A script for finding the smallest lossless encoding of a PNG or GIF input image (that I know of).

sisyphus.sh

#!/usr/bin/env -S unshare --pid --mount-proc --kill-child --map-root-user /bin/sh
# shellcheck shell=sh

# Licensed under the Zero-Clause BSD terms: https://opensource.org/license/0bsd
## Requires: find, awk, oxipng, and gifsicle
## Optional: pngquant, cwebp & gif2webp, gif2apng, and (perl-image-)exiftool


## -C: Fail if redirects try to overwrite an existing file.
## -e: Fail if any command fails (with exceptions).
## -u: Fail if an unset variable tries to be expanded.
## -f: No glob expansion.
set -Ceuf

perr() { printf "\a\033[1;31m‽\033[0;39m %s\033[0;39m\n" "${1-"$(cat)"}" >&2; }
clear_prog() { printf "\033]9;4;0\007" >&2; }
exit_clear() { clear_prog; exit 1; }
osc777() { printf "\a\033]777;notify;Sisyphus;%b\033\\" "$1" >&2; }
nexit() { osc777 "${1-"$(cat)"}"; exit_clear; }
pquit() { arg="${1-"$(cat)"}"; perr "$arg"; nexit "$arg"; }
pstat() { printf "\033[1;34m%20b\033[0;39m%s\033[0;39m\n" "$1" "${2-"$(cat)"}"; }


## `kill` handles background jobs, but `exit` is required for normal processes.
### The second trap handles unexpected signals, where a notification IS desired.
#### Trying to trap SIGTERM leads to a "Segmentation fault" error
trap 'kill "$$"; exit_clear' INT
trap 'kill "$$"; nexit "SIGQUIT or SIGABRT received! Was operating on: $1"' QUIT ABRT


help() {
	cat <<"HELP"; exit 0
Usage: sisyphus [OPTION]... SRC [DEST]
Losslessly optimise PNGs & GIFs by all known means.

If DEST is omitted, DEST is set to '{SRC wo/ext}.new.EXT'. Possible output
formats include: PNG, GIF, WebP, and APNG.

Options:
 -a, --all-oxi <bool>	Optionally accepts a boolean value. Controls whether to
			  always or never try all OxiPNG variations. When this
			  option is omitted, a heuristic determines whether any
			  OxiPNG variations are attempted.
 -b, --brute-lines <int> Accepts an integer between 1 and the SRC image's height.
			  Sets the maximum '--brute-lines' value for the OxiPNG
			  variations. The default is '16'.
 -d, --dry-run		Perform all operations except for writing the output.
 -f, --force		Allow existing destination files to be overwritten.
 -m, --max-procs <int>	Accepts an integer greater than 0. Limits the number of
			  simultaneous processes for 'xargs'. The default is '8'.
 -n, --no-webp		Do not attempt to use CWebP nor GIF2WebP.
 -N, --no-apng		Do not attempt to use GIF2APNG.
 -q, --quiet		Do not print to STDOUT or send notifications. This option
			  implies '--results 0', but this can be overridden after.
 -r, --results <arg>	Accepts a boolean value or an integer greater than 0.
			  Controls how the results list is printed, if at all.
			  If <arg> is an integer, only the top <arg> trials will
			  be printed. The results are printed to STDERR.
 -s, --size <int>	Accepts an integer in bytes. Sets the minimum size target.
			  If the best encoding is greater or equal to the minimum
			  size, then DEST is not created.
 -S, --stdout		Write the best output to STDOUT. This option semi-implies
			  '--quiet' except it does NOT also imply '--results 0'.

 -h, --help		Display this help message, and then exit.


Booleans:
 -a, --all-oxi		Disable: '0', 'no', or 'false'.
			  Enable: '1', 'yes', or 'true'.
 -r, --results		Hide: 'no', 'false', 'hide', or 'none'.
			  Show: 'yes', 'true', 'show', or 'all'.

Warning:
  HDR SRC images will NOT be losslessly optimised!
HELP
}




# Setup a private TMPFS for this script to use, which is what 'unshare' is for.
## 'nr_inodes' must be >= to 'max number of files + 1'. This is adjusted later
work="/tmp"
mount --types tmpfs --options nosuid,nodev,noexec,size=50%,nr_inodes=16 \
	sisyphus "$work"/ || pquit "Failed to overmount '$work/'!"




# Argument parsing
ALL_OXI=""	MAX_BLIN="16"	DRY=""		FORCE=""	MAX_PROCS="8"
NO_WEBP=""	NO_APNG=""	STDOUT=""	RESULTS="0"	SIZE=""		
INTERNAL=""	ARG_COUNT="0"	SKIP=""
while [ "$ARG_COUNT" -lt "$#" ]; do
	if [ -z "$SKIP" ]; then
		case "$1" in
			-a|--all-oxi)
				case "${2:-}" in
					0|no|false) ALL_OXI="0"; shift ;;
					1|yes|true) ALL_OXI="1"; shift ;;
					*) ALL_OXI="1" ;;
				esac
				;;
			-b|--brute-lines)
				case "${2:-}" in
					[1-9]*) MAX_BLIN="$2"; shift ;;
					*) pquit "No positive integer greater than zero was given for '-b|--brute-lines'!" ;;
				esac
				;;
			-d|--dry-run) DRY="1" ;;
			-f|--force) FORCE="1" ;;
			-m|--max-procs)
				case "${2:-}" in
					[1-9]*) MAX_PROCS="$2"; shift ;;
					*) pquit "No positive integer greater than zero was given for '-m|--max-procs'!" ;;
				esac
				;;
			-n|--no-webp) NO_WEBP="1" ;;
			-N|--no-apng) NO_APNG="1" ;;
			-q|--quiet|-S|--stdout)
				## Only allow writing to STDOUT on FD 8.
				### Don't do `exec 8>&1` twice or else FD 8 will
				### point to `/dev/null` too
				if [ "$(readlink /proc/self/fd/1)" != "/dev/null" ]; then
					exec 8>&1
					exec 1>/dev/null
				fi

				## '--results 0' should only be set when
				## '--quiet' is used; '--stdout' doesn't do this
				case "$1" in
					-S|--stdout) STDOUT="1" ;;
					-q|--quiet) RESULTS="0" ;;
				esac

				## Functions have to be redefined to be updated.
				### This `perr()` omits `\a`
				perr() { printf "\033[1;31m‽\033[0;39m %s\033[0;39m\n" "${1-"$(cat)"}" >&2; }
				osc777() { :; }
				pquit() { perr "$1"; exit_clear; }
				;;
			-r|--results)
				case "${2:-}" in
					no|false|hide|none) shift ;;
					yes|true|show|all) RESULTS="ALL"; shift ;;
					[0-9]*) RESULTS="$2"; shift ;;
					*) pquit "An unknown value was given for '-r|--results'!" ;;
				esac
				;;
			-s|--size)
				case "${2:-}" in
					[0-9]*) SIZE="$2"; shift ;;
					*) pquit "No positive integer was given for '-s|--size'!" ;;
				esac
				;;
			-h|--help) help	;;
			--_internal)
				## DO NOT USE MANUALLY. For self-execution
				INTERNAL="1"
				;;
			--) SKIP="1" ;;
			-*) pquit "'$1' is not a known option!" ;;
			*) set -- "$@" "$1"; ARG_COUNT="$(( ARG_COUNT + 1 ))" ;;
		esac

		else set -- "$@" "$1"; ARG_COUNT="$(( ARG_COUNT + 1 ))"
	fi

	shift
done




# Validate arguments

## `[ cond ] && cmd` will carry over non-zero exit codes; always use `||`
[ "$#" -le 2 ] || pquit "Too many arguments given!"

## Don't try WebP if `cwebp` & `gif2webp` aren't available
command -v cwebp >/dev/null || NO_WEBP="1"
command -v gif2webp >/dev/null || NO_WEBP="1"

## Don't try APNG if `gif2apng` isn't available
command -v gif2apng >/dev/null || NO_APNG="1"

## Don't try `pngquant` if it's not available
NO_QUANT=""
command -v pngquant >/dev/null || NO_QUANT="1"

## Don't try anything if `find` or `awk` are unavailable
command -v find >/dev/null || pquit "The 'find' command is required!"
command -v awk >/dev/null || pquit "The 'awk' command is required!"




## SRC
src_real="${1:?$(pquit "No source file was specified!")}"
[ -s "$src_real" ] || pquit "'$src_real' is not a non-empty file!"

### Assign the SRC a file descriptor
exec 3<"$src_real"
src="/proc/self/fd/3"
src_real="$(realpath -- "$src_real")"


### Size detection
find_size() {
	# Handle non-existent files by giving them arbitrarily huge sizes
	if [ -f "$1" ]; then
			wc --bytes <"$1"
		else
			printf "4294967296"
	fi
}

src_size="$(find_size "$src")"


### Minimum size target validation
check_number() {
	[ "$1" != "" ] || return 0

	num="$1" oldnum="$num" option="$2"

	## Strip leading zeros and avoid integer overflow
	num="$(printf "%s" "$num" | sed "s/^0*//")"

	## If `sed` trimmed '0' to nothing, undo that
	num="${num:-"0"}"

	if ! printf "%d" "$num" >/dev/null 2>&1; then
		perr "The value given for '$option' ($oldnum) is not an integer, or is far beyond the integer limit!"

		return 1
	fi

	if [ "$num" -ne "$(printf "%.12s" "$num")" ]; then
		perr "The value given for '$option' ($oldnum) is over the integer limit!" \

		return 1
	fi

	printf "%d" "$num"
}

SIZE="$(check_number "$SIZE" "-s|--size")"


### Max processes validation
MAX_PROCS="$(check_number "$MAX_PROCS" "-m|--max-procs")"


### Max `--brute-lines` validation
MAX_BLIN="$(check_number "$MAX_BLIN" "-b|--brute-lines")"


## Results validation
[ "$RESULTS" = "ALL" ] || RESULTS="$(check_number "$RESULTS" "-r|--results")"


### Mimetype detection
mime="$(file --dereference --brief --mime-type -- "$src")"
mode="PNG"
safe="all"
case "$mime" in
	"image/png") : ;;
	"image/gif") mode="GIF" safe="safe" ;;
	*) pquit "The mimetype of '$src_real' is neither PNG nor GIF!" ;;
esac

#### When '$INTERNAL' is set, the input is APNG and must be safely stripped
[ -z "$INTERNAL" ] || safe="safe"

src_height="$(file --dereference --brief -- "$src" |
	sed -n "s/.*x \([[:digit:]]\+\).*/\1/p")"
src_height="$(check_number "$src_height" "\$src_height")"




## DEST
dest_exists() {
	if [ -n "$FORCE" ] || [ -n "$STDOUT" ]; then return 0; fi

	## Allow globbing for just this for-loop
	set +f

	for similar in "$1"*; do
		[ -e "$similar" ] || continue

		if [ "${similar%.*}" = "$1" ]; then
			perr "'$similar' shares a filename with '$1'!"

			return 1
		fi
	done

	set -f
}

dest_path="${2:-}"
if [ -n "$STDOUT" ]; then
		[ -z "$dest_path" ] ||
			pquit "A destination, '$dest_path', and '--stdout' were both specified!"
	else
		dest_path="${dest_path%.*}"
		dest_path="${dest_path:-${src_real%.*}.new}"
		dest_path="$(realpath -- "$dest_path")"

		dest_exists "$dest_path" ||
			nexit "'$similar' shares a filename with '$dest_path'!"
fi




# Print known information
printf "%s  \033[2m(%s)" "$src_real" "$mime" | pstat "Input ← "
printf "%s\033[2m.???" "$dest_path" | pstat "Output template ⇨ "
[ -z "$SIZE" ] || pstat "Size target ↘ " "$SIZE bytes"




# Optimisation
create_png_bases() {
	cwebp_base="$work/cwebp.webp"

	if [ "$NO_WEBP" != "1" ]; then
		## `-z 9` implies `-q 100 -m 6 -lossless`;
		## `-z 9 -q 100` disables lossless mode
		cwebp -quiet -mt -z 9 -alpha_filter best -o "$cwebp_base" \
			-- "$src" ||
				perr "Passing '$src_real' through 'cwebp' failed!"
	fi &




	## Values above the SRC height aren't useful for `--brute-lines`.
	## For the baseline, limit it to a sensible value: 8.
	baseline_max_blin="$((
		src_height > 8
		? 8
		: src_height
	))"

	## If `--brute-lines` was set below 8, respect that too
	baseline_max_blin="$((
		baseline_max_blin > MAX_BLIN
		? MAX_BLIN
		: baseline_max_blin
	))"

	oxipng_base="$work/oxipng + zc12 + f9 + level=5  + lines=$baseline_max_blin.png"

	{
		oxipng --opt max --filters 9 --brute-level=5 \
			--brute-lines="$baseline_max_blin" --strip "$safe" \
			--alpha --out "$oxipng_base" -- "$src" \
				>/dev/null 2>&1 ||
					perr "Creating '$oxipng_base' failed!"
	} &




	if [ "$safe" != "safe" ] && [ -z "$NO_QUANT" ]; then
		quant_src="$work/quant.png"

		## '$quant_src' won't be created if it can't be quantised
		## losslessly (`--quality 100-100`).
		### In that case, it'll exit with code 99
		pngquant --quality 100-100 --speed 1 --strip \
			--output "$quant_src" -- "$src" ||:

		## If '$src' was already quantised beforehand, '$quant_src' will
		## just be a duplicate that should be removed.
		### `unset` works after `||/&&`, unlike `quant_src=""`
		[ -f "$quant_src" ] || unset quant_src

		if cmp -- "$src" "${quant_src:-}" >/dev/null 2>&1; then
			rm -- "$quant_src" ||
				perr "Failed to remove '$quant_src'!"

			unset quant_src
		fi
	fi

	wait




	oxipng_base_size="$(find_size "$oxipng_base")"
	cwebp_base_size="$(find_size "$cwebp_base")"

	smallest_heuristic="$((
		oxipng_base_size < cwebp_base_size
		? oxipng_base_size
		: cwebp_base_size
	))"

	## '$smallest_known' gets read by `try_oxi_vars`
	smallest_known="$((
		smallest_heuristic < src_size
		? smallest_heuristic
		: src_size
	))"

	## If '$SIZE' was given, factor it into the smallest-known size value
	[ -z "$SIZE" ] ||
		smallest_known="$((
			smallest_heuristic < SIZE
			? smallest_heuristic
			: SIZE
		))"
}

gen_list() {
	# Create the command list `xargs` will parse

	# For GIF mode
	if [ "$mode" = "GIF" ]; then
		gif2webp_dest="$work/gif2webp.webp"
		gif2apng_prefix="$work/gif2apng + z"
		gifsicle_prefix="$work/gifsicle + o"
		nproc="$(nproc)"


		# Skip `gif2webp` and or `gif2apng` if requested.
		[ -n "$NO_WEBP" ] || cat <<-GIF2WEBP
$src	$gif2webp_dest	gif2webp -quiet -mt -min_size -m 6 -q 100 -metadata none
		GIF2WEBP

		[ -n "$NO_APNG" ] || cat <<-GIF2APNG
$src	${gif2apng_prefix}0.apng	gif2apng -i20 -z0
$src	${gif2apng_prefix}1.apng	gif2apng -i20 -z1
$src	${gif2apng_prefix}2.apng	gif2apng -i20 -z2
$src	${gif2apng_prefix}0 + kp.apng	gif2apng -i20 -z0 -kp
$src	${gif2apng_prefix}1 + kp.apng	gif2apng -i20 -z1 -kp
$src	${gif2apng_prefix}2 + kp.apng	gif2apng -i20 -z2 -kp
		GIF2APNG


		cat <<-GIFSICLE
$src	${gifsicle_prefix}3.gif	gifsicle --optimize=3 --optimize=keep-empty --threads="$nproc"
$src	${gifsicle_prefix}2.gif	gifsicle --optimize=2 --optimize=keep-empty --threads="$nproc"
$src	${gifsicle_prefix}1.gif	gifsicle --optimize=1 --optimize=keep-empty --threads="$nproc"
		GIFSICLE

		return 0
	fi




	# For PNG mode
	pre="$work/oxipng"
	osrc="$src"

	## Duplicate the list for '$quant_src' (if set). This is `break`ed later
	while :; do


	## `--brute-{level,lines}` only applies to the brute filter (`-f 9`).
	zc="1"
	while [ "$zc" -le 12 ]; do
		pzc="$(printf "%-2s" "$zc")"

		cat <<-ZC
$osrc	$pre + zc$pzc + f0-8.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8
$osrc	$pre + zc$pzc + f0-8 + nb.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb
$osrc	$pre + zc$pzc + f0-8 + nc.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nc
$osrc	$pre + zc$pzc + f0-8 + nb + nc.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --nc
$osrc	$pre + zc$pzc + f0-8 + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --ng
$osrc	$pre + zc$pzc + f0-8 + nb + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --ng
$osrc	$pre + zc$pzc + f0-8 + nc + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nc --ng
$osrc	$pre + zc$pzc + f0-8 + nb + nc + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --nc --ng
$osrc	$pre + zc$pzc + f0-8 + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --np
$osrc	$pre + zc$pzc + f0-8 + nb + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --np
$osrc	$pre + zc$pzc + f0-8 + nc + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nc --np
$osrc	$pre + zc$pzc + f0-8 + nb + nc + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --nc --np
$osrc	$pre + zc$pzc + f0-8 + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --ng --np
$osrc	$pre + zc$pzc + f0-8 + nb + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --ng --np
$osrc	$pre + zc$pzc + f0-8 + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nc --ng --np
$osrc	$pre + zc$pzc + f0-8 + nb + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=0-8 --nb --nc --ng --np
		ZC

		zc="$(( zc + 1 ))"
	done

	cat <<-ZOP
$osrc	$pre + zop  + f0-8.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8
$osrc	$pre + zop  + f0-8 + nb.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb
$osrc	$pre + zop  + f0-8 + nc.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nc
$osrc	$pre + zop  + f0-8 + nb + nc.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --nc
$osrc	$pre + zop  + f0-8 + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --ng
$osrc	$pre + zop  + f0-8 + nb + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --ng
$osrc	$pre + zop  + f0-8 + nc + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nc --ng
$osrc	$pre + zop  + f0-8 + nb + nc + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --nc --ng
$osrc	$pre + zop  + f0-8 + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --np
$osrc	$pre + zop  + f0-8 + nb + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --np
$osrc	$pre + zop  + f0-8 + nc + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nc --np
$osrc	$pre + zop  + f0-8 + nb + nc + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --nc --np
$osrc	$pre + zop  + f0-8 + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --ng --np
$osrc	$pre + zop  + f0-8 + nb + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --ng --np
$osrc	$pre + zop  + f0-8 + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nc --ng --np
$osrc	$pre + zop  + f0-8 + nb + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=0-8 --nb --nc --ng --np
	ZOP


	## `--brute-lines` only makes sense up to the height of the SRC image
	blin_max="$((
		src_height > MAX_BLIN
		? MAX_BLIN
		: src_height
	))"


	## If '$src_height' is 1, ensure at least 1 line is tried.
	blin="2"
	if [ "$blin_max" -lt "$blin" ]; then
		blin="1"
		blin_max="$blin"
	fi

	while [ "$blin" -le "$blin_max" ]; do

		blev="1"
		while [ "$blev" -le 12 ]; do
			## Ensure numbers like '1' and '12' are the same length
			pblin="$(printf "%-2s" "$blin")"
			pblev="$(printf "%-2s" "$blev")"

			zc="1"
			while [ "$zc" -le 12 ]; do
				pzc="$(printf "%-2s" "$zc")"

				## `${pblin% *}` prevents "... lines=N .png"
				cat <<-ZCF9
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=${pblin% *}.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=${pblin% *}
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nc.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + nc.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --ng
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --ng
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nc + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --ng
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + nc + ng.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --ng
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nc + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + nc + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --ng --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --ng --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --ng --np
$osrc	$pre + zc$pzc + f9 + level=$pblev + lines=$pblin + nb + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zc=$pzc --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --ng --np
				ZCF9

				zc="$(( zc + 1 ))"
			done

			## Don't duplicate the `--zopfli` lines per `--zc` level
			cat <<-ZOPF9
$osrc	$pre + zop  + f9 + level=$pblev + lines=${pblin% *}.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=${pblin% *}
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nc.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + nc.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --ng
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --ng
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nc + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --ng
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + nc + ng.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --ng
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nc + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + nc + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --ng --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --ng --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nc --ng --np
$osrc	$pre + zop  + f9 + level=$pblev + lines=$pblin + nb + nc + ng + np.png	oxipng --opt max --alpha --strip $safe --zopfli --zi=1024 --ziwi=16 --filters=9 --brute-level=$pblev --brute-lines=$pblin --nb --nc --ng --np
			ZOPF9

			blev="$(( blev + 1 ))"
		done

		blin="$(( blin + 1 ))"
	done


	## If '$quant_src' is set, generate the list for each of '$src' and
	## '$quant_src' sequentially
	if [ "$osrc" = "${quant_src:-}" ] || [ -z "${quant_src:-}" ]; then
		break; fi

	osrc="$quant_src"	pre="$work/quant > oxipng"

	done	## This `done` is for the `while :;` statement
}

gen_script() {
	cat <<-"SCRIPT"
## `noexec` on `$work` prevents having `#!/bin/sh` here instead
set -Ceuf

## Redirect this STDERR to FD 9. This is reverted outside `xargs`
exec 2>&9

perr() {
	arg="${1-"$(cat)"}"
	printf "\a\033[1;31m‽\033[0;39m %s\033[0;39m\n" "$arg" >&2
	printf "\033]777;notify;Sisyphus;%b\033\\" "$arg"
}

## `exit 255` prevents `xargs` from processing any more line batches (the
## batches from `--max-procs`)
pquit() { perr "${1-"$(cat)"}"; exit 255; }


smallest_known="$1"	total_lines="$2"	oxipng_base="$3"


## Parse the line
IFS="	" read -r index input output cmd <<-LINE ||:
	$4
LINE

## Check if the output (e.g. '$oxipng_base') already exists
[ ! -f "${output:-}" ] || exit 0


## Update the progress bar now. Otherwise, long-running '$cmd's will cause the
## bar to jump backwards on their completion.
### Without `>&2`, and when `--stdout` is used, the updates get redirected too
printf "\033]9;4;1;%d\007" "$(( ( index * 100 ) / total_lines ))" >&2


## Handle each command differently where needed
case "$cmd" in
	quant*|oxipng*)
		env --split-string "$cmd" --out "$output" -- "$input" \
			2>&1 || pquit "'$cmd --out $output -- $input' failed!"
		;;
	gif2webp*|gifsicle*)
		env --split-string "$cmd" -o "$output" -- "$input" \
			2>&1 || pquit "'$cmd -o $output -- $input' failed!"
		;;
	gif2apng*)
		## `gif2apng` only accepts relative paths for its arguments:
		## https://sourceforge.net/p/gif2apng/discussion/1022150/thread/8ec5e7e288
		input_rel="$(realpath --relative-to="$PWD" -- "$input")"
		output_rel="$(realpath --relative-to="$PWD" -- "$output")"

		env --split-string "$cmd" -- "$input_rel" "$output_rel" \
			2>&1 || pquit "'$cmd -- $input_rel $output_rel' failed!"

		exiftool -overwrite_original_in_place -all= -- "$output" 2>&1 ||:
		;;
	*) pquit "Unknown command: $cmd" ;;
esac >/dev/null

## If the output doesn't exist, don't continue
[ -f "$output" ] || exit 0

## Also, don't continue if '$oxipng_base' is unset
[ -n "${oxipng_base:-}" ] || exit 0


discard() {
	trial="$(realpath -- "$1")"
	baseline="$(realpath -- "$2")"

	[ -s "$trial" ] || return 0
	[ -s "$baseline" ] ||
		perr "'$baseline' does not exist, or has no content, but should!"

	if cmp "$1" "$2" >/dev/null; then
			## This handles otherwise truncated trials which are
			## ordered before '$baseline' in the final list
			ln --force --symbolic "$baseline" "$trial" \
				2>/dev/null ||
					perr "Couldn't replace '$trial' with a symlink to '$baseline'!"
		else
			trial_size="$(wc --bytes <"$trial")"
			[ "$smallest_known" -gt "$trial_size" ] ||
				fallocate --punch-hole --length="$trial_size" \
					-- "$trial"
	fi
}


## If '$quant_src' was the input, the output should be compared to that instead

[ "$output" = "${output#quant > }" ] || oxipng_base="$input"

discard "$output" "$oxipng_base"

	SCRIPT
}

optimise() {
	if [ "$mode" = "PNG" ]; then
			# Create the PNG-mode baselines, and decide on whether
			# further variants need to be created
			create_png_bases

			## If requested (`-a 0`), skip the OxiPNG variants
			[ "$ALL_OXI" != "0" ] || return 0

			## Skip the OxiPNG variants if:
			## 1. It's not been explicitly requested to try them.
			## 2. And, if the baseline OxiPNG size is greater than
			##    256 bytes.
			## 3. And, if the input image is greater in size than
			##    the smallest known encoding.
			if [ "$ALL_OXI" != "1" ] &&
			   [ "$oxipng_base_size" -gt 256 ] &&
			   [ "$src_size" -gt "$smallest_heuristic" ]; then
				return 0
			fi


			# Calculate '$total_lines' for use in progress tracking.
			## Levels index from 2, so subtract 1
			levels="$((
				(src_height > MAX_BLIN ? MAX_BLIN : src_height)
				- 1
			))"

			## Also, ensure '$levels' isn't less than 1
			levels="$(( levels > 0 ? levels : 1 ))"

			zcs="12"
			oxis="16"
			lines="12"

			## The `+ 1`'s account for the Zopfli variants adding an
			## extra '$oxis' for brute and non-brute variants each
			total_lines="$((
				(zcs + 1) * (lines * levels + 1) * oxis
			))"

			[ -z "${quant_src:-}" ] ||
				total_lines="$(( total_lines * 2 ))"
		else
			smallest_known="$src_size"

			## At minimum, the 3 `gifsicle` trials are attempted
			total_lines="3"

			[ -n "$NO_WEBP" ] || total_lines="$(( total_lines + 1 ))"
			[ -n "$NO_APNG" ] || total_lines="$(( total_lines + 6 ))"
	fi


	# Remount '$work' with a high enough `nr_inodes` limit
	## '16' is a random, safe number to ensure non-`xargs` files can exist
	mount --options remount,nosuid,nodev,noexec,size=50%,nr_inodes="$(( total_lines + 16 ))" \
		"$work"/ || pquit "Failed to remount '$work/'!"


	# Create the script `xargs` will execute
	tmp_oxi="$work/oxi"
	gen_script >"$tmp_oxi"


	# Parse & execute each line in the command list
	## `9>&2` redirects the script's STDERR to FD 2, while redirecting the
	## STDERR of `xargs` to `/dev/null`. `exit 255` causes the STDERR logs.
	### `cat --number` indexes the lines for progress tracking
	gen_list | cat --number | xargs \
		--max-procs "$MAX_PROCS" \
		--delimiter "\n" \
		--replace="%" \
		-- sh "$tmp_oxi" \
			"$smallest_known" \
			"$total_lines" \
			"${oxipng_base:-}" \
			"%" \
		9>&2 2>/dev/null ||
			exit_clear

	clear_prog

	## The script isn't useful anymore, and `find` will see it, so remove it
	rm "$tmp_oxi" || pquit "Failed to remove '$tmp_oxi'!"




	# Optimise the `gif2apng` outputs with this script
	## ...unless told otherwise
	[ -z "$NO_APNG" ] || return 0

	## Allow globbing for just these for-loops
	set +f

	## De-duplicate the APNGs beforehand
	for dup in /tmp/gif2apng*; do
		[ -f "$dup" ] || continue
		[ ! -L "$dup" ] || continue

		for dup2 in /tmp/gif2apng*; do
			[ "$dup" != "$dup2" ] || continue

			! cmp "$dup" "$dup2" >/dev/null 2>&1 ||
				ln --force --symbolic "$dup" "$dup2" 2>/dev/null ||
					perr "Failed to symlink '$dup2' to '$dup'!"
		done
	done

	for apngsrc in /tmp/gif2apng*; do
		## Don't optimise duplicate SRC images; the output is identical
		if [ -L "$apngsrc" ]; then
			printf "'%s' and '%s' are identical. Skipping." \
				"$apngsrc" "$(readlink -- "$apngsrc")" |
					pstat "APNG ⇦ "

			continue
		fi

		[ -f "$apngsrc" ] || continue

		apngdest="${apngsrc%.*}.oxipng.apng"


		## Each `sisyphus` instance has its own '$work' directory;
		## therefore, pass the input as a file descriptor
		exec 4<"$apngsrc"
		apngsrc_real="$apngsrc"
		apngsrc="/proc/self/fd/4"


		pstat "APNG ← " "Optimising '$apngsrc_real'…"

		## `--stdout` implies `--quiet`
		"$0" --_internal --max-procs "$MAX_PROCS" --no-webp \
			--all-oxi "${ALL_OXI:-"1"}" --stdout -- "$apngsrc" \
			>"$apngdest" ||:


		## If '$apngdest' is still empty, substitute it with '$apngsrc'
		if [ -f "$apngdest" ] && [ ! -s "$apngdest" ]; then
			ln --force --symbolic "$apngsrc_real" "$apngdest" \
				2>/dev/null ||
					perr "Failed to symlink '$apngdest' to '$apngsrc_real'!"
		fi
	done

	set -f
}

optimise




# Output listing & selection
tmp_smallest="$work/smallest"

## 0. Gather all the results, while ignoring any possible empty files that can
##    be created by failed APNG optimisations.
## 1. Prefix lines with their length followed by a space, e.g. '20 8 ...'.
## 2. Sort the lines by their size (2n) and then by their length (1V).
## 3. Remove the first, length field (2-), e.g. '8 ...'.
find "$work"/ -maxdepth 1 -follow -type f -size +0c -printf "%s\t%f\n" |
	awk '{ printf "%d\t%s\n", length, $0 }' |
	sort --field-separator="	" --key="2n" --key="1V" | {
# | Start of the inside of this pipe |

## Read the first line from STDIN, which will be for the smallest result, while
## preserving the lines following it
IFS="	" read -r _ smallest_size smallest
smallest_read="$smallest_size	$smallest"

if [ "$RESULTS" != "0" ]; then
	pstat "Size order ↯ " ""

	## Make sure `cut` & `sed` only filter the results by using `{}`
	{
		## Print the smallest result
		### `0\t` is a placeholder for the length
		printf "0\t%s\n" "$smallest_read"

		## Print the remaining results
		if [ "$RESULTS" = "ALL" ]; then
				cat
			else
				## The smallest result was already printed, so
				## print one fewer lines than specified
				head --lines="$(( RESULTS - 1 ))"
		fi
	} |
		cut --delimiter="	" --fields=2- |
		sed "s/\t/ bytes\t/g ; s/^/\t/g" >&2

	## This will only be printed if '--quiet' is omitted; it redirects FD 1
	printf "\n"
fi

## Variables set inside pipelines are isolated from the script, so write the
## smallest result to a file.
### Do the write at the end in case `find` tries to include '$tmp_smallest'.
### After the RESULTS-ALL if-statement, it wouldn't matter if it were found
printf "%s\n" "$smallest_read" >"$tmp_smallest"

# | End of the inside of this pipe |
}


## Read '$tmp_smallest' for the best known result
IFS="	" read -r smallest_size smallest <"$tmp_smallest" ||
		pquit "Failed to read the smallest result from '$tmp_smallest'!"

smallest="$work/$smallest"
[ -f "$smallest" ] ||
	pquit "'$smallest' is the best result, but it doesn't exist or isn't a file!"


## Check for an improvement
[ "$smallest_size" -le "${SIZE:-"$smallest_size"}" ] ||
	printf "'%s' is equal to, or larger than, the minimum size target: %d bytes." \
		"$smallest" "$SIZE" | pquit

if [ "$smallest_size" -eq "$src_size" ]; then
	pstat "\033[32mAlready optimal ✓ " "The input is as small as the best result!"

	## '--quiet' redefines `osc777()` to be `true`; this is already handled
	osc777 "'$src_real' is already optimal!"

	exit 0
fi

[ "$smallest_size" -le "$src_size" ] ||
	pquit "'$smallest' is larger than '$src_real'."


## Symlinks also have an apparent size of '0'; exclude them from this check
if [ "$(stat --format "%b" -- "$smallest")" -le 0 ] && [ ! -L "$smallest" ]; then
	pquit "'$smallest' was truncated at some point; it cannot be used!"; fi


## Output the best result (unless '--dry-run' was specified)
if [ -n "$STDOUT" ]; then
		## '--stdout' implies '--quiet'; therefore:
		### FD 8 prints to STDOUT
		[ -n "$DRY" ] || cat "$smallest" >&8

		### And, the proceeding steps are irrelevant
		exit 0
fi

final_dest="$dest_path.${smallest##*.}"
if [ -e "$final_dest" ] && [ -z "$FORCE" ]; then
	pquit "'$final_dest' already exists!"; fi

[ -n "$DRY" ] || cp -- "$smallest" "$final_dest"


## Print the final statistics.
### "> ": as in the "quant > ..." prefix of the filename
method="${smallest##*"> "}"
method="${smallest##*"/"}"
method="${method%.*}"

printf "%s  \033[2m(%s)" "$final_dest" "$method" | pstat "Final output → "
printf "%d bytes \033[2m↘\033[0;39m %d bytes" "$src_size" "$smallest_size" |
	pstat "Size diff ↘ "

osc777 "'$final_dest' is finished! Size diff: $src_size bytes -> $smallest_size bytes ($method)"

Sometimes running FO multiple times on the same file (Shift+F5,▶️) back-to-back gives better results, too, so that's kind-of expected.