create any filesystem in disk image as a non-root user

09 Mar 2022

[update 2022-03-11]

just found out that ‘virt-make-fs’ has already implemented same function.
so please use ‘virt-make-fs’ instead for most cases:

$ virt-make-fs -s $size -t $fstype $dir_or_tar $image --partition --label=Label

why

Sometimes I have to create some virtual disk image and create filesystems in it(e.g:
Windows answerfile image) and attach it to my test libvirt/KVM Guest, but when I do
creating filesystem I must do losetup and mkfs with root privileges(sudo). and I don’t
always get root permissions,, sh*t

how

by googling I got some ideas, for example mke2fs, but it only could be used to create
ext* filesystem. so seems I have to figure it out myself..

luckily I got one idea: split disk head and partition after creating partition done,
and got two files(assume only created one partition): img-head, img-part; then do:

mkfs.$fstype img-part
cat img-head img-part >disk.img

and the exciting thing is that it actually works, here’s the full script:
(now it only create one partition and satisfy me)

create_vdiskn() {
	local path=$1
	local dsize=$2
	local fstype=$3
	local imghead=img-head-$$
	local imgtail=img-tail-$$
	local fn=${FUNCNAME[0]}

	echo -e "\n[$fn:info] creating disk and partition"
	dd if=/dev/null of=$path bs=1${dsize//[0-9]/} seek=${dsize//[^0-9]/}
	printf "o\nn\np\n1\n\n\nw\n" | fdisk "$path"
	partprobe "$path"

	read pstart psize < <( LANG=C parted -s $path unit B print | sed 's/B//g' |
		awk -v P=1 '/^Number/{start=1;next}; start {if ($1==P) {print $2, $4}}' )
	echo -e "\n[$fn:info] split disk head and partition($pstart:$psize)"
	dd if=$path of=$imghead bs=${pstart} count=1
	truncate --size=${psize} $imgtail

	echo -e "\n[$fn:info] making fs($fstype)"
	mkfs.$fstype $MKFS_OPT "$imgtail"

	echo -e "\n[$fn:info] concat image-head and partition"
	cat $imghead $imgtail >$path
	rm -vf $imghead $imgtail
}

[[ $# -lt 3 ]] && {
	cat <<-COMM
	Usage: [MKFS_OPT=xxx] $0 <image> <size> <fstype>

	Examples:
	  $0 usb.img 256M vfat
	  $0 ext4.img 4G ext4
	  MKFS_OPT="-f -i attr=2,size=512" $0 xfs.img 4G xfs
	COMM
	exit 1
}
create_vdiskn "$@"

[update 2022-03-11]

just found out that ‘virt-make-fs’ has already implemented same function.
so please use ‘virt-make-fs’ instead for most cases:

$ virt-make-fs -s $size -t $fstype $dir_or_tar $image --partition --label=Label

tcler's blog --- 其实我是一个程序员
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