Millet is a command line tool for Xiaomi Mi Band. It was created to reverse engineer the protocol but can also be useful for end users. Requires BlueZ and expect.
First of all you need to find out the Bluetooth MAC address of your band:
$ sudo hcitool lescan
LE Scan ...
88:0F:10:EC:70:22 (unknown)
88:0F:10:EC:70:22 MI
Then use it to connect:
$ ./millet 88:0F:10:EC:70:22
spawn gatttool -I -b 88:0F:10:EC:70:22
connect
[88:0F:10:EC:70:22][LE]> connect
Attempting to connect to 88:0F:10:EC:70:22
Connection successful
[88:0F:10:EC:70:22][LE]> char-read-uuid ff01
handle: 0x0012 value: 88 9d 87 b8 00 00 02 8e 00 06 00 02 0e 0a 00 01
[88:0F:10:EC:70:22][LE]> Firmware: 1.0.10.14
char-read-uuid ff0c
handle: 0x002c value: 33 11 0a 19 11 02 34 0c 00 04
[88:0F:10:EC:70:22][LE]> Battery: 51%, 12 cycles, last charge 2017-11-25
char-read-uuid ff06
handle: 0x001d value: b7 30 00 00
[88:0F:10:EC:70:22][LE]> Steps: 12471