You'll need:
- An Ubuntu ISO downloaded from here
- USB stick
$ hdiutil convert -format UDRW ~/Downloads/ubuntu-18.04.1-desktop-amd64.img -o ~/Downloads/ubuntu-18.04.1-desktop-amd64.iso$ diskutil unmountDisk /dev/disk3$ sudo dd if=~/Downloads/ubuntu-18.04.1-desktop-amd64.img.dmg of=/dev/rdisk3 bs=1m$ diskutil eject /dev/disk3
Convert the iso a bootable image using hdiutil
user:~$ hdiutil convert -format UDRW -o ~/Downloads/ubuntu-18.04.1-desktop-amd64.img ~/Downloads/ubuntu-18.04.1-desktop-amd64.iso
Reading Driver Descriptor Map (DDM : 0)…
Reading Ubuntu 18.04.1 LTS amd64 (Apple_ISO : 1)…
Reading Apple (Apple_partition_map : 2)…
Reading Ubuntu 18.04.1 LTS amd64 (Apple_ISO : 3)…
.............................................................................
Reading EFI (Apple_HFS : 4)…
.............................................................................
Reading Ubuntu 18.04.1 LTS amd64 (Apple_ISO : 5)…
..............................................................................
Elapsed Time: 18.745s
Speed: 99.4Mbytes/sec
Savings: 0.0%
created: /Users/user/Downloads/ubuntu-18.04.1-desktop-amd64.img.dmg
Find your USB stick that's mounted on the machine:
user:~$ diskutil list
/dev/disk1 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *3.0 TB disk1
1: EFI EFI 209.7 MB disk1s1
2: Apple_APFS Container disk2 3.0 TB disk1s2
...
/dev/disk3 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: GUID_partition_scheme *30.8 GB disk3
1: EFI EFI 209.7 MB disk3s1
2: Microsoft Basic Data UBUNTU 30.5 GB disk3s2
Then you'll unmount it and use dd to write the image to the USB
user:~$ diskutil unmountDisk /dev/disk3
Unmount of all volumes on disk3 was successful
user:~$ sudo dd if=~/Downloads/ubuntu-18.04.1-desktop-amd64.img.dmg of=/dev/rdisk3 bs=1m
Password:
1862+1 records in
1862+1 records out
1953349632 bytes transferred in 143.480153 secs (13614075 bytes/sec)
Finally, eject the disk and you'll be able to boot into the install menu
user:~$ diskutil eject /dev/disk3
Disk /dev/disk3 ejected