automate filesystem expansion - booting from a USB Drive (8GB Image burned to 32GB USB)

Question

I have an 8GB image that is burned to a 32GB USB drive.

How can I automate this filesystem expansion process? Is there a script I can create to expand the USB correctly and fully? How can I find the exact partition size needed? I noticed when I manually expand the filesystem it only increases the size in /dev/sda2 not /dev/sda1.

1) I have seen a suggestion to use

parted /dev/sda resizepart 1 102400 # resize /dev/sda1 to 100GB
resize2fs /dev/sda1 # resize filesystem

In this method, I am unsure how to calculate the value needed to ensure full expansion. I have used fdisk -l but unsure how to use the data here for the command above.

2) I have looked at the /usr/bin/raspi-config file and don't fully understand what I need to extract from the --expand-rootfs section of the file for my needs.

--- RESULTS FROM fdisk -l ---

8GB Image manually expanded on 32GB USB

Device         Boot Start      End  Sectors  Size Id Type
/dev/mmcblk0p1       2048 62332927 62330880 29.7G  c W95 FAT32 (LBA)

Disk /dev/sda: 28.7 GiB, 30752000000 bytes, 60062500 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x8f1eafaf

Device     Boot  Start      End  Sectors  Size Id Type
/dev/sda1         8192   131071   122880   60M  c W95 FAT32 (LBA)
/dev/sda2       131072 60062499 59931428 28.6G 83 Linux

8GB Image awaiting expansion on 32GB USB

Device         Boot Start      End  Sectors  Size Id Type
/dev/mmcblk0p1       8192 62333951 62325760 29.7G  c W95 FAT32 (LBA)

Disk /dev/sda: 28.7 GiB, 30752636928 bytes, 60063744 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0xb9da48fc

Device     Boot  Start      End  Sectors  Size Id Type
/dev/sda1         8192   131071   122880   60M  c W95 FAT32 (LBA)
/dev/sda2       131072 15523839 15392768  7.3G 83 Linux

Answer 1

On the very first boot of a virgin Raspbian image there is running a script that you can find in /boot/cmdline.txt before booting first time. This is from Rasbian Buster Lite 2020-02-13:

console=serial0,115200 console=tty1 root=PARTUUID=738a4d67-02 rootfstype=ext4 elevator=deadline fsck.repair=yes rootwait quiet init=/usr/lib/raspi-config/init_resize.sh

As you see there is /usr/lib/raspi-config/init_resize.sh called but after the first boot this entry in /boot/cmdline.txt is removed so you will not find it on following bootups. If your image does not resize the partition on first boot then either it isn't a Raspbian image or it has already booted before.

You may have a look at this script how resizing of a partition works. Maybe you can just copy it?