addswap.sh

#!/bin/bash
set -e  # bail if anything goes wrong

is_root(){
   if [ "$( id -u )" -ne 0  ] ; then
      return 1
   fi
   return 0
}

get_swap_amount(){
    # Obtain amount of swap in Gigabytes
    awk '/SwapTotal/{printf "%.2f",($2/1048576)}' /proc/meminfo
}

make_swap_file(){ 
   # This is the function that does the job of creatig swap file
   # and enabling it.  All files are timestamped
   printf "Current swap ammount: %f\n" "$(get_swap_amount)"
   printf "Working on creating swap file\n"
   DATE=$(date +%s) # append date of creation to filename
   filename="/swapfile.""$DATE" # File will be /swapfile.$DATE
   dd if=/dev/zero  of="$filename" bs=1"$2" count="$1"
   chmod 600 "$filename"
   mkswap "$filename" && 
   swapon "$filename" && {
       printf "\nCreated and turned on %s\n"  "$filename"
       printf "Current swap ammount: %f\n" "$(get_swap_amount)"
   }
}


ask_to_enable_on_boot(){
   # Prompt user to enable this new swap file on boot. If user
   # enters y, the swap file will be added to /etc/fstab
   printf "Do you want to turn on this file at boot?[y/n]\n"
   read ANSWER
   case "$ANSWER" in
    [Yy]) printf "\n%s none swap defaults 0 0\n" "$filename"  >> /etc/fstab &&
       printf "\n %s added to /etc/fstab successfuly\n" "$filename"
       exit 0 ;;
    [Nn]) printf "Exiting\n" && exit 0 ;;
    *) printf "Wrong input: %s . Exiting. /etc/fstab not altered\n" "$ANSWER" && exit 1 ;;
   esac

}

bad_arguments(){
     # check if second argument is a character 
     case "$2" in 
         [A-Z]) return 1;;
         *) return 0;;
     esac

    # Check if first argument is a digit. 
    # https://stackoverflow.com/a/3951175/3701431
    case "$1" in
        ''|*[!0-9]*) return 0;;
        *) return 1 ;;
    esac 

}

main(){

    # Check if we're root and if args are OK. If everything is ok, do stuff

    if is_root 
    then
        if [ $# -ne 2   ] ||  bad_arguments "$@"
        then
            printf "%s\n" ">>> ERR: $0: bad or insufficient arguments" > /dev/stderr
            printf "%s\n" ">>> Usage: $0 INTEGER LETTER" > /dev/stderr
            exit 2
        fi
        make_swap_file "$@" && ask_to_enable_on_boot
    else
        printf ">>> ERR: $0 must run as root\n" > /dev/stderr
        exit 1
    fi
}

main "$@"