file initrd.img-4.18.0-16-generic

$ file initrd.img-4.18.0-16-generic
initrd.img-4.18.0-16-generic: ASCII cpio archive (SVR4 with no CRC)

(cpio -ivd && gunzip | cpio -ivd) <initrd.img-4.18.0-16-generic

$ (cpio -ivd && gunzip | cpio -ivd) < initrd.img-4.18.0-16-generic 
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
56 блоков

gzip: stdin: not in gzip format
cpio: преждевременный конец архива

(cpio -ivd && cat >/tmp/second.img) <initrd.img-4.18.0-16-generic
file /tmp/second.img

$ (cpio -ivd && cat >/tmp/second.img) <initrd.img-4.18.0-16-generic
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
56 блоков
$ file /tmp/second.img
/tmp/second.img: ASCII cpio archive (SVR4 with no CRC)
$

(cpio -ivd && cpio -ivd && cat >/tmp/third.img) <initrd.img-4.18.0-16-generic

file /tmp/third.img
/tmp/third.img: gzip compressed data, last modified: Thu Mar 21 21:17:46 2019, from Unix

(cpio -ivd && cpio -ivd && gunzip | cpio -ivd) < initrd.img-4.18.0-16-generic

gzip -dc initrd.img-3.2.0-4-686-pae | cpio -iv

$ gzip -dc initrd.img-4.18.0-16-generic | cpio -iv

gzip: initrd.img-4.18.0-16-generic: not in gzip format
cpio: преждевременный конец архива
$

echo "TEST" > test.txt
gzip test.txt
cat test.txt.gz initrd.img-3.2.0-4-686-pae > initrd.img-3.2.0-4-686-pae.gz
gzip -dc initrd.img-3.2.0-4-686-pae.gz | cpio -iv

#!/bin/sh

unpack()
{
	local comp_alg decomp_cmd

	comp_alg=$( file --brief --mime-type "${1}" )
	case "${comp_alg}" in
		"application/x-gzip")
			decomp_cmd="gunzip"
		;;

		"application/octet-stream")
			decomp_cmd=""
		;;

		*)
			echo "Unknown compression: ${comp_alg}" >&2
			exit 1
		;;
	esac

	(
		rm --force "${TMPNEXT}"
		set -xeu

		if [ -n "${decomp_cmd}" ]; then
			$decomp_cmd | cpio -ivd
		else
			cpio -ivd
		fi

		cat >"${TMPNEXT}"
	) <"${1}"
}

set -xeu

TMPDIR=""
trap '[ -n "${TMPDIR}" ] && rm --force --recursive "${TMPDIR}"' EXIT
TMPDIR=$( mktemp --directory --tmpdir="/tmp" --suffix=".unpackinitcpio" )
TMPNEXT="${TMPDIR}/next"

next="${1}"

while [ -s "${next}" ]; do
	unpack "${next}"
	next="${TMPNEXT}"
done

$ ./_uinitrd initrd.img-5.0.2-pc
+ TMPDIR=
+ trap [ -n "${TMPDIR}" ] && rm --force --recursive "${TMPDIR}" EXIT
+ mktemp --directory --tmpdir=/tmp --suffix=.unpackinitcpio
+ TMPDIR=/tmp/tmp.UJKYFGUx5y.unpackinitcpio
+ TMPNEXT=/tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ next=initrd.img-5.0.2-pc
+ [ -s initrd.img-5.0.2-pc ]
+ unpack initrd.img-5.0.2-pc
+ local comp_alg decomp_cmd
+ file --brief --mime-type initrd.img-5.0.2-pc
+ comp_alg=application/octet-stream
+ decomp_cmd=
+ rm --force /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ set -xeu
+ [ -n  ]
+ cpio -ivd
.
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/AuthenticAMD.bin
56 блоков
+ cat
+ next=/tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ [ -s /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next ]
+ unpack /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ local comp_alg decomp_cmd
+ file --brief --mime-type /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ comp_alg=application/octet-stream
+ decomp_cmd=
+ rm --force /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ set -xeu
+ [ -n  ]
+ cpio -ivd
kernel
kernel/x86
kernel/x86/microcode
kernel/x86/microcode/.enuineIntel.align.0123456789abc
kernel/x86/microcode/GenuineIntel.bin
3080 блоков
+ cat
+ next=/tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ [ -s /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next ]
+ unpack /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ local comp_alg decomp_cmd
+ file --brief --mime-type /tmp/tmp.UJKYFGUx5y.unpackinitcpio/next
+ comp_alg=application/gzip
+ echo Unknown compression: application/gzip
Unknown compression: application/gzip
+ exit 1
+ [ -n /tmp/tmp.UJKYFGUx5y.unpackinitcpio ]
+ rm --force --recursive /tmp/tmp.UJKYFGUx5y.unpackinitcpio
$ 

diff --git a/unpackinitcpio b/unpackinitcpio
index c575bff137e9..f0c1c3b8a051 100755
--- a/unpackinitcpio
+++ b/unpackinitcpio
@@ -6,7 +6,7 @@ unpack()
 
 	comp_alg=$( file --brief --mime-type "${1}" )
 	case "${comp_alg}" in
-		"application/x-gzip")
+		"application/x-gzip" | "application/gzip")
 			decomp_cmd="gunzip"
 		;;
 

$ cat `which unmkinitramfs`
#!/bin/sh

set -eu

usage()
{
	echo "Usage: $(basename "$0") [-v] <initramfs file> <directory>"
}

# Extract a compressed cpio archive
xcpio()
{
	archive="$1"
	dir="$2"
	shift 2

	if zcat -t "$archive" >/dev/null 2>&1 ; then
		zcat "$archive"
	elif xzcat -t "$archive" >/dev/null 2>&1 ; then
		xzcat "$archive"
	elif bzip2 -t "$archive" >/dev/null 2>&1 ; then
		bzip2 -c -d "$archive"
	elif lzop -t "$archive" >/dev/null 2>&1 ; then
		lzop -c -d "$archive"
	# Ignoring other data, which may be garbage at the end of the file
	fi | (
		if [ -n "$dir" ]; then
			mkdir -p -- "$dir"
			cd -- "$dir"
		fi
		cpio "$@"
	)
}

# Read bytes out of a file, checking that they are valid hex digits
readhex()
{
	dd < "$1" bs=1 skip="$2" count="$3" 2> /dev/null | \
		LANG=C grep -E "^[0-9A-Fa-f]{$3}\$"
}

# Check for a zero byte in a file
checkzero()
{
	dd < "$1" bs=1 skip="$2" count=1 2> /dev/null | \
		LANG=C grep -q -z '^$'
}

# Split an initramfs into archives and call xcpio on each
splitinitramfs()
{
	initramfs="$1"
	dir="$2"
	shift 2

	# There may be a prepended uncompressed archive.  cpio
	# won't tell us the true size of this so we have to
	# parse the headers and padding ourselves.  This is
	# very roughly based on linux/lib/earlycpio.c
	offset=0
	while true; do
		if checkzero "$initramfs" $offset; then
			offset=$((offset + 4))
			continue
		fi
		magic="$(readhex "$initramfs" $offset 6)" || break
		test $magic = 070701 || test $magic = 070702 || break
		namesize=0x$(readhex "$initramfs" $((offset + 94)) 8)
		filesize=0x$(readhex "$initramfs" $((offset + 54)) 8)
		offset=$(((offset + 110)))
		offset=$(((offset + $namesize + 3) & ~3))
		offset=$(((offset + $filesize + 3) & ~3))
	done

	if [ $offset -ne 0 ]; then
		# uncompressed archive
		(
			if [ -n "$dir" ]; then
				mkdir -p -- "$dir/early"
				cd -- "$dir/early"
			fi
			cpio -i "$@"
		) < "$initramfs"

		# main archive
		subarchive=$(mktemp ${TMPDIR:-/var/tmp}/unmkinitramfs_XXXXXX)
		trap "rm -f '$subarchive'" EXIT
		dd < "$initramfs" bs="$offset" skip=1 2> /dev/null \
			> $subarchive
		xcpio "$subarchive" "${dir:+$dir/main}" -i "$@"
	else
		xcpio "$initramfs" "$dir" -i "$@"
	fi
}

OPTIONS=`getopt -o hv --long help,list,verbose -n "$0" -- "$@"`
# Check for non-GNU getopt
if [ $? != 0 ] ; then echo "W: non-GNU getopt" >&2 ; exit 1 ; fi

cpio_opts="--preserve-modification-time --no-absolute-filenames --quiet"
expected_args=2
eval set -- "$OPTIONS"

while true; do
	case "$1" in
        -h|--help)
		usage
		exit 0
	;;
	--list)
		# For lsinitramfs
		cpio_opts="${cpio_opts:+${cpio_opts} --list}"
		expected_args=1
		shift
	;;
	-v|--verbose)
		cpio_opts="${cpio_opts:+${cpio_opts} --verbose}"
		shift
	;;
	--)
		shift
		break
	;;
	*)
		echo "Internal error!" >&2
		exit 1
	esac
done

if [ $# -ne $expected_args ]; then
	usage
	exit 2
fi

splitinitramfs "$1" "${2:-}" $cpio_opts
$

$ time unpackinitcpio /boot/initrd.img-4.18.0-16-generic
...
real	0m0,746s
user	0m0,705s
sys	0m0,233s

$ time unmkinitramfs /boot/initrd.img-4.18.0-16-generic .
...
real	0m1,895s
user	0m1,622s
sys	0m0,262s