How to use sh script to send hex data to /dev/ttyUSB0?

Question

I'm working with a paired down embedded Linux installation that does not support bash, only sh across a ssh terminal. I need to send hex data to a connected USB device /dev/ttyUSB0. I'm able to do this easily enough with bash (on a desktop installation of Ubuntu 18.04) using echo -en '\x01\x02\x03' > /dev/ttyUSB0, but it seems that the backslash escapes are not supported by sh. Is there another relatively easy (or even not-so-easy) way to do this with sh?

Unfortunately, $SHELL --version is not returning anything. Kernel version is 4.1.0.

@StéphaneChazelas, argh, right, of course – ilkkachu May 25 '22 at 19:12 — ilkkachu, May 25 '22 at 19:12

score 1 · Answer 1 · answered May 25 '22 at 19:24

This is one of the reasons why printf is better than echo. Implementations of echo vary in if and how they support options and backslash-escapes, but all versions of printf should support backslash-escapes.

The only catch is that only octal escapes, like \001 are standard, so \x01 might not be available everywhere even with printf. You could start by trying printf "\x01" though.

If that doesn't work, you'll have to switch to octal. Either do the conversion off-line with any programmer's calculator, or use something like printf "%o\n" "0x61" to convert or use something like this to batch-convert your string:

echo 'foo\x01\x61' | perl -pe 's/\\x([0-9a-fA-F]{2})/sprintf("\\%03o", hex($1))/ge'

(it should give foo\001\141)

Thanks, but unfortunately my target (running ash shell) doesn't support perl. — Jim Fell, May 25 '22 at 22:16
@JimFell, mostly I meant to do the hex-octal conversion offline — ilkkachu, May 26 '22 at 08:35

score 0 · Answer 2 · answered May 24 '22 at 22:25

0

You are mixing slash / and backslash \.

Escaping is done with backslash:

echo -en '\x01\x02\x03' > /dev/ttyUSB0

answered May 24 '22 at 22:25

White Owl

5,129

Kudos for catching the typo, but sh echo still doesn't support the backslash escaping. – Jim Fell May 25 '22 at 02:06
Any ideas why I have to echo twice before the device actually accepts the hex being sent? – Got To Figure Jul 12 '23 at 17:20

score 0 · Accepted Answer · answered May 25 '22 at 18:43

In the end, I used my Ubuntu desktop to create binary files.

echo -en '\x01\x02\x03' > binary.file

Then, after copying the binary file to my system running ash shell, I cat that file in my script to the USB.

cat binary.file > /dev/ttyUSB0

It'd be nice if there were a better way to do this, without the intermediary step of creating a set of binary files, but this'll work for now.

How to use sh script to send hex data to /dev/ttyUSB0?

3 Answers3