using bash: write bit representation of integer to file

Question

I have a file with binary data and I need to replace a few bytes in a certain position. I\'ve come up with the following to direct bash to the offset and show me that it fou

Answer 1

If you're willing to rely on bc (which is fairly common)

echo -e "ibase=16\n obase=2 \n A1" | bc -q

might help.

Answer 2

printf is more portable than echo. This function takes a decimal integer and outputs a byte with that value:

echobyte () {
    if (( $1 >= 0 && $1 <= 255 ))
    then
        printf "\\x$(printf "%x" $1)"
    else
        printf "Invalid value\n" >&2
        return 1
    fi
}

$ echobyte 97
a
$ for i in {0..15}; do echobyte $i; done | hd
00000000  00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f  |................|
00000010

Answer 3

I have a function to do this:

# number representation from 0 to 255 (one char long)
function chr() { printf "\\$(printf '%03o' "$1")" ; return 0 ; }
# from 0 to 65535 (two char long)
function word_litleendian() { chr $(($1 / 256)) ; chr $(($1 % 256)) ; return 0 ; }
function word_bigendian() { chr $(($1 % 256)) ; chr $(($1 / 256)) ; return 0 ; }
# from 0 to 4294967295 (four char long)
function dword_litleendian() { word_lilteendian $(($1 / 65536)) ; word_litleendian $(($1 % 65536)) ; return 0 ; }
function dword_bigendian() { word_bigendian $(($1 / 65536)) ; word_bigendian $(($1 % 65536)) ; return 0 ; }

You can use piping or redirection to catch the result.

Answer 4

xxd is the better way. xxd -r infile outfile will take ascii hex-value in infile to patch outfile, and you can specify the specific position in infile by this: 1FE:55AA

Answer 5

You can use echo to emit specific bytes using hex or octal. For example:

echo -n -e \\x30 

will print ascii 0 (0x30)

(-n remove trailing newline)

Answer 6

With bash, "printf" has the "-v" option, and all shell has logical operators.

So here is simplier form in bash :

int2bin() {
  local i=$1
  local f
  printf -v f '\\x%02x\\x%02x\\x%02x\\x%02x' $((i&255)) $((i >> 8 & 255)) $((i >> 16 & 255)) $((i >> 24 & 255))
  printf "$f"
}