下载镜像 server 版本 或者 desktop 版本
转换镜像（从 iso 文件格式转换为 img 格式）命令如下
hdiutil convert -format UDRW -o (.img文件输出全路径) (.iso文件的全路径)
eg:
$ hdiutil convert -format UDRW -o /Users/cason/Downloads/ubuntu-18.04.4-live-server-amd64.img /Users/cason/Downloads/ubuntu-18.04.4-live-server-amd64.iso
Reading Driver Descriptor Map (DDM : 0)…
Reading Ubuntu-Server 18.04.4 LTS amd64 (Apple_ISO : 1)…
Reading Apple (Apple_partition_map : 2)…
Reading Ubuntu-Server 18.04.4 LTS amd64 (Apple_ISO : 3)…
...........................................................
Reading EFI (Apple_HFS : 4)…
...........................................................
Reading Ubuntu-Server 18.04.4 LTS amd64 (Apple_ISO : 5)…
...............................................................................
Elapsed Time: 6.209s
Speed: 140.1Mbytes/sec
Savings: 0.0%
created: /Users/cason/Downloads/ubuntu-18.04.4-live-server-amd64.img.dmg
查看你的目标盘信息（要制作启动盘的U盘或者移动硬盘）
diskutil list
eg:
$ diskutil list
...
/dev/disk7 (external, physical):
#: TYPE NAME SIZE IDENTIFIER
0: FDisk_partition_scheme *31.5 GB disk7
1: Windows_FAT_32 SUPERME 31.5 GB disk7s1
取消挂载
diskutil unmountDisk 外置盘路径
eg:
$ diskutil unmountDisk /dev/disk7
Unmount of all volumes on disk7 was successful
制作启动盘
sudo dd if=转换成img格式的镜像路径 of=启动盘目标路径(注意是rdisk) bs=1m
eg:
$ sudo dd if=/Users/cason/Downloads/ubuntu-18.04.4-live-server-amd64.img.dmg of=/dev/rdisk7 bs=1m
870+0 records in
870+0 records out
912261120 bytes transferred in 35.409260 secs (25763349 bytes/sec)
补充如果提示： dd: /dev/rdisk7: Operation not supported 请确认你的启动盘格式为 MS-DOS(FAT) 格式 再尝试一次