Gfilui

gksudo for Baobab Disk Usage Analyzer doesn't show /boot. I recently removed several old kernels. How can I verify that I've freed up space in /boot?

Backstory —
I started spring cleaning because Ubuntu wasn't booting properly without workarounds. I diagnosed the error when my computer warned me that /boot was almost full. The warning opened Disk Usage Analyzer without root permission so /boot wasn't scanned. Now the analyzer can't seem to find the directory at all.

What I Did —
After updating to most recent kernel, I removed old kernels one at a time with:

sudo apt purge linux-image-[VERSION].

When I was done, I updated grub2.

Each time the purge command said it would free up 0 Bytes of space. They are gone tho according to:

dpkg --list | grep linux-image

df -h shows you space on all partitions, not just /home, and to see all kernels, do dpkg -l linux-image*

You can remove multiple old kernels at once with a command like
sudo apt-get remove linux-image-2.6.32-21,37,38,39,40,41,42,43,44-server (substitute your numbers based on what you find with dpkg) but be very careful not to remove the current kernel, nor the latest kernel!

uname -r shows you what you are currently using.

Note:
sudo apt-get autoremove is good to run after removing old kernels as it removes obsolete dependencies.

If /boot is a separate file system and if you run baobab with sudo, it will not list /boot as part of the full disk analysis. Instead you have to select the file system you want to analyse at the startup screen of baobab.

Alternatively you can run baobab like this: sudo baobab /boot to immediately open the disk usage for the /boot folder.

However, if you are on the command line already there are more ways (see other answer) to query the system disk usage.

Easiest way is to use df command:

$ df -h
Filesystem Size Used Avail Use% Mounted on
udev 3.7G 0 3.7G 0% /dev
tmpfs 759M 1.9M 757M 1% /run
/dev/nvme0n1p7 44G 19G 24G 44% /
tmpfs 3.8G 52M 3.7G 2% /dev/shm
tmpfs 5.0M 4.0K 5.0M 1% /run/lock
tmpfs 3.8G 0 3.8G 0% /sys/fs/cgroup
/dev/nvme0n1p8 9.1G 49M 9.0G 1% /mnt/e
/dev/nvme0n1p2 95M 28M 68M 29% /boot/efi
/dev/nvme0n1p4 391G 130G 262G 34% /mnt/c
/dev/sda3 920G 42G 878G 5% /mnt/d
tmpfs 759M 64K 759M 1% /run/user/1000
/dev/sdb5 6.3G 101M 5.9G 2% /media/rick/casper-rw
/dev/sdb1 6.4G 43M 6.4G 1% /media/rick/usbdata
/dev/sdb4 1.4G 1.4G 0 100% /media/rick/Ubuntu 18.04 LTS amd64

On my system /boot is embedded in root (/) directory. So I can see it's on partition /dev/nvme0n1p7 which is 44% full (24G available). This is enough to store 48 new kernels without purging any of the existing ones.

If you have a separate /boot partition you will see it on the df listing.

To see how much disk space, not only in /boot but also in /src/lib and /lib/modules each kernel consumes you can use rm-kernels to display storage used without removing anything:

In this case there are 6.9 GB used in kernels not only in /boot but across rest of /. The display shows a separate column for how much is used in /boot directory.

You can also run the du (Disk Usage) command:

$ du /boot -h -s
1.2G /boot