Small utility bash script to mount a qcow2 disk using qemu-nbd

mount-qcow2.sh

#!/bin/bash
set -euo pipefail

THIS_SCRIPT="$0"
DEFAULT_MOUNTPOINT="$(dirname $0)/mountpoint"

function errcho
{
    echo "$@" >&2
}

function fail
{
    if [[ $# -gt 0 ]];
    then
        errcho "$@"
    fi
    exit 1
}

function usage_error
{
    if [[ $# -gt 0 ]];
    then
        errcho "$@"
        errcho
    fi
	errcho "Usage: ${THIS_SCRIPT} [OPTIONS] QCOW2_FILE"
    errcho
    errcho "Options:"
    errcho "  -n, --nbd NBD_INDEX      index of the net block device to use"
    errcho "  -t, --target MOUNTPOINT  path to target mountmoint"
    errcho "  -p, --part PART_INDEX    index of partition to be mounted"
    fail
}

function check_parameter
#arg1: param descr
#...args: remaining script parameters
{
    if [[ $# -lt 3 ]];
    then
        usage_error "Expecting $1 as $2 parameter, got nothing"
    elif [[ $# =~ ^- ]];
    then
        fail "Expecting $1 as $2 parameter, got '$3'"
    fi
}

function check_parameter_decint
#arg1: param descr
#...args: remaining script parameters
{
    check_parameter "$@"
    if [[ ! "$3" =~ ^[[:digit:]]+$ ]];
    then
        fail "Expecting $1 as $2 parameter to be decimal integer, got '$3'"
    fi
    local n=$(("$3"))
    if [[ "$3" != "$n" ]];
    then
        fail "Expecting $1 as $2 parameter to be decimal integer, got '$3'"
    fi
}


if [[ $# -eq 0 ]]; then
    usage_error
fi

NBD_INDEX=0
MOUNTPOINT="$DEFAULT_MOUNTPOINT"
PART_INDEX=1

# load nbd kernel module if /dev/nbd0 is not available
if [[ ! -b /dev/nbd0 ]];
then
    modprobe nbd
fi

while [[ $# -gt 1 ]]; do
    case "$1" in 
        '--nbd'|'-n')
            check_parameter_decint 'net block device index' "$@"
            shift
            NBD_INDEX=$(("$1"))
            shift
        ;;
        '--target'|'-t')
            check_parameter 'mounting point' "$@"
            shift
            MOUNTPOINT="$1"
            shift
        ;;
        '--part'|'-p')
            check_parameter_decint 'partition index' "$@"
            shift
            PART_INDEX=$(("$1"))
            shift
        ;;
        *)
            usage_error "Invalid option '$1'"
    esac
done

if [[ $# -eq 0 ]];
then
    usage_error 'Missing compulsory argument <path_to_qcow2>'
fi
QCOW_PATH="$1"
if [[ ! -f "$QCOW_PATH" ]];
then
    fail "No such qcow2 file '$QCOW_PATH'"
fi

NBD_DEV="/dev/nbd${NBD_INDEX}"
if [[ ! -b "$NBD_DEV" ]];
then
    fail "No such nbd device '$NBD_DEV'"
fi

if [[ ! -d "$MOUNTPOINT" ]];
then
    fail "Mountpoint '$MOUNTPOINT' is not a directory"
fi

echo "Mounting '$QCOW_PATH' as $NBD_DEV."
qemu-nbd --connect "$NBD_DEV" "$QCOW_PATH"

PART_PATH="${NBD_DEV}p${PART_INDEX}"
if [[ ! -b "$PART_PATH" ]];
then
    errcho "No such partition ${PART_PATH}"
    echo "Unmounting ${NBD_DEV}."
    qemu-nbd --disconnect "${NBD_DEV}"
    fail
fi

echo "Mounting ${PART_PATH} to '${MOUNTPOINT}'."
mount "${PART_PATH}" "${MOUNTPOINT}"