Write a single byte to the serial port with Bash

Question

I have an Arduino which I have coded to read from a USB serial port and power an LED. I know it is working because it works on the built serial monitor. Now I want to write a Bash script which writes to the serial port.

Here is the command:

 echo 121 > /dev/cu.usbmodem411

It outputs the string "123". How can I instead write a single byte with a value of 121?

Thanks that was right I found my problem. If I pass 121 it writes an integer. If I put "y" it will it will write 121 as a byte which is what I want. — DJRyan
– DJRyan, Commented Nov 19, 2011 at 4:32

ruakh · Accepted Answer · 2011-11-19 04:31:57Z

11

echo 121 > /dev/cu.usbmodem411

will write four bytes: 0x31 (meaning '1'), 0x32 (meaning '2'), 0x31 again, 0x0A (meaning a newline).

If your goal is to write a single byte, with value 121, you would write this:

echo -n $'\171' > /dev/cu.usbmodem411

where 171 is 121 expressed in base-8, and -n tells echo not to print a newline character.

If that's not your goal, then please clarify.

answered Nov 19, 2011 at 4:31

ruakh

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1 Comment

fduff Over a year ago

or you can also use the hex notation when sending the value: echo -en '\x79' > /dev/cu.usbmodem411

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