Command line interface to BLE (UART)

I've been able to use an Arduino Nano 33 BLE and create a UART connection to the unit with an iPhone (using Bluefruit Connect). I can send and receive messages; this is great. Relatively simple setup.

Now, the trick is to do so with a Linux box (Ubuntu 20.04).

From what I gather, using 'sudo hcitool -i hci0 lescan' can do an LE Scan, and I can see the various devices around. For example:

FB:0D:FF:43:44:3F (unknown)
5B:5D:F1:A3:D5:60 (unknown)
5B:5D:F1:A3:D5:60 Echo
1B:A8:4E:B8:03:7D (unknown)
7B:FF:94:C5:60:74 (unknown)

The device I have has been able to connect to via the phone. The question is, how does one connect to this device (which has a UART capability) from the Linux command line? Again, a simple 'Echo' program (which is in the Arduino HardwareSerialBLE examples) works fine with the phone.

I've tried the gatttool command in interactive mode:

gatttool --device=5B:5D:F1:A3:D5:60 -I

which returns:

[5B:5D:F1:A3:D5:60][LE]>

This is a good sign. Then, connecting:

[5B:5D:F1:A3:D5:60][LE]> connect
Attempting to connect to 5B:5D:F1:A3:D5:60
Connection successful
[5B:5D:F1:A3:D5:60][LE]> 

Now, looking at the primary and characteristics:

[5B:5D:F1:A3:D5:60][LE]> primary
attr handle: 0x0001, end grp handle: 0x0005 uuid: 00001800-0000-1000-8000-00805f9b34fb
attr handle: 0x0006, end grp handle: 0x0009 uuid: 00001801-0000-1000-8000-00805f9b34fb
attr handle: 0x000a, end grp handle: 0x000f uuid: 6e400001-b5a3-f393-e0a9-e50e24dcca9e
[5B:5D:F1:A3:D5:60][LE]> characteristics
handle: 0x0002, char properties: 0x02, char value handle: 0x0003, uuid: 00002a00-0000-1000-8000-00805f9b34fb
handle: 0x0004, char properties: 0x02, char value handle: 0x0005, uuid: 00002a01-0000-1000-8000-00805f9b34fb
handle: 0x0007, char properties: 0x20, char value handle: 0x0008, uuid: 00002a05-0000-1000-8000-00805f9b34fb
handle: 0x000b, char properties: 0x04, char value handle: 0x000c, uuid: 6e400002-b5a3-f393-e0a9-e50e24dcca9e
handle: 0x000d, char properties: 0x10, char value handle: 0x000e, uuid: 6e400003-b5a3-f393-e0a9-e50e24dcca9e
[5B:5D:F1:A3:D5:60][LE]> 

(The primary and characteristic commands show values that show that the serial communication stuff is there; a search on the uuid of 6e400002-b5a3-f393-e0a9-e50e24dcca9e and 6e400003-b5a3-f393-e0a9-e50e24dcca9e show that these are the UART things.

But what is the next step in connecting, so strings can can be sent back and forth?

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