convert a filename eg file01010101 from decimal to binary using two's complement, without using built in binary function in bash

Question

This is the binary evaluation and conversion part of the shell script.

elif [[ $file == file[0-1][0-1][0-1][0-1][0-1][0-1][0-1][0-1] ]]
then
    first=`echo $file | cut -c 5`
    second=`echo $file | cut -c 6`
    third=`echo $file | cut -c 7`
    fourth=`echo $file | cut -c 8`
    fifth=`echo $file | cut -c 9`
    sixth=`echo $file | cut -c 10`
    seventh=`echo $file | cut -c 11`
    eighth=`echo $file | cut -c 12`
    newname=file`expr $first \* 128 + $second \* 64 + $third \* 32 + $fourth \* 16 + $fifth \* 8 + $sixth \* 4 + $seventh \* 2 + $eighth \* 1` #this is converting the binary into decimal one bit at a time starting from the leftmost number
    while [ ! -d CATEGORY1 ]
    do
            mkdir CATEGORY1 
    done 
    mv $1/$file CATEGORY1/$newname
    echo "renamed - $file (now named  $newname) so it has been moved to the CATEGORY1 directory."

This is what I've got, but it doesn't incorporate two's complement and I can't use built-in binary features of bash.

Answer 1

I can't use built-in binary features of bash

I'm not sure what it means, arithmetic expressions?

First, to simplify a little, I would use a bash regex for capturing the binary number:

if [[ $file =~ ^file([01]{8})$ ]]

then you just have to convert it in whatever way you prefer:

newname=file$((2#${BASH_REMATCH[1]})) # bash builtin conversion
# or
newname=file$(echo "ibase=2;${BASH_REMATCH[1]}" | bc) # POSIX conversion

---

Update

For the two's complement you could do:

if [[ ${BASH_REMATCH[1]} == 1* ]]
then
    echo "ibase=2; - ( $(tr 10 01 <<< "${BASH_REMATCH[1]}") + 1)"
else
    echo "ibase=2; ${BASH_REMATCH[1]}"
fi | bc