Base16 and swap endianness in bash

user_5300329 0
Viewed 215

I need to perform an operation in bash. I have this

401f

I like to perform these operations in bash:

  • swap endianness
  • from base16

In this way: https://gchq.github.io/CyberChef/#recipe=Swap_endianness('Hex',4,true)From_Base(16)&input=NDAxZg

So the result should be 8000. And I like to do it using the less dependencies as possible. I mean if that can be done using just linux core utils, then nice... I guess that something will be neede. Nnot sure what, maybe xxd, awk and that's ok, but I'd like to avoid the use of bc and stuff like that. Thanks.

endianness hex bash
1 Answers

Easy to do in shell using arithmetic expansion, which supports bitwise operations.

num=401f
# Add a 0x prefix so it's treated as base 16 in shell arithmetic.
num="0x$num"
# Swap the bytes in a 16-bit number and print the result in base 10
printf "%d\n" $(( ((num & 0xFF) << 8) | (num >> 8) ))
# Or assign to a variable, etc.
newnum=$(( ((num & 0xFF) << 8) | (num >> 8) ))

Handy bash functions for 16-bit and 32-bit byte swaps:

bswap16() {
    # Default to 0 if no argument given
    local num="0x${1:-0}"
    printf "%d\n" $(( ((num & 0xFF) << 8) | (num >> 8) ))
}

bswap32() {
    local num="0x${1:-0}"
    printf "%d\n" $(( ((num & 0xFF) << 24) |
                      (((num >>  8) & 0xFF) << 16) |
                      (((num >> 16) & 0xFF) <<  8) |
                        (num >> 24) ))
}

bswap16 401f # 8000
bswap32 401f # 524288000

An alternative using bash's parameter substring expansion (Unlike the above, this version requires that the number have 4 hex digits to work right):

num=401f
echo $(("0x${num:2:2}${num:0:2}"))
user_99521960
Related