create ]]; then
    # require exactly two args
    if (( $# != 2 )); then
      echo "Usage: ramdisk create <size_in_GiB>" >&2
      return 1
    fi

    # numeric check (digits only)
    if ! [[ $2 =~ ^[0-9]+$ ]]; then
      echo "Error: size must be an integer (no letters or symbols)" >&2
      return 1
    fi

    # strip leading zeros and convert to number
    size=$((10#$2))

    # enforce range 1–128 GiB
    if (( size < 1 || size > 128 )); then
      echo "Error: size must be between 1 and 128 GiB" >&2
      return 1
    fi

    # prevent duplicate mount
    if [[ -d $mp ]]; then
      echo "Error: RAMDisk already exists at $mp" >&2
      return 1
    fi

    # calculate block count (GiB × 2^21 blocks of 512 bytes)
    blocks=$(( size * 2097152 ))

    echo "→ Attaching ram://$blocks (512-byte blocks → ${size}GiB)…"
    device=$(hdiutil attach -nomount ram://$blocks 2>/dev/null \
      | awk '/^\/dev/{print $1; exit}') || {
      echo "Error: hdiutil attach failed" >&2
      return 1
    }
    echo "  → assigned device: $device"

    echo "→ Creating APFS container & volume 'RAMDisk' on $device…"
    diskutil apfs create "$device" RAMDisk >/dev/null || {
      echo "Error: formatting failed" >&2
      hdiutil detach "$device" -force >/dev/null
      return 1
    }
    echo "  → volume 'RAMDisk' mounted at $mp"

    echo "✔ RAMDisk (${size}GiB) ready at $mp (device: $device)"
    return 0
  fi

  # Destroy 
  if [[ $cmd == destroy ]]; then
    # no extra args
    if (( $# != 1 )); then
      echo "Usage: ramdisk destroy" >&2
      return 1
    fi

    # must be mounted
    if [[ ! -d $mp ]]; then
      echo "Error: no RAMDisk found at $mp" >&2
      return 1
    fi

    echo "→ Locating device for $mp…"
    device=$(df "$mp" 2>/dev/null | awk 'NR==2{print $1}') || {
      echo "Error: cannot determine device for $mp" >&2
      return 1
    }
    echo "  → device is $device"

    echo "→ Unmounting $mp"
    diskutil unmount "$mp" >/dev/null || { echo "Error: unmount failed" >&2; return 1; }

    # find the RAM-backing device (Physical Store of the APFS container)
    echo "→ Finding backing store for ${device%s*}…"
    backing=$(diskutil info "${device%s*}" \
      | awk '/Physical Store/{print $NF; exit}') || {
      echo "Error: cannot determine backing device" >&2
      return 1
    }
    echo "  → backing device is /dev/$backing"

    echo "→ Detaching /dev/$backing"
    hdiutil detach "/dev/$backing" >/dev/null || { echo "Error: eject failed" >&2; return 1; }

    echo "✔ RAMDisk at $mp destroyed"
    return 0
  fi
}