Generate SSH Key in Dropbear with some options (by default it uses default id_dropbear as the name and ed25519 as the type)

generate-dropbear-key

#!/bin/sh +ux
# We set the sh +ux flags so that we error on undefined variables and error on bad commands

help() {
  echo >&2 "$0 [-f] [-p] [-q] [<priv_key_file>] [<key_type>] [<key_comment>]"
  echo >&2
  echo >&2 "-q / --quiet to silent all output (except -p if passed)"
  echo >&2 "-p / --pubkey to output public key after generation"
  echo >&2 "-f / --force to force replacing existing key"
  echo >&2
  echo >&2 "<priv_key_file> can be any valid filename [default: '$HOME/.ssh/id_dropbear']"
  echo >&2 "<key_type> can be 'rsa', 'dss', 'ecdsa' or 'ed25519' [default: 'ed25519']"
  echo >&2 "<key_comment> can be be any valid ascii string [default: '$USER@$(hostname)'"
  echo >&2
}

REPLACE_KEY=""
OUTPUT_PUBKEY=""
QUIET=""
for ARG in "$@"; do
  case "$ARG" in
    '-h'|'-help'|'--help') help; exit 255;;
    '-f'|'-force'|'--force') REPLACE_KEY="Y"; shift;;
    '-p'|'-pubkey'|'--pubkey') OUTPUT_PUBKEY="Y"; shift;;
    '-q'|'-quiet'|'--quiet') QUIET="Y"; shift;;
    '-'*) echo >&2 "ERROR: unknown argument '$ARG'"; echo >&2; help; exit 255;;
  esac
done

# Ensure that dropbearkey is installed
command -v 'dropbearkey' >/dev/null 2>&1 || { echo >&2 "I require dropbearkey but it's not installed.  Aborting."; exit 1; }

# Will accept private key file name as first argument
PRIVATE_KEY_FILE="${1:-"$HOME/.ssh/id_dropbear"}"
PUBLIC_KEY_FILE="${PRIVATE_KEY_FILE}.pub"

# Will accept key type as second argument
KEY_TYPE="${2:-"ed25519"}"

KEY_COMMENT="${3:-"$USER@$(hostname)"}"
KEY_DIR="$(dirname "$PRIVATE_KEY_FILE")"

# Ensure the directory exists
mkdir -p "$KEY_DIR"
# Set permissions for directory
chmod 700 "$KEY_DIR"

[ -f "$PRIVATE_KEY_FILE" -a -z "$REPLACE_KEY" ] && { echo >&2 "ERROR: $PRIVATE_KEY_FILE already exists. Pass -f/--force to override"; exit 1; }
rm -f "$PRIVATE_KEY_FILE"

# Generate an RSA key using dropbear
if [ -z "$QUIET" ]; then
  dropbearkey -t "$KEY_TYPE" -f "$PRIVATE_KEY_FILE" >/dev/null || { echo >&2 "ERROR: failed generating private key"; exit 1; }
  echo >&2 "Key generation complete"
else
  dropbearkey -t "$KEY_TYPE" -f "$PRIVATE_KEY_FILE" 2>/dev/null >/dev/null || { echo >&2 "ERROR: failed generating private key"; exit 1; }
fi
[ -f "$PRIVATE_KEY_FILE" ] || { echo >&2 "ERROR: private key file $PRIVATE_KEY_FILE does not exist"; exit 1; }

# Set permissions for private key
chmod 600 "$PRIVATE_KEY_FILE"

# Output Public Key (override if exists)
PUBLIC_KEY="$(dropbearkey -y -f "$PRIVATE_KEY_FILE" 2>/dev/null | grep "ssh-${KEY_TYPE} " | cut -f2 -d ' ')"
echo "ssh-${KEY_TYPE} ${PUBLIC_KEY} ${KEY_COMMENT}" > "$PUBLIC_KEY_FILE"
# Set permissions for public key
chmod 644 "$PUBLIC_KEY_FILE"

if [ -n "$OUTPUT_PUBKEY" ]; then
  # Show Public Key
  if [ -z "$QUIET" ]; then
    echo >&2 "Private Key:"
  fi
  cat "$PUBLIC_KEY_FILE"
fi

Thanks @stokito , updated to set the correct default location.

Have also added some extra arguments, help and the ability to customize the filename and won't override the key by default without the generate-dropbear-key -f flag.

You can get the help with generate-dropbear-key -h now.

If you just want to only generate (and overwrite anything thats there) and not output anything:
generate-dropbear-key -f -q

You can change the name and type here (e.g. to create a rsa dropbear key:
generate-dropbear-key -p "$HOME/.ssh/id_dropbear_rsa" "rsa" "mycomment"

I made a small simplification https://gist.github.com/stokito/e43054c71572d77a665e2d8ec1096749/revisions

I created a PR to the Dropbear to generate a .pub with a comment mkj/dropbear#267
Please like, review and test.