ISO images are distributed in ISO 9660 format. The format is for optical discs, which could contain both data and the boot sector, making it bootable. Suppose you burn the ISO images directly into an external drive such as a USB stick or portable hard drive; you can directly boot from it.
ISO images are typically used to distribute operating system installers. The image should be bootable, automatically launching the operating system's installation program upon system boot.
Many Linux tools can be used to create a bootable USB drive from ISO image, and dd is the most universally available. dd is a command-line tool and is installed by default in most Linux distributions, such as Ubuntu, Fedora, CentOS, or SUSE.
Related: How to backup optical disk in Linux
$ file Downloads/ubuntu-21.04-desktop-amd64.iso Downloads/ubuntu-21.04-desktop-amd64.iso: ISO 9660 CD-ROM filesystem data (DOS/MBR boot sector) 'Ubuntu 21.04 amd64' (bootable)
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 55.4M 1 loop /snap/core18/1997 loop1 7:1 0 219M 1 loop /snap/gnome-3-34-1804/66 loop2 7:2 0 64.8M 1 loop /snap/gtk-common-themes/1514 loop3 7:3 0 32.3M 1 loop /snap/snapd/11588 loop4 7:4 0 51M 1 loop /snap/snap-store/518 loop5 7:5 0 65.1M 1 loop /snap/gtk-common-themes/1515 sda 8:0 0 20G 0 disk sdb 8:16 0 20G 0 disk ├─sdb1 8:17 0 1M 0 part ├─sdb2 8:18 0 513M 0 part /boot/efi └─sdb3 8:19 0 19.5G 0 part / sr0 11:0 1 1024M 0 rom
$ lsblk NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT loop0 7:0 0 55.4M 1 loop /snap/core18/1997 loop1 7:1 0 219M 1 loop /snap/gnome-3-34-1804/66 loop2 7:2 0 64.8M 1 loop /snap/gtk-common-themes/1514 loop3 7:3 0 32.3M 1 loop /snap/snapd/11588 loop4 7:4 0 51M 1 loop /snap/snap-store/518 loop5 7:5 0 65.1M 1 loop /snap/gtk-common-themes/1515 sda 8:0 0 20G 0 disk sdb 8:16 0 20G 0 disk ├─sdb1 8:17 0 1M 0 part ├─sdb2 8:18 0 513M 0 part /boot/efi └─sdb3 8:19 0 19.5G 0 part / sdc 8:32 1 58.6G 0 disk ├─sdc1 8:33 1 2.5G 0 part /media/user/Ubuntu 20.04 LTS amd64 └─sdc2 8:34 1 3.9M 0 part /media/user/1079-24A3 sr0 11:0 1 1024M 0 rom
$ sudo umount /dev/sdc1 /dev/sdc2 [sudo] password for user:
$ sudo dd if=Downloads/ubuntu-21.04-desktop-amd64.iso of=/dev/sdc conv=fdatasync 5505348+0 records in 5505348+0 records out 2818738176 bytes (2.8 GB, 2.6 GiB) copied, 628.961 s, 4.5 MB/s
$ sudo blkid /dev/sdc /dev/sdc: BLOCK_SIZE="2048" UUID="2021-04-20-11-16-16-00" LABEL="Ubuntu 21.04 amd64" TYPE="iso9660" PTUUID="af8737a9-1e23-4373-b87a-c8b16199d461" PTTYPE="gpt"
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