Read, store, and reproduce binary data, including nulls, in pure bash with no external executables or even subshells.

binary_read_tty.sh

#!/usr/bin/env bash
# Read binary data, including null bytes, from a serial port,
# in pure bash with no external executables nor even subshells,
# even though it's not possible to store a null in a shell variable.
# Uses globals to avoid forking subshells.

# Example, echo -e "\0\0\0" |read FOO  or  printf -v FOO '%b' "\0\0\0"
# FOO will not contain any 0x00 bytes.
# Same goes for mapfile/readarray.

# What you CAN do is read and store every byte other that null normally,
# and you can DETECT when you read a null and record that fact instead of the null byte itself.

# Read bytes one at a time from a file or device node until the data stops.
# Store each byte as a hex pair in it's own array element.
#
# When "read" gets a null byte, the variable will be empty, but it will
# also be empty when there was no data to read.
#
# The exit value from "read" discloses when a null-byte was consumed as a delimiter
# vs when there was no data and the read command timed out.
# $?=0 we got a non-null byte normally
#    1 we got a 0x00, and ate it as a delimiter, x is empty but we know to use '00' for this byte
#  142 we timed-out, x is empty, and we know to terminate the loop and NOT fill x='00'

read_tty () {
	local -i e ;local LANG=C x ;HEX=()
	while : ;do
		IFS= read -t0.1 -r -d'' -n1 -u3 x ;e=$?
		((e>1)) && break
		((e)) && x='00' || printf -v x '%02x' "'$x"
		HEX+=($x)
	done
}

###############################################################
# main
typeset -a HEX=()
LANG=C PORT="/dev/ttyUSB0"
exec 3<>"$PORT"
stty -F "$PORT" 19200 raw pass8 clocal cread -echo crtscts -ixon -ixoff -ixany

read_tty

# We now have an array HEX[], where each element is a hex pair representing one byte, including nulls.
# The binary data can be reconstituted with printf.

# copy the array to a string plus one leading space
# produces x=" 63 61 71..."
x=" ${HEX[*]}"

# substitute all the spaces with "\x"
# produces "\x63\x61\x71..."
# printf that with \x## interpretation
# produces "cat..."
printf '%b' "${x// /\\x}" >capture.bin

# To reproduce the original binary data, the printf output must go to a file
# or a device node or piped to another process etc.