Creating a bootable Ubuntu USB flash drive - terminal commands

1. Keep the ubuntu.iso file in any one of your hard disk partition
2. Next to mount the ubuntu.iso file. Run the below commands in terminal,
sudo mkdir /media/iso
sudo mount -o loop /path/to/ubuntu.iso /media/iso

3. Next to copy all the files from /media/iso to your mounted usb flash drive folder in /media
and insert your usb flash drive
4. Now its automatically mounted, after processing created the mounted folder,
like /media/installer
5. Run command,
cp -a /media/iso/. /media/installer
6. Then paste the ldlinux.sys file inside your USB flash drive partition
(In my case /dev/sdd1) to make the usb bootable. Run the below commands,
sudo apt-get install syslinux mtools
syslinux -s /dev/sdd1
7. Now open the /media/installer and rename the isolinux directory to syslinux.
Then go into the renamed syslinux folder and rename the file isolinux.cfg to syslinux.cfg
8.Reboot your system and change the boot-order in bios to USB.
Now your ubuntu usb flash drive will booted up and you can install it.