Recently I setup a raspberry pi as my home wireless router. It worked great, until I accidently broke the sd-card. I had a spare SD-Card but it was only a 1GB memory. Considering the 1.8GB raspbian image (after decompressing), something had to be done to shrink the image size to fit into a 1GB SD-Card. Here's the How To but I should warn, the following instructions to be executed very carefully or you might wipe the wrong storage and lose the precious data.
Note: Most of the following commands are to be executed with root privileges. Please ensure you understand them before trying to execute them.
First I needed to create an image, with the right size to fit exactly into my
1GB SC-Card. To check the size of the media, simply execute fdisk -l. My SD-Card
has around 940MB capacity, So the following command creates an image file
with the exact size:
truncate -s 940M sd-card.img
The instructions to create partition table on the image, should be done
on a block device.
The next step is to create block devices from the raspberry image and my
own image. Here I choose /dev/loop1 for the rasbian image file and /dev/loop2
for the custom image file.
losetup /dev/loop1 2020-05-27-raspios-buster-lite-armhf.img
losetup /dev/loop2 sd-card.img
So, from now on, the /dev/loop1 device, represents the raspbian image and the
/dev/loop2 device should represents my custom image device.
Then, I needed to properly copy the boot instructions from the Raspbian into the image. The boot instructions are located after the partition table at sector zero and ends before the start position of the first partition.
fdisk -l /dev/loop1
<sniped>
Units: sectors of 1 * 512 = 512 bytes
<sniped>
Device Boot Start End Sectors Size Id Type
/dev/loop1p1 8192 532479 524288 256M c W95 FAT32 (LBA)
/dev/loop1p2 532480 3620863 3088384 1.5G 83 Linux
From the fdisk output, it's clear that I need to copy the sectors from 0 to
8191, just before the start of the first partition.
sudo dd if=/dev/loop1 of=/dev/loop2 bs=512 count=8192
Then, I needed to recreate the boot and root partitions according to the
size of the media. So, I reserved 55MB for the boot partition and the reset for
the root partition. The following script, did just that:
sudo fdisk /dev/loop2 <<EOF
d
1
d
2
n
p
1
8192
+55M
t
c
n
p
2
120832
p
w
EOF
After creating the new paritions with the right size, I needed to force the kernel to re-read the new partition table containing the new partition sizes and then reformat.
partprobe /dev/loop2
sudo mkfs.fat /dev/loop2p1
sudo mkfs.ext4 /dev/loop2p2
Now, all left to do is to copy all the files to the new image excluding the ones that's not actually useful, just to spare some space on SD-Card. First the boot partition:
mkdir -p /tmp/raspbian-boot
mkdir -p /tmp/sdcard-boot
mount /dev/loop1p1 /tmp/raspbian-boot
mount /dev/loop2p1 /tmp/sdcard-boot
cp -a /tmp/raspbian-boot/* /tmp/sdcard-boot
# Enable ssh automatically on boot
touch /tmp/sdcard-boot/ssh
umount /dev/loop1p1
umount /dev/loop2p1
rmdir /tmp/raspbian-boot
rmdir /tmp/sdcard-boot
Then, to Copy the root filesystem contents, I decided copy all the files except the files that are not necessary, like man-pages, linux kernel modules for different kernel versions and so one. The following commands copies exactly the needed files to boot on regular Raspberry pi 3:
mkdir -p /tmp/raspbian-root
mkdir -p /tmp/sdcard-root
mount /dev/loop1p2 /tmp/raspbian-root
mount /dev/loop2p2 /tmp/sdcard-root
mkdir -p /tmp/sdcard-root/lib/modules
mkdir -p /tmp/sdcard-root/usr/{games,src,local,include,share}
# Copying necessary files
cp -a /tmp/raspbian-root/{boot,media,mnt,proc,srv,sys,tmp,dev,root,run,home,etc,sbin,bin,var} /tmp/sdcard-root
cp -a /tmp/raspbian-root/lib/{arm-linux-gnueabihf,cpp,dhcpcd,ifupdown,klibc-fAGGTaZfOmYXUytsXgfSuL5MT48.so,ld-linux.so.3,modprobe.d,resolvconf,terminfo,console-setup,crda,firmware,init,ld-linux-armhf.so.3,lsb,systemd,udev} /tmp/sdcard-root/lib/
cp -a /tmp/raspbian-root/lib/modules/4.19.118-v7+ /tmp/sdcard-root/lib/modules/
cp -a /tmp/raspbian-root/usr/{sbin,bin,lib} /tmp/sdcard-root/usr/
cp -a /tmp/raspbian-root/usr/lib /tmp/sdcard-root/usr/
sudo cp -a /tmp/raspbian-root/usr/share/{GeoIP,X11,aclocal,adduser,alsa,applications,apport,apps,apt-listchanges,avahi,base-files,base-passwd,bash-completion,binfmts,bug,build-essential,ca-certificates,calendar,cmake,common-licenses,console-setup,consolefonts,consoletrans,dbus-1,debconf,debhelper,debianutils,dhcpcd,dict,distro-info,doc-base,dpkg,file,gcc-8,gdb,gettext,glib-2.0,gnupg,groff,hal,i18n,icons,info,initramfs-tools,iptables,iso-codes,java,keyrings,keyutils,libc-bin,lintian,luajit-2.1.0-beta3,man,man-db,menu,metainfo,mime,misc,nano,nfs-common,openssh,pam,pam-configs,perl,perl5,pixmaps,pkg-config-crosswrapper,pkg-config-dpkghook,pkgconfig,polkit-1,publicsuffix,pyshared,python,python-apt,python3,raspi-config,readline,sensible-utils,sounds,systemd,tabset,tasksel,terminfo,usb_modeswitch,vim,vl805fw,xml,zoneinfo,zsh} /tmp/sdcard-root/usr/share/
# Removing unnecessary files to make enough free space available
rm /tmp/sdcard-root/var/cache/apt/srcpkgcache.bin
rm -rf /tmp/sdcard-root/usr/share/{GeoIP,i18n,man,bash-completion,sounds,misc}
umount /dev/loop1p2
umount /dev/loop2p2
rmdir /tmp/raspbian-root
rmdir /tmp/sdcard-root
That's it. Hopefully after boot, there's enough space for upgrade and installing missing softwares. If it's not, try removing other unnecessary files. Lucky for me, the home wireless router worked (almost) out of the box and very little installation was needed.