Example for BASH two's complement with Hex values?

Question

I have a routine that is collecting a hex value via SNMP. Here is a real collection from my bash script 08 01 18 00 FF FF. The value is base on expr $((16#${array[4]})) - $((16#${array[5]})) so the results are 0, how do I introduce two is complement? The correct value for expr $((16#${array[4]})) - $((16#${array[5]})) is -1 based on the example I am working on.

Unsigned value of 255 in twos complement is -1, how do you use twos complement in bash so if I had 255 how do I convert to -1 — justZito
– justZito, Commented Aug 4, 2015 at 0:11

John1024 · Accepted Answer · 2015-08-04 00:22:15Z

2

For convenience, let's create a bash function:

twos() { x=$((16#$1)); [ "$x" -gt 127 ] && ((x=x-256)); echo "$x"; }

Now:

$ twos FF
-1
$ twos FE
-2
 $ twos 01
1

Converting multiple values in one call

Define an eXtended two's complement function:

$ twosx() { for x in "$@"; do x=$((16#$x)); [ "$x" -gt 127 ] && ((x=x-256)); printf "%s " "$x"; done; echo ""; }

Sample usage:

$ twosx 00 01 7F 80 FE FF
0 1 127 -128 -2 -1

edited Aug 4, 2015 at 0:22

answered Aug 4, 2015 at 0:16

John1024

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2 Comments

justZito Over a year ago

is it possible to use echo for twosx() so I can combine the output with another string in the script like echo 'device 1=' twos & twosx 00 01 7F 80 FE FF

John1024 Over a year ago

How about: echo "device 1=$(twosx 00 01 7F 80 FE FF)" ?

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