「Bluetooth Mouse Won't Connect after Reboot - Ubuntu 18.04 LTS」
じっさいに入力したコマンドと、そのコマンドの動作を確認するために重要なメッセージだけ、以下に書き出しておきます。
username@machinename $ bluetoothctl
[bluetooth]# list
Controller XX:XX:XX:XX:XX:XX machinename [default]
[bluetooth]# select XX:XX:XX:XX:XX:XX
[bluetooth]# show
Controller XX:XX:XX:XX:XX:XX (public)
......
[bluetooth]# power on
Changing power on succeeded
[bluetooth]# scan on
Discovery started
[CHG] Controller XX:XX:XX:XX:XX:XX Discovering: yes
[NEW] Device YY:YY:YY:YY:YY:YY Bluetooth Mouse
......
[bluetooth]# scan off
[CHG] Controller XX:XX:XX:XX:XX:XX Discovering: no
Discovery stopped
[bluetooth]# agent on
Agent registered
......
[bluetooth]# pair YY:YY:YY:YY:YY:YY
Pairing successful
......
[bluetooth]# connect YY:YY:YY:YY:YY:YY
Connection successful
......
[bluetooth]# trust YY:YY:YY:YY:YY:YY
Trust succeeded
......
[bluetooth]# exit