Get value of particular bit in hex digit

mackowiakp · 04-24-2017, 10:43 AM

I have unit connected to USB. It is 4 digital input device, so I can read state of inputs by command

Code:

./arco

It returns one hexadecimal digit representing state of inputs.
My question is. How can I in most simplest way, know value of particular bit in this digit? The answer should be 0 or 1.
Any idea?

AwesomeMachine · 04-24-2017, 11:01 AM

One hex digit is 4 bits. Therefore, it cannot be represented as 0 or 1. The entire value could be 0 or 1, but it could be greater than that. If you're looking at one bit of a hex digit, usually people just memorize the bit positions.

Code:

sundialsvcs · 04-24-2017, 11:06 AM

You simply need to write your script in a programming language that knows how to do this:

Convert the hexadecimal-encoded string to its binary result.
Shift the value '1' to the left n-1 bits.
Logic-AND the two results together.
Test whether the result is or is not zero.

Although I am quite sure ...

... that some future respondent will show us how "bash scripting" can do this," I would instead recommend that you write your script in "a real programming language" (I can think of more than half-a-dozen, right off the bat) that really knows how to do this.

Then, by prefixing your script with #!language_processor_name, aka "shebang," you can use that language to "write your script."

hydrurga · 04-24-2017, 11:26 AM

If $test contains the hex digit (in numeric form),

Code:

echo $((($test & 2#0010) != 0))

will give you 0 or 1 indicating whether the second least significant bit is set.

mackowiakp · 04-24-2017, 11:37 AM

Quote:

Originally Posted by hydrurga

If $test contains the hex digit (in numeric form),

Code:

echo $((($test & 2#0010) != 0))

will give you 0 or 1 indicating whether the second least significant bit is set.

I use sh not bash and probably because of that this line returns:

Code:

./test: line 7: arithmetic syntax error

pan64 · 04-24-2017, 11:59 AM

based on post #2 I would try to make a switch/case:

Code:

case <your input> in
 0) a=0000 ;;
 1) a=0001 ;;
....
esac
and you will need to pick the particular bit of string a

suicidaleggroll · 04-24-2017, 12:58 PM

Code:

echo "obase=2; ibase=16; $var" | bc -l

will convert $var from hex to binary and print it out, then you just need to grab the appropriate digit.

smallpond · 04-24-2017, 01:40 PM

In Perl:

Code:

BIT=0  # or 1, 2, 3
perl -e '$bit = hex(`./arco`) >> $ARGV[0]; print 1 & $bit,"\n"'  $BIT

dugan · 04-24-2017, 02:20 PM

If you're using BASH and DIGIT has the hex digit as a string (e.g. "A"):

First bit:

Code:

if "$(( 0x${DIGIT} & 1 ))" == "1"

Second digit:

Code:

if "$(( 0x${DIGIT} & 2 ))" == "2"

Third bit:

Code:

if "$( 0x${DIGIT} & 4 ))" == "4"

Fourth bit:

Code:

if "$(( 0x${DIGIT} & 8 ))" == "8"

Doug G · 04-24-2017, 09:55 PM

Simplest for me would be to refer to this table: http://ascii.cl/conversion.htm

astrogeek · 04-24-2017, 11:21 PM

This will give the bit state as 0 or 1 for each bit, where HEXD is a single hex digit:

Code:

#!/bin/sh
HEXD=A;
echo "Bit 3 = $(( (0x${HEXD} & 8) > 0))"
echo "Bit 2 = $(( (0x${HEXD} & 4) > 0))"
echo "Bit 1 = $(( (0x${HEXD} & 2) > 0))"
echo "Bit 0 = $(( (0x${HEXD} & 1) > 0))"

You could put it into a loop, but if you only need the lower four bits this is still simple and explicit.

As your arco function returns one hex digit, simply substitute $1 for HEXD, save to script, say hexbits, then suppose arco returns 'A':

Code:

./arco | hexbits
Bit 3 = 1
Bit 2 = 0
Bit 1 = 1
Bit 0 = 0

AwesomeMachine · 04-25-2017, 12:26 AM

Quote:

Originally Posted by Doug G

Simplest for me would be to refer to this table: http://ascii.cl/conversion.htm

If you can handle the octal and binary running together.

mackowiakp · 04-26-2017, 04:53 AM

Quote:

Originally Posted by astrogeek

This will give the bit state as 0 or 1 for each bit, where HEXD is a single hex digit:

Code:

#!/bin/sh
HEXD=A;
echo "Bit 3 = $(( (0x${HEXD} & 8) > 0))"
echo "Bit 2 = $(( (0x${HEXD} & 4) > 0))"
echo "Bit 1 = $(( (0x${HEXD} & 2) > 0))"
echo "Bit 0 = $(( (0x${HEXD} & 1) > 0))"

You could put it into a loop, but if you only need the lower four bits this is still simple and explicit.

As your arco function returns one hex digit, simply substitute $1 for HEXD, save to script, say hexbits, then suppose arco returns 'A':

Code:

./arco | hexbits
Bit 3 = 1
Bit 2 = 0
Bit 1 = 1
Bit 0 = 0

THX. That resolved my problem !