How to manipulate hexadecimal value in Bash? - bash

Question

I have a variable with an hexadecimal value: in this example, a byte with value 0xfe:
echo $MYVAR | hexdump
0000000 0afe
0000002
I want to use this value on my bash script, in particular I need to:
use as a string (echo X$MYVAR should give me Xfe)
increment it, (0xff)
convert back to the original format (I need to save the incremented value for future use)
Maybe it would be easier if I convert it into integer format?
EDIT: here is how I initialize the var:
printf "\xfe" | dd bs=1 of=tempfile seek=8001
MYVAR=`dd if=tempfile skip=8001 count=1 bs=1`

Answer 1

To print hex number as string you can
printf 0x%X $MYVAR
to increment it and print it back in hex you can do, for example
printf 0x%X `echo $(( 0xfe + 1 ))`
For "convert back" to the original format I think you mean keep the integer value, in this case you can simply use $MYVAR without format conversion.
Hope this helps,
Regards.
EDIT :
To follow your question editing, I'll add my answer below.
You could set MYVAR in this way:
read dummy MYVAR <<EOF
`dd if=tempfile skip=8001 count=1 bs=1|od -x`
EOF
Now you have hex value of the byte read from file stored in MYVALUE.
You can now print it directly with echo, printf or whatever.
$ echo $MYVAR
00fe
You can perform math on it as said before:
$ printf %X $((0x$MYVAR + 1))
FF
(thanks to fedorqui for the shortest version)
Regards.

Answer 2

Assuming you stream represents constantly changed numeric parameter, it may be converted by hexdump or od from coreutils. Note, stdbuf is highly recomended for real-time stream processing.
# Print decimal byte values per line
stdbuf -oL hexdump -v -e '1/1 "%u\n"'
# Floating point values
stdbuf -oL hexdump -v -e '1/4 "%f\n"'
Using od from coreutils, for input endianess correction:
# unsigned 8-bit
stdbuf -oL od -v -An -w7 -tu1 --endian=little
# signed 32-bit
stdbuf -oL od -v -An -w4 -tu4 --endian=little
# Float 32-bit
stdbuf -oL od -v -An -w4 -tf4 --endian=little
Unless endianess is issue, hexdump could be more preferable due to good format support - almost like printf:
cat /dev/urandom \
| stdbuf -oL hexdump -v -e '1/4 "%i" 1/4 " %i" 1/4 " %f\n"' \
| stdbuf -oL awk '{ print "("$1" + "$2") * "$3" = ", $1 + $2 * $3 }'
some pointless 8-bit stream math, for example
# Replace the >/dev/null safeguard to actually try it
cat /dev/urandom \
| stdbuf -oL hexdump -e '1/1 " %i" 1/1 " %i" "\n"' \
| {
while read a b; do
r=$(( (a + b) / 2 ))
printf "printf \"\%03o\"" $r
done
} | sh >/dev/null