Reverse a hexadecimal number in bash

Question

Is there a simple command to reverse an hexadecimal number?

For example, given the hexadecimal number:

The output should be:

Using the rev command, I get the following:

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

@cat I added the answer because the one that worked for me, is at the comments of the selected answer. But I'm ok removing it. — Iñaki Murillo, Nov 08 '16 at 15:47
You could add it as an answer by clicking the "Answer this question" button below the answer box (self-answers are encouraged), and you should, just don't put it in the question. — cat, Nov 08 '16 at 15:53
The C programmer in me wants to say "010203" is an octal number, not a hex number (0x10203) — infixed, Nov 08 '16 at 16:18
@infixed You are not wrong, but I wanted an answer that treats `010203` as an hexadecimal, even though I do not use `0x` — Iñaki Murillo, Nov 08 '16 at 16:22

score 12 · Answer 1 · edited Nov 08 '16 at 15:40

12

With fold + tac + tr :

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

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

edited Nov 08 '16 at 15:40

GAD3R

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answered Nov 08 '16 at 13:03

Ipor Sircer

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Stéphane Chazelas · Answer 2 · 2021-10-25T16:06:54.863

11

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")

edited Oct 25 '21 at 16:06

answered Nov 08 '16 at 12:57

Stéphane Chazelas

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3

I was going to suggest `perl -F'(..)' -lane 'print reverse(@F)'` :) – Sundeep Nov 08 '16 at 12:59
1

@Sundeep, nice. I didn't know one could use `-F` like that. (I can see it described in the `split()` manual now). – Stéphane Chazelas Nov 08 '16 at 13:22
1

as far as I know, `-F` is basically splitting on `$_`.. apart from using regex like `-F'/"\K\|(?=")/'` one can specify number of splits as well... like `-F'/:/,$_,2'` ... use `()` if separator has to be captured as well – Sundeep Nov 08 '16 at 13:27
Instead of using paste you could use `tr -d '\n'` – AKHolland Nov 10 '16 at 21:35

l0b0 · Accepted Answer · 2021-10-25T18:49:43.867

8

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.

edited Oct 25 '21 at 18:49

answered Nov 08 '16 at 12:37

l0b0

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I get `0102030a` instead of `010203` – Iñaki Murillo Nov 08 '16 at 12:48
@l0b0 and too me,get `0102030a` – Baba Nov 08 '16 at 12:49
@IñakiMurillo Strange. Did you run *exactly* the command above? If so, which `bash`/`xxd`/`rev` versions are you using? – l0b0 Nov 08 '16 at 12:50
@l0b0 I run it exactly. I updated the question with software versions – Iñaki Murillo Nov 08 '16 at 12:56
A `0a` at the end must mean that a newline was introduced at some point (`0x0a` is newline in ASCII). – l0b0 Nov 08 '16 at 12:56
2

@IñakiMurillo in your `rev` version `2.20.1`; use this formate `xxd -revert -plain <<< '030201' | LC_ALL=C rev | tr -d '\n'| xxd -plain` – Baba Nov 08 '16 at 12:59
2

`rev` before version `2.24` have a new line bug . more info https://github.com/karelzak/util-linux/commit/d267fdb8540202f77efb28fdf5dddd9826626757 – Baba Nov 08 '16 at 13:33
2

It does not work if the hex string contains the '00' NUL byte. – Sylvain Leroux Nov 09 '16 at 08:43

score 6 · Answer 4 · answered Nov 08 '16 at 15:52

6

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

This reverts each hexadecimal line:

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

answered Nov 08 '16 at 15:52

JJoao

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score 4 · Answer 5 · answered Nov 08 '16 at 16:25

4

(for the sake of completeness)

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

answered Nov 08 '16 at 16:25

zeppelin

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*This* works if the '00' NUL byte is in the input. – Sylvain Leroux Nov 09 '16 at 08:54

score 1 · Answer 6 · answered Nov 15 '19 at 10:12

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

Reverse a hexadecimal number in bash

6 Answers6