check if the specific device has been mounted

is_mounted.sh

is_mounted () {
    if [ $# -eq 1 ]; then
        if [ ! -b "$1" ]; then
            echo "error: invalid device:$1"
            return 1
        elif df | awk '{print $1}' | grep "$1" &>/dev/null; then
            return 0
        else
            return 1
        fi
    elif [ $# -eq 2 ]; then
        if [ ! -b "$1" ]; then
            echo "error: invalid device:$1"
            return 1
        elif df | awk '{print $1,$6}' | grep "^$1 $2$" &>/dev/null; then
            return 0
        else
            return 1
        fi
    else
        echo "error: invalid arguments"
        echo "Usage: is_mounted /dev/sdxN [directory]"
    fi
}