base64.bash

One of the fastest* pure bash base64 decoder out there.

This implementation is fully self-contained and does not rely on any external programs.

Tested with GNU Bash 5.1 down to 4.1, striving for full support from 4.1+.

_{* Compared to alternative implementations}

Usage

The command line interface follows base64(1) from coreutils, but only decoding mode -d is supported.

USAGE: base64 -d [FILE]

Library Mode

The library is meant to be sourced without prerequisites, it does not alter shell options or flags.

#!/usr/bin/env bash
set -euE -o pipefail
source base64.bash

base64 -d <<<"aGVsbG8gd29ybGQhCg=="

hello world!

Standalone Mode

There is also support for a standalone mode, where the library can act as an executable.

$ bash base64.bash -d <<<"aGVsbG8gd29ybGQhCg=="

Alternatively the file can be installed to PATH.

$ install -m u+w,a+rx base64.bash ~/.local/bin/base64
$ base64 -d <<<"aGVsbG8gd29ybGQhCg=="

Why it is "fast"

This implementation makes use of the base#n notation, an arithmetic feature common to bash, zsh or ash (busybox).

The base can be set as high as 64, using a consistent order of symbols up to base 62 starting with the digits 0-9, followed by lower case letters a-z, then upper case letters A-Z. A base of 64 adds two more characters, @ (at-sign) and _ (underscore), representing values of 62 and 63, respectively. That is, both encodings share the same symbols for the values of 0 to 61, there are only two symbols left that need to be translated in order to satisfy the bash encoding. Once a symbol has been read, the decoding work greatly reduces to remapping the obtained value from one enconding to another.

	base64	64#n
0-25	A–Z	0-9	0-9
26-51	a–z	a-z	10-35
52-61	0–9	A-Z	36-61
62	+	@	62
62	/	_	63

There is a dual use to this approach: Input data can, with subtle modification, be directly converted to numbers - and it can be fed in using a proper word size. As base64 encodes three bytes (3×8 bit) of data into four symbols (4×6 bit) and vice versa, the input word size should be four symbols wide. The decoder can be tuned to read four characters in a row, remap + and / and convert it to a 24 bit integer to do some math.

Requirements

Only GNU Bash 4.1+ is required with array support enabled at compile time.