Reverse a hexadecimal number in bash

Question

Is there a simple command to reverse an hexadecimal number?

For example, given the hexadecimal number:

030201

The output should be:

010203

Using the rev command, I get the following:

102030

Update

$ bash --version | head -n1
GNU bash, version 4.3.11(1)-release (x86_64-pc-linux-gnu)
$ xxd -version
xxd V1.10 27oct98 by Juergen Weigert
$ rev --version
rev from util-linux 2.20.1

Answer 1

With fold + tac + tr :

$ echo 030201|fold -w2|tac|tr -d "\n"
010203

• fold - split every 2 byte
• tac - reverse cat
• tr - remove newlines

Answer 2

If your system has a rev command.

hex=030201
new_hex=$(printf %s "$hex" | dd conv=swab 2> /dev/null | rev)

If it has a tac or tail -r command:

new_hex=$(echo "$hex" | fold -w 2 | tac | paste -sd '\0' -)

With zsh:

setopt extendedglob
new_hex=${(j[])${(s[]Oa)${hex//(#b)(?)(?)/$match[2]$match[1]}}}

(like in the dd approach: swap pairs of characters, split into list of individual characters (s[]), reverse the order (Oa) and join (j[])).

Or:

printf -v new_hex '%2$s%1$s' ${(s[]Oa)hex}

POSIXly:

new_hex=$(
  awk '
    BEGIN {
      hex = ARGV[1]; l = length(hex)
      for (i = 1; i < l; i += 2)
        new_hex = substr(hex, i, 2) new_hex
      print new_hex
    }' "$hex"
)

Or

new_hex=$(echo "$hex" |
  sed -e 'G;:1' -e 's/\(..\)\(.*\n\)/\2\1/;t1' -e 's/.//')

With perl:

new_hex=$(perl -le 'print reverse(shift =~ /../g)' -- "$hex")

Answer 3

You can convert it to binary, reverse the bytes, optionally remove trailing newlines rev <2.24, and convert it back:

$ xxd -revert -plain <<< 030201 | LC_ALL=C rev | tr -d '\n' | xxd -plain
010203

Using

$ bash --version | head -n1
GNU bash, version 4.3.42(1)-release (x86_64-redhat-linux-gnu)
$ xxd -version
xxd V1.10 27oct98 by Juergen Weigert
$ rev --version
rev from util-linux 2.28.2

This does not work if the string contains 00 (the NUL byte), because rev will truncate the output at that point, or 0a (newline), because rev reverses each line rather than the entire output.

Answer 4

perl -nE 'say reverse /(..)/g'

This reverts each hexadecimal line:

• /(..)/g buils a list with the captured matches

Answer 5

(for the sake of completeness)

$ echo 030201 | grep -o .. | tac | paste -sd '' -
010203

Answer 6

Based on Ipor Sircer's answer https://unix.stackexchange.com/a/321867/337458 I would recommend this in your ~/.bashrc to have a nice command that you simply can call:

function hex_inverse() {
    echo ${1} | fold -w2 | tac | tr -d "\n" ; echo "" 
}

$> hex_inverse 030201

010203