How to create a bootable USB from a bootable ISO with the command line on Linux?

Ok after some research I've figured out a solution, and I'll go through it step by step. Problem was two-fold.

1. Plug in the USB flash drive and determine the device it's mounted on with the command:

   sudo fdisk -l
   

   This time around it was /dev/sdc1 for me, so I'll use that as my example.

2. Umount the device

   umount /dev/sdc1
   

3. Not sure if necessary but I formatted the drive in FAT32, just in case

   sudo mkdosfs -n 'USB-Drive-Name' -I /dev/sdc -F 32
   

4. Now my ISO was using isolinux not syslinux. I knew it worked with CDs so I figured out that I needed to call the isohybrid command, which allows for an ISO to be recognized by the BIOS from a hard drive.

    isohybrid filename.iso
   

   You can find out more about this command here, but this was the cause of the message "Missing Operating System" The first problem was fixed, but now it said "isolinux.bin was missing or corrupt"

5. The next step is to copy the iso. My second problem lay here, where I was copying to the partition, sdc1, not the device, sdc.

   sudo dd if=filename.iso of=/dev/sdc bs=4k
   

   This seems to work just fine, but the forum where I got the last fix, it was recommended to do the following before unplugging the device:

   sync
   sudo eject /dev/sdc
   

This is a common issue with SanDisk USB sticks, or sticks not formatted in FAT32.

If not either of those it is most certainly an issue with your stick partition order or the syslinux.cfg file.

isohybrid may not always work. For example, I had an .iso with FreeDOS and isohybrid was not able to find some important files there (I don't know whether they should had been put there by syslinux, which I used too, or anything else). I propose several alternatives here.

1) Install another bootloader there such as GRUB. It is explained here:

"Assume your USB stick's first partition is FAT32 and its partition is /dev/sdy1" (I had grub2 on my Fedora Core, so I changed the commands a bit):

# mkdir -p /mnt/usb ; mount /dev/sdy1 /mnt/usb
# grub2-install --target=i386-pc --recheck --debug --boot-directory=/mnt/usb/boot /dev/sdy
# grub2-mkconfig -o /mnt/usb/boot/grub2/grub.cfg

# optional, backup config files of grub.cfg
# mkdir -p /mnt/usb/etc/default
# cp /etc/default/grub /mnt/usb/etc/default
# cp -a /etc/grub.d /mnt/usb/etc

# sync; umount /mnt/usb

2) FreeDOS wiki offers a compound method with GRUB and syslinux here (though I don't understand how they launched grub> - I couldn't boot from the usb up to that stage).

3) Here is a post which may be useful - it says 'As long as the command.com, kernel.sys, syslinux.cfg, ldlinux.sys and fat32lba.bss files are in the root of the drive and the MBR and boot sector are not rewritten the drive should remain bootable.'

4) Here it is explained how to generate a bootable .iso file with a syslinux bootloader. They don't even use isohybrid. Unfortunately that didn't help me (maybe because of syslinux).

5*) Use a windows program via wine. I tried rufus, however that didn't work, it couldn't find the device.

I warn you that unfortunately I couldn't solve this problem, my device was unbootable, but I hope that this may be useful for other people (also those who want to install not a linux on the usb). The usb image generated by chtaube though worked for me, so I think these methods are correct.

UPD: The 3rd method really works (with a correction for a custom installation file).

UPD2: (fixed links). The problem with isohybrid was probably because the versions of isolinux.bin on iso and my system were different. Recompiled the iso using genisoimage as suggested here:

genisoimage -l -r -J -b isolinux/isolinux.bin -no-emul-boot -boot-load-size 4 -boot-info-table -c isolinux/boot.cat -o fd11new.iso fd11new

There were some problems too, they were solved by the method here before genisoimage:

cp /boot/extlinux/*.c32 fd11new/isolinux/
extlinux --install /boot/isolinux

== end UPD2 ==

You could use bootiso utility, which does exactly that, securely:

bootiso -d /dev/sdb filename.iso

bootiso will check that selected device /dev/sdb is connected through USB and fails if it doesn't, which is a big security plus. It will also autodetect USB device if there is exactly one USB drive connected to your system, see it in action:

If you want to install syslinux bootloader to circumvent issues mentioned by @xsdf, use -b option:

 bootiso -b -d /dev/sdb filename.iso

To install it:

curl -L https://rawgit.com/jsamr/bootiso/latest/bootiso -O
chmod +x bootiso