Generate a temporary cloud-init.yml file that is fed directly to multipass to launch a VM

multipass-launcher.sh

#!/bin/sh
# WIP experiment

VM_NAME="$1"
$SSH_PUBLIC_KEY="Your Public SSH Key"

# The following method for handling temp files is adapted from
#
#   https://unix.stackexchange.com/a/181938/458942
#
# Flow:
#   1.  Create file in /tmp directory
#   2.  Attach read and write descriptors to the file
#   3.  Delete the file
#
# The descriptors keep the "file" open (as a stream) until the
# script closes (which automatically releases them).  Deleting the
# file now removes it's entry from the directory list so that, even
# if the script errors out early, the file doesn't clutter up /tmp

# 1.  Create temporary file
tmpfile=$(mktemp /tmp/cloud-init.$VM_NAME.yml.XXXXXX) || exit 1

# 2a.  Create file descriptor 3 for writing to a temporary file so that
# echo ... >&3 writes to that file
exec 3>"$tmpfile"

# 2b.  Create file descriptor 4 for reading from the same file so that
# the file seek positions for reading and writing can be different
exec 4<"$tmpfile"

# 3.  Delete temp file; the directory entry is deleted at once; the reference 
# counter of the inode is decremented only after the file descriptor has been 
# closed.  The file content blocks are deallocated (this is the real deletion) 
# when the reference counter drops to zero.
rm "$tmpfile"

# At this point, we can reference our temp "file" via >&3 (to write) and 
# <&4 (to read) and not have to worry about cleaning up /tmp

# "write" out the cloud-init.yml file we want to use.  In this demo, we're
# prepping for a vscode-enabled VM
cat >&3 <<EOF
#cloud-config  
groups:  
  - vscode  
users:  
  - name: ubuntu  
    groups:  
      - vscode  
    sudo: ['ALL=(ALL) NOPASSWD:ALL']  
    ssh-authorized-keys:  
      - ssh-rsa $SSH_PUBLIC_KEY
EOF

# Launch Multipass, passing the cloud-init.yml data in via stdin
head -n 1 <&4 | \
	multipass launch \
		-n $VM_NAME \
		... \
		--cloud-init -

# close the file descriptor (done automatically when script exits)
# see section 2.7.6 of the POSIX definition of the Shell Command Language
exec 3>&-