[ZTE Axon 7 Mini Questions & Answers] Core2idiot:
[core2idiot@Core2idiot-PC ~]$ sudo fastboot flashing get_unlock_ability
...
FAILED (remote: unknown command)

I don't have a "request" so the next command fails before it even gets to the phone.

[core2idiot@Core2idiot-PC ~]$ sudo fastboot flashing unlock_bootloader
usage: fastboot [ <option> ] <command>

commands:
update <filename> Reflash device from update.zip.
flashall Flash boot, system, vendor, and --
if found -- recovery.
flash <partition> [ <filename> ] Write a file to a flash partition.
flashing lock Locks the device. Prevents flashing.
flashing unlock Unlocks the device. Allows flashing
any partition except
bootloader-related partitions.
flashing lock_critical Prevents flashing bootloader-related
partitions.
flashing unlock_critical Enables flashing bootloader-related
partitions.
flashing get_unlock_ability Queries bootloader to see if the
device is unlocked.
flashing get_unlock_bootloader_nonce Queries the bootloader to get the
unlock nonce.
flashing unlock_bootloader <request> Issue unlock bootloader using request.
flashing lock_bootloader Locks the bootloader to prevent
bootloader version rollback.
erase <partition> Erase a flash partition.
format[:[<fs type>][:[<size>]] <partition>
Format a flash partition. Can
override the fs type and/or size
the bootloader reports.
getvar <variable> Display a bootloader variable.
set_active <suffix> Sets the active slot. If slots are
not supported, this does nothing.
boot <kernel> [ <ramdisk> [ <second> ] ] Download and boot kernel.
flash:raw boot <kernel> [ <ramdisk> [ <second> ] ]
Create bootimage and flash it.
devices [-l] List all connected devices [with
device paths].
continue Continue with autoboot.
reboot [bootloader] Reboot device [into bootloader].
reboot-bootloader Reboot device into bootloader.
help Show this help message.

options:
-w Erase userdata and cache (and format
if supported by partition type).
-u Do not erase partition before
formatting.
-s <specific device> Specify a device. For USB, provide either
a serial number or path to device port.
For ethernet, provide an address in the form <protocol>:<hostname>[:port] where <protocol> is either tcp or udp.
-p <product> Specify product name.
-c <cmdline> Override kernel commandline.
-i <vendor id> Specify a custom USB vendor id.
-b, --base <base_addr> Specify a custom kernel base
address (default: 0x10000000).
--kernel-offset Specify a custom kernel offset.
(default: 0x00008000)
--ramdisk-offset Specify a custom ramdisk offset.
(default: 0x01000000)
--tags-offset Specify a custom tags offset.
(default: 0x00000100)
-n, --page-size <page size> Specify the nand page size
(default: 2048).
-S <size>[K|M|G] Automatically sparse files greater
than 'size'. 0 to disable.
--slot <suffix> Specify slot suffix to be used if the
device supports slots. This will be
added to all partition names that use
slots. 'all' can be given to refer
to all slots. 'other' can be given to
refer to a non-current slot. If this
flag is not used, slotted partitions
will default to the current active slot.
-a, --set-active[=<suffix>] Sets the active slot. If no suffix is
provided, this will default to the value
given by --slot. If slots are not
supported, this does nothing. This will
run after all non-reboot commands.
--wipe-and-use-fbe On devices which support it,
erase userdata and cache, and
enable file-based encryption
--unbuffered Do not buffer input or output.
--version Display version.
-h, --help show this message.